Multihull Structure Thoughts

The following article is from a guy called Dave Culp. A real estate agent (may be retired) in California who built a series of small kite powered cats, tris and proas for fun. Most worked well. He eventually promoted kite powered boats to the point where a 50 foot tri who had broken its mast, sailed from the Caribbean to Florida using a kite.

“I have been asked about a quick easy way to produce one-off foam sandwich hulls I used to build one of my boats. It's just a simple foam strip plank set-up, using CAD-generated mould sections. The trick is that a) the hull half-sections are small enough that the moulds can each be got out of a half-sheet (or less) of plywood (so you're just cutting holes in square pieces of 1/8" doorskin ply), and b) with CAD, it's no big deal to build a mould every 6-8", so you don't need stringers.

It is then put together on a large jig. In my case, 2 x 2's were screwed to my workbench, and to saw-horses set beyond each end. The foam strips (3/8" x 1.5", for a 20' hull), are laid in, and just hot-glued to the moulds. Very little tapering is done; what's necessary is done by edge-bending the foam, on the bench, and cutting with a straightedge and knife. (Very elegant tapers result, and it takes only seconds to do).

Once the foam is in, the inside is glassed (and left very rough--no finish resin yet, and no sanding). The half-hull is braced (half a dozen sticks, again hot-glued across the hull), and popped/cut out of the moulds. The whole mould/jig is disassembled and re-assembled, front-for-back, resulting in a "mirror" image in three dimensions, and the whole thing is repeated to get the other half-hull.

Both halves are carefully trimmed to centreline marks, and the hull glued together (not hot glue this time, but resin). We used scrap foam "tabs," glued alternately on the inside of the hull-halves to assure alignment. When the resin set, the tabs were removed and the inside joint taped with 2" glass tape. The stern got a foam transom later, and the bow a solid block of foam, carved to get the shape.

I now had a "solid" hull, glassed/taped together inside, and raw foam on the outside. I cut a bunch of moulds down to make a cradle, and began fairing the foam until I liked it - Surform and a long sanding block. (I'd eliminate the Surform next time, and just use a long block with 36-grit paper). I filled the gaps in the foam with microballoons (less than two quarts, on a 20' hull, and 3/4 of that ended up on the floor as sawdust), faired again, and glassed it.

Lastly, I finished off the cockpit rim, put in the seat-back/bulkhead, and a hatch for the beer. The hull weighed 40lbs, before we added mounting pads and hardware.

I did all the above, completely alone in a single weekend - about 18 hours total, from gluing the plotted paper stations to doorskin ply with rubber cement (much faster than transferring the lines) to final sanding. The only power tools I used were a table saw to rip the foam strips, and a sabre saw to cut the doorskins. I did all the sanding by hand. Newly cured glass is as soft as pencil eraser, if you get it at just the right time. As I had laid up the boat with small batches of resin, I could follow this cure state around behind myself. This trick I learned from Jim Brown on Searunner tris.

Mind you, I have built a hull or two, but never before with either the CAD stations, nor of foam strips. I have hand-carved hulls from solid foam, so am not afraid of shaping to eye, but I used no tricks beyond mixing the resin pretty hot, and moving fast. I expected a pretty rough finish, and would have allowed broad tolerances (speed boats are throw-aways, after all, and this hull was not meant to touch water at anything over 12-15 kts), but was astonished at the quality it produced.

I give complete credit to this technique to Greg and Dan Ketterman. -- They used it to prototype all the TriFoilers, Avocets, and for LongShot herself.”

The first jpeg is his proa "Whigmaleerie" during a speed run at the 1987 Johnnie Walker/RYA Speed Trials in Weymouth, England. She's powered here by a 5-stack of 12 ft Flexifoil kites on 150' flying lines (the fifth kite is pale yellow, and can just be seen). The second jpeg is of Dave Culp.

 

Great Britain 3 was in 1974 the largest foam glass trimaran in the world. Its 80 x 38 foot displacing 31,000 lbs carrying an 85 foot mast with 2,260 foot of sail in the main and fore triangle. This boats racing career was limited and variable, it was fast but it also had its problems. A float bow broke off after being hit by a ship and the tri capsized. The water pressure in the centreboard case got so great in one high speed run that the case top blew open flooding the main hull. After this insurance companies refused to cover it which limited its racing. Eventually it became a charter boat in 1984 in the Caribbean. The tri was designed, tank tested and structurally tested prior to the final build and the majority of the structure has lasted well. The hulls etc were 3 mm polyester glass 20 mm PVC foam 3 mm polyester glass. The rigging was Kevlar. A good cruiser but a moderate racer. Jpegs give an idea.

 

The following PDF is very informative about multihull capsizes. The document was done in 2002 by the Wolfson research unit in the UK. Of the 124 stability incidents they documented 33 involved cats, 67 involved tris the remainder were proa’s and other undefined types. The interesting findings included most accidents happened with multis under 11 meters (36 foot) and a secondary peak occurred at 18 meters (60 feet) mainly racing multi’s. Most of the catamaran casualties occurred in winds of force 6 to 9, with none documented in more severe conditions. The trimaran casualties occurred over a wider range of conditions, with a large proportion in force 8 to 12 conditions. It was assumed that in severe conditions the catamarans would have little or no sail set and their vulnerability would be low. Trimarans on the other hand, appear to be more vulnerable to breaking wave capsize in severe conditions.

Wolfson unit then did a series of test models for tank testing to understand the characteristics of a multihull that can cause capsize by hull shape and wave height etc. A summary of the results is hard and you need to read the document fully. But if the speed ratio (speed in knots divided by square root of the waterline) is below 1 the capsize is unlikely. If a multi has fine bows and its LCG is forward its more likely to pitchpole. Tri’s with small floats are likely to broach and cartwheel at higher speed ratio’s in larger wave conditions. The height of the LCG has an effect IE an open bridgedeck cat is safer than a full deck house cat in difficult conditions. As I said there is more but the trends may give you hints as to the type of multihull you may desire.

 

Splinter is a 25 foot self righting trailable trimaran that was designed to compete in the OSTAR. The tri is 25 foot long x 28 foot beam with a weight of 1000 lbs. The 35 foot wing mast is 1 .33 foot wide and is built of 0.8 mm aircraft ply either side if a 12 mm Verticel core. Carrying 360 square foot of working sail area. The tri has 150% floats (1500 lbs displacement) and has been tested to be self righting. The hulls are tortured ply construction, probably 4 mm ply, with everything from the wing, deck, cab etc are thin ply backed by Verticel cores. The boat won the 240 mile 1981 port Huon to Mackinac race being first over the line beating much larger boats. The tri can go upwind at 9 knots and can hit 18 knots. The jpegs give the idea. This was a boat way ahead of its time and would be a pain to disassemble for trailing but its still a very good idea. I suspect the F boats folding capability and internal room were the reason the idea wasn’t developed further.

 

The Oceanic 30 catamarans were a very early version of real cruising catamarans in the 60’s. Yes they were slow, heavy and had minimal sail but they introduced a generation to the advantages of accommodation that did not heel over. Many spouses were won over. Anneliese was probably the most famous sailing around the world via the capes in the 70's with a young family on board. Anneliese was built in 1969 by Bill O'Brien as an Oceanic Mk 3. The cat is 30 x 14.1 foot with a weight of 5500 lbs and a displacement of 9500 lbs with 637 square foot in main, genoa and mizzen. Hull Material Solid GRP to waterline, foam sandwich above the waterline with decks of 12 mm marine ply 2 oz CSM on either side. Keels were GRP foam core designed by Bill O'Brien, of Bill O'Brien's Southhampton Caravan & Catamaran Company. Now the interesting part there were dozens of these boats built ranging in size from 22 footers to 34 footers. The Oceanic 30 was the most popular size with 5 developmental versions. The first versions were 12 mm ply with 2 oz (600 gram) CSM mat on either side for most of the structure. The final boats were nearly full fiberglass versions with minimal ply. The Oceanic 30’s later versions had 15.5 foot beams and bit more sail but they were 6 knot peaking at 12 knot surfing sort of boats. They especially liked good motors for any real upwind work. Bill O’Brien major contribution to cats was his understanding that accommodation and comfort was as important to a lot of people as speed was to others. He built bullet proof boats to meet the “I want comfort” market.

 

I don’t know much about the structure of this tri but the idea of the boat is worth looking at. The tri was designed by Dennis Lobb who did a few tri designs mainly in plywood in the 80’s and 90’s. The tri is 26 x 16.9 foot and can be folded to 8 foot. The weight is 2000 lbs and displacement is a maximum of 3200 lbs. Sail area is 292 square foot in main and fore triangle. The main hull has a 7.3:1 length to beam ratio. The accommodation for a 26 foot boat is interesting, I would suggest it will be a little tight for 4 people with some narrow berths and a lack of headroom. Its a good accommodation idea that would take some refinement. The boat is mainly plywood with metal cross beam components. The folding system would work but would take some effort. Dudley Dix did a similar folding system of a 20 foot Threefold tri he designed. Also Jim Browns Searunner 25 is another variant of the system.

 

The Pahi 52 is a ”production” model that is exclusive to a Philippines boat builder. The boat is 51.7 x 24 foot weighing 12300 lbs and displacing 22000 lbs. Sail area in its gaff schooner wing sail rig is 1000 square foot. The length to beam as designed is 14:1. Now like all boats the reality is not always as designed. One version is 53 x 25 foot weighing 16800 lbs displacing 22000 lbs carrying 1050 square foot of sail. The 2 masts are hollow, laminated, glassed over, cedar strips; top section filled w/crumpled aluminium foil. Stays in stainless 10mm 7x7 wire, forestay 10mm 1x19. The hulls are epoxy Gaboon plywood glassed on solid ironwood timbers. Up to 14 layers of glass near the keel. Deck construction plywood glass epoxy with 2” glass wool insulation & iron wood beams. Painted with 2 part polyurethane finish throughout. Slatted deck & companion way ladders: ironwood. All trim in varnished red cedar. Reason for showing this design? One day calculate the surface area of the hulls, any deck cabins and decks of a Pahi 52. Then calculate the same shell surface area of a Looping 50 or a Raku 52. The surface area will be surprisingly close between the designs. Weight and surface area are often good guides as how long a home build will take. People build a Wharram thinking they will be easier than to build than “complex” bridgedeck cats. Reality is the cross beam structures on a Wharram are often harder to build than cross beam structures on a full bridgedeck cat. The interior furniture, electric and mechanical system fitout is about the same. Most Wharram builders take just as long to build a good similar sized boat as a Grainger, Woods or Hughes design.

 

This little tri is very similar to Dennis Lobb's Rhapsody which was a 41 footer. Same proportions and style.

 

The Tiki 46 is one of Wharram’s more popular designs. The 46 x 24 foot cat weighs 10,000 lbs and displaces 20,000 lbs carrying 1000 square foot in a soft gaff wingsail schooner rig. The rig has 42 foot masts which can either be circular 200 mm diameter with 6 mm wall aluminium or same diameter with 32 mm wood walls. The hulls are 12 mm ply (one version has 3 layers of 3 mm plus a 5 mm veneer) with 65 x 22 mm stringers and 12 mm ply bulkheads. Decks are 12 mm ply. The crossbeams are 450 x 140 mm max tapering to 350 x 140 mm at ends. The jpeg gives the idea. These crossbeams are lashed to hull sides but the hide a secret. An 18 mm stainless steel bolt through the centre of crossbeam that crosses the hull and bulkheads on either side of that crossbeam. The rudders are attached to the hull with low stretch rope (this is genius). The timber and epoxy (and microballoons) required at the keel line of these hulls is significant. The build of a Tiki 46 is documented at Building of the Tiki 46 n°2 "GRAND PHA" Part 1 https://wharrambuilders.ning.com/profiles/blogs/building-of-the-tiki-46-n2-pha

To do these boats well will take you as long as any other design especially if you build your own masts etc. The next problem is build it to plan for 2 reasons. First Wharram does integrated designs that depend on the furniture etc for strength. Secondly a modified Wharram generally does not sell as well as a standard build. Finally the last jpeg is of a biplane junk rig Tiki 46 cat that served its owner well, after they rebuild the initial masts and reengineered other parts of the rig.

 

Spirita was a Crowther one off 60 foot build for a guy who had already had a Crowther 44 but wanted more performance. The cat was designed at 60 x 29.5 foot displacing 27,000 lbs and 72 foot mast carrying 2000 square foot of sail. The built boat was 62 foot long. It was initially to be a private yacht but was used as a charter cat in Port Douglas in Queensland. The boats actual name was Synergy 2. The boat was built in Melbourne to survey specifications in 1997 at a cost of over $1 million. In 2015 the boat appeared on an auction site and was sold for $300,000 in full survey (that is cheap). The cat is still being chartered in Port Douglas. Why is the boat of interest? Because of its cross beams. The main Cross beam is 3.25 foot high in the centre with a 225 x 100 mm timber insert in the centre to act as the mast post. The web of the full width main cross beam is 100 mm thick balsa with 3 mm thick of multi layers of triaxle on each face, all in epoxy. The top and bottom flange’s are 80 x 100 mm unidirectionals in the centre tapering to 40 x 100 mm at the ends before tapering down 30 x 100 mm down the inside face of the hull. The rear cross beam is of a lower height but the same basic structure. The hulls were foam glass with 30 mm PVC foam all done in epoxy. An interesting variation was on the outside and inside of the hulls, from the main beam to the bow, was a solid glass deck edging 200 mm wide and 13 layers of 500 gsm unidirectionals deep to limit bow bending of the relatively fine hulls. The length to beam is 15:1. This boat sails well in all conditions.

 

Greene F40 trimaran was built in 1986 to compete in the F40 class. The tri was 40 x 33 foot. It weighed initially 5000 lbs and carried a 65 foot mast with 1000 square foot of sail area. The boat after modification in 2004 weighed 6000 lbs. This boat was fast winning European One star in 1992. Offshore it was very good, inshore it could be beaten by “bay only” F40’s. A person who helped built the boat said “I learned a lot but I was sceptical at the time about the 'pinnings' of the boats hull/beam/amas as that was what he tasked me with. As she was originally designed as a F-40, requiring that she be dismountable for transport to different racing venues. I had my doubts about the beams being so close together when I was installing the beam end fixations for the amas on the original build. Walter had the strip plank coring with Kevlar inner skins and S glass outer skins pretty well figured out but I thought that was sort of inside out. Walter explained the theory to me at the time and time seems to have proved it out. We used a limited amount of CF uni mostly to load map and 'harden' attachment points for localized loads that hardware and the beam attachments imposed. When you look at the long history of that boat and the mid ocean abandonment and subsequent rebuilds it was really way ahead of its time.” The blue and white boat is the original build. The yellow boat is after her 2004 rebuild. The boat was picked up really cheap and was rebuilt in 2004 with new carbon beams, more widely spaced for improved platform torsional rigidity aided by wing accommodation boxes. The new class the boat was to be raced in required inboard diesel and the boat was fitted out as a family cruiser/racer. The boat capsized in the 2011 Route De Rum and was recovered.

The following description is of the build of the boat as it was originally raced. The hulls were 260 gsm unidirectional S glass 15 mm WRC 300 gsm Kevlar on the interior. The cross beams were initially timber ply as described. The jpegs give more information.

 

Seaclipper 28 is 28.5 x 18.5 weighing 1900 lbs displacing 2650 lbs with a 32 foot mast carrying 166 square foot main and a 205 square foot genoa. The main hull beam at the gunnel is 5.25 foot and in standard build format has 5.25 foot of headroom. The length to beam on the main hull is 8.5:1. The floats have 140% buoyancy. This is a love hate boat. The love is it is a simple tri to build, its relatively fast for its size (17 plus knots) and can handle almost any weather thrown at it. One with Jim Brown aboard went through a hurricane. The hate part. Its accommodation is always a few inches short. It does not have full headroom, the double bunk is really only a wide single, the floor space is too narrow to stand comfortably, the payload (750 lbs) is small, 2 thin people is the absolute limit for any longer than a day sail etc. Some people have modified their boats to slightly improve things and Marples has included some mods in later plan releases.

The boats build is mainly 9 mm ply with a few stringers and bulkheads, the timber ply cross beams are a simple box structure. All above water ply is covered with 120 gsm cloth and epoxy, bottoms are covered with 330 gsm cloth and epoxy. The crossbeams can be built independently and bolted onto the boat for disassembly later. There is a second version of the design that has folding crossbeams as with a Seaclipper 24 or DC3 tri. Of the people who have sailed the boat, all love the boats sailing capability, even though it is wet to windward, and if correctly build like its longevity. The issues are about the centreboard case initial build and maintenance of the boat overall. West system type products are almost mandatory to ensure a good life and reasonable maintenance routine. The builders all recommend good quality ply and Douglas fir framing and stringers. Floats are a simple V shape with bog and tape at the keel line. Floats have 2 12 mm bulkheads with 100 x 100 mm deck beams at deck level for float attachment. The float decks are 12 mm ply. Build this boat to plan and keep it simple and light and you will have a superb small sailing boat with accommodation a Wharram Tiki 26 owner would love to have.

 

The attached Word document ( should have been a PDF but it will not copy to this thread) is comparing hand layups versus resin infusion processes in making a solid glass 12 foot dingy. The outcomes make sense but the results are counter intuitive. The solid glass 12 foot dingy done in a female mould layup was a surface tissue, 300 gram CSM, 600 gsm woven roving and a 600 gram CSM. The result showed that the resin infusion technique produced better result upon ultimate tensile strength (27% better) but resin infusion produced slightly less satisfactory results for compressive stress (12% lower than hand laid) and flexural stress (34% lower than hand laid). Even though resin infusion was only better in tensile strength, physically the product is more lightweight with a better resin-to-fiber ratio.

The reason for the results of lower compressive and flexural stress is of resin infusion is told in the thickness of the laminate. A hand laid laminate is about 3.5 mm thick, a resin infusion layup is about 2.5 mm thick. The thinner resin infusion laminate is lighter and has a better glass resin ratio providing a stronger tensile strength but because its thinner it lower in compressive and flexural stress.

Now this applies to the same build materials in a solid laminate. The reason resin infusion and a sandwich laminate work so well is that 2 thinner skins separated by foam, balsa or WRC (which adds stiffness by having a thicker panel) allow each material to provide the best structural solution to providing a strong skin that is light weight, thick and stiff.

In short, don’t just improve your laminate by using resin infusion unless you have designed for a resin infusion layup in the first place. Remember that a major requirement of a hull structure is panel stiffness. Thinner skins due to improved construction techniques without increased thickness gained by a core material could result in weaker hull structure.

Please look up the PDF for the full text and tables. It is under several research organisations and should be free to download. Sorry about the limited word version.

 

Okay - now you have me interested. Some of the interior pics are of Dan's Seaclipper and he used to have a Wharram - A Pahi 31 - IIRC but he sails that tri up and down the coast in a seasteader fashion.

 

Those people who made comparison had time to waste, You don't need measurement to find that applying pressure to avoid excess resin will give a thinner laminate with less panel stiffness !
Anyway thanks for all those useful data