The Seakeeping Characteristics ai a Small Waterplane Area, Twin-Hull (SWATH) Ship Jairies A. Foin, Miircjarel D. Oohi and Kaih iyn K. tv'icCreighl

David W. Taylor Naval Ship llesea'ch c,nd Development Conler

Boihesda. Maryland, U.S.A.

ABSTRACT

Tl i is paper addresses ti l t: rnot io i in a Smal l WaierplcTiti A r e o , T w i n - r i u l ! (SWATH) shiu in a scov/ay, The ci)rii:.r!t s t a t u i of fu l l scriie t r i a l s , mo ' ie l expe r ience and p r o J ' c t i o n c -pc . l ; i i i t y is preser i isd ond the (v/ t l rou) ivTr i ic f ac to rs c o n t r i h u f i n q to Un- Qcne ia ' i y low i i i r / i io r i i af SV/ATH ships Gie i d e n f i f i o d , . C o r r e l a t i o n bei'.veen I rons fe r func t ions f r o m m o d i f i o d s t r i p theory pcedic-i ion.s. ur,d I M : I .scale r(;sa!t,s fo r SSP l<Ai.V..ALINO, a •SWATH ship, is g iven c ionq w i t i i response spec t ra f r n m fuli scale i t i a i s inc lud iny ; l ie o f f e c r of auto iy io t ic n^ot ion c o n i r o i . Compar isons of niodid and fu l l scale mo t i ons in Se;.i S t o i r s '( ond 5 are shown. P red i c t i ons o f na tu ra l periods a ie g iven as o f u n c t i o n of speed and are compared to t r i a l resu l ts . T i ie design phi losopf iy fo r SV/ATH na tu ra l periods is dascr i i jed and f i t od i c t i on ot severest mo t i ons to be ec ted in an encourdercd .seav/ey is made by a 3loi i . , , !cc!l c s t i i r i o t i nn leehn 'oue. The overa l l good niot ions o;vd ho.b i tob i l i ty c p o r c c t e r i s t i c s to be expec ted o f br- SW.'XTr! t ype of ship are sb.ov.Ti tor a range of speeds end headings l o tb.e sea.

. '-•-i^fliiODUadON

Ttie analysis of rhe seakeeping pe; f o r r r o n c e o f any ï i . ip design depends on good rheo re r i ca l p red i c t i ons va l ida ted tiy rnodei expe r imen ts and fu l l scole i r i c i s . For

n^'w t ype of ship tbie depenciei'ce on e x p e r i m e n t a l da ta is vnr, | lo bu i id confici'-^nee in l i ie o n o i y i i c a i tools and to if ' j in !ns:gi;t i n t o t i i c t i ydrcdynarn ic ph'.H'iomena un iq i ie t o l ' ia t sf-iip t>'pe. In rliis paper a fu l i scale t r i a l o f a p i ' c io l ype is used t o .ricin con f i t i cnce in on c n a i y t i c a l i;".erhod i h o t is the.n cp i i i i ed to tb.e design p rob lems of exi f e m e vol us p r e d i c t i o n ond nmui-oi p.eriods e s t i n i o ï i o n .

Th is paper has t w o p r i m a r y ob jec t i ve . : . One is io pre.se.nt the SMwor ih i ness 1r:al da ta on SSP KA i iV iA t Jb lO • f l ic ship hyd ;odynomics c o m m u n i t y v / i t l i on ' fV 'e rpre ta i ion of the shipbs m o t i o n be'^iayior. r i i s ofbioi' is

.'t..! nnn 5 i r a t e how t r i a l ; , mode l c-sperirnents and l i ca l p red i c t i ons enhance r l ie deve lopr . i rn t o f

Slv. ;coping design cons idero i ions fo r .SVv'-\Tt 1 sh ips.

2. THt:' g.A/ATH CONCEPT

The SWATbl ship presents a uniqne chalb. 'nge to naval a r c h i t e c t s and h y d i o d y n a n i i c i s t s , s ince i ts seai<eeping c h o r c c t e r i s l ies car, i)e d i c i o l e d by the designer to a for g reo te r e x i e n t than for convenf ior ;a l i r iono i iu i ls . SVA\TH ships con o f f e r exce l l en t moiio'- 's and su.>tai;-:ed speed c a p a b i l i t y in a seaway. The ,SWATI"i concepif was de r i ved f r o m conven t i ona l ca tamarans and ocean o i l - d r i l l i n g p l a i f o n n s . i t combines tb.c speed and iarge desk area of the conven t i cna l c u f a m a t e n w i t h fhe seakindl iness end p l a t f o r m .s tab i l i ty of Hie d r i l l i n g r i g . A SV.'ATid s:-,;p consis is of i w o s i r e a m ü n e d sunmerged hul ls t h a t are t o r p e d o d i b e in sbcpe connec ted t o an a i jove w a t e r s t r u c t u r u l box by one cr i w o t h i n s t ru ts on each s ide. P rope l l e rs loca ted behind each hui l p rov ide the p ropu ls ive f o r c e . C o n t r o l sur faces -on the submerged lower l iu l ! enhance s f a b i l i t y and can be a c t i v a t e d to c o n t i o l t r i m and f u r t he r reduce mot ions vvf;en t i ie ship is unde rway .

A l yp i co l SWATH design v/ou!d have on ly 20 p e r c e n t o f the w a l e r p l o n e area of a conven t i ona i rnono l i i j l l . The reduced wa te rp l ane orea and r e d i s t r i b u t i o n o f ouoyent vo l ume in to s i ib i r ierged bul ls reduce tbie e x c i t o t i o n f o r ces o f t t ie seav/ay ond increase t l ie ne tu ro l per iods of m o l i o n of l l i e c r a f t . The r e l a t i v e l y in f iegu 'ent o c c u n e n c e of s t o r m v/aves v / i t h long per iods makes i t possibl . i for snips w i t h et iuo i lv long no tu ra i per iods to avo id synch.ronous respoiise in the most commo, i l> ' o c c u r r i n g scnv/ays, T i i i s decoup l ing of tbe ship f r o m wove e x c i t a t i e ; ) forces is the f undamen ta l idea behind tr io SVv'ATH c o n c e p i . The reduced v /a terp iane area also o l lows the SWATr i s i i ip l o be m o r e responsive t c Ihe fo rces gene ra ted by c o n t r o l su i f oces t i ian a conven t i ona l sh ip.

O fhe r p a r a m e i e r s >A/b!cIi s t r ong l y inf l i . ience seakeeping are long i tud ina l m e t a c e n t r i c l ieiglvt (CrMi_'), t ransverse r n e t a c e n r r i c heigf i r (GM-|-), sepa ia f i on of l ong i tud ina l cen te r o f buoyancy ond c e n t e r of f l o l o f i o n ( l . C B - L C r ) , s l ï u t c o n f i g u r a t i o n ond s ize, end area ond pos i t i on of ihe c o i i t ; o l sur ' facss. The i n f l uence of t l iese pa rame ie r s as r e f l e c t e d by tho na tu ra l per ioos of a SVv'A'lH sl i ip v / i l i be discussed l a te r .

The U.S. Navy 's dcvclapn-,e; i f of Ihe SVAMi-l ccncepJ began in 1369 w i l h the desion o f SSP K A I M A L i b i O by 1 .

V - 4 - 1

G. Lang of the Nnva l Ocean Systems CenTer (NOSC) in C a l i f o r n i a and the M O D C A T p r c g r a m a t the D a v i d T a y l o r Nava l Ship Research and D e v e l o p m e n t Cen te r ( D T N S R D C ) . M O D C A T v/as renair ied SWATH (Smal l Waterp lane A r e a T w i n Hu l l ) in l o d i f f e r e n t i a t e the concept f r o m ccc iven l iona l co tamorans . Over t l ic last t en years progress has been made in tlie p r e d i c t i o n o f d rag , s t a b i l i t y , m a n e u v e r i n g , m o t i o n s , and loads fo r SWATH ship designs, A summary of th is w o r k is con ta ined in Larnb and Fe in (Ref . I ) . in p a r t i c u l a r two -d imens iona l s t r i p t heo ry has been app l ied to SWATH ship mo t i ons p r e d i c t i o n by Lee and Curp f iey (Ref . 2). A c o m p u l e r p r o g r a m app ly ing th is t h e o r y has s l iown good c o r r e l a t i o n w i t h mode l tes ts for m o t i o n s , v/ave e x c i t a t i o n fo rces and osc i l l a t i on c o e f f i c i e n t s . Tho p red i c t i ons can be made fo r regular or i r regu la r v/aves. Th is techn ique is ra the r t i m e - c o n s u m i n g so s i m p l i f i e d tec lm iques fo r e s t i m a t i n g the p o t e n t i a l f l o w added mass and damp ing fo r two -d imens iona l SWATH sec t ions hove been deve loped by D a l z e l l (Re f . 3) und L e e (Ref . ' l ) . These p rog rams can be used to screen a w ide range of p o t e n t i a l designs in a shor t t i m e .

SSi^ K A I M A L I N O was the f i r s t e x p e r i m e n t a l p r o i o i y p e for SWATH ships. The goal of the U.S. Navy 's SWATH p r o g r a m is la rger p r o t o t y p e s tho t con d e m o n s t r a t e the advantages of SWATH .ships in r e l a t i o n to f-lava miss ions. T l ie M i t s u i Semi -Submerged C a t a m a r a n (SSC) Mesa-80 (Re f . 5) is a recent add i t i on to the f a m i l y o f SWATH p r o t o t y p e s . Its a t t r i b u t e s w i l l g r e a t i y advance the deve lopment o f SWATH teehno iogy ,

3. SEM 1-SUBMERGËD P L A T F O R M (SSP) K A I M A L I N O

3.1 D e s c r i p t i o n

Semi -Submerged P l a t f o r m (SSP) K A I M A L I N O is the f i r s t SWATH ship b u i l t in the U n i t e d S ta tes . Des igned by T . G. Lang ( f^ef. 6), i t was in tended as a w o r k b o a t for the fdaval Ocean Sys iems Cen te r (NOSC) Hav /a i i L a b o r a t o r y , i t has p roven va luab le bo th as a w o r k b o a t and as a p l a t f o r m for d e m o n s t r a t i n g t l i e seakeeping advantages of the aVATbl concep t . C o n s t r u c t e d in 1973 by tbe C u r t i s Bay Coast Guard S|-iipyard in B a l t i m o r e , M a r y l a n d , the ship was m o d i f i e d at D i l l i n g h a m Sh ipya rd in H a w a i i by t he a d d i t i o n o f d i sp lacemen t inc reas ing buoyancy b l i s te rs in 1975. The c u n ent d i sp lacemen t is 220 tonnes .

The p a r t i c u l a r s of SSP K / M M A L I N O are p resen ted in T a b l e I. O r i g i n a l des ign va lues and values foi- the most recen t seakeeping t r i a l s are g i v e n . The a d d i t i o n o f t he b l i s te rs is respons ib le for mos t of the changes. As ske tched in F i g u r e I , the sfi ip cons is ts of two c y l i n d r i c a l lov/er hu l ls connec ted to fhe upper box by t w o s t ru ts per h u l l . Buoyoncy b l i s te rs ore l oca ted on the inboard side o f each h u l l , e x t end ing v / i th cons tan t th i ckness f r o m a p p r o x i m a t e l y 7 to 16 m e t e r s a f t o f t he nose and t e r m i n a t i n g a t 21 m e t e r s a f t o f the nose. T a p e r e d all m o v a b l e c o n t r o l f ins (ca l led canards) o re l oca ted inboard just o f t o f the noses. A cons lan f chord f l opped h y d r o f o i l .spans t h e space b e t w e e n t h e hul ls a f t o f amidsh ips . F^udders are m o u n t e d in the p rope l l e r s l i p s t r e a m behind each h u l l . The f o r w a r d s t r u t s increase in cho rd and th ickness f r o m a p o i n t jus t beiov/ the w a t e r l i n e to the c o n n e c t i o n to the box , the oft s t r u t s also increase in th ickness as tlie box is approached . B o t h s t a r b o a r d s t ru t s are o u t f i t t e d w i t h spray ra i l s above the w a t e r l i n e t h a t help in d e f l e c t i n g sheets o f w a t e r t h a t m i g h t c l i m b the s t r u t s in waves . The above w a t e r box is f l a t b o t t o m e d e x c e p t fo r s l an ted , shaped sec t ions on the f o r w a r d end. These sec t ions tend t o cushion s lams in head s e a s .

SSP K A I M A L d N O has n a t u r a l l y good m o t i o n c h a r a c t e r i s t i c s in m a n y sea c o n d i t i o n s . A n a u t o m a t i c

T a b l e 1 Cl i a r a c t e r i s t i c s o f t h o S S P KAIA / IAL i r ' JO

Original Design

As Tostod 1976 and 1979

Length, Undeiwater, m 24.8 24.8

Length, Overall, m 26.9 26.9

Displacement, M T S W 192.2 220.0

Draft, m .t.3 4.8

Beam, IViaximum Submerged, m 15.1 15.1

KG, Height of Center ot Cïravity above Baseline, m 4.7 4.6

Diameter of Lower Hulls, m 1.98 1.E8

KB, Heiglit of CB above Baseline 1.75 1.65

G M l , m 4.42 3.84

GMj , m 2.01 1.01

Waterplane Area, 23.0 23.0

Longitudinal Distance CG Forward of Aft Perpendicular, m 12.9 13.4

Longitudinal Distance CF Forward of Aft Perpendicular, ni 13.5 13.5

Pitch Radius of Gyration, m 7.1 7.0

Roll Radius of Gyration, m 4.8 5.5

Aft Foil, Projected Area, m^ 25.1 25.1

Fnn/i/ard Canards, Total Area, m^ 7.1 7.1

F ig . 1 S k e t c h o f S S P KAil\;LAL!rslO S h o v / i n g

i n s t r u m e n t L o c a t i o n s

m o t i o n c o n t r o l sys tem designed to reduce mot ions in all sea cond i t i ons was designed and documen ted by Higdon (Ref . 7). The sys tem uses t l ie f o r w a r d canards end the s te rn fo i l f laps to m i n i m i z e p i t ch and ro l l mo t i ons , r ieave is not r n i n i i n i zed but i t can be c o n t r o l l e d so as to m i n i m i z e r e l a t i v e m o t i o n v / i th respec t to t l ie v/ove in order to reduce v /ater c o n t a c t s in low encounter f requency s i t ua t i ons such as f o l l o w i n g seas. This con t ro l mode re l ies on pressure (he igh t ) sensors loca ted in the u n d e r w a t e r hul ls in con junc t i on w i t h min imi ized iner t ia l p i t c h and r o l l . Pleading c o n t r o l is m a i n t a i n e d by the rudders keyed to a yaw r a te gyroscope. The ship has 153 k i l o w a t t s (20Ü horsepov/er) ava i l ab le fo r f in and rudder a c t u a t i o n , v /h ich scales up to values t ha t are unacceptab le for la rger SWATH designs. Hov /ever , m o r e e f f i c i e n t a c t u a t o r s cou ld p rov ide a c c e p t a b l e c o n t r o l de f l ec t i on

\ ' - 4 - 2

rates for less power on larger ships as shown by H igdon

'^^'^'^SSP K A l M A L l I d O is o u t f i t t e d w i f h two 1660 k i l o w a t t (2230 horsepower) a i r c r a f t t ype gas tu rb ine engines in t he inner box t l i o t dr ive c o n t r o l l a b l e p i t c h p rope l le rs th rough

a chain d r i ve sys tem. SSP K A I M A I J N O reoched 25 kno ts in 1974 be fo re the b l i s te rs we re added. C u r r e n t to rque l im i t a t i ons on d r i ve t r a i n componen ts reduce the top speed to aboul 18 kno ts .

SSP K A I M A L I t T O does no t represent c u r r e n t design phi losophy fo r SWATH ships; fo r examp le , t l ie f u l l span s iern fo i l is larger t han e i the r s t a b i l i t y or c o n t r o l requ i res. The smal l o v e r a l l size o f SSP K A I M A L I N O makes her m o t i o n in h igh sea s l a tes worse than larger SWATH ships. In a d d i t i o n , her w a t e r p l a n e area and m e t a c e n t r i c heights resu l t in na tu ra l m o t i o n per iods a t low speeds tha t are not o p t i m u m fo r la rger SWATH ship designs. These d i f f e rences ore discussed in de ta i l in L a m b and Fe i n (Ref . I) w i t h i n the c o n t e x t of c u r r e n t SV/ATH design p r a c t i c e . Neve r the less SSP K A I M A L I N O does & -instrate the advantages of t l ie SWATH c o n c e p t . I t ol prov ides a means for r e l a t i n g fu l l scale p e r f o r m a n c e to t h e o r e t i c a l and e x p e r i m e n t a l mode l w o r k , ^ ffius v e r i f y i n g the design too ls that apply to any SWATH ship.

3.2 Mode l E x p e r i m e n t s

Mode l expe r imen ts we re conduc ted a t D T N S R D C on G 1/7.8 scale mode l of SSP K A I M A L I N O in 1971 as the design was being f i n a l i z e d . The o b j e c t i v e of the exper iments v/as to eva lua te t l ie design and i d e n t i f y p rob lem areas. Ttie 3.35 m e t e r (11 f o o t ) long mode l bu i l t üi D T N S R D C v/as o u t f i t t e d fo r d rag , p ropu ls ion , s t a b i l i t y , seakeeping, and s t r u c t u r a l measu remen ts . Lxper iments were c o n d u c t e d on C a r r i a g e 2 (DTNSRDC's Deep Water S t ro igh t l i ne Basin) and in the seakeeping tank of i h e Maneuver ing and Seakeeping F a c i l i t y ( M A S K ) . For the seokeeping expe r imen ts t l ie model was s e l f - p r o p e l l e d and te the red by power cables and s a f e t y l ines to the t o w i n g ca r r iage . P i t c h and roll angles were measured by gyroscopes. R e l a t i v e bow m o t i o n was ob ta ined f r o m on u l t rason ic t ransducer moun ted on the bow. A n occe le rome te r p laced a t the f o r w o r d m o s t po in t o f the model measured bow v e r t i c a l a c c e l e r a t i o n . Head ing was r - - ' n t a i n e d by s e r v o - c o n t r o l l e d rudders t h a t r e a c t e d to

Y and yov/ ra te feedback . P o w e r i n g was c o n t r o l l e d f r o m the ca r r i age and v a r i e d to keep s lock in the cab les and sa fe ty l ines regard less of the wave induced surge. Ttie a f i f o i l and canards w e i e f i xed at angles requ i red to give level t r i m and design s inkage in c a l m w a t e r a t the speed under s tudy . The i r regu la r waves w e r e p rov ided by pneumat ic wavemokers d r i ven by m a g n e t i c t ape , f he waves a p p r o x i m a t e d P i e r s o n - M o s k o w i t z sea spec t ra f o r Sea Sta tes ') and 5. A l l headings v/ere i nves t i ga ted over a range o f speeds f r o m k.2 to 21 kno ts f u l l sca le . The bov/ acce le ra t i on was higl iest in head seas wh i l e p i t c h m o t i o n was largest in f o l l o w i n g seas. I^o s i g n i f i c a n t s l a m m i n g was noted at any head ing. These ea r l y resu l t s d id not g i ve a comp le te p i c t u r e of seakeeping o f SSP K A I M A L I N O bu t they did shov/ t ha t there were no ma jo r p rob lems . L a r g e mot ions in s te rn q u a r t e r i n g and f o l l o w i n g seas w e r e o f some concern though they v/ere assoc ia ted w i t i i long per iods. The conc lus ions o f these e x p e r i m e n t s v/ere genera l ly v e r i f i e d by subsequent f u l l scale t r i a l s .

ATsecond ser ies o f m o d e l e x p e r i m e n t s was c o n d u c t e d in 1973 as c o n s t r u c t i o n of the ship v/as end ing . The ob jec t i ve v/as to i nves t i ga te synehronous cond i t i ons v/here the encounter per iod in waves cor responded to t i ie ship's • - i u ra l per iods . A 1.5 m e t e r long r a d i o - c o n t r o l l e d mode l

dgned by HOSC was u t i l i z e d . This mode l a l l o w e d for more t i m e a t speed in i h e M A S K f a c i l i t y and cou ld be run

w i i f i o u t power f r o m the tov / ing c a r r i a g e . The mode l v/as i n s t r u m e n t e d t o measure i m p a c t pressures, p ' t c l i , r o l l , r e l a t i v e bow m o t i o n , and bow v e r t i c a l a c c e l e r a t i o n . The e x p e r i m e n t s cons is ted o f t he d e t e r m i n a t i o n o f c a l m w a t e r na tu ra l pe r iods a t z e r o speed, evo lua t i on of regu la r wave responses and i nves t i ga t i ons in a seaway r e p r e s e n t i n g a f u l l scale moda l pe r iod of 9.5 seconds and s i g n i f i c a n t wave he igh t o f ' t m e t e r s .

The na tu ra l pe r iods at ze ro speed were 0.49 seconds fo r p i t c h , 8.06 seconds fo r heave and 15.7 seconds fo r r o l l in f u l l scale t e r m s . The regu lar wave work emphas ized encounter f r equenc ies t h a t m i g h t e x c i t e synchronous m o t i o n s . La rges t m o t i o n s were found in f o l l o v /mg seas when ship speed was c lose to the wave speed. In t h a t case a large bow down s t a t i c t r i m occu r red due to i he a c t i o n of the v/ave on the f u l l span a f t f o i l . Th is c o n d i t i o n , w h i c h cou ld lead to the upper s t r u c t u r e bow be ing bu r ied in the wave and i h e p rope l l e r b roach ing , was la te r observed in f u l l scale t r i a l s .

In t he i r r egu la r wave expe r imen t s s i g n i f i c a n t s l a m m i n g was expe r i enced in head Sea S ta te 6 a t soeeds above 10 kno ts f u l l sca le . It should be no ted that al l these mode l e x p e r i m e n t s v/ere conduc ted w i t h c o n t r o l s f i x e d . Severe m o t i o n s and i m p c c i s a t speed cou ld be e x p e c t e d to be a l l e v i a t e d by the use o f a u t o m a t i c c o n t r o l .

3.3 Fu l l Scale T r i a l s

SSP K A I M A L I N O has undergone e x i e n s i v e t r i a l s beg inn ing in I97'4. The f i r s t series ot t r i a l s v/as c o n d u c t e d (as were i h e mode l expe r imen ts ) on t l ie o r i g i na l c o n f i g u r a t i o n . These we re c o m p l e t e d in 1975 ond inc luded i nves t i ga t i ons of pov /e r ing , s t r u c t u r a l loads, c o n t r o l response, m a n e u v e r i n g , and seakeepi r iq , i h e t r i a l cond i t i ons v/ere la te r repea ted a f l e r SSP K A I M A L I N O was o u t f i t t e d w i t h buoyancy b l i s te rs . In a d d i t i o n , c o n t r o l sys tem e v a l u a t i o n t r i a l s , he l i cop te r landing t r i a l s , and compar i son i r i o l s w i t h t w o Coast Guard ships i iave been c o n d u c t e d . The pov/er ing resu l ts arc d o c u m e n t e d by Stenson (Re f . 9) and Woo and M.ouck (Re f . 10). The maneuve r i ng resu l ts are s u m m a r i z e d in Larnb and F e i n (Re f . 1). F i g u r e 2 is a pho tog raph o f SSP KA1MAI . . IN0 undergo ing t r i a l s .

F ig . 2 SSP KAII ' i/1ALir\!0 U n d e r g o i n g T r i a l s

The f i r s t ser ies of seakeeping t r i a l s in 1975 v/as conduc ted in the K c i w i Channel b e t w e e n the islands of Oahu and M o l o k o i (See F igu re 3) , Measu remen ts v/ere made of t he ship's r i g i d body mo t ions ( p i t c h , r o l l , r e l a t i v e bow m o t i o n ) , a c c e l e r a t i o n s (surge, sway , heave , bow

\ ' - 4 - 3

v e r t i c a l and po r t v e r t i c a l ) , ra tes {yaw, p i t c h , and r o l l ) , and i m p o c t oressures a t 20 loca t ions . The seaway was measured by a f r ee f l o a t i n g buoy tha t was dep loyed by the chase boa t . The buoy con ta ined a double i n t e g r o t m g o c c e l e r o m e t e r and t e l e m e t e r e d the wove he igh t s ignal back to t he ship. The buoy tended to d r i f t and a t t i m e s was far enough f r o m the ope ra t i ng area t o m a k e t h e re levancy of some of the wave i r ieasurernents ques t ionab le . S i g n i f i c a n t wave he igh t was about 1.75 m e t e r s .

T a b l e 2 S e a k e e p i n g T r i a l C o n d i t i o n s

160°V/

Niihau

Trials Sites

t 1975 Trial

2 1976 Trial

3 First 3 Days

4 Last Day -

158^W Predominant ' North Wave Direclion

(All Trials)

?2°N

•i- — ^ - - ^ 21°t

~ 1979 Trial

19/9 Trial 50 km

F ig . 3 H a w a i i a n I s l a n d s T r i a l s L o c a t i o n s

T r i a l s v/ere conduc ted a t nomina l speeds o f 16.5, 12.5 and 9.5 kno ts . A v e r a g e speeds, s i gn i f i can t wave he igh ts , and t r i a l cond i t i ons are l i s ted in Tab le 2. The c o n t r o l sys tem v/as not emp loyed for these t r ia ls so canards and the s te rn f o i l w e r e f i x e d at d e f l e c t i o n s des igna ted for mean t r i m a t the g iven speed. The t r i a l s p rocedure cons is ted o f ad jus t i ng t l ie ship to level f r i m a t ze ro speed and reco rd i ng d i s p l a c e m e n t . B e f o r e each run Ihe ship was set on course o t t he des i red heading t o fhe waves as d e t e r m i n e d by a deck observer . As w i t h any f u l l scale t r i a l , t h e r e was some doubt about t he d i r e c t i o n a l i t y o f the seav/ay a l t hough the d i r e c t i o n of p r i m a r y energy was c a r e f u l l y obse rved . A t least 30 m inu tes o f da ta we re taken for each t r i a l c o n d i t i o n . Mo changes in p ropu ls ion or c o n t r o l d e f l e c t i o n w e r e p e r m i t t e d du r i ng the da ta c o l l e c t i o n pe r i od . Few impac ts were n o t e d , fhe on ly case whe re deck wetness o c c u r r e d was in f o l l ow ing seas when t l ie wave speed approached the ship speed and a la rge a m p l i t u d e bu t g e n t l e bow " p l o w - i n " o c c u r r e d . P rope l l e r b roach ing o c c u r r e d in s im i l a r cond i t i ons in g u a r t e r i n g seas. As r e p o r t e d by K a l l i o (Re f . 1 I ) , the da ta we re inadequate fo r power spec t ra l ana lys is ; nowever , s i g n i f i c a n t va lues of t he mo t i ons and a c c e l e r a t i o n s w e r e ob ta i ned a t a l l head ings.

In Nove inbe r o f 1976 low speed t r i a l s v /ere c o n d u c t e d by D T N S R D C on SSP K A I M A L I N O to o b t a i n bending m o m e n t s in beam seas. In c o n j u n c t i o n w i t h t ha t e f f o r t , t he m o t i o n s , bov/ v e r t i c a l a c c e l e r a t i o n , and seav/ay were measured a t a l l headings a t the nomina l speed o f 5 kno ts . These resu l t s p rov ided i n f o r m a t i o n about SSP K A I M A L I N O m o t i o n s in a h igh Sea S ta te 5 a t low speed. The ship had been m o d i f i e d p r io r t o these expe r imen t s by the a d d i t i o n o f buoyancy b l i s te rs on the inboard lovver hu l l s . The ship was o p e r a t i n g on on ly one p rope l l e r dur ing these t r i a l s . P i t c h and ro l l w e r e measured a t t he s tab le t a b l e ( l o ca ted about 6 m e t e r s f o r w a r d o f the C G on the c e n t e r l i n e ) w h i l e bow m o t i o n was d e t e r m i n e d f r o m an u l t r a s o n i c he igh t sensor loca ted 1.0 m e t e r f o r w a r d o f the bow on the ship c e n t e r l i n e . Bow v e r t i c a l a c c e l e r a t i o n was measured as in the e a r l i e r t r i a l s in the p i l o t house. Wave s p e c t r a v /ere aga in ob ta i ned f r o m a f r e e f l o a t i n g buoy and in these t r i a l s the buoy v/as dep loyed f r o m the ship and was v e r y near the t r i a l l o ca t i on near Oahu Is land.

Title Sea

Dato Location g , ^ , ^

Significant Wuva Ht

m

r,'uiTil)er of Conditions

Speeds Control

Evaluated

Seakcfiping Evalu.ilton

7/28/75 KAIWI STRAIT

'1 1.8 6 9.8.12.8 no

7/31/75 KAIWI STRAIT

•i 1.7 10 9.8,17 No

Structural Evaluation

11/26/76 North-East of Oahu

5 3.fl 6 6 No

Control Evaluation

1/21/79 South-East of Kauai

5 2.9 10 15.5 Yes

1/23/79 South-East of Kauai

4 2.5 4 0,10 Yos

1/25/79 South-East of Kauai

1 2.0 18 3.5,10,15.5 Yes

1/31/79 r-lorih-East of Oahu

5 3.2 19 7,10,15.5 Yes"

• One blisUT mihsinj, data not leportod herein. ^

Sign i f i can t wave he igh t is g iven in Tab le 2. The t r ia l s we re conduc ted w i t h o u t the use of the c o n t r o l s y s t e m , as the speed v/as too lov/ for s u f f i c i e n t c o n t r o l e f f e c t i v e n e s s . Spec t ra f r o m these t r i a l s w i l l be disccssed in Sec t ion 5.

The most recen t seakeeping t r i a l s were conduc ted in January of 1979 in order ro ex tend the range o f seakeeping resu l t s , to ob ta in da ta t t ia f cou ld be used to genera te t r a n s f e r func t ions , and to q u a n t i f y the e f f e c t of a u t o m a t i c c o n t r o l on the mo t i ons of SSP KAIMALI I^JO. These t r i a l s v/ere conduc ted o f f Kaua i and Oahu as no ted in F igu re 3. Mea.surernents were made of r i g i d body m o t i o n s , acce le ra t i ons and the seaway. I m p a c t pressures and s t ra ins we re recorded bu t not ana lyzed since there w e r e f ew i m p o c t s . The t ronsducer loca t ions in th is t r i a l are shown in F i g u r e I . Waves were again mea.sured by a f r ee f l o a t i n g buoy in c lose p r o x i m i t y to the t r i a l a rea . When not in t he wa te r the buoy was a t t a c h e d fo the deck and t h e r e b y p rov ided a v e r t i c a l d isp lacernent measu remen t near the l ong i tud ina l cen te r of g r a v i t y (CG) t ha t was a l m o s t equ iva len t t o heove .

In p repa ra t i on for each p a r t i c u l a r r u n , the ship was s tead ied on course a t a p p r o x i m a t e l y t he des i red speed. The speed v a r i e d s l i gh t l y due to w i n d ond v/uvo cond i t i ons . The ship course v/as set to m a i n t a i n a heading w h i c h was cons tan t r e l a t i v e to the p redom inan t seciv/oy as d e t e r m i n e d by observa t ions . Once tine head ing and speed w e r e se t , manua l c o n t r o l se t t i ngs we re made or the c o n t r o l sys tem was t u r n e d on, and t hen the da ta c o l l e c t i o n po r t i on of a run began. C o l l e c t i o n t i m e was governed by the need fo r .su f f i c ien t encoun te rs a t t he g iven speed and heading and va r i ed f r o m 20 to 50 m i n u t e s . No chonges in manua l c o n t r o l s u r f a c e d e f l e c t i o n or p ropu ls ion set t ings were made dur ing the runs. Resu l ts f r o m these t r i a l s w i l l be discussed in f o r t h c o m i n g sec t ions .

In a d d i t i o n , SSP K A I M A L I N O p a r t i c i p a t e d in a three ship c o m p a r a t i v e t r i a l t h a t was conduc ted in M a y of 1978. It is documen ted by Woolover and p e t e r s (Re f . 12). SSP K A I M A L I N O v/as run a longs ide a 100 ton.no pa t ro l c r a f t .30 m e t e r s long, and a 3000 tonne Coast Guard C u t t e r . The t r i a l s w e r e c o n d u c t e d in Sea S ta te 3. Emphasis v/as on human f a c t o r s and c o m p a r a t i v e re la t ionsh ips ra ther than on a t « o l u t e d a t a . The resul ts d e m o n s t r a t e t h a t SSP K A I M A L I N O has super ior mo t ions to a ship o f 10 t i m e s g r e a t e r d i sp lacemen t tn the same Sea S ta te 3.

I

J A O J I O N S RESULTS F O R SSP K A I M A L I N O

The resu l ts o f seakeep ing e x p e r i m e n t s a re descr ibable in severa l ways . One is in the f o r m of_ l l i e r a t i o of s i gn i f i can t responses to s i gn i f i can t v/ave h e i g l i t . A s ign i f i can t value is the overage of t f ie o n e - t h i r d h ighest values and may be ob ta ined f r o m a t i m e h i s to ry of the mo i i ons or wave excurs ions . A n o t h e r m e l h o d of analys is is the power spec t ra l dens i t y d i s t r i b u t i o n w h i c h re l a tes the energy of the m o t i o n oi- wave to the f r equency . SSP KAIMAL l l x lO fu l l sca le t r i a l s s p e c t r a l da ta are p resen ted in the next sec t i on . A t h i r d m e t h o d of analys is used in Sect ion 6.3 invo lves the t rans fe r f u n c f i o n s . "fhe t r a n s f e r func t ions prov ide the un i t response of a m o t i o n to a un i t v/ave height th roughou t t l i e f r equency range.

The ra t ios o f s i gn i f i can t responses to s i g n i f i c a n t v/ave he igh t are presented in F igu res 4 to 13 for the model and fu l l scale sh ip . The data were ob ta ined in Sea Sta les ' l and 5. The wave spec t ra for the mode l exper imen ts v/ere d i f f e r e n t f r o m those in the f u l l sca le t r i a l s , The e f f e c t o f c o n t r o l a t h igh speed is also inc luded

p i t ch and ro l l in F igu res 10 t h r o u g h 13.

F igures 4 th rough 7 show the e f f e c t of speed on f r ee body mot ions w i l h o u l any f o r m of c o n l r o l in head seas. The resul ts show l i n e a r i t y t h roughou t the wave l ie igh t ronge exam ined . C o n e i a t i o n b e t w e e n mode l and niW scale is good for p i t c h t h roughou t the .speed range, i h e model resul ts we re ob ta i ned fromi the 1971 and 1973 expe r imen ts . F r o m a m a x i m u m at zero speed, p i t c h decreases to a m i n i m u m nt 10 kno ts , then peaks at abou t 14 kno ts . B e t w e e n 10 and 14 knots t l ie v/ave drag hump occurs . A t 10 knots t he ship is " c l i m b i n g " i ts .se l f -gencra led bow wove and thus i t is heav i l y damped in p i t c h . A t 14 kno ts the s l i i p -gene ro ted wave is beh ind the ship ond may ac t in a des tab i l i z i ng manner . T r i a l cond i t ions a f t e r t h e add i t i on of t l ie buoyancy b l i s te rs ore ind ica ted by so l id po in ts . T l ie da ta w i t l i buoyancy b l i s te rs shov/ good ag reemen t w i t h the ea r i i e r da ta v /h ich ind ica tes t ha t t he b l i s te rs , w h i c h ore c e n t e r e d a t the long i tud ina l C G , have l i t t l e e f f e c l on p i t c h m o t i o n .

F igu re 5 presents ro l l in head seas. The mode l expe r imen ts v/h ich v/ere c o n d u c t e d in a u n i d i r e c t i o n a l head sea show m i n i m a l ro l l m o t i o n wh i l e the fu l l sca le t r i a l resu l ts shov/ s i g n i f i c a n t r o l l . Th is may be a t t r i b u t e d l o the f a c t t h a t v/ave components are present in t he real

v i r o n m e n t f r o m o the r than the d i r e c t i o n of the . edorninont energy . The v e r t i c a l a c c e l e r a t i o n in F i g u r e 6 remains r e l a t i v e l y cons tan t w i l h speed, s l i g h t l y increas ing a t the h igher speeds. The i-atio o f s i g n i f i c a n t re la t i ve bow m o t i o n to s i gn i f i can t wove he igh t in F i g u r e 7 shows a lmos t no speed e f f e c t . Excep t for a s ing le po in t near 16 kno ts , t he mode l da ta ag ree qu i t e we l l w i t i i t he f u l l scale resu l t s .

F igu res 8 and 9 show .speed dependence of the p i t c h and ro l l mo t i ons in beam seas. P i t c h in beam seas a t speeds belov/ 5 knots is s l i g h t l y h igher fo r the f u l l scale t r i a l s t l ian fo r t he mode l resu l t s . The d i f f e r e n c e is probab ly due to u n c e r t a i n t i e s in heading and v/ave d i r e c t i o n , but even so i h e mode l da ta do show the presence of p i t c h in beam seas. The p i t c h is j i r o b a b l y due to the f o re and a f t a s y m m e t r y of the s l i ip . I he la rge o f t f o i l e xc i t es p i t c h as the ship responds to the beam waves , ond since the re is no area fo r v /a rd to cance l th is e f f e c t a riel p i t c h m o t i o n ensues. The mode l ro l l m o i i o n s ag ree qu i te we l l v / i t i i t he fu l l sca le over the speed range in F igure 9. A g a i n the b l i s te rs seem t o hove l i t t l e e f f e c t on s i gn i f i can t r o l l . The peak in p i t c h and ro l l a t abou t 14 !;nots is aga in p resent in beam seas. R o l l f o l l o w s the same t r e n d w i t h speed as p i t c h , dec reas ing to abou t 10

ots and then showing on increase in t l i s v/ave d rag hump •-lime.

O Full Scale Trials

e Full Scale Trials w i th Üuoyai icy [Blister

A Model Experimonts

8 12

Speed (knots)

Ficj. 4 P i t c h M o t i o n f o r S S P K A I M A L i r v I O in H e a d

S o o s

Fig.

0 4 8 12

Speed ( id iots)

5 Ro l l fV Io t i on f o r S S P KAI fV IAL I fMO in H e a d S e a s

Spaed (knots)

F ig . G V e r t i c a l A c c e l e r a t i o n f o r S S P K A I M A L l P v O i n

H e a d S e a s

1.5

CQ DC «

XJ z>

e <

3 O n

1.0

0.5 h

A

n

4 8 12 1 6 20

Speed (knots)

F ig . 7 R e l a t i v e B o w M o t i o n f o r 3 S P K A i M A L i N O i n

H e a d S e a s

V - 4 • 5

O)

-Q

Q .ü D.

O Full Scale Trials

O Full Scale Trials w i th Buoyancy Blister

A Model Experiment

1 / } A

a 12

Speed (knots)

16 20

F i g . 8 P i t c h i V i o t i o n fo r

S e a s

S S P KAI IVIALI i^JO i n B e a m

0 1 6

F i g .

4 8 12

Speed (Knots)

Ro l l M o t i o n f o r S S P K A I M A L I N O in B e a m S e a s

4,0

O! XJ +j x: O) '03 ;c > 10

.3.0

2.0

•o

71 t-<

O Q

1.0

A Mode! Expenments — Uncontrol led

O Full Scale Trials - Uncontrol led

• Full Scale Trials

Q

Automat ic Control

: /\ •

1

Heading (deg)

180

Head

F i g . 10 P i t c h M o t i o n fo r S S P K A l M A L i i - y O a t 10 iCno ts

The e f f e c t s of a u t o m a t i c m o t i o n c o n t r o l are c l e a r l y d e m o n s t r a t e d in F igu res 10 th roug t i 13 whe re the da ta is p l o t t e d as a f u n c t i o n of head ing . The fu l l scale da ta were ob ta i ned dur ing the c o n t r o l sys tem e v a l u a t i o n t r i a l s o f 1979. The mode l da ta in F igu res 10 and I I are f r o m e x p e r i m e n t s in Sea Sta tes 5 and 6 ( w i t h o u t b l i s te rs ) . T h e r e are no mode l da ta w i t h a c t i v e c o n t r o l . A g r e e m e n t b e t w e e n mode l and fu l l scale p i t c h m o t i o n is exce l l en t a t a l l headings fo r 10 kno ts . R o l l m a g n i t u d e is low fo r the mode ls in head and f o l l o w i n g seas because of the pure ly u n i d i r e c t i o n a l e n v i r o n m e n t in the t o w i n g tank but agrees v / i t h t r i a l s qu i t e w e l l of the other headings. The e f f e c t o f m o t i o n con t ro l at 10 knots is s i g n i f i c a n t . M a x i m u m p i t c h m o t i o n w h i c h occurs in q u a r t e r i n g seas is reduced to abou t o n e - t h i i d i ts u n c o n t r o l l e d va lue . R o l l is I 'educed by a p p r o x i m a t e l y one-ha l f a t a l l head ings. The c o n t r o l e v a l u a t i o n t r i a l s resul ts show a la rge r e d u c t i o n in s i g n i f i c a n i va lues o f mo t i ons at 10 and 15.5 knots and some r e d u c t i o n at 7 kno ts . A t lower speeds the c o n t r o l su r f ace e f f e c t i v e n e s s is not s u f f i c i e n t t o a f f e c t the m o t i o n s .

Fol lowing

F i g . 11 R o i ! M o t i o n f o r S S P K A I M A L I N O a t 10 K n o t »

F i g .

Fol lowing Heading (dog)

12 P i ^ c h M o t i o n f o r Ö-SP K A I M A U i M O a t 15.5

K n o t s

V - 4 • 6

R g .

Fol lowing Heading (deg)

13 Ro l l M o t i o n f o r S S P K A I i V l A L I I M O a t V3.5 K n o t s

A t 15.5 kno ts (F igures 12 and 13) l o l i and p i t c l i a re both ma>dmum in f o l l o w i n g seas. The c o n t r o l sys tem reduces s i g n i f i c a n t ro l l m o t i o n by 90 percent a t i h i s speed in f o l l o w i n g seas and by about 70 pe rcen t at o the r headings, P i i c l i is reduced by over 50 pe rcen t a t a l l l ieodings. These s i g n i f i c c n i va lues d e m o n s t r a t e the e f fec t i veness o f m o t i o n c o n t r o l . S i gn i f i can t values are impor tan t in es tab l i sh ing seakeeping c r i i e r i a for designs since m o t i o n l i m i t s fo r weapon sys iems or a i r c r a f t operat ions o ie ge i ie ra l l y expressed in t e r m s of s i gn i f i can t responses.

5. DISCUSSION O F F U L L S C A L E T R I A L S P E C T R A

Pov/er spec t ra l dens i t ies as f unc t i ons of encoun te r f iequency were ob ta ined by Fou r i e r analys is of t i ie mot ions da ta and are p resen ted fo r t h r e e speeds in Fil :s IA, 15 and 16. The response spec t ra in como ina t i on v / i th the wave spec t ra o f encoun te r p rov ide a good ins igh l in to t l ie sources o f the m o t i o n s . The 5 knot speed data we re ob ta ined in 1976 and the 10 and 15.5 kno t speeds are f r o m the c o n t r o l e v a l u a t i o n t r i a l s of 1979. For tha t w o t i igher speeds, spec t ra w i f h and w i t h o u t a u t o m a t i c m o t i o n c o n t r o l a re g i v e n . R e l a t i v e bow m o t i o n (RB/y\) v/as measured in 1976 but no t in 1979. V e r t i c a l mo t ion near the C G was ob ta ined fo r most cond i t i ons in 1979 and is p resen ted in p lace of f^BM for t he i i igher speeds. Wave spec t ra are p resen ted in F igu res 17, 18 and 19 for each t r i a l day fo r v /h ich m o t i o n spec t ra we re obta ined. T i ie moda l peiTod o f t he wave spec t ra is 'na ico ied on each m o t i o n s p e c t r u m by the symbo l T Q to show the l oca t i on of the m a x i m u m wove energy in the encounter f r eguency d o m a i n .

The resu l ts fo r a 5 kno t speed f r o m the 1976 t r i a l s in " f i igh Sea S ta te 5 appear in F i g u r e Rl . For each head ing R B M , p i t c h and ro l l are p resen ted . The R B M depends on Pdch, heave and the e n c o u n t e r e d v/ave a t t h e bow. A.t ' l igh f requenc ies and lov/ speed SSP K A I M A L I N O tends to p l a t f o r m the waves . T f ia t is, t he ship tends not to '•-Mierienee m o t i o n as i t encoun te rs t he waves . Since the ' ' 'BM s p e c t r u m reaches a m a x i m u m in t he h igh f r equency Jf l where p i t c h and ro l l a re s m a l l , th is i nd i ca tes t h a t '•5 r e l a t i v e m o t i o n may be due p r i m a r i l y fo heave and the

m o t i o n o f the shor ter v/aves. A t lower f requenc ies c o n t o u r i n g occurs as the ship f o l l ows the v /ave. Th is leads to lower RBf/\ energy a t those f requenc ies even though s i gn i f i c an t p i t c h and ro l l energy mioy be p resen t . In a l l cases the R B M peak energy occurs at f requenc ies higher than the v/ave moda l f requency ( i nd i ca ted by T ^ on the f i gu res ) . The RBfv^ is la rges t in head and bow seas and shov/s double or secondary peaks of these headings.

The p i t c h spec t ra a t 5 l<nofs are a l l s ingle peaked and the peaks fend fo occur a t oig = 0.65 r ad / sec , w h i c h is c lose to t he e s t i m a t e d na tu ra l f r equency in p i t c h a t t h i s speed, f 'o r head and bov/ seas the peak cor responds to the peak of the encoun te red wove spec t ra i n d i c a t i n g cons iderab le e x c i t a t i o n near the na tu ra l f r equency for these headings. P i t c h response is m i n i m u m in beam seas as e x p e c t e d .

The ro l l spec t ra ore not as u n i f o r m as the p i t c h spec t ra and tend to bc somev/ l ia t b roader . The p r i m a r y peak in the ro l l response occurs of t f ie peak in t he wave spec t ra at a l ! headings v / i t h the excep t i on of I iead seas. A secondory peak is ev iden t a t 0.8 rad /sec in q u a r t e r i n g seas. The head sea to l l s p e c t r u m is f la t v / i i h srnui l peaks above and below the v/ave modal f r e q u e n c y .

The spec t ra fo r 10 knots ere c o n t a i n e d in F i g u r e 15. The spec t ra are p resen led for v e r t i c a l m o t i o n near fhe C G , p i t c h , and r o l l . The data w e r e ob ta ined du r i ng f h e c o n t r o l e v a l u a t i o n t r i a l s on January 25, 1979, in a Sea S ta te 4. T l ie resul ts are p resen ted w i t h and w i t h o u t u t i l i z a t i o n of a u t o m a t i c c o n t r o l . The c o n t r o l l e d cases ore i nd i ca ted by the dcshed l ines. The c o n t r o l sys tem on SSP K A I M A L I N O is in tended to m i n i m i z e p i i c h and ro l l but not v e r t i c a l m o t i o n . The v e r t i c a l m o t i o n componen t due to p i t c h and /o r ro l l is reduced in some cases, bu t heave is sm.all in a l l c o n d i t i o n s , even v / i t hou t c o n t r o l . The v e r t i c a l m o t i o n peak cor responds to the wave moda l pe r i od in al l cases excep t head seas. A secondary peak a t lower f r equency in head and bov/ seas cor responds to a peak in fhe ro l l s p e c t r u m .

The p i t c h spec t ra at 10 knots show m a x i m u m energy in q u a r t e r i n g seas. Wove moda l per iods co r respond to p i t c h energy peaks in beam, q u a r t e r i n g and f o l l o w i n g seas. In head seas the p i t c h response is ve r y f l a t , w i t l i a sma l l response over a v/ ide range of f requenc ies . In bow seas t he re is a secondary peak o l the moda l f r e q u e n c y o f fhe waves a t about 0.85 rad /sec wh i l e t he p r i m a r y peak is a t 0.72 rad /sec . In a l l cond i t i ons the c o n t r o l sys tem g r e a t l y decreases fhe p i t c h response. For e x a m p l e , in f o l l o w i n g seas the c o n t r o l s ore least e f f e c t i v e bu t s t i l l reduce t l ie peak value by over 50 p e r c e n t . A t lov/ f r equenc ies in q u a r t e r i n g ond f o l l o w i n g seas the c o n t r o l l e d response exceeds fhe u n c o n t r o l l e d response. These long pe r iod mo t i ons are o f t r i b u l o b l e t o t he c o n t r o l system's feedback loop response.

The ro l l spec t ra a t 10 knots for SSP K A 1 M A L 1 I 4 0 shov/ m a x i m u m response in I jeam seas w i t h o i m o s t as g rea t a response in q u a r t e r i n g and bow seas. C o n t r o l is e f f e c t i v e in reduc ing ro l l mo t i ons in a l l sea headings. In a l l cases the m a i n r o l l peak is assoc ia ted w i t h t h e wave moda l pe r iod and t l ie re is a secondary peak a t u lower f r equency t h a t can be a t t r i b u t e d fo t he n a t u r a l p e r i o d in r o l l . The low f requency peak is in the f r equency range w h e r e measu remen ts o re d o u b t f u l v /h ich leads t o d i f f i c u l t i e s in f i x i n g i ts m a g n i t u d e . A t f i i r d pealc a t a h igher f r equency than Ihe wave peak is no ted fo r bow and q u a r t e r i n g seas. I h i s is due to a s i gn i f i can t c o m p o n e n t of wave energy a t those f requenc ies of e n c o u n t e r .

It is also o f i n te res t to po in t out t h a t in beam seas at 10 knots t he wave m o d a l pe r i od v/as a p i ) r o x i m a t e l y t f ie some as the na tu ra l pe r iods o f heave and p i t c h , caus ing subs tan t ia l response in t l iese modes . The n a t u r a l p e r i o d in ro l l of the ship a t th is speed is about 21 seconds or

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app rox ima te l y t v / i ce it.s pitci- i pe r i od . A l t h o u g h l i t t l e wove energy ex is ts at fhe ro i l na tu ra l f r e q u e n c y , some small ro l l m o t i o n does appear . Hov/ever the p r i m e response in ro l l occurs v / i l i i i n l! ic some f i e q u e n c y r e g i m e as p i t c h v/h ich lias large e x c i t a t i o n at i ts na tu ra l f reouency. This behavior is exh ib i t ed in bow and qil . 'r ing seas as v /e l l . Thus, at least f o r th is speed, i l iere appeors l o be s i g n i f i c a n i coup l ing be tween the t h ree modes ( p i t c h , heave and ro l l ) of m o t i o n .

The response spec t ra at 15.5 knots ob ta ined in a Sea State 5 (on January 2 1 , 1979) du r ing the c o n t r o l evaluat ion t r i a l are g iven in F igu re 16. Response of ver t i ca l m o l i o n near t he C G , p i t ch and ro l l arc i nc l uded , though v e r t i c a l m o t i o n was noi ava i lab le for the head and fo i iow ing sea cases. A m o n g i l ie cases ova i l ab l e , v e r t i c o l mot ion response is g rea tes t in q u a r t e r i n g seas. C o n t r o l of p i tch and ro l l has l i t t l e e f f e c t on the v e r t i c o l m o t i o n excepi in c |uar ter ing seas. In o i l coses the p r i m a r y peaks for v e r t i c a l m o t i o n co r respond i o t he f r equency o f m a x i m u m wave energy . A secondary peak in q u a r t e r i n g seas at oig = 0.72 rad /sec may resu l t f r o m i h e f a c t t ha t Ihere is s i g n i f i c a n t wave energy neor the heave and p i t c h noturai per ioas .

P i t c h response is more than on order of m a g n i t u d e larger in fo l l ov / i ng and q i j o r i e r i n g seas than in head, bow or beam seas. P i l c i i response is p r i m a r i l y v/ove induced in l>eum, q u a r t e r i n g and f o i i o w i n g seas, and in f o l l o w i n g seas ihe encoun te red v/ove s p e c t r u m has cons iderab le energy near ihe na tu ra l p i t c h pe r iod at i h e 15.5 kno t speed. The Icirge responses in q u a r t e r i n g end f o l l o w i n g seas occu r at lov scjuencies where the waves t r a v e l a t speeds c lose t o the ^,iip speed. The large s te rn c o n t r o l fo i l in the SSP con induce large mo t i ons under sucl i c o n d i t i o n s . In bow seas pircf i response is f l a t over a broad range w i t l i the most energy a t = O.A rad /sec . The head sea p i t c h response specirum, is d i s t i n c t i v e in t h a t the re is l i t t l e response, of liie wove moda l pe r iod but ins tead peak values of response occur a t encoun te r f r equenc ies o f O.A and 1.8 rad / sec . Tlie higti f r equency peak is in a reg ion v/here t he re is ' d i l a p i t c h response fo r any o ther head ing . A c t i v e -on t ro l s i g n i f i c a n t l y decreases the p i t c l i response fo r a l l headings.

Ro l l spec t ra a t 15.5 knots exh ib i t wave moda l p e r i o d induced ro l l i ng f o r a l l sea f ieodings. A l so t f i e re is a •ccondary peak in heod, i iow and beam seas of a l owe r ' requency of encoun te r v / l i i ch is a p p r o x i r n a f e i y 0.35 •ad/sec, c lose to the e s t i m a t e d ro l l natu i 'a l f r e q u e n c y . In ' o l l ow ing seas i h i s same f r equency is c lose to t l i e i nodo l i'<-'i'iod o f the encoun te red waves , r e s u l i i n g in an a m o u n t '•'f ro l l response g r e a t e r than i h e ro l l response in beam J^Qs and only s l i g h t l y less t f ian the ro l l in q u o r f e r i n g seas. • ont ro l o f r o l l m o t i o n is ve r y e f f e c t i v e a t a l l headings as ' ' ' n t r o l l e d responses a te a lmos t i ns i gn i f i can i c o m p a r e d to j ' ls u n c o n i r o l l e d cases. In p a r t i c u l a r , the severe r o l l i n g in

'ng seas is v i r t u a l l y e l i m i n a t e d by the c o n t r o l

F i gu res 17, 18 and 19 g ive the v/ave spec t ra in the v/aye f r equency doma in fo r each o f the t h r e e days on w h i c h t l ie data in the response spec t ra were o b t a i n e d . There is ve r y l i t t l e wave energy i ie lov /o jg = 0.3 rad /sec w h i c h causes d i f f i c u l t i e s in de f i n i ng lov/ f r equency e f f e c t s in fhe m o t i o n spec t ra . .Each wave s p e c t r u m is s ing le peaked though the re are some ind ico f ions of seconda i y humps. The s i g n i f i c o n i wave he ights for these spec t ra ore g iven in Tab le 2.

O v e r a l l , the .seakeeping t r i a l s of SSP K A I M A L I N O d e m o n s t r a t e d t l i a t the ship cou ld ope ra te over l i s f u l l speed range in v/ave cond i t i ons up to o high Sea S ta te 5. Excess ive s l a m m i n g v/as not encoun te red a t any head ing e i t he r v / i t h or w i t h o u t a u t o m a t i c m o t i o n c o n t r o l . S l amming a t 15.5 knots in a head Sea S ta te 5 v/as v i r t u a l l y e l i m i n a t e d by t r i m m i n g the ship bov/ up tv/o degrees. The ship's tendency to heave ( f o l l o w i n g the waves) l iel [)ed in m o d e r a t i n g t t ie impac ts . A t no l i m e did i l ie ship s low down due l o sea cond i t i ons even though t l ie ave rage magn i t ude of i l ie o n e - t e n t h h ighest v/aves (double a m p l i i u d e ) was more i hon t w i c e the c lea rance he ight o f 1.8 m e t e r s . Du r i ng i l ie Sea S ta te 5 t r i a l s on January 2 1 , 1979, the average of the o n e - t e n t h h ighest waves v/os 3.65 me te r s w h i c h is 15 pe rcen t of the leng th of Ihe ship. On tha t doy SSP K A I M A L I I J O o p e r a l e d to r over 10 hours a t speeds around 15.5 knots at a v a r i e t y of headings v / i th no deg rada t i on of c r e w or ship pe r fo rm iance .

6. C O M P A R I S O N WITH T H E O R E T I C A L M O T I O N

PREDICTIO! -

6.1 SWATH Ship P r e d i c t i o n M e i l i o d o l o q y

C o n f i d e n c e in the a b i l i t y of t heo ry to p red i c t t rends and genera l magn i tudes of m o t i o n is p a r t i c i j l a r l y i m p o r t a n t for i he SWATH ship designer s ince the SW/\TFI concept locks the ex tens ive exper ience in design ava i l ab le to t he monohu l l designer . Whi le mode l e x p e r i m e n t s are requ i red b e f o r e a design is f i n a l i z e d , t h e o r e t i c a l p red i c t i ons must be used to ana lyze the m o t i o n o~' p o t e n t i a l c o n f i g u r a t i o n s a t an ea r l y s tage of des ign.

The deve lopmen t of the t h e o r y used at DTNSRL')C to descr ibe i h e ino t ions of SWATH ships is p resen ted by L e e and Cu rphey (Ref . 2) . S t r i p t heo ry prov ides five basis t o r th is deve lopmen t . Tha t is, i t is assumed t h a i the f i o w c l one t ransverse two -d imens iona l sec t i on of fhe sl i ip Is independent of the f l o w ot ono ihe r s e c t i o n , Ti i is app roach has been used success fu l l y to p r e d i c t i h e mo t i ons o f conven t i ona l d i sp lacement ships; however , SWATI-i ships d i f f e r f r o m conven t i ona l d i sp lacemen i ships in s i g n i f i c a n t v/oys, c r e a t i n g a m o r e c o m p l e x p r o b l e m . For e x a m p l e , the i n i e r a c f i o n e f f e c t s r esu l t i ng f r o m t l ie closeness of t h e t w o hul ls must be desc r ibed . The re l a t i ve i ) - sma l l wa te rp l ane area of SWATH ships imp l ies a sma l l heave

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23 H e l a t i v e R o w IV io t i on T r a n s f e r F u n c t i o n f o r

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r es to r i ng f o r c e . Consequen t l y , v iscous damp ing e f f e c t s w h i c h may be ignored in m o d e l l i n g the v e r t i c a l plane mo t i ons of conven t i ona l d isp lacement ships become r e l a t i v e l y i m p o r t a n t fo r SV/ATH ships. The e f f e c t of f o r w a r d speed on the v/ave t r a i n is not mode l l ed fo r e i ther SVV'ATH or canven t i ono l ships. For the SWATH ship fhe wai<e c r e a t e d by a s t ru t w i l l a f f e c t the f l o w at sect ions a f t of i t . It con be e x p e c t e d tha t th is e f f e c t w i l l be g rea te r for ships v / i th t w o s t ru t s per hul l than fo r ships w i t h one long s t ru t and t h a t slender s t r u t s w i l l genera te less wal<e t l ian t h i cke r ones.

C o r r e l a t i o n o f t h e o r y v / i t h model e x p e r i m e n t s has shown genera l l y good a g r e e m e n t for a range of speeds end headings. A ser ies of e x p e r i m e n t s for the SWATH 6 series v/as c a r r i e d out f^y K a l l i o ( R e f . 13). These were models for a 2900 t o n n e , 73 m e t e r design w i t h var ious G M L ' s . The c o n f i g u r a t i o n s , denoted / \ , B , and G , hod t h e same lower hul ls bu t d i f f e r e n t s t r u t c o n f i g u r a t i o n s . The 6A and fhe 6B are s ingle s t r u t c o n f i g u r a t i o n s whereas Ihe 6G is u t w i n s t r u t c o n f i g u r a t i o n . C o r r e l a t i o n be tween t heo ry and mode l e x p e r i m e n t s is gene ra l l y good; however , c e r t a i n p rob lems e m e r g e . Z e r o speed p red i c t i ons are somewhat incons is ten t in q u a l i t y . Th is is p robab ly due l o prob lems in the genera l m o d e l l i n g o f the non l inear v isccus d a m p i n g . A t h igher .speed, behav ior is w e l l descr ibed but for some cond i t i ons some d isc repanc ies in magn i tude ex i s t . Com,parisons of Ka l l i o ' s model e x p e r i m e n t s arid p red i c t i ons fo r the 6C c o n f i g u r a t i o n s are pi-esented in F igu res 20 to 23 fo r zero and 20 kno ts in beam seas, f i e o v e and r e l a t i v e bow m o t i m response per u n i t v/ave a m p l i t u d e are g iven as o f u n c t i o n of w o v e l eng th . M zero speed the peak value o f heave is v/el l p r e d i c t e d though it is p r e d i c t e d to occur a t a s l i gh t l y g r e a t e r wave length than found in the measu remen ts . The d isc repancy in heave afso seems to be r e f l e c t e d in the r e l a t i v e bow m o t i o n p red i c t i ons . The c o r r e l a t i o n for 20 knots is very good as shown in F igures 20 and 22.

6.2 E x c i t a t i o n F o r c e Compar i sons for SSP K A I M A L I N O

The good c o r r e l a t i o n v / i th mode l scale t ransfer f unc t i ons for SWATH 6C imp l ies t ha t p r e d i c t i o n s fo r SSP K A I M A L I N O , also a t w i n s t ru t des ign , should ba reasonab le . U r i f o r t u n o f e l y , the re are no mode ! scale t r ans fe r f unc f i ons ava i l ab le for SSP K A I M A L I N O w i th v /h ich to ,mal';e t h e o r e t i c a l compar isons . F lov/evcr , morJel scale resu l ts do ex is t for t he v/ave e x c i t i n g heave force and p i t c h m o m e n t . Since f i i ey are on i m p o r t a n t e lement in m o t i o n p r e d i c t i o n , t hey g ive some i n d i c a t i o n o f the re l evancy of the m o t i o n p red i c t i ons for SSP K A i M A L I N O , The fo rces and m o m e n t s on the rnodei in head seas were ob ta ined as p a r t of on e x p e r i m e n t a l p r o g r a m docutr iented by Fe in and Stoh l (Re f . RO. The mode l d id no t have b l i s t e r s , whereas the p red i c t i ons inc lude the e f f e c t of b l i s te rs . The b l i s te rs we re shown to have smal l e f f e c t on the head sea m o t i o n in F igu res h and 5; even though mo t i ons are t h e same, e x c i t i n g forces are not necessari ly t l ie same.

The nond imens iona l heave fo rce (F3o(e)) and phase («3) v / i t h respec t to the v/ave ore g iven in t e rms o f nond imens iona l encounter f r e q u e n c y . The ncndimensio i i -

a l i z a i i o n for encoun te r f r equency is:

N/17 ( t )

w h e r e

L.

is t h e encounter f requency in rad / sec , is nomina l l eng th in me te r s and is g r a v i t a t i o n a l cons tan t in m/.sec2.

4 - 12

|s)orio'iiriensional heave f o r c e is g iven by:

mgh

wl iere

300

(2)

F 3 0 is fhe measured f o r c e a m p l i t u d e in newfons , m is mass of the ship in k i l og rams and h is wove height in m e t e r s .

la

3

0

SSP Model

O Model

ThGor>'

O

Q

o

4

[•iq. 24 H e a v o E x c i t i n g F o r c e i n H e a d S e a s a t 7 K n o t s

2,5 H o a v e F x c i t i n g F o r c e in H e a d S a a s a t 15.5

K n o t s

F ig . 26 H e a v e F o r c e P h a s e A n g l e in F ioad Seas a t 7

K n o t s

F ig . 27 H e a v e F o r c e P h a s e A n g l e in H e a d S e a s a t 15.5

K n o t s

The resu l l s for v/ave e x c i t a t i o n f o r c e and phase at 7 and 15.5 knots ore g iven in F igu res 2't to 27. These resu l ts ore t y p i c a l of the c o r r e l a t i o n a t o the i ' speeds. A g r e e m e n t be tween theo ry and mode l resu l ts for heave e x c i t i n g f o r c e we re qu i t e good a t a l l speeds. M a g n i t u d e and pos i t i on of fhe peaks ore we l l p r e d i c t e d in F i g u r e 25 fo r t he h igh speed. For the 7 kno t speed, F i g u r e 24 shows good ag reemen t up tot.'.'^ = 2.6, v /h ich is equ i va l en t to on encoun te r f r equency o f 1.7 rad /sec fo r t l ie s f i ip . This covers the range of f requenc ies found in mos t sea s pec t r a . Phases agree c lose ly f o r bo th speeds over the e n t i r e f r equency range,

6 . 3 T rans fe r F u n c t i o n Compar isons for SSP K A I M A L I N O

A l t h o u g h mode l scale t rans fe r f unc t i ons ai'e not ava i l ab l e fo r SSP K A I M A L I t ^ O , t rans fe r f u n c t i o n s can , in p r i n c i p l e , be e x t r a c t e d f r o m the f u l l sca le t r i a l du ta , a l t hough th is is not an ideal approach because of p rob lems ossoc ia fed w i t h wove measurements and d e t e r m i n a t i o n of d i r e c t i o n a l i t y aspecis of the seaway.

P r e d i c t i o n of t he mo t i ons for SSP K A I M A L I N O is moi-e d i f f i c u l t than fo r ships in fhe .SWATH 6 ser ies . The 6 C resu l ts i nd i ca te t ha t i l ie presence o f two s t r u t s per hul l does not in i t se l f c r e o i e p rob lems in p r e d i c t i n g SWATFI responses. H o w e v e r , a doiTiinant f e a t u r e o f SSP K A I M A L I N O is i ts la rge a f t miocinred h y d r o f o i l . The s te rn fo i l s used on the SWATH 6 ser ies do n o i span the

V - 4 - 1 3

sepa ra t i on between hul ls and are mucli siTialler in a rea r e l a t i v e to t he ship s ize. Observa t ions du r i ng f u l l scale SSP K A I M A L I N O t r i a l s i nd i ca te (hat v e n t i l a t i o n can occur as the he ight o f fhe f ree su r face over the f o i l va r ies a t some speeds, resu l t i ng in r e d u c t i o n of t l ie e f f e c t i v e n e s s o f t ho f o i l . Th is is not m o d e l l e d c u r r e n t l y in the SWATH p r e d i c t i o n p rog rams .

Wi th these rese rva t i ons , p r e d i c t i o n s for heave and p i t c h for SSP K A I M A L I N O t r a v e l l i n g in beam sees are p resen ted in F igures 28 to 34. Fu l l scale resu l l s were ava i l ab l e for th is heading a t 7, 10 and 15.5 kno ts fo r u n c o n t r o l l e d heave ( v e r t i c a l m o t i o n c o r r e c t e d to the C G ) and p i t c h m o t i o n . Heave is p resen ted in nond imens iona l f o r m and p i t d i in degrees per m e t e r . B o t h mo t i ons ore p resen ted as a f u n c t i o n o f t h e wave f r e q u e n c y , P r e d i c t i o n s are shov/n w i f h so l id l ines, fu l l scale resu l ts w i t h dashed l ines. The degree o f c o r r e l a t i o n for heave is no t as good as for the SWATH 6 C . The heave p red i c t i ons fo r SSP K A I M A L l h f O ore heav i l y damped v/hereas t h e f u l l sca le resul ts a i e no t . This p r e d i c t e d c h a r a c t e r i s t i c is p robab ly due to the m o d e l l i n g of t l ie e f f e c t o f the a f t f o i l . T l ie l i f t cu rve slope values were e s t i m a t e d by t heo ry and c o n f i r m e d by mode l resu l t s . D e g r a d a t i o n in l i f t due t o chonges in the f ree su r face cond i t i ons ore not inc luded in fhe t h e o r y . For 10 knots tv/o sets of resu l ts for n o m i n a l l y fhe same t r i a l cond i t i ons are g iven in F igu re 2 9 . 1 he t r ia l s v/ere conduc ted on the same day bu t separa ted by a numiber of hours . The resu l ts d i f f e r s o m e w l i ü t in m a g n i t u d e , a l t hough t l i ey exh i l ) i t t he some t rends . The ma jo r d i f f e r e n c e s occur over the f requency range where t h e r e v/as l i t t l e wove ene rgy , i l l u s t r o t i n g the d i f f i c u l t i e s in ob ta in i ng t r a n s f e r f unc t i ons f r o m fu l l scale t r i a l da ta ,

Trie c o r r e l a t i o n for p i t c h mict ion of SSP K A I M A L I N O is b e t t e r than for heave m o t i o n . The c o r r e l a t i o n fo r zero and 7 l<nots is qu i te good, though again the ag reemen t b e t w e e n t heo ry ond t r i a l s of 10 knots is no t . | -or this speed a large f ree su r face d e f o r m a t i o n was observed du r i ng the t r i a l s . The 10 knot resu l ts in F igu re 33 show

<

O LO

Q. f: <

0.5

Beam Seas

7 Knots

Trials

Theory

t ü e

F i g . 20 H e a v o T i a n s f a r F u n c t i o n f o r SSP K / M M A L I N O

a t 7 K n o t s

1.5

a. E <

•o

Q.

E

<

1,0

O.B

Beam Seas

10 Knots

—• • Trials

Theon/

9.5 1.0 1.;>

F ig . 29 H e a v e T r a n s f e r F u n c t i o n f o r S S P K A I M A L I N O a t 10 K n o t s

l.S

E -r

a E

0.5

0

Bcarn Seas

15.5 Knots

• ™- Trials

Theory

/ \

0 1.5 0.5 1.0

cüe

F ig . 30 H e a v o T r a n s f e r F u n c t i o n f o r S S P K A I M A L i i M O

a t 1:5.5 iCnots

d i f f e r e n c e s be tween t l ie t w o e x p e r i m e n t a l cond i t i ons as large as the d i f f e r e n c e s b e t w e e n the theory and one of the t r i a l resu l t s . The 15.5 kno t c o r r e l a t i o n is b e t t e r than at 10 knots .

The ove ra l l a c c u r o c y o f the SWA I H ship mictions p rog ram has been es tab l i shed using rnodei expe r imen ts . Its a p p l i c a b i l i t y t o SSP K A I M A L I N O m o t i o n p red i c t i on can be i n f e r r e d f r o m f h e reasonable c o r r e l a t i o n in the coses g i ven .

V l !

O) <D •O

O

O.

ra 5

a E <

Bearn Seas

Zero Speed

Trials

Theoiy

0.5 1.0 1.5

l ig. 31 P i t c h T r a n s f e r F u n c t i o n f o r S S P KAlIVlALirvlO

a t 7.0X0 S p e e d

5 1 ,

n. 'O

i l

Beam Seas

7 Knots

O O 0.5 1.0 1.5

OJf;

Figi >. P i t c h T r a n s f e r F u n c t i o n f o r S S P KAI [ ^ , / iAL INO

a t 7 K n o t s

4 r

>

V,-;

F -> C) <U <u Xi •O

O. de

2 O.

< "Q. E

< 1

/ »

Beam Seas

10 Knots

Trials

Theor\ '

0.5 1.0 1.5

W e

• iy. 33 P i t c h T r a n s f e r F u n c t i o n f o r S S P ICA i fv ' iA IJWO

I a t 10 K n o t s

D) CD

V xs

fc <̂ < | .

I < 1

O

Beam Seas

15.5 Knots

Trials

Theory

O 1.5 0.5 1.0

F ig . 34 P i t c h T r a n s f e r F u n c t i o n f o r f i S P K A i l V l A i m . l O

a t 15.5 K n o t s

7. DESIGlNl C O N S I D E R A T I O N S

7.1 l^iatural P e r i o d Resu l t s

Know ledge of the no tu ro ! per iods of m o t i o n in i ieove, p i t c h and ro l l o ie v i t a l to ono l yz ing the seakeep ing behav ior of a l l ships, espec ia l l y SWATH ships. T l ie designer of o SWATH sl i ip desires to avo id syncl i ronous m o t i o n cond i t i ons under norma l ope ra t i ons . As no ted ea r l i e r t l ie per iods are h igh ly dependent on p a r a m e t e r s l l i o l va ry w i d e l y in a SWATH des ign. The s tandard re la t ionsh ips fo r na tu ro l per iods as g iven in C o m s t o c k (Re t . 15) a re : heave pe r i od ,

Tz = 2ti A (1 + Cz)

g eg A \

( 3 )

p i t c i l pe r i od ,

Te = 2 rr

and ro l l pe r i od ,

lif = 2n

whore

K ^ d + Cp)

g G M l

K| + d2C,|,

g G M t

(5)

Avv A g p

C 0

is t he wa te rp l ane area in m?-, is t he sl i ip d i sp lacement we igh t in kilp^qr ams , is t he g r a v i t a t i o n a l cons lan t in i n / sec^ , is t h e dens i ty of v/ater in kg /n i ^ ^ is t l ie added mass c o e f t i c i e n r , is i h e added p i l c h m o m e n t o f i n e r t i a c o e f f i c i e n t , is t he added ro l l m o m e n t of i n e r t i a c o e f f i c i e n t , is f he p i t c l i radius of g y r a t i o n in .meters, is i h e ro l l radius o f g y r a t i o n in me te rs ond is ha l f the hul l c e n t e r l i n e sepa ra t i on in m e t e r s ,

hose equat ions i n d i c a t e , severa l f a c t o r s a f f e c t t he na tu ra l per iods , The added moss and m o m e n t c o e f f i c i en i s are a func I ion of lower hul l geornetry.^ The w a t e r p l a n e area and m e t a c e n t r i c l ie ig l i ts are a f u n c t i o n o f the g e o m e t r y near ihe w a t e r l i n e . The rad i i o f g y r a t i o n are dependent on ihe ship's mass d i s t r i b u t i o n .

Equat ions 3, 4 and 5 ignore coup l i ng be tween modes and damp ing bu t do p rov ide an e s t i m a t e of n a t u r a l pe r ­iods. Assum ing s imp le g e o m e f r i c o j adde^ mass o n d ^ n o -m e n t va lues fo r SSP K A I M A L I N O of C ^ = C , 0.9 as an a p p r o x i m a t i o n f o l l o w i n g N u m a f o (Re f , 16) and

V - 4 - 15

us ing t l ie data in Tab le I for the as - tes ted ship g ives: ~\\ = 8.6 seconds, T<#, = 9.7 seconds, and Ty^ 15.8 seconds. The per iods also we re c a l c u l a t e d using Lqua f 3, 't and 5 in wh i ch C,, C „ , and C^ , the added mass and

ob ta ined f r o m s t r i p added i n e r t i a c o e f f i c i e n t s , w e r e t i i e o r y . increases w i t h speed wh i l e and are independent o f speed. These values are p l o t t e d in F igu res 35, 36 and 37 a long v / i th t l ie na tu ra l per iods ob ta ined f r o m the 1976 t r i a l s and the 1979 c o n t r o l eva lua t i on Tr ia ls. The ship Ixid buoyancy b l i s te rs fo r these t r i a l s . The t r i a l n a t u r a l per iods are the per iods assoc ia ted w i t h the peaks of the t rans fe r f u n c t i o n s . Ana lys i s of da ta f o r some cond i t i ons resu l ted in d i f f e r e n t na tu ra l per iods . These have been inc luded in fhe f i gu res to d e m o n s t r a t e the a c c u r a c y w i t h v /h ich na tu ra l per iods may be ob ta ined by th i s t echn ique . I '^atural per iods ob ta ined in 1973 by app l y i ng impulses to a scale mode l o f the o r ig ina l design a t ze ro speed also ore inc luded .

Heave na tura l pei iods as d e t e r m i n e d f r o m the t r i a l data (F igure 35) are f a i r l y cons tan t w i t h speed as t heo ry p r e d i c t s . The s l igh t va r i a t i ons betv /een t l ieory and t r i a l s m o v be due to changes in t l ie v /o te r l i ne at var ious speeds w h i c h are not accoun ted for in the theory . P i t c l i na tu ra l pe r iod (F igure 36) tends to increase v / i th speed. The per iod based on added i ne r t i a f r o m s t r i p t heo ry shows this speed e f f e c t reasonably v /e l l . The ro l l pe r i od in F i g u r e 3 / is p r e d i c t e d by theory fo be i8 .5 seconds, i r r espec t i ve of sneed. Tl ie t r i a l resu l ts for ro l l o re sparse since of these long per iods there was ve ry l i t t l e v/ave energy and gene ra l i y un re l i ab le da ta . The t r i a l da ta t h a t are ava i l ab le agree f a i r l y we l l v / i th the t h e o r y . T l ie mode l test resu l t is lov/er than the t r i a l va lue . The mode l rep resen ted the o r i g ina l design c o n f i g u r a t i o n , and does not f u l l y represent the ship as tes ted . The e s t i m a t e d ro l l pe r i od is low. In th is case the value of Cih is based on the assumpt ion tha t r o l l i n g is equ iva len t to t i ie heav ing o f one hu l l up wh i l e the o ther moves d o w n .

There are c e r t a i n s t a t i c e r ro rs inheren t in c o m p a r i n g n a t u r a l per iods f r o m t r i a l s w i t h p r e d i c t e d va lues. One is u n c e r t a i n t y about t he exac t fue l and ba l las t load du r ing the t r i a l . Though the ship was ba l l as ted to the same c o n d i t i o n eve ry day , fue l use and we igh t s h i f t i n g con o c c u r . This e f f e c t on m e t a c e n t r i c he igh t and momen ts of i n e r t i a cannot be e s t i m a t e d . A lso viscous damp ing has a s t rong i n f l uence on p i t c h na tu ra l pe r iod unde rway . A b e t t e r m e t i i o d fo r o b t a i n i n g n a t u r a l per iods is to app ly a f o r c e impulse t o the ship v/h i le u n d e r w a y , w h i c h u n f o r t u n a t e l y has no t been done to SSP K A I M A L I N O since the a d d i t i o n o f the b l i s t e r s .

30

7.0

(I) (/)

O Estimated

• Model Experiment (1973)

V Full Scale Tnals (1976 and 1979)

—™-=" Theory

Speed (Knots)

F ig . .30 H o a v o IMatura l P e r i o d f o r S C P iCA! i \ / lAL I i \ iO

20

c o u 0) CO a>

H

Q Estimated

• Model Experiment (1973)

y Full Scale Trials (1976 and 1979)

~ ~ — Theoiy

' ° 8

5 1 0

Speed (Knots)

F ig . 3(3 P i t c h N a t u r a l P e r i o d f o r S S P K A I i V f A L I W O

3 0

Q Estimated

• Model Expcnment (1973)

V Full Scale Trials {1976 and 1979)

, Ti ieory

F ig . 37

10

Speed (Knots)

Ro l l N a t u r a l P e r i o d f o r S S P K A I I V i A L I N O

7.2 N a t u r a l Per iods and Des ign

The advantage of the long na tu ra l pe r iods at ta inable by SWATH ship designs is t ha t resonant cond i t i ons leading to large m o t i o n s and degraded o p e r a b i l i t y v / i l l not occur f o r mos t v/ove c o n d i t i o n s . The des igner desires p l a t f o r m i n g cond i t i ons to m i n i m i z e p i t c h and heave m o t i o n in head seas. If the na tu ra l per iods in p i t c h and heave ore about 20 pe rcen t g rea te r than fhe encountered m o d a l pe r i od of fhe seaway th is s u p e r c r i t i c a l behav io r can be a c c o m p l i s h e d .

If na tu ra l per iods are s u f f i c i e n t l y long the ext reme mo t i ons assoc ia ted w i t h resonance v/ i l l no t lead fo severe v e l o c i t i e s or acce le ra t i ons s ince encoun te r per iods v/ould be lov/. This was found to be t r ue in the case o f SSP K A I M A I _ 1 N 0 when ope ra t i ng in f o l l o w i n g seas a t 15.5 knots w i t h o u t c o n t r o l . P i t c h m o t i o n s o f 20 degrees c r e s f " t o - t r o u g h were reco rded bu t o p e r a b i l i t y was not adverse ly a f f e c t e d as long as the p rope l l e r remained submerged and the bov/ was not i m p a c t i n g . Simple a u t o m a t i c or even manua l c o n t r o l c o u i d eas i ly prevent

V - 4 - 1 6

,,rh ex t r emes of m o l i o n . S im i l a r t f i i nk ing appl ies to . ( i in sea r o l l i n g . A ve r y long t o l l pe r iod assures t f i a t

., •.r,nont cond i f i cns w i l l not occur in beam seas under ',i,C)5f al l cond i t i ons . ResonanI cond i t i ons t h a t m i g h t , riir in f o l l o w i n g seas a l m o d e r a t e speeds con be

, , ,n i ro l led by a u t o m a t i c m.otion r e d u c t i o n . Tf ius the t r end 1 SV\'ATH ship design is to tal<e advantage of long na tu ra l

• r r iods for good inherent seokeeping in most sea !( , ; idi t ions wliile re l y ing on o u t o m o i ic c o n t r o l and o ther nic-ans to decrease mot ions when resonant cond i t i ons ore nached in f o l l o w i n g seas at m o d e r a t e speeds.

The SWATH ship designer must also be aware o f the in jer re la t ionsh ips among the var ious n o i u r a l per iods , i ' i tch and ro l l per iods should be separa ted so th t i t , incomfor tab le " c o r k s c r e w i n g " mo t i ons w i l l not occur in i juar ter ing seas as in the case of SSP K A I M A L I M O at 15.5 !-iots when JQ (16 seconds) and T^ (21 seconds) were ln)*!i exc i ted a i i he some t i m e . The change in per iods w i t h -.•Ktpd under l ines ihe impo r tance of hov ing a good m o t i o n predict ion fechn iq t ie t ha t con be used in the ea r l y stages ,.: design. Ro l l and l ieove pei iods and p i t ch and heave fo r f i st iould also be separa ted if poss ib le . The iü ier re la t lonsh ips among the na tu ra l per iods is par t i cu la r ly i m p o i t o n t at low speeds v/here c o n t r o l f ins ,:;c not e f f e c t i v e in m o d i f y i n g m o t i o n s . The cho ice of i iMural pet iods mioy be d i c t a t e d by low speed behav io r , par t i cu la r ly i f ihe sli ip's miss ion requ i res a long d u r a t i o n at low speeds,

The na tu ra l per iods o i e c lose ly r e l a t e d i o t h r e e o the r 'lesign issues; the amoun t of v /o te rp lane a rea , the l.i idginq s t r u c t u r e cleat once he ig l i l and the m e t a c e n t r i c h'dghts. The l ieove na tu ra l pe r iod can be increased by decreasing wa te rp lane o rea . Sucl i a decrease olso w i l l ('rerease the hoove e x c i t i n g fo rce in v/aves, and p robab ly Gocreose res is tance as w e l l . I t must be r e m e m b e r e d riawever t ha t cs the v /a te rp lone area is decreased, the restoring f o r c e also is decreased. Th is con resu l t in the S ' iuot ion t h a i , s ' lould resonance occur at Ihese longer periods, i he m o i i o n s con be ra the r large even though the exci t ing f o r c e is smal l as shown in P ien and f_ee (Re f . 17). The f ina l se lec t i on of w a t e r p l a n e area must r e f l e c t a t'olance betv/een ihe above r e q u i r e m e n t s . O i l i e r issues tl.at v / i l l com.e i n t o the dec is ion ore s t r u c t u r a l and i r rongements cons idera t ions .

Long i t ud ina l m e t a c e n t r i c he igh t is a f u n c t i o n o f the :'iSl' ion of the v /a ie rp ione a rea . Inc reas ing G M L J-'cret.ses the p i t c h na tu ra l pe r i od bu t p rov ides a " s t i f f e r " '•liip in t e rms of p i t c h r e s t o r i n g f o r c e fo r s u r v i v i n g in txtreme seas. Increased c lea rance he igh t tends t o 7vcrease bo th m e t a c e n t r i c he igh ts v/hi le impos ing a -̂^ vere s t r u c t u r a l we igh t p e n a l t y . P l o t f o r m i i n g (not •'esponding in) a l l waves wou ld requ i re a h igh c l e a r a n c e , i ' l is is the approach t o k e n by designers o f c o l u m n ' ' ab i l i zed p l a t f o r m s . Ghang ing f r o m p l a t f o r m i n g to • inTouring as seaways ge t h igher w i t h longer assoc ia ted

•tiods is the des i rab le approach for SWATH ships. This ' " ' Is an uppei l i m i t on fhe des i red n a t u r a l pe r iods .

J to ina t ic c o n t r o l and bow up t r i m a re f u r t h e r ^ 'osiderat ions t l i a t may i n f l uence i h e f i na l c l ea rance

• -^ign o f a S.VATH ship.

Overa l l seakeeping is bo th a c o n s t r a i n t and on ' pe r tun i i y to tiie designer as h y d r o s t a t i c s , s t r u c t u r e s , ' tangemients, p o w e r i n g and oilier f a c t o r s are ba lanced to

ia in a good design., The dependence of seokeep ing on ^decentr ic he igh ts , m o m e n t s o f i n e r t i a and c iea ronce

' ' ints out t ha t t l i e ustiol v /e ight g rov / t h of ships can be a ' 'ioi.is p r o b l e m fo r a SvVATH h u l l . Thus , un t i l expe r i ence

•i'Jined in design and c o n s t r u c t i o n , t l i e SWATH ship v / i l i ' Ju i re larger marg ins than c o n v e n t i o n a l hulls. "•'S Teless the S'NAIH c o n c e p t o f f e r s the o p p o r t u n i t y

^-:. ign a l iu l i v / i t h low m o t i o n s end t h e r e f o r e enhances

i ts c a p a b i l i t y f o r naval opera t ions such os a i r c r a f t land ing and t a k e o f f , v/eopons f i r i n g , and tov / ing sonar .

7.3 PREDICTIO iM O F E X T R E M E M O T I O N S

In prev ious sec t ions the mo t i ons of SSP K A I M A L I N O have been p resen ted as ob ta ined in seas o f a spec i f i ed s e v e r i t y c h a r a c t e r i z e d by the spec i f i c wove spec t re g iven in F igu res 17, 18, and 19, The shape o f wove s p e c t r a , h o w e v e r , con va r y cons iderab ly (even though the s i g n i f i c a n t he igh ts o re the some) depend ing on d u r a t i o n and f e t c h of w i n d , s tage of g r o w t h and decay of a s t o r m , or i h e ex i s tence of s w e l l . Thus, du r ing her l i f e t i m e , a SWATH ship may encoun te r a vast v a r i e t y o f w o v e s i t u a t i o n s . For design c o n s i d e r a t i o n , t h e r e f o r e , i f is i m p o r t a n t to examine the e f f e c t of wave s e v e r i t y as we l l OS wove spec t ra l f o r m u l a t i o n s on t h e mogn i t t i do o f responses o f SWATH ships. For i h i s , c o m p u t a t i o n s of the p robab le e x t r e m e va lues of heave and p i t c h o f SSP K A I M A L I N O in o seaway hove been c a r r i e d out in th ree d i f f e r e n t m a t h e m a t i c a l spec t ra f o i i o w i n g the m e t h o d f o r shor t t e r m response p r e d i c t i o n p resen ted in R e f e r e n c e 18. The p robab le e x t r e m e value is the lai 'gest va lue l i ke l y t o occu r In a spec i f i ed ship ope ra t i on t i m e , l iere taken as i h e d u r a t i o n o f t he sea c o n d i t i o n . The B re t schne ide r , P i e r s o n - M o s k o w i t z and the Och i s i x - p a r a m e t e r spec t ra l f o r m u l a t i o n s w e r e used in the c o m p u t a i i o n s , and these w e r e app l ied to ihe t t ons fe r f unc t i ons ob ta ined f r o m on ana lys is o f SSP K / \ I M A L l f ^ O data ob ta ined in beam seas a t a speed of 10 kno ts .

G o m p u t o t i o n s using the B re tschne ide r and ihe Och i s ix -porome+er rep resen ta t i ons were mode for a i o m i l y of v/ave spec t ra f r o m w h i c h the upper and lower bounds of responses w i l h c o n f i d e n c e c o e f f i c i e n t o f 0,95 v/ere d e t e r m i n e d as v/el l as those responses most l i ke l y to o c c u r ; i h o t is. the responses in t he m o s t p robab le v/ave s p e c t r u m fo r a g iven sea seve r i t y . These responses v/ere t f i en c o m p a r e d v / i i h those c o m p u t e d using micasured spec t ra a t S t a t i o n India in the Idor th A t l a n t i c CvVeafher S t o t i o n I) in order i o d e t e r m i n e how v/ei l the bounds cover the v a r i a t i o n of responses in the measured s p e c i r a . The resu l t s are sliov/n in F igures 38 and 39 for p i t c l i and in F i gu res AO and 41 fo r heave for wove he ights up to 4.9 m e t e r s . Inc luded also in the f igures are ihe responses in the P i e r s o n - M o s k o w i t z s p e c t r u m . Vv'ove he igh ts g rea te r t h a n 4.9 m e t e r s (Sea S ta te 6) we re not i nves t i ga ted s ince c r a f t l i n e a r i t y has not been v e r i f i e d in seas of s e v e r i t y g r e a t e r t han t h i s .

The s c a t t e r o f the responses c o m p u t e d in l l ie measured S ta t i on India spec t ra sl iov/n by the crosses ind ica tes t ha t the p robab le e x t r e m e va lues o f bo t l i p i t c h and heave a m p l i i u d e va ry cons ide rab ly for a g i ven s i g n i f i c a n t wove he igh t . The P ie r son -Moskov / i t z s p e c i r u m unde rp red i c t s the mo t i ons in the lower wave he ights wh i l e i t tends fo p r e d i c t va lues somev/hat h igh in t ho h igher s i g n i f i c a n t v/ave he igh ts , 11 is also apparent f. iorn the f i gu res t h a t , in g e n e r a l , for bo th p i t c h and hoove m o t i o n s , t he upper and lower bounds of the values c o m p u t e d using t h e s i x - p o r o m e i e r spec t ra l f o r m u l a t i o n b e t t e r encompass fhe da ta f r o m tha measured spec t ra in t he N o r t h /"<t!arit ic t h a n do t i iose ob ta i ned using the B r e i s c i m e i d e r f o r m u l a t i o n . Wl i i le the f o r m e r appears to cover ttie range o f magn i t udes c o m p u t e d using the meosured spec t ra reasonab ly w e l l , the l a t t e r tends to o v e r p r e d i c t i he lov/er bounds and u n d e r p r e d i c t the upper bounds of bo th modes o f m o t i o n . The s i x - p a r a m e t e r spec t ra l f o r m u l a t i o n con b e t t e r descr ibe ihe shape of spec t ra (e .g . , double pecks) t han the B r e t s c hne i de r f o r m u l a t i o n . Th is a i l ows for o b e t t e r d e s c r i p t i o n o f some seaways, and p robab l y accoun ts for ihe good c o r r e l a t i o n be tween tha responses

V - 4 • 1?

28

24

20

SSP KAIMALINO Beam Seas

10 Knots Stat ion India

tl)

16 11) a : 1

E <

o

12

0

Max imum Probable

0

-f^.v.i',y~~"--piet3on-Moskowil/-

2

Minirnum

Ficj.

S i gn i f i can t W o v a He ig l i t ! m )

28 P r o b a b l e Ex t r tM t i e P i t c l i A r n n l i t u d f i s f o r

r v l t i a su red S p e c t r a a t S t a t i o n i n d i a , P i o r s o n -

M o s k o w i T / . a n d Uound.o o f B r e t s c h n e i d e r

S p e c t r a

20

20

a>

Ij

"D. E < x : o

I / ,

0

SSP KAIMALIMO Beam Seas

10 Knots Stat ion India

Most Probable M in imum

Max imum

-Pierson-Moskovi/ itz

_ „ l J _ _ _ L

0 6

F ig .

S i g n i f i c a n t W a v e H e i g h t (rn)

39 P r o b a b l e E x t r o m o P i t c h A r n p l i t i i d o s f o r

| \ / i sa3u red S p e c t r a a t S t a t i o n I n d i a f i n d B o u n d s

o f O c h i C-Parar i iQtar S p e o t r f i

SSP KAIMALINO Beam Seas

10 Knots Stat ion India

0)

•a

"5. E <

> ID 0) •X.

S ign i f i can t W a v e H e i g h t (m)

F ig . 40 P r o b a b l e E x t r e m e H e a v o A m p l i t u d o s for M e a s u r e d S p e c t r a a t S t a t i o n I n d i a , P ieraon-I v i oskov ' / i t z a n d B o u n d s o f R r o t s c h n e i d o r S p e c t r a

<

SSP K A I M A Ü N Ü Beam Seas

10 Knots Stat ion india

M in imum

F ig . 41

Pie ison-Moskowi tz

I I 4 6

S i g n i f i c a n t W . ^ V G H e i g h t (rn)

P r o b a b i o C K t r c m a H e a v a A m p ü t i ê d o fo '

M e a s u r e d S p e c t r a a t S t a t i o n I n d i a a m i ESouncls

o f O c h i B - P a r a m o t o r S p o c t r a

V • 4 - 1 8

based on the s i x - p a r o m c t e r f o r m u l a t i o n and the S t a t i o n

India da ta . . Vv'hiile t l ie most probable e x t r e m e value o f a m o t i o n in

n spec i f ied seaway is t l i a t w h i c i i is most l i ke ly l o occu r , tli= p robab i l i t y Iho t the e x t r e m e value w i l l exceed the most probable va lue is q u i t e large ( t h e o r e t i c a l l y 63 r je rcent ) . Hence , i t is h igh l y des i rab le to p r e d i c t the ex t reme va lue fo r w h i c h fhe p r o b a b i l i t y of be ing exceeded Is a preassigned smal l va lue fi. Och i (Re fe rence 18) developed a ^ fo rmu la fo r p r e d i c t i n g th is e x t r e m e va lue which he co l ls the "des ign e x t r e m e va lue , " and i t is expressed as f o l l o w s ;

Yn(/3) \ / 2 l n "{60)H , rm; \ s T ^ (6)

2T i ( / 3 / k ) ^ n i o /

v/here

1 is the obse rva t i on t i m e in hours, is the area under t h e response s p e c t r u m , is ihe second m o m e n t of the response s p e c t r u m , is the 1 isk p a r a m e t e r and is i hs number of encounters w i t h a spec i f i ed sea in a ship's l i f e t i m e .

The r isk f a c t o r , /3, can be assigned at i l ie designer 's d i sc ie t i on but is g iven a va lue of 0.01 if i t >s des i red t o design for 99 percen t assurance tha i th is (s ingle amp l i t ude w i l l noi be exceeded . The nunnber of encounters w i t h a spec i f i ed sea, k, invo lves the ship opera t ion t i m e a t the spec i f i ed speed in i h e sea considered and t l ie m a x i m u m du ro l i on of t i ie sea.

As noted p rev ious ly , an i m p o r t o n t cons ide ra t i on in the design of SWATH sii ips is t l ie s iz ing of i he b r i dg ing s i r u c t u r e c lea rance he igh t , s ince i l must r e f l e c t a ba lance be tween requ i r emen ts t o l i m i t we igh t end to assure odequote t ransverse m e t a c e n i r i c he igh t , and t l ie requ i remen t for s u f f i c i e n t c lea rance t o m i n i m i z e w a t e r con tac t and s l amming on t l ie b r idg ing s t r u c t u r e .

T h e r e f o r e , i t may be o f i n te res i to examine how the e x t r e m e r e l a t i v e bow i n o i i o n t h a i rnay be expe r i enced by SSP K A I M A L I M O w i t h o u t any f o r m of c o n t r o l r e l a tes t o its c l ea rance he igh t of 1.83 m e t e r s . C o m p u t a t i o n s using Equa t ion 6 v/ere t h e r e f o r e c a r r i e d out for o head Sea Sta te 5 ( s i gn i f i can t wave he igh t of 3.05 me te r s ) o i a speed

7 knots using t l ie B re tschne ide r f o r m u l a t i o n for a range 1̂ moda l per iods a p p r o p r i a t e for the spec i f i ed wave

f ie ight . The r isk f a c t o r fi v/as token as 0.01 and the l i f e t i m e exposure , k, or Ihe number o f encoun te rs w i t h the spec i f i ed sea, head ing , and speed v/as assumed t o be a p p r o x i m a t e l y t e n . The c a l c u l a t i o n s were made fo r t he c e n t e r l i n e of fhe cross s t r u c t u r e a t the f o r w a r d m o s t loca t ion of the f la t b o t t o m just be fo re the s t a r t o f fhe cu rved bow. Tl ie re.sults ere p resen ted in F i g u r e 42_where they are shown as a f u n c t i o n of ope ra t i on t i m e . I f is seen that t he e x t r e m e va lue increases s i g n i f i c a n t l y du r i ng t h e f i r s t severa l hours and t h e r e a f t e r ii ici-eoses v e r y s lov / ly i h roughou t t he 44 hour pe r i od w h i c h is the e s l i i n a t e d du ra t i on of a sea of the .specif ied .sever i ty. The f i gu re olso shows tha t the e x t r e m e r e l a t i v e bov/ i n o t i o n of t he ship w i l l exceed the cross s t r u c t u r e c l ea rance ne igh t w i t h i n four hours ope ra t i on l i m e i f no m o t i o n c o n f r o i is used. Hov/ever , i ts p e r f o r m a n c e dur ing the rema inde r of the s t o r m (about AO hours in ih is cose) should noi degrade much a f t e r the i n i t i a l t i m e pe r i od .

A l t h o u g h v/ater c o n t a c t can be expec ted i t does no t necessar i ly imp ly a s l a m . Indeed, expe r ience o l joo rd SSP K A I M A L I N O dur ing t he seakeeping t r i a l s d id show i h a t wh i l e t he re was o f a i r amoun t of v/ater c o n t o c i v / i th the '••ridging s t r u c t u r e in bend Sea S ta te 5, ihese c o n t a c t s ended a t the lov/er speeds to be gen t l e wave siaps f h o f

T.

•: '

Q. E <

o ca

2.5

2.0

1.5

1.0

0 . 5

Design Extreme Value (/3= .01)

Height of Bridging Structure Clearance (1.83m)

10 20 30 40

Time in l-lours

50 Ö0

F ig . 42 E x i r o r n e V a l u e s o f R e l a t i v e B o w M o t i o n o f

SSP ICAir.'IALIiMO i n

K n o t s

iaaci bea b ta to o at 7

d id not i m p a r l on a r res t i ng m o t i o n to the ship. Though some v a r i a b i l i t y may exist b e t w e e n the assumpt ions mode i iere rega rd ing SSP K A I M A L I N O opera t i ons and t t ie a c t u a l o p e r a t i o n a l exper ience over her l i f e t i m e , i h i s analysis does d e m o n s t r a t e the i m p o r t a n c e of i h e r e l a l i onsh ip b e t w e e n c lea rance he igh t and r e l a t i v e bow m o t i o n .

8 ^ C O N C L U D I N G R E M A f ^ K S

The resul ts of the f u l l scale t r i a l s of SSP K A I M A L I N O d e m o n s t r a t e t he good m o t i o n s c h a r a c t e r i s t i c s o f SWATH ships and the l i e n e f i i s of o u l o m o t i c miot ion c o n t r o l . The ag reemen t b e i w e o n inc s i g n i f i c a n t values of mo t i ons f r o m mode l and fu l l scaie resul ts is reassur ing for the a p p l i c a t i o n of f u t u r e rnodei scale resu l t s . The t h e o r e t i c a l p r e d i c i i o n s agree l y i j h the t w i n - s t r u t SWATFI 6C mode l d a t a . Fo r SSP K A l M A i - l N O the ag reemen t be tween t f i eo ry and miodel e x c i t i n g fo rces is reasonab le , wh i l e fhe t r a n s f e r f u n d ions de r i ved f r o m the t r i a l s da ta hove on inheren t u n c e r t a i n t y t ha t miokes f i r m conc lus ions d i f f i c u l t . The m o t i o n resu l ts f o r SSf^ K A I M A L I t ^ O i l l u s l r o t e ihe i m p o r i o n c e of sepa ra t i on of notui-al per iods and t i ie i n f l uence o i these per iods on t l ie m o t i o n response. The e x t r e m e va lue p r e d i c t i o n s o f f e r a means o f app l y i ng exper i rments and t h e o r y t o design and ope ra t i ona l p rob lems .

The SWATH ship o f f e r s g r e a t p o t e n t i a l f o r ach iev ing good seakeep ing . To the designer th i s means inc reased o p e r a b i l i t y and e f f e c t i v e n e s s . T l ie smal w a t e r p l a n e area a l te rs t he seakeeping c h a i a c t e r i s t i c s and a l lows for a r e l a f i v e l y sma l l c o n t r o l f o r c e fo mo'-.e large changes in the m o t i o n responses. The SWATH c o n c e p t locks the long design h i s to ry o f rnonohul ls ; hov /ever , the too ls requireci for p r e d i c t i n g m o t i o n s , n a t u r a l pe r iods , and e x t r e m e va lues ore we l l deve loped . T i i r ough these p r e d i c t i o n i echn iques , t he e f f e c t s on seokeep ing o f n a i u r o l per iods , m e t a c e n t r i c he igh ts , w a t e r p l a n e o rea , and o the r p a r a m e t e r s are b e c o m i n g unde rs tood .

V 1')

REFEF<ENCES

1. L a m b , G . R. and F e i n , J . A . , "The D e v e l o p i n g Techno looy fo r SWATH Ships", p resen ted at A l A A / S N A M E Advanced" ^Aarine V e h i c l e s G o n f e r e n c e , B a l t i m o r e , M d , O c t . 1979.

2. L e e , C . iv\. a m d R. M . Gu rphey , " P r e d i c t i o n o f M o t i o n , S t a b i l i t y and Wove Loads o f S m a l l -W o t e r p l o n e - A r e a , Tv. ' in-Hul l Ships", T rans . Soc. N a v a l A r c h i t e c t s and M a r i n e Eng ineers , V o l . 85, 1977, pp. 9 4 - I 3 Ü .

3. D a l z e l l , J . F., " A S i m p l i f i e d E v a l u a t i o n M e t h o d fo r V e r t i c a l P l a n e , Ze ro Speed, Seakeep ing C l i a r a c t e r i s t i c s o f SWATH Vesse ls " , D L - 7 8 - 1 9 7 0 , Ju l y 1978, Stevens i n s t i t u t e o f Techno iogy , H o b o k e n , IM.J.

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