Cylinder Mold Construction Manual

Cylinder Mold Construction Manual 11

CYLINDER MOLD MULTIHULL CONSTRUCTION Cylinder Molded multihulls have come a long way in thelast twenty nine years. By now in 2012 there are hun-dreds of CM built multis out there building or sailing now.The original KAMANU, the first C.M. USCG certified char-ter catamaran has been sailing nonstop for nearly twentyfive years. Dozens of C.M. cats are now USCG certifiedto carry passengers. Builders on all continents are findingout that Cylinder Molding can give them the very highestquality wood/epoxy multi hulls in less time than any otherone-off construction system there is. With the economyin shambles, the fact that a CM hull cost about 40% lessto build than a foam/glass/epoxy one probably explainsthe recent surge in interest in CM multihulls. And, unlikeother molding systems, CM lets builders get the hullshapes they need, not the ones the system allows. Hereis how it evolved. While it started all those years ago,builders have continuously helped with improvements. Ithas been constantly evolving. Many of my friends and clients share a curious dilemmawith me; we crave sleek, high speed multihulls, but wecannot possibly afford them. Over the years we havedeveloped a dodge to get into these fantastic boats at alower cost and in much less time than usual. The dodge isa rapid hull construction system, usually called cylindermold, though nothing about it resembles a cylinder. Thehull sections that we use are conic sections like ellipses andparabolas, but not cylinders. I have been more accu-rately calling it vacuum-formed plywood, but the name“cylinder mold” seems to have stuck, so it will be used. Cylinder molding is an odd way to build boats. It is aprocess where vacuum pressure and epoxy are used toform thin sheets of plywood into thicker panels that areused to build hulls. Instead of the project progressingslowly in an incremental, linear, cumulative fashion, youstart with the skin and work inward. Instead of being ableto take continual stock of your progress, you will meta-morphose raw materials into a hull in three major steps.Each major step will last only long enough for the epoxyto cure. Then, except for detailing and trimming, like it ornot, it is done. So curmudgeons beware! Not everyone issuited to build this way. Instead of carefully followingdirections in a repetitive haze, a builder needs to makerapid eyeball judgements, with confident calls. To helpbuilders make those eyeball judgements, we offer aConstruction Manual on Video or DVD. It is available tobuilders of CM multihulls.

Kamanu

Cylinder Mold Construction Manual12

and chop it into bits. Then they apply many many hours toconvert it back to plywood again. Rough plywood. It makesno sense.I strongly believe in using the high quality industrial materialsall around us. To take fullest advantage of the quality, onehas to stay in the nature of materials. For example, mostCM hulls are made from 3mm Asian plywoods. These thin

“If you want curved plywood-curve plywood!”Stig HoffstedtSweden THE NEED I feel that multihull builders need a system of generatinghulls, uncompromising hulls at very low cost and with far lesswork than usual. A fast building system that creates slowhulls is of no use. They would have to be of the mostefficient round bilge shapes.Also, I have always been curious about why many multihulldesigners force builders to work outside the nature ofmaterials. Manipulating a blob of fiberglass resin into asmooth, fair, thin shape using typical composite constructioncan only be done by applying many many hours. Coldmolded plywood or Constant Camber? tm ?constructionsystems typically take thin, smooth, fair sheets of plywood

plywoods are fantastically smooth and fair. Hulls made fromthem are smooth and fair. Builders need a constructionsystem that lets them take advantage of the industrialprocesses already out there. CM does let builders work inthe nature of the plywood. The results are lower cost,fewer hours required, and better quality.The technology of modern multihulls is changing very rap-idly. A design that was called “state of the art” two or threeyears ago, is obsolete and overweight now. The implicationsare several. If you survey the common methods of hullconstruction, there are a lot of expensive wrong turnsavailable to one, both in dollars and in product. Lets look atsome of them. Buying a very sophisticated multihull design, such as a one-off composite craft, will cost a great deal of money. Also,typical composite one-off multihulls are extremely timeconsuming to build. By the time an amateur builder finished,the design that was the latest state-of-the-art when hebegan might be obsolete. Still, buying a less expensive boat, such as a productionrun composite boat, gets a product that is by definitionobsolete. The accountants will still want to get as manyboats as possible off of those ten or 20 year old molds.


had a pair of panels done. Thataccomplishment would be about 5hours into a CM hull. I do noticethat they never used to count thetime to build a bagging table in thetime-to-build. Now they notebagging table time to build as 5hours I think. I’d like to see that in5 hours. Its basically cabinet-making. In CM, the panel is itsown bagging table, as it were. One

is not needed for the hull.

Next I saw the chop saw come out and the hull panelsgot chopped along the keel into short darts. These bitswere then bent into a profile and glassed over. Theprimary structure was secondary bonded. Their basicpremises, chopping the full hull length fibers, and depend-ing on secondary bonding for primary strength, arecontrary to modern composites practice. (see ProfessionalBoatbuilder #66, page 78) The best study shows that apolyester secondary bond with a 12/1 taper ratio hasabout 20% the strength of the same thickness of originallamiminate. See tensile PDF. In Miami I saw no taper, sothe strength must have been even less than 20%. Thefull length, properly oriented fibers, used with epoxy,(seeepoxy) that are so important to multihull rigidity andstrength are ignored as I understand KSS. The level ofsmoothness and fairness that I saw in Miami, especiallybelow the waterline, would be utterly unacceptable in amodern boatyard. KSS is actually a variation on platenmolding, which has been used to build mega-yachts fordecades. KSS has been changing constantly, while declar-ing each version of itself to have “no downsides”. It ispleasing to see composite panels apparently being devel-oped similar to CM. I believe there are more elegantways to do that. So, I did publish and present a paper onrapid composite construction in 1992. See the paper, and

Kit Panel? tm by Kelsall seems to be rapid to build. Infact, in one of the recent issues of a magazine, it lookedlike he had just discovered cylinder molding, but withfiberglass. In the pictures below, one is a recent KSS hulland the other is a CM hull from 15 years ago. Both aredeveloped and both look to be wired along the keel.Sweet. The CM hull is the one on the right. It does looka little straighter along the keel to me.

At a recent Multihull design confer-ence, in Miami, I watched severalpeople working late into the night tobuild a little KSS hull. By the middleof the second day they basically

Which one is the KSS andwhich is the CM hull?


other articles. A custom cold-molded boat may be seen as a compro-mise between the two above choices. It may be moremodern than the factory run boat, and less expensive thana composite one-off boat. However that compromiseignores one of the major reasons that multihulls are betternow than they were three years ago, that is fiber orienta-tion. The modern boats have the strongest fibers orientedwith the lines of stress. The older designs are what is calledquasi-isotropic, meaning for practical purposes, equallystrong in all directions. But hull stresses do not run equally inall directions, so that would be an inefficient way to build. Atriple diagonal hull would be considered quasi-isotropic. Adouble diagonal hull would be even less structurally efficient.It would have only a third the bending strength in the planeof plies, compared to a properly oriented plywood skin ofequal thickness. For me, a cold-molded hull is too clumsy inits structural attributes alone, ignoring for now the largeamount of work required to build it. Strip plank might be a good choice. Where complex hullshapes are called for I specify these. It is again howevermore time consuming. A strongback must be set up, thestation patterns lofted, beveled and lined up. Then thestripping material has to be beveled and scarphed to fulllength. Each ofthese pieces hasto be individuallyepoxied on thejoining faces.Then every piecehas to be carefullyfastened to themold with struc-tural bog spreadbetween everypiece. Theexcess bog issqueeged awayand the wholething is faired.Finally rovingreinforcement is laid up on both side of the hull, and it issurfaced with bog and faired. Note however that the strip pieces can’t just be laid down.Since the hull gets skinnier at the ends, some strips have tobe cut back at the ends. To resist those multihull globalloads, the sheer and keel areas must be full length. Thatmeans that many of the pieces between have to be care-

Strip plank Imaginesanding this hull


fully fitted in. It is a fussy, time consuming job requiring lotsof bog. For complicated or fat wood/epoxy hulls I usually callfor strip plank foam. Fusion Hulls can be a combination of CM and core materials.They can be a CM bilge with composite topsides or a full onCM hull with core bagged inside to eliminate stringers. Another choice might be sheet plywood hulls. Face grainoriented plywood could develop more structural performancethan cold molded hulls but the choices are limited. They wouldhave to be a “V” section, a chine hull, or a developed com-pound shape. A”V” section is absolutely out of the question.There may not be any other way to come up with less boatusing more material. A chined hull has poor hydrodynamics,and enough structural discontinuities to be an unacceptablechoice for an optimum hull. Also, aside from the structure, even the simplest chine hull,even a “V” section will be noticeably more work than the samesize cylinder molded hull. The developed hull or Stressform tm, compounded fromsheet plywood, has the most promise; but there are difficultieshere too. Boats bigger than daysailers need hulls of a certainminimum thickness. Thick plywood, however, is very difficult tocompound. Imagine springing a full, high prismatic shape into asheet of 12mm marine plywood. It won’t go. So the choicesare to keep the skin thin, and consequently, the boat small, orwork something else out. Cylinder Mold Technology is thesomething else. And it works out very well. HISTORY Cylinder molding is a relatively recent invention. It requires thesimultaneous combination of several rather moderntechnologies:the use of plywood, of modern epoxy, and ofvacuum bagging. The earliest record that I find of this sort of boat constructionseems to be by Uffa Fox with his tortured plywood hulls. Whilehe did not use epoxy or vacuum bagging, our molding technol-ogy owes a good deal to his developed or tortured ply boats.(The Gougeon brothers book on boat construction has a verycomplete chapter on the construction of developed plywoodboats.) Certainly cylinder mold hull building was around before Ibegan using it. The problem was, the panels that I saw beingbuilt didn’t look like anything that I would want on my boat.They were made with rough, nasty veneers, and the sectioncurve was the same for the topsides as it was for the bilge. Iknew boats shouldn’t look like that.


In 1980, when I got a high quote to build two new 30'amas for my tri, I decided that I had to find a way to geta pair of amas for a lot less money. They had to be goodlooking amas, and had to be built quickly.I managed it, using cylinder mold technology. Now,between myself and other builders, at least 400 hulls later(maybe as many as 500) it can be called a very success-ful system. Here is what cylinder molding can do. POTENTIAL In a reasonably equipped woodshop, you can expect tobuild a cylinder mold in six or eight hours. To build a 35'hull for example, you can expect to build a full length moldfor seventy or eighty dollars. The mold for a full standingheadroom hull will be just over a hundred dollars. On thatmold you will be able to lay up a full length hull panel in twoor three hours. Preparing for the layup will take up to ahalf day, depending on the size of boat. The panel forthis 35' hull will be made from a dozen or so sheets of thin(usually 3mm) plywood. These sheets will be coated withepoxy, laid on the mold two or three thick, and vacuum-bagged together into a panel. The panel will become onecontinuous sheet of plywood, four or more feet wide andthirty five long, with the transverse curve already in it. When the panels are compounded into hulls, they willhave several important characteristics. The face grains willbe oriented along the lines of greatest bending stress; thesurface will be smooth already, and because the processof developing a hull treats it like a three dimensional battenspline, it will be automatically fair. You can expect a two layer (6mm.) hull skin with theoutside glassed, to weigh .9 to .95 lbs. per square foot.That is very close to a comparable cold-molded or com-posite hull weight using E glass. The hull costs will rangefrom $4.00 per square foot for Asian plywood to $5 ormore per square foot for Bruynzeel plywood. Cold-moldedhulls will be in a similar cost range, but composite onecould be double the price. A composite hull using highmodulus exotic fabrics could run ten times that price persquare foot. A moderately experienced builder can expect to deliver asimple hull, an ama for instance, decked and glassed, atthe rate of 12 to 15 minutes per square foot of hull. Forcomparison, a cold-molded oneoff hull will run between 2and 3 hours per square foot, without decks. Paint readyrequires the usual smoothing time, but the developedplywood surfaces are already several orders smoother


than cold molded or composite surfaces. Fairing and filling willbe needed at the keel area, but the large, flatter surfacesusually only need smoothing. Using this evolution from Uffa Fox’s fold up boats, one canbuild hulls, even large hulls, at labor and cost savings thatare unattainable by other systems. At the same time thestructural integrity is much better. CONSTANT CAMBER I am still asked if this is like Constant Camber tm.Except for the fact that we both use wood, epoxy andvacuum bagging, the answer was no. There was no similar-ity. Building the Constant Camber tm mold alone looked tome like a major operation. My sense was that the hullsmade that way have more in common with cold- moldedhulls. Just count the number of pieces, and what has to bedone to them. In fact, over the last few years CC hasbeen adopting many of the techniques that we started with,including full length molds, elliptical mold sections, and onone boat, full sheets of plywood. I used to note that theystill have not been using scarphs, proper longitudinalfacegrain fiber orientation, and I had yet to see a CC hullthat has a good modern hull shape to my eyes. One withthe minimum wetted surface round sections that I like, andthe nice wide transoms that dampen pitching and givepowerful reaches. I stand corrected. I just saw a CC 65catamaran that used full sheets of properly orientedbagged plywood, came from a mold with no surface, hadnice sections, used stringers andlots of bulkheads. Except for thesheets of ply not being scarphed, itwas exactly what we have beendoing in CM for about a quartercentury. Again, sweet. Welcomeaboard. CONSTRUCTION Lets take a run through the stagesof hull construction. We can buildthat 35' ama as an example.I will not include the very significanttime required to set up your shop,or the time required to collectmaterials. There are too manypossible differences betweenbuilders and situations.Building the mold will be done first. To do that, cut outenough identical mold sections to allow one for every two

The Mold


feet of panel length.Locating the sections atexactly two feet is notnecessary, but makingthe sections identical is.Once they are cut out,from waferboard, stackthem together andcompare them. If theyare at all different fromeach other, a grinder willmake them all match. Alumber cleat is addedtop and bottom to finishthe mold. The firstmajor step is to buildpanels. Each hull will

have two full length panels, port and starboard. A scar-phed edge must first be put on every thin sheet ofplywood to be used on the entire hull. That is easy. Stackthe sheets in an even 8 to 1 step back, using a grinder togive them identical scarphs. It will take about an hour.Consider that every piece in a panel will be both lappedand scarphed to the others near it.A vacuum system must be set up to bag a panel. Iusually use a shop vac. It can both clean your shop andsuck a vacuum to make panels. For the bagging materialwe use 4 or 6 mil polyethylene. For the bag edge seal,storm window snap bead has proven to cause the lowestgrief at best cost. It can be used a dozen times, isclean, rapid, and not bothered by wrinkles and dust. Asa diffusion layer inside the bag, bubble-pak is best.

I have laid up panels by myself, but it is many timesfaster to have a helper or two. Enough sheets of 3mmplywood to build a panel will be coated with epoxy on thescarphs and mating faces only. When this is done, thesheets of plywood are laid on the mold. Finally close thebag and start the vacuum pump up. To build their lastpanel of the 36’KAMANU, Schooner Creek Boatworksspent only 38 minutes. It takes me an hour or two work-ing alone. Remember again that every sheet of thinplywood forming a panel is both lapped and scarphed tothe others. The result is a panel that behaves, whencompounded, like the huge, fair, batten spline it is. Natu-rally that will make a stronger hull. After the bag ischecked for leaks, the vacuum pump must run longenough for the epoxy to cure, usually eight or ten hours.If you have not done vacuum bagging before, it is quiteamazing. Your own mild mannered shop vac will suckenough vacuum that you cannot physically lift the bubble

Vacuum Bagging

Apply Plywood


pak off of the panel.

After completing the two panels (port & starboard)necessary to build a hull, a pattern called the flat sur-face profile is laid out on each panel. Cutting along thepattern lines gives two identical half hulls. Identical sheertimbers are next glued to each profile.The reflected profiles are wired together along the keel.The sheers are spread to the distance noted on theplans. At the bow area we install a composite stem.The stem unit allows one to keep the stem area undercontrol, a condition that was a bit lacking in previousdeveloped hull schemes.The next major step is the keel pour. The hull halves willnow be fully joined. We build the keel from a mixture ofepoxy and fillers, under biaxial roving.To create the proper developed fullness, and to insure thatport matches starboard, a temporary deck flange is to bebuilt. The final major step in hull building is when the deckflange is set in place. The bulkheads and stringers may nowbe added. Since the hull skin has already assumed asmooth fair surface, the bulkheads must be fitted lightly.They should be a littleunder size to keep fromdeforming the hull skin.With proper planning, theinside of the hull maycoated at the same timethat the bulkheads arecoved in and the stringersare pushed in through theopen transom. If theentire job is done in one“wet out”, no insidesanding or bonding prepa-ration is needed. I likethat.The best decks seem tobe built with compositecores bagged onto thinplywood. There is timesaving over using plywoodstringers, and the coreddecks reduce themicrofissuring that oftenoccurs on the underside of epoxied plywood/stringer decks.After the bilge wires are removed, and the edges trimmed,the keel has a layer of biaxial tape added to the outside.Any necessary filling is done at the sheers or keel, and thehull is sheathed with a layer of 4 oz. or 6 oz. glass cloth.A light, high performance hull can be built very quickly at low

Panels

Installing bulkheads and stringers.


cost. This technologyshould give one moreof a chance to finish aboat before it becomesoutdated, or the moneyruns out.This is not to suggestthat cylinder molding willmake boats inexpen-sive. Hulls are only asmall part of the cost ofa boat. Multis that arealmost all hull will see bigsavings. Bridgedeckcats for example willsee smaller relative timeand cost savings fromrapidly built hulls.

PROBLEMS Every building system has limitations. CM is no exception.There are some restrictions on the possible hull shapes.To keep building time down, CM hulls should be simplecurve sections. Flares, steps, and hollows are not easilypossible. These hulls must have slenderness ratios 10:1 or thinnerat the waterline if they have reasonable amount of rockerin the profile. Within these constraints however, virtuallyany hull shape is possible. Also, as a designer, I cannot locate things in a cylindermolded hull with absolute certainty. For instance, I couldspecify that a seat be located 14" below the sheer level. Icould not also state that it would be some exact distanceup from the keel. I can come close, but there is still someuncertainty. Two hulls built from identical profiles butdifferent plywood might come out slightly different. Youcould not see the difference, but a tape measure mightshow it. It is eyeball boat building, with ‘make port reflectstarboard’ being the dominant rule. If you must haveexact accuracy in all aspects, this is not for you. I’mhappy to design boats for you to be built in other con-struction systems. We do many composite boats forclients where time and cost are less important. CM hulls are better suited for some types of multis thanothers. I see CM as being ideal for hulls such as tri amasor charter cats where interior living in the hull is not

Smooth and Fair Hulls


needed. I see CM as having the least advantage in hullssuch as livaboard cats where interior is everywhere in thehull and the structure must be carefully finished. Finally, one must be careful about which plywood to use.Some types of excellent plywood might be too stiff tocompound. With the great number of useful plywoods outthere, that does not need to be a problem. THE FUTURE The system is moving toward unprecedented dimensionalcertainty as I collect an ever-increasing body of data fromthe boats that have been built. A mature CADD systemmakes that even easier. In fact CADD (computer aideddesign and drafting) is a natural fit with CM. Changing thelength, beam, or freeboard on a building project is as easyas a rescale is on the computer. Adding eighteen moreinches freeboard on a 40' tri main hull panel was done inabout three minutes, by simply sliding the flat surfacepattern up 18". That is about the same difficulty as adrawing rescale on the screen. Of course all KHSD multisare drawn using CADD now, as they have been for 20years.Experiments with strategies to allow fatter length/beamratios on boats with larger amounts of rocker have turnedout very well. The result has been higher displacementcylinder molded cruising boats than we ever thought possibleyears ago. At this point as we keep exploring the envelopeof possibilities, I must repeat, it looks like virtually anyslender, positive curve hull shape wanted is possible.A mold system that we call the air mold is being developed.It consists of no mold at all, beyond the lower channel.Hence the name.Several rapid composite hulls have been built using thevarious rapid construction systems we have been develop-ing. I will get around to publishing them soon. Designsupport takes so much time that publicity has to take backseat. CONCLUSION This construction system, developed by people too poor todeserve boats, got us into sleek, high speed multihulls thatwould be out of our reach otherwise. They are also thehighest quality boats we could have chosen. CylinderMolded hulls may be the best possible choice for manybuilders of multihulls.

Cylinder Mold Construction Manual

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