Multihull Structure Thoughts

I used to drool over photos like these when I was a kid.
(Martin Cooper's Kraken-25 Jabberwock, from the April 1965 edition of "Catamaran And Trimaran International News")

 

The Bunyip was designed in 1960 by Lock Crowther. This design was the simplified Kraken type trimaran that was easier to build with chine sheet plywood hulls. The Bunyip 20 was 20 x 11 foot weighing 250 lbs and displacing 450 lbs. The mast is 27 foot high carrying a 227 square foot rig (including a 100 square foot genoa). The main hull had a 10 to 1 length to beam and the floats had a 13.5 to 1 length to beam ratio. The floats are 14.5 foot long.

The Kraken 18 was designed after the Bunyip with the same rig but with round bilge hulls. The Kraken 18 was slightly faster and slightly lighter due to her 4.2 mm thick cold molded hulls.

Bunyip hulls are from 4.8 mm sheet plywood chine hulls with timber stringers and frames. There ae 3 plywood bulkheads with timber edging under the crossbeams and mast mounting points in the main hull. There also is a crash bulkhead forward. The crossbeams that have a top and bottom timber (oregan) flanges separated by 4.8 mm plywood webs and forward wing fairings. There were 2 versions of cross beams, one set were designed to fold the floats in and the other set of crossbeams were one piece units that were bolted onto the boat. There were only 8 nuts required on the one piece crossbeams to assemble/disassemble the tri. The most time consuming part of the one piece beams was the lacing/removal of trampolines for trailing.

The daggerboard is 5 foot deep which helped its windward performance. The rudder was initially small at 1.25 foot deep but was modified by owners to be deeper to get better control.

According to one owner “The best performance of the Bunyip was in low to medium winds. In high wind, the big genny needed a very strong crewman and the low beam float connection dragged in waves and slowed the boat.” The Bunyip was designed to be sailed with 1 or 2 people using a trapeze in stronger winds.

A lot of these tri’s were built globally by a lot of the early pioneers of multihull movement as they were very advance for the time. The design was the fore runner of the Buccaneer 24 which had relatively longer floats, bigger rig and similar hull construction. The Buccaneer 24 had aluminium cross beams. The Bunyip would be a great day sailor cruising design. A lot of fun.

 

Easy 12 cruising catamaran is 39.33 x 20 or 21 foot and weighs between 9000 and 10,000lbs. The displacement would be about 15,000 lbs. The 46 foot mast carries about 800 square foot of sail. This item is mainly about the build process and structure. An Easy build blog for “Selah” is at Building Selah https://pauseandreflect.blog/the-build/

The Easy 12 is designed by Peter Snell who has now retired. He designed many plywood cats from32 to 43 foot. The plywood construction is relatively easy to do for a home builder but the real joy is the skill of Peter to design to the material. By designing around standard material sizes he keeps waste material down to the minimum. For example, all the bulkheads and frames are based upon the standard 1220 x 2440 plywood sheet. The 22 degree stem means no timber lengths exceed 39.33 foot, which means only one scarf joint is required for the chines and stringers. Many of the hull and deck components are full, or lightly trimmed standard panels. There are 27 sheets of 1220 x 2440 mm ply panels for each hull. By careful use of the off-cuts, each hull (including the keels) can be planked with 15 standard size sheets with almost no wastage. This helps to keep building costs and build time down.

The hull are basically 9 mm plywood. The hull chine stringers are 68 x 19 mm, the hull stringers are 43 x 19 mm. The hull bulkheads are 9 mm ply with 43 x 19 mm timber edging. Decks are mainly 9 mm ply on 42 x 19 mm deck stringers at 300 mm centre lines. Now a small warning each sheet of the slightly compounded deck ply takes about 200 screws to lock onto the deck stringers. About 30 sheets of ply for the deck requires you to put in 6000 screws. All external surfaces are covered with 200 gsm e-glass in epoxy. The low aspect ratio keels required 3 layers of 300 gsm biax e-glass with a 450 gsm dynel final layer in epoxy over the top. The cabin turret top requires a ply timber sandwich with 68 x 19 mm support structure beneath. Any corners of the deck/turret are reinforced with a 450 gsm biax tape in epoxy.

The underwing is 12 mm plywood with 7 stringers of 60×40 mm douglas fir on the underside covered with 200 gsm e-glass over outside in epoxy. The forward curved wind deck is 2 layers of 6 mm. The underwing hull joint are covered with 2 layers of 450 gsm biax tape in epoxy over a wide fillet. The cross beam structure comprises of 6 ply timber beams.

The 440 mm wide rudders are 80 mm thick shaped plywood on a 48 mm stainless steel pipe with a flange. The rudder tube sits inside a 60 mm pvc pipe covered by 2 layers of 450 gsm biax e-glass. There are 6 mm plastic bearing materials in the 60 mm PVC pipe.

The build blog of “Selah” gives more hints. “Selah” home builder is an efficient fast builder of a simple good design. Peter Snell could put a 40 footer together in under a year. A good design. The 2 sets of jpegs give the idea.

 

Second set of Jpegs for Easy 12 cruising cat.

 

The first major commercial design Peter Snell did was Jessica (named after a daughter) that was known as an Easy 32. The Easy 32 is 32.8 x 15.9 foot and weighed about 6500 lbs and displaced 9000 lbs. The 37 foot aluminium mast carries 390 square foot of in the main and foretriangle. The main is 175 square foot, the staysail is 95 square foot and the furling genoa is 285 square foot. Most 32’s had low aspect ratio keels.

The hull is 9 mm ply with timber stringers and 9 mm ply bulkheads that have timber frame edging. Notice this is the same as the Easy 12 (39.33 foot) which displaces 50% more. Reality of boat design is you often design to the available materials not always the strength required. EG the Easy 32 could probably be skinned with 6 mm plywood if you had additional stringers and did not sail it to hard. But it would be a little on edge for cruising, the next available size in Australia that is easily available is 9 mm plywood.

The underwing is 12 mm ply with stringers. Deck is 9 mm plywood with deck stringers at 300 mm centre lines. The main crossbeam bulkheads in the Easy 32 I walked over, was a 9 mm ply web, 19 mm timber framing and 9 mm plywood panel web. There are 4 crossbeam bulkheads. The entire external surface is covered with 200 gsm e-glass cloth in epoxy.

Since the structure is very similar to the Easy 12, why would you build an Easy 32. Simple, it is a hell of lot less work. An Easy 12 has about 1900 square foot of external surface and 6 major crossarm bulkheads. An Easy 32 has about 1200 square foot of external surface area and 4 major crossarm bulkheads. If you have ever faired and painted a cat each square foot of work takes time. Notice the surface area of an Easy 32 is about 65% of an Easy 12 and the Easy 32 weight is about 65% of the Easy 12. There is a real almost linear relationship between the build hours and the weight of a cat for the same type of build materials. Also, smaller interior spaces require less fitout time than say a larger 40 foot cat. Also much of the rig and equipment for an Easy 32 is smaller and cheaper. EG two 9.9 HP outboard engines will drive the Easy 32 at 10 knots.

The Easy 32 demonstrates that a smaller cat can still have 4 berths, dining area, good galley and loo and still be a very good cruiser that can do long ocean distances. The owner I have spoken to said he could do 6 to 8 knot averages single handed and over 10 knots in favorable conditions. The peak speed he ever sailed at was 15 knots with a crew in very favorable conditions, but he thought it could go faster. He said it was a very easy boat to sail and maintain. If these cats are well built from good materials and West type epoxy systems, they will also generally be low maintenance. The jpegs give the idea.

 

Second set of Easy 32 cat jpegs.

 

Patterson’s boat Buddy.
2001 Patterson custom trimaran sailboat for sale in Outside United States https://m.sailboatlistings.com/view/81744

 

Today we will talk about a certified cruising cat that only has one issue. You can capsize it in 7 knots of real wind. The upside it can average (not peak speed, average) 20 plus knots in 12 knots of wind. This cat can reach peak speeds in the high 20's. The catamaran is 38 x 18 foot and carries a 55 foot rotating aluminium mast with a 500 square foot main, a 300 square foot jib and an approximately 650 square foot screacher. The rig is a bit aggressive, but not unreasonable. So how does this cat, off the wind, average twice wind speed until about 12/13 knots true wind speed? A displacement in day sailing mode of 1700 lbs helps, but when the cat is set up for ocean racing, with its cabin attached, it weighed in on the scales at 2114 lbs.

The owner and part designer was Steve Dashew. Steve’s father started him on a fast boat journey when his father got Rudy Choy to design him a large catamaran in 1959. Steve was inspired and built a series of day sailing cats resulting in Beowulf VI in 1974. In 1976 Beowulf VI created the race record for the 125 nautical mile Newport Ensenada race in 10 hours 15 minutes (average of 12.5 knots). To allow Beowulf VI to enter the Newport Ensenada race Beowulf had to be a valid cabin catamaran which it achieved by the addition of a deck cabin. The ORCA Board inspected the cat and went for a small sail before they certified it as a valid cabin catamaran. The cabin contained two bunks, a stove and ice box, and a Portapotti. What more could you ask? The add on cabin was built mainly with ply, some timber and some fabric coverings.

So, what was this cruising cat built from? 6 mm plywood, high density foam and e-glass. The cross beams are aluminium tubes with a dolphin striker on the main beam. You could build a boat like this at your home. The hulls are basically a plywood boxes with a shaped foam bottom added on covered by e-glass and epoxy. At the daggerboard box exit a ply support V structure was built to reinforce the daggerbox bottom. The hulls had 6 watertight ply bulkheads and deck beams. The final hulls weighed 375 lbs a piece. The approximately 22 to 1 length to beam hull shape was done by Norm Riise who, in his spare time, used a mainframe computer at Jet Propulsion Labs to optimized a boat hull shape. This was the first time a computer optimization was done on a boat like this.

Such a low displacement comes from 2 things. Minimal structural dimensions and small surface area. Beowulf Vl has thin hulls, very low freeboard and minimal internal comfort reduces weight. This cat may be suitable for “ocean racing” in California but would not be suitable for ocean racing in New Zealand or Australia where the average wind speeds are often 50% higher than California.

Bob Handel sailed his 60 foot Double Bullet to NZ from California with the intention of sailing it to Australia, Bob visited Sydney prior to bringing Double Bullet across the Tasman and went sailing with Lock Crowther when “a freak gust” hit, according to Bob. Lock told Bob this was normal sailing conditions for Australia. Bob sent Double Bullet home to the US from NZ.

Steve Dashew’s previous cats included Beowulf V a 32 foot day sailing racing cat that was built from 4.8 mm tortured ply hulls. This cat is still sailing after 40 years and has recently been upgraded by its latest owner to a larger fat head rig. Yes, you can build light long lasting multihulls out of plywood if they are built from good quality materials.

If Steve Dashews name is familiar, its because he gave up fast sailing cats in the late 70’s due to politics of race committee’s etc. He then went cruising on a series of aluminium monohulls. They were called the Deerfoot range and his favourite was the 78 footer that he, his wife and 2 daughters sailed 300 mile days around the Pacific without any other crew. He subsequently developed a range of long distance displacement power boats.

 

There has been an interesting discussion about crossbeam going on Boatdesign.net thread “Changing crossbeam from wood to composite”. A man wants to improve his Phai 63 cat 6 timber and ply crossbeams for a more rot resistant lower maintenance version. A Phai 63 is a plywood cat 63 x 28 foot with a 50 foot waterline displacing 28,000 lbs and carrying 1400 square foot of sail on a schooner rig on 55 foot masts.

I wanted to see if there were any other similar cats to see what crossbeams they had. I found some guy called Nigel Irens who appears to be good multihull designer (60 foot tris, ENZA global cat etc) who designed “Rustic” (“Sandpiper”). Rustic is a plywood cat of 64 x 28 foot with a 50 foot waterline that weighs/displaces 28,000 lbs carrying on 57 foot schooner rigged masts 1250 square foot of sail. We will talk about “Rustic” tomorrow.

The cross section of the Phai 63 crossbeams are in jpeg 1. Basically a 76 x 146 mm timber flange on a centre ply web of 2 layers of 19 mm plywood with a height of 355 mm in the centre. There are 6 crossbeams. The crossbeams are “tied” on by 6 loops of 10 mm low stretch rope on each crossbeam at each gunnel. The ropes were wearing into the beams causing some maintenance issues.

Nigel Rustic has 3 crossbeams. The beams are 510 x 533 mm boxes with top and bottom flanges of 4 layers of 19 mm timber for a thickness of each flange of 76 mm x 470 mm wide. The box has fore and aft plywood faces of 2 layers of ply cut at 45 degrees. There are timber bulkheads every approximately 300 mm centre lines. The 3 crossbeams are locked into position by carbon fibre straps.

Translation. The Phai 63 is a “flexible” cat whose beams can move up to 150 mm across their width. The Rustic 64 is basically a solid cat with minimal movement between the hulls and beams.

John Perry from the AYRS suggested an alternative composite crossbeam for the Phai 63 crossbeam in the final PDF. An interesting alternative.

The jpegs give some idea’s. Many ways to do the same crossbeam task for the same size and type of boat.

 

“Rustic (or Sandpiper)” is a Nigel Irens design (60 foot racing tris, ENZA global cat etc). Rustic is a plywood cat of 64 x 28 foot with a 50 foot waterline that weighs 28,000 lbs and displaces 38,500 lbs. It carries two 240 mm diameter 57 foot tube aluminium masts in a schooner rigged with 1250 square foot of sail in a gaff-rig. The hulls length to beam is 11.75 to 1. The numbers indicate a good performance cruiser.

“Rustic” was conceived as a ‘classic catamaran’, her architecture is based on the Polynesian Wharram style design with improved construction design and style. This cat is a serious long distance offshore sailing cat that can average 10 knots. There are daggerboards in the design and the boats draught is 4 foot. The cat is designed so the propellers and rudders are protected to allow the cat to be beached at any time.

The entire structure is basically plywood with timber stringers and chines. There is a thick plywood frames or bulkhead every 3 foot. The timber deck beams are attached to every frame. The decks are plywood. The entire external hull shell is covered with biaxial glass and epoxy. This cat is built from good materials and has a West type finish throughout. The daggerboard and rudders are timber construction.

Rustic has 3 crossbeams. The beams are 510 x 533 mm boxes with top and bottom flanges of 4 layers of 19 mm timber for a thickness of each flange of 76 mm x 470 mm wide. The box has fore and aft plywood faces of 2 layers of ply cut at 45 degrees. There are timber bulkheads every approximately 300 mm centre lines. The 3 crossbeams are locked into position by carbon fibre straps. The cat is basically designed as one solid unit.

There are two 100 hp engines with 500 litres of fuel. Due to the smaller sail areas the deck gear is smaller than you would expect but very appropriate.

Now for those who would like one professionally built according to the builders think about $1 million plus. This is a big cat with 3 very comfortable sleeping cabins. The galley and dinette area are also comfortable. The downside is this boat is a warm climate vessel as you can get cold shifting from one hull to another.

The jpegs give some idea’s. I would love to see a race between Rustic and a Phai 63 one day (unlikely to happen) to see which is faster. The Phai 63 has narrower hulls, slightly less displacement and a slightly better wing sail rig. The Rustic has a better hull shape with less wetted surface and fuller sterns, daggerboards and a more rigid structure which helps performance. It would be fun to sail on.

 

Second set of Jpegs for Rustic. The build pictures.

 

Cheers for the photos on Rustic. I interviewed Irens once and he said he would rather cruise on a heavy mono or something like a Wharram. I had not kept up with his designs (silly me). I really like the idea of holding the beams down with uni tape, it is so sensible. If Nigel Irens thought that this was a good way to go, then us mere mortals need to sit back, shut up and learn. Thanks a heap Old Multi.

 

The discussion on crossbeams in the boatdesign thread “changing catamaran beams from wood to composites” is raising a few interesting points. One is torsional twisting of open bridge deck cats when sailing hard. Catsketcher sailed Afterburner at one point and the windward hull was bow down as the aft shroud pulled up the stern of the windward hull. Jpeg 1.

Now I give you Tony Grainger’s 1986 solution to the problem. It was called the “Air Bulb”. It was radical for its day with not only an X beam (Marc Pajot had a 60 foot x cat about the same time) but the real variations were a very wide beam and central pod that could provide accommodation with a very low windage design concept. I am working on memory here but I think it was 40 x 30 foot with a displacement of about 6000 lbs. The 55 foot wing mast had a 720 square foot main and a 320 square foot foretriangle. The length to beam on the hulls is 18 to 1. This boat was never built to my knowledge but if it could be built to that weight with that sail area it could have been very fast.

The X beam structure came from discussions Tony was having with a few people to try and reduce torsional twisting in both cats and tri’s as sails were losing shape due to slack forestays as multi’s loaded up.

Tony wanted to improve the aerodynamics of cats as well resulting in minimum freeboard, rounded sections and a low drag cabin. The cabin structure also acted as a beam that has the forestay attached at one end and runners/main sheet attached to the stern section of the accommodation pod. This will ensure a tight forestay and good windward performance. The accommodation pod has only 5 foot headroom and minimal space due to the X beam structure but it would be a viable space for a racer.

The boats structure was never finalised but at that stage tony was into duracore or foam glass solutions.

The final 2 jpegs give the idea.

 

Jeff Schionning designed an X-beam (in this case X-bulkheads) cat called Growler many years ago. Low freeboard rounded hulls, streamlined pod, forward longeron for forestay attachment. He drew a massive spokeless steering wheel that surrounded the pod entry hatch.

One was built, photos showed incredibly sleek lines with beautiful strip planking. I don't know if it was completed and sailed, don't know if they built the big wheel. Jeff never mentioned the idea again, so I guess the wheel was a non-starter.

Schionning now uses the Growler name for a range of powercats.

See also John Hitch's X-it. (Exit? Something like that.....)

 

It was Elf Acquitaine 2 , design by Philippe Briand (1984) and then, after a capsize, by Gilles Ollier (1986, same cross beams on new hulls). More details and photos of the construction ( it is said "2,5 t of carbon to build the 2 hulls, the cross beams and the mast" ) :
Document sans titre http://www.histoiredeshalfs.com/50%20multis/G%20DF9%20Elf.htm
https://www.ultimboat.com/elf-aquitaine-ii

Now proposed to charter by Caseneuve maxi catamaran under the name "Catamaran Elle et Moi" :
Croisière Cap Taillat en Méditerranée | Caseneuve Maxi Catamaran https://www.caseneuvemaxicatamaran.com/croisiere-cap-taillat-mediterranée