Power prediction of WIG craft

International Shipbuilding Progress 58 (2011) 219–238 219DOI 10.3233/ISP-2012-0076IOS Press

Power prediction of WIG craft

M.A. Mosaad, M.M. Gaafary, H.S. El-Kilani and I.A. Amin ∗

NAME Department, Port Said University, Port-Said, EgyptE-mail: [email protected]

Received 18 November 2011

Accepted 3 February 2012

Power prediction for WIG crafts is challenging. This is due to the lack of information about its perfor-mance data, and hence, there is no simple method for power prediction yet available that can be used in thepreliminary design stage. In this paper, a method to predict the required power for WIG craft is presented.The authors assume that WIG craft is a prismatic planing hull facilitated with wings and longitudinalstabilizer. The hydrodynamic and aerodynamic characteristics which describe the pitching moment, lift,drag, and different centers of each force are represented in a developed mathematical model. An iterativecomputer program is structured. The output of this proposed method gives the logic and acceptable per-formance of the WIG craft compared to the related planing hull. An illustrative example is included todetermine the required horsepower, running trim and take-off speed.

Keywords: Power prediction, aero and hydrodynamic of WIG crafts

Nomenclature

a distance between Df and CG (measured normal to Df )AR aspect ratio

b hull beamC distance between CG and center of normal pressure force Nc airfoil or wing chord

CL0 lift coefficient at angle of attack zeroCLβ lift coefficient at angle of attack β

CL gr lift coefficient of wing in ground effectCL wing lift coefficientClα airfoil lift coefficientd1 distance between Da1 and CG

*Corresponding author: Islam Amin, NAME Department, Port Said University, Port Said, Egypt. Tel.:+20 1007895000; Fax: +20 66 3231901; E-mail: [email protected].

0020-868X/11/$27.50 © 2011 – IOS Press and the authors. All rights reserved

220 M.A. Mosaad et al. / Power prediction of WIG craft

d2 distance between Da2 and CGD depth

Da1 airfoil drag force with ground effect correctionDa2 tailplane drag forceDf hull frictional resistanceDah main hull air drag forceDp hull pressure resistancef distance between T and CG (measured normal to T )

F1 horizontal distance between CG and center of propeller forceF2 vertical distance between CG and center of propeller force

h ground clearance between lower point of hull bottom and water surfaceK1 distance between CG and superstructure aerodynamic force centerK2 distance between CG and hull aerodynamic force center of effortL hull length

La1 airfoil lifting force with ground effect correctionLa2 tailplane lifting force

LCG longitudinal center of gravityLCG1 horizontal distance of center of afford of main wing from transomLCG2 horizontal distance of center of afford of stabilizer from transom

Lc wetted chain lengthLk wetted keel lengthN component of resistance force normal to bottom

PLCG horizontal distance of propeller center from transomPVCG vertical height of propeller center from keel

s distance between Dah and CGT propeller thrustt2 distance between La2 and CGt1 distance between La1 and CG

VCG vertical center of gravityVCG1 vertical center of effort of main wing from keelVCG2 vertical center of effort stabilizer from keel

W craft weightε inclination of thrust line relative to keel lineτ trim angle of planing areaΔ craft displacementα angle of attack in radians.

1. Introduction

Wing in Ground (WIG) effect crafts are unique vehicles that operate at altitudesof several meters above the sea surface to take advantage of favorable aerodynamicinteraction between the wing and ground. These crafts are characterized by aerody-

M.A. Mosaad et al. / Power prediction of WIG craft 221

namic support beside the static and dynamic supports. There are many aerodynamicconfigurations for WIG crafts depending on the method of satisfying the longitudinalstability requirements [7].

This paper deals with the airplane configuration, which is the most common shapefor WIG unit; it consists of a fuselage (hull), wings, an air propeller and a longi-tudinal stabilizer. The WIG hull have the same shape of a planing boat such as afttransom and hard chine shape to generate a high dynamic support to help in accel-erating the take-off from water and the beginning of air born. Wings and stabilizershelp in generating an aerodynamic support and controlling the craft during air andwater born.

There is no simple power prediction method available for WIG craft. In this work,the authors consider WIG craft as a planing hull boat fitted with wings and stabilizer.Therefore, the method can treat WIG hull with available planing boat power predic-tion method before take-off occurs. The aerodynamic force can be predicted at allspeed regimes by estimating the lift and drag forces from wing and stabilizer at aspecified angle of attack.

Useful research works which have been published in the field of hydrodynamicsof planing hull and aerodynamics of wings are reviewed. The experimental work ofClement et al. [4], on the resistance and performance of Series 62 models is con-sidered. The work developed by Savitsky [11,12] on the prediction of planing hullperformance and the development of Blount factor to modify Savitsky method forbetter prediction of Series 62 models resistance is reviewed [2].

Savitsky method on planing hull hydrodynamics has been modified several times.The present work investigates the power prediction of the WIG craft, which has itshull similar to the planing hull form. Savitsky mathematical model of planing hullhydrodynamics, predicts the required hull power and performance [11]. Savitsky andBrown [12] had modified that mathematical model by adding fixed slope flap to theprismatic hull in order to control the hull trim angle and eliminate porpoising phe-nomenon. The flap lift and drag forces were added to the equilibrium equation, wherethe flap moment is applied to control trim. Denko [5] had modified that mathemati-cal model by adding aerodynamic lift and drag forces for planing hull and its super-structure. Hadler [6] had also modified Savitsky planing hull mathematical model byadding the effect of hull appendage and the propeller lift and drag forces to the hullequilibrium equation. Fox and Blount [3] had also modified the method by adding acorrection factor to apply when the hull has a non-prismatic form. Mosaad, Gaafaryand Amin [10] had modified the method by adding the effects of:

• a controllable flap with variable slope;• the controllable longitudinal shift of weight, and• a proposed wind spoiler

to control the trim angle and consequently reduce the porpoising phenomenon of aplaning prismatic hull.


Kerwin [8] had presented empirical equations to determine the lift and drag forcesfor the airfoil sections at different angles of attack. The authors have used this equa-tion to determine the lift and drag forces for the wing and tailplane.

WIG craft operates close to sea surface which causes an overall improvement inlift to drag ratio. Tomasz [1] had represented a numerical investigation about airfoil inground effect, showing the increase in lift coefficient in ground effect. An empiricalequation was used to correct the lifting coefficient of the free stream model to theground effect model.

A method is proposed to predict the power required for the WIG craft. This methoddepends on a mathematical model based on the sum of forces and moment about thecenter of gravity. All forces acting on the WIG body was considered, such as theaerodynamic force generated from wing and longitudinal stabilizer, hydrodynamicforces and static forces. The equilibrium trim angle, which causes the balance to theforces system, can be detected by use of an equilibrium equation which has beenderived from the presented mathematical model. The equilibrium equation is usedin an iterative procedure to determine the equilibrium trim angle, other forces andmoments, and then determine the total drag which leads to determine the requiredeffective horse power for the WIG craft.

The computational procedure is carried out using Visual Basic language to makethe iteration method easier to solve. An illustrative example is included as an appli-cation on the proposed method for determining the required powering for WIG craftat different operating speeds. Also, the application aimed to show the favorable effi-ciency of WIG configuration (planing hull with wing and stabilizer) compared withthe basic planing boat especially at high speed regime.

2. Hydro- and aero-dynamic equation of WIG’s hull

2.1. Prediction of planing hull performance

There had been many developments and published work on planing hull perfor-mance. Savitsky is one of the peak pioneers in developing a milestone work on thehydrodynamics and performance of planing hulls [1,8,11,14]. He had developed amethod for predicting the performance of prismatic planing hulls. This method in-volves the determination of the performance of planing hull as the running trim andthe resistance which provide the equilibrium condition of the hull at a given run-ning speed, load and location of LCG. For simplicity, Savitsky considered prismaticplaning hull geometry without any appendage or propeller force.

Considering the equilibrium of forces and moments acting on a planing hull asshown in Fig. 1, the vertical and horizontal equations of forces are given as:

Δ = N ∗ cos τ + T ∗ sin(τ + ε) − Df ∗ sin τ , (1)

T ∗ cos(τ + ε) = Df ∗ cos τ + N ∗ sin τ. (2)


Fig. 1. Equilibrium of forces in planing hull [11]. (Colors are visible in the online version of the article;http://dx.doi.org/10.3233/ISP-2012-0076.)

A simple equilibrium equation is given by [11],

Δ{[[1 − sin τ ∗ sin(τ + ε)]C/ cos τ ] − f ∗ sin τ} + Df (a − f ) = 0. (3)

Savitsky [11] had discussed the various components of the hydrodynamic forcesacting upon a planing boat hull and how they can be estimated. The results had beensummarized in the form of equations and charts. To make it simple and applicable tothe designer, they must be combined with computational procedures for predictingthe power requirements. The method presents a procedure for computing the runningtrim, wetted length, resistance, and power requirement of any planing hull, whichwill be determined for the specific equilibrium condition of the hull at a given speed,load and center of gravity location.

2.2. Danko equilibrium of hull forces

Danko [5] had presented a mathematical model based on Savitsky [11] method forthe performance prediction of planing hull of prismatic form in calm water. Dankoadded air drag force to Savitsky equilibrium equation. Empirical air resistance forceswere presented in Danko work and its center of pressure is determined. Figure 2shows the forces and moments acting on the planing hull with an air drag force.

T ∗ cos(τ + ε) − Df ∗ cos τ − N ∗ sin τ − F1 − F2 = 0, (4)

T ∗ sin(τ + ε) − Df ∗ sin τ − N ∗ cos τ − Δ = 0, (5)

T ∗ f + F1 ∗ k1 + F2 ∗ k2 − Df ∗ a − N ∗ C = 0. (6)


Fig. 2. Equilibrium of planing hull. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

After elimination of propeller thrust, T , and pressure resultant forces, N , the basicequilibrium planing equation becomes:

Df (f + C ∗ sin ε − a ∗ cos ε) + Δ[f ∗ sin τ − c ∗ cos(τ + ε)]

+ (F1 ∗ k1 + F2 ∗ k2) cos ε + (F1 + F2)[f ∗ cos τ + c ∗ sin(τ + ε)]

= 0. (7)

Air resistance forces, F1 and F2, are calculated empirically for small planing hullsaccording to Michelsen [9]:

F1 = 0.0003716 ∗ A1 ∗ V 2, (8)

F2 = 0.0000929 ∗ A2 ∗ V 2. (9)

2.3. WIG ground effect and airfoil lift and drag forces

In the present work, it is assumed that logically the WIG crafts do not trim morethan 15◦, so that the relation between the lift coefficient and the angle of attack isconsidered to be linear. Kerwin [8] had presented in his notes an equation to deter-mine the lift and drag forces for the airfoil sections at different angles of attack as alinear relation, i.e., as a 2D case. The authors have used this equation to determinethe lift and drag force for the wing and the stabilizer.

The lifting line theory has been basically developed on the basis of the potentialflow linear theory. Hence, the flow is assumed to be inviscid and the only force


produced due to the section camber is the lift force. Nevertheless, the airfoil sectiondrag is derived as an empirical correction for each airfoil section form with specificsection parameters such as (area, chord, thickness, angle of attack and others).

It is known from the literature of lifting line theory that lift is generated due to theform of the airfoil camber line curvature, and has nothing to do with the thicknessdistribution of the airfoil section itself. Hence, the airfoil drag forces may be esti-mated by using the airfoil empirical equations. The drag forces are affected by thesection thickness, distribution, the position of maximum (t/c) ratio, angle of attackand other parameters.

Therefore, it is the core hypothesis of the lifting line theory to concentrate the liftforce along the camber line into the point of the airfoil section center of effort.

According to Kerwin [8], the lift coefficient for airfoil section can be calculated asfollows:

CL0 = 2πα. (10)

According to the lifting line theory, the lift coefficient can be calculated for a 3Dcase in the following way:

CL−3D = CL0

(AR

AR + 2

)α. (11)

A WIG craft operates close to the sea surface which causes overall improvement inlift to drag ratio. Tomasz [1] had presented a numerical investigation about an airfoilin ground effect, showing the increase in the lift coefficient due to the ground effect.An empirical equation has been used to correct the lifting coefficient of a free streammodel to the ground effect model:

CL gr = CL

(h

c

)−0.11

. (12)

According to Wieselsberger [13], the drag coefficient with the effect of ground canbe calculated as follows:

CD = (1 − σ)CL ∗ S

π ∗ b2 , (13)

where: σ is the “influence coefficient”.

σ =1 − 0.66h

b

1.05 + 3.7hb

. (14)


2.4. Center of effort of wing

The center of lift of most wings beyond the ground effect is located about onequarter of the wing chord from the leading edge of the wing. The aerodynamic char-acteristics of some NACA wing profiles can be found in Kerwin [8]. From this re-ported data, it can be seen that the centre of lift of most wing profiles is located closeto 25% of the chord from the leading edge of the wing. The reason for this is that themajority of lift is generated by the suction pressure profile over the upper surface.

3. Developed mathematical model for power prediction of WIG craft

Different modes of forces are acting on WIG crafts during operation, namelystatic, dynamic and aerodynamic forces. First, when the craft is at rest or at a veryslow speed, the only force acting on the WIG craft is the buoyancy force (staticforce). By increasing the speed, the craft transfer to the dynamic mode where a hy-drodynamic force is being applied to craft. The craft meets a hump resistance beforetaking-off from water. At the take-off speed, the craft will lift off from the watersurface to reach the true flying mode in ground effect. After take-off, a daylightclearance will exist under the hull bottom, and the resistance will suddenly drop,allowing swift acceleration to cruising mode.

A proposed method is presented in this work to predict the power required to WIGcraft. The method depends on a mathematical model to sum up forces and momentsabout the center of gravity, as shown in Fig. 3. There is one running trim angle whichcauses the balance of forces system. An iteration method is applied to determine theequilibrium trim angle which leads to determine the other forces and moments.

The proposed mathematical model is constructed by adding the aerodynamic forcegenerated from the wing and the longitudinal stabilizer to the hydrodynamic andstatic forces acting on the WIG hull. An equilibrium equation is used to detect theequilibrium trim angle which causes the balance between all forces and then calcu-late the total drag which leads to the required effective horse power for WIG crafts.The proposed computational procedure is programmed in Visual Basic language toallow iterative method calculations.

The basic equilibrium equation in the present proposed method has been derivedfrom the following equations.

For vertical equilibrium:

W = N ∗ cos τ + T ∗ sin(ε + τ ) − Df ∗ sin τ + La1 + La2. (15)

For horizontal equilibrium:

T ∗ cos(ε + τ ) = Df ∗ cos τ + N ∗ sin τ + Dah + Da2 + Da1. (16)


Fig. 3. Proposed mathematical model. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

For equilibrium of pitch moment (the total equilibrium equation):

N ∗ C + Df ∗ a + T ∗ f − Dah ∗ s − Da1 ∗ d1 − Da2 ∗ d2

+ La2 ∗ t2 − La1 ∗ t1 = 0. (17)

Assuming that cos(ε + τ ) ≈ 1, since the thrust acts at a line parallel to keel.For equilibrium in keel direction:

T ∗ cos(ε + τ ) ≈ T

= W ∗ sin τ + Df + Dah ∗ cos τ + Da1 ∗ cos τ + Da2 ∗ cos τ

− La2 ∗ sin τ − La1 ∗ sin τ. (18)

Therefore, using equation (18), the vertical equilibrium equation becomes:

W = N ∗ cos τ − Df ∗ sin τ + La1 + La2 +[W ∗ sin τ sin(ε + τ )

+ Df ∗ sin(ε + τ ) + Dah ∗ cos τ sin(ε + τ ) + Da1 ∗ cos τ sin(ε + τ )

+ Da2 ∗ cos τ sin(ε + τ ) − La2 ∗ sin τ sin(ε + τ )]. (19)

Let

P1 = 1 − sin τ ∗ sin(ε + τ ), (20)

P2 = 1 − cos τ ∗ sin(ε + τ ). (21)


Then, the normal pressure force is expressed as:

N =[W − La1 − La2](P1) − [Dah + Da1 + Da2](P2)

cos τ. (22)

The total drag in the general condition before take-off condition is:

Dtotal = Df + DP + Dah + Dwing + Dstb. (23)

A computer program is carried out to detect the WIG take off speed by the followingcondition of vertical equilibrium of forces:

Total hull weight = Wing lift + Stabilizer lift

+ Thrust vertical component. (24)

The proposed method is sensitive to each operating mode by using conditional sen-tences in the program to control the canceling of all hydrodynamic forces and mo-ments acting on WIG hull after the craft take-off from water. At take-off condition,there is no contact with water surface, so that, frictional drag (Df ), pressure drag(DP ), and dynamic and static lift (N ) will equal zero.

N = 0, (25)

Df = 0, (26)

DP = 0. (27)

The proposed method depends on an iteration loop to determine the equilibriumtrim angle, by determining all moments about the center of gravity and detectingwhich trim angle will satisfy equilibrium condition for hull. The determination ofthe distance between the center of gravity and the center of effort of all force actingon WIG craft is a big challenge. Figures 4–6 illustrate the required distances. Thearms as measured from the center of gravity are expressed as:

F1 = (LCG − PLCG) ∗ cos τ + (PVCG − VCG) ∗ sin τ , (28)

F2 = (PVCG − VCG) ∗ cos τ − (LCG − PLCG) ∗ sin τ , (29)

t1 =LCG1 − LCG

cos τ− [d1 ∗ tan τ ], (30)

t2 =LCG − LCG2

cos τ− [d2 ∗ tan τ ], (31)

S = [(0.5D − VCG) + (0.75L − LCG) ∗ tan τ ] ∗ cos τ. (32)


Fig. 4. Detection of the center of effort of the wing and stabilizer lift forces. (Colors are visible in theonline version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

Fig. 5. Detection of the center of thrust force. (Colors are visible in the online version of the article;http://dx.doi.org/10.3233/ISP-2012-0076.)

All forces acting on the WIG craft and its center of effort are detected, as well as thearm of each force about the center of gravity as a function of trim angle. Equilibriumequations is derived in order to collect all moments around the center of gravity andfind out the equilibrium trim angle and hence estimate all drag force components andthe required effective power.

4. Developed computer program

The equations presented in the past two sections are programmed in Visual Basiclanguage, to allow the required iterations procedure. The flow chart given in Fig. 7summarizes the iteration procedure to illustrate the steps of the program. It starts


Fig. 6. Detection of the center of aerodynamic hull drag force. (Colors are visible in the online version ofthe article; http://dx.doi.org/10.3233/ISP-2012-0076.)

from the input data of the hull, estimates the different forces and centers of effort,iteration loop to determine the equilibrium trim angle which satisfies the equilibriumequation then check whether the take-off occurs or not, finally the required EHP andthe total resistance are the determined output.

5. Illustrative examples

A case study is used to illustrate the proposed computational method to predictthe required power and take off speed for WIG craft. The application illustrates theimprovement in performance at high speed range due to adding wings to a planinghull boat (WIG configuration). The equilibrium trim angle at each operating speed,and all drag forces acting on the WIG craft (frictional, pressure and air drag forces)are estimated. Also the program estimates the draft, wetted keel and chine length, andall lifting forces such as wing lift, stabilizer lift and vertical component of propellerthruster forces. The effective power required to the WIG craft is the main aim ofrunning the program.

1 – Planing boat without wingsBoat length = 24 m, B = 3.75 m,Weight = 11.3 ton, Deadrise angle = 10◦,LCG = 9 m, VCG = 1.25 m.

2 – Same planing boat with wings and longitudinal stabilizerWing span = 12.5 m,Wing center of pressure LCp = 10.93 m,VCp = 1.9 m,Stabilizer center of pressure = 0.625 m, VCp = 6.25 m.


Fig. 7. Flow chart of proposed program. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

Analysis of the results

The results in the example show that the wing added additional drag in slow andmedium speed until the speed reaches the planing speed; after this speed regime thewing generates a high lifting force that can take off the boat from water and the dragrapidly decreases until the boat hull exits from water and begins the flying mode.

The results are presented graphically as shown in Figs 8–15.Figure 8 shows the variation of the total resistance and propeller thrust with speed.

The propeller thrust is considered as a given data. The propeller thrust should cover


Fig. 8. Total resistance and propeller thrust versus speed. (Colors are visible in the online version of thearticle; http://dx.doi.org/10.3233/ISP-2012-0076.)

Fig. 9. Different drag components versus speed.

all operating speeds specially the hump regime or maximum resistance before take-off. The program detects the hump resistance point required for the selection of the


Fig. 10. Draft versus speed. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

Fig. 11. Weight, static, dynamic and aerodynamic lift forces versus speed. (Colors are visible in the onlineversion of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

propulsion system. The figure shows the huge drop in total resistance (about 70%)after the craft takes off from water (70 knots).


Fig. 12. EHP for basic hull and hull with wing and stabilizer (WIG configuration). (Colors are visible inthe online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

Fig. 13. Draft for a basic hull and a hull with wing and stabilizer (WIG configuration). (Colors are visiblein the online version of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)


Fig. 14. Frictional drag for a basic hull and a hull with wing and stabilizer. (Colors are visible in the onlineversion of the article; http://dx.doi.org/10.3233/ISP-2012-0076.)

Fig. 15. Trim angle for boat and WIG configurations. (Colors are visible in the online version of thearticle; http://dx.doi.org/10.3233/ISP-2012-0076.)

Figure 9 shows different drag components and total resistance versus speed. Itis observed that, the dominant drag component before take-off is the friction drag(about 55% from total drag), the contribution of the aerodynamic drag is about 30%,and that of the pressure drag is about 15%. After take-off, the aerodynamic compo-


nent presents all the drag acting on the craft. It should be noticed that at the take offspeed, where no contact exists between WIG hull and water, friction and pressuredrag disappear.

From Fig. 10 it is observed that, at slow and medium speeds the draft is slightlyincreased due to suction effect. By entrance in the planing regime, the draft decreasesrapidly until take off occurs.

According to Fig. 11, at rest and at slow speed the only lifting force supporting thehull is the buoyancy force as a static force (from 100 to 90% of total craft weight).With increase of the speed, the dynamic force starts to appear while the aerodynamicforce is still not sufficient to support the WIG (almost half of craft weight is sup-ported by dynamic lift force at 40 knots). The normal force (N ) in Fig. 11 consistsof the magnitude of static and dynamic lift force acting on hull bottom. At highspeed range, the aerodynamic lift supports the craft until the take-off occurs; neitherdynamic nor static forces act upon the hull, only aerodynamic force and the WIG isoperating in flying mode (take off occurs at 70 knots).

The results in Figs 12–15 show some of performance characteristics for WIG con-figuration (hull with wings) compared with a planing boat of the same dimensionsof the WIG hull without wings. The authors assume that the total weight of the boatand the WIG is the same. The comparison aims to illustrate the advantage of usingthe aerodynamic support and the favorable effect of ground effect. Figure 12 showsthe comparison between the effective horse power (EHP) for the boat and the WIGconfiguration. The EHP required for the boat at slow and medium speed decreasesfor the WIG configuration due to the additional drag caused by wings and longitudi-nal stabilizer. After the WIG craft takes-off from water, there is a sudden drops in therequired EHP due to the absence of frictional and pressure drag when the boat is freefrom water. It can be observed that, after take-off the WIG craft can consume thirdof the power required for a planing boat having the same weight and dimensions ofWIG craft.

Figure 13 shows the comparison between draft in case of WIG configuration andbasic planing boat without wings. The draft for WIG craft increases compared tothat of the basic planing boat at slow speed due to the wings moment acting on hullwhich causes an increase in trim angle and an increase in draft aft. After the WIGtakes its speed, the aerodynamic lift causes a decrease in draft compared to basicplaning boat.

Figure 14 shows the comparison between frictional drag force for WIG craft andplaning boat. The use of a WIG configuration aims to escape from the large frictionaldrag force in high speed range. This is clear since the frictional drag force is smallerthan that of a planing boat in slow, medium and high speed ranges. A sudden drop infrictional drag after take-off occurs due to getting free from water.

Figure 15 shows the comparison between the trim angle of a WIG craft and aplaning boat. The same behavior is observed for both units, but a WIG craft shows asmall increment in the trim angle than a planing boat at slow and medium speed dueto the wings aerodynamic negative moment acting on the craft. At high speed range


the trim decreases for both units, until the WIG take-off from water. This correspondsto the trim equilibrium operating condition.

6. Conclusions and future recommendations

WIG craft is a marine craft that depends on the aerodynamic support in its operat-ing regime to increase the speed range and decrease the resistance facing the vesselwhich leads to increased transport efficiency. Aerodynamic support for marine craftis a new evolution in marine field, which is necessary in the near future to improveits overall efficiency. There are no empirical or mathematical procedures to deal withWIG craft. The authors consider the WIG craft as a planing hull fitted with wingsand stabilizers. The output of the proposed method gives the logic and acceptableperformance characteristic of a WIG craft compared to the corresponding planinghull.

Applications of the method using Visual Basic program show that the proposedmethod can be used as a simple method to predict the power of WIG crafts effec-tively. The program predicts the required power at any speed. Also, the program canpredict the running trim angle and determine the take off speed. The program givesa good indicator to the designer in the preliminary design stage.

The application results show that after take-off the WIG craft from water can con-sume third of the power required for a planing boat having the same weight anddimensions, The drop in the required power for a WIG compared to a planing boatis then about 70%.

Also, the dominant drag component before take-off is the friction drag (about 55%from total drag), the contribution of the aerodynamic drag is about 30%, and that ofthe pressure drag is about 15%. Many devices can be used to eliminate the frictiondrag during hump resistance, such as (flap, hydroski and air lubricating).

References

[1] T. Abramowski, Numerical investigation of airfoil in ground proximity, Journal of Theoretical andApplied Mechanics 45(2) (2007), 425–436.

[2] D.L. Blount and D.L. Fox, Small craft power prediction, Navel Ship Engineering Center, 1975.[3] D.L. Blount and D.L. Fox, Small craft power prediction, Marine Technology 13 (1976), 14–45.[4] E.P. Clement and D.L. Blount, Resistance tests of a systematic series of planing hull forms, SNAME

Transactions 71 (1963), 491–579.[5] D. Gugle and T. Prolic, The design space of hydrodynamically adequate projects of planing hulls,

in: IMAM’95, Dubrovnik, 23–27 April 1995.[6] J.B. Hadler, The prediction of power performance on planing craft, SNAME Transactions 74 (1966),

563–610.[7] R.D. Irodov, Criteria of Longitudinal Stability of Ekranoplan, Ucheniye Zapiski TSAGI, Moscow

1(4) (1970), 63–74.[8] J.E. Kerwin, Lecture Notes Hydrofoils and Propellers, January 2001.


[9] F.C. Michelsen et al., Small craft engineering resistance, propulsion and seakeeping, College ofEngineering, The University of Michigan.

[10] M.A. Mossad, M.M. Gaafary and I.A. Amin, Energy saving and dynamic stability of planing hulldue to hydrodynamic control of trim angles, in: IMAM Conference, Portugal, October 2005.

[11] D. Savitsky, Hydrodynamic design of planing hulls, Marine Technology 1(1) (1964), 71–95.[12] D. Savitsky and P. Brown, Procedures for hydrodynamic evaluation of planing hulls in smooth and

rough water, Marine Technology 13(4) (1976), 381–400.[13] C. Wieselsberger, Wing resistance near the ground, NASA (non Center Specific), NACA-TM-77,

USA, 1922.[14] L. Yun, A. Bliault and J. Doo, WIG Craft and Ekranoplan, Springer, Berlin, 2010.

Power prediction of WIG craft

Documents

Transcript of Power prediction of WIG craft