Multihull Structure Thoughts

Drifter trimarans are designed by Mark Gumprecht. This one is the Drifter 17 which is a small trailable cabin trimaran for bay and river cruising etc. The tri is 17 x 12 foot with a 22 foot plywood and timber wing mast that can be home built. The main is 115 square foot with a 60 square foot fore triangle. I do not know displacement figures. Mark designs light boats that require good building practice, but these boats structurally are very good. Follow the plan and you will have a good boat.

The design main hull has 6 mm bulkheads, frames and stringers as per jpegs. The hull skin is 3 mm ply with 130 gsm e-glass in epoxy on the outside doubled over the bottom to the waterline. The "chine" on the main hull is strip planked to provide a smoother water flow. Additional glass layers are also put on the stem. The wing deck and decks are 6 mm ply covered with 170 gsm glass. There is a short 40 x 60 mm bow spirit for a gennaker. Bunk boards are 6 mm with 18 x 38 mm cleats glued to the boards. The cross beams are box sections with top and bottom timber flanges and 6 mm plywood sides. The beams have bulkheads inside. The beams are glassed over with 170 gsm e glass in epoxy. The cross beam hinges are stainless steel. The floats are the same basic structure as the main hull.

The builder of the mast describes its build. “Drifter 17 mast is relatively easy to build, if you have good woodworking skills. I would not recommend it for those who do not know how to make scarfs. It is built in 1/8 ″ CP (~ 3 mm) and the beams in spruce, laminated on the outside with a layer of 6 oz (170g) glass and epoxy. I use PVC foam to make a rounded leading edge. There is a 1/2 ″ ”(13 mm) fiberglass tube on the trailing edge, split, for the sail headline. You can add carbon if you want to make it stronger, but I think the base mast with glass cloth is strong enough for a boat of this size. The basic section without rigging weighs 27lbs (12.25 kg) for 22 '(6.70m) long. The profile cord is 6 1/2 "" (16.5 cm), before adding the mainsail rail, which ultimately gives 7 ″ "(17.8cm) long, and 2 5/8" "( 66.7 mm) thick ( external dimensions ). The rigging is stainless steel cable 1/8 "" (3mm) 1 × 19.”

The jpegs give some hints. An interesting design.

 

Oldmulti, Thank you so much for the tremendous wealth of information you have been providing. It is greatly appreciated!
Just one quick question. in your last post, about the wing mast you state "It is built in 1/8 ″ CP (~ 3 mm)" What is "CP"? I'm not familiar with that abbreviation. Thank you again for sharing your wealth of knowledge!

 

Nolan. Sorry, I am use to reading abbreviations from European sources. CP = plywood (good quality for the mast). Jarcat's 20 foot catamarans have slightly simpler wing masts with 1.2 mm aircraft birch plywood skins with a 75 x 25 mm Spruce centre spar and a 25 x 20 mm aft spar, all covered with 84 gsm cloth.

 

Final boat from Mark Grumpecht. A small cruising catamaran. The cat is 22 x 8.5 foot weighing 1900 lbs with 24 foot wing mast as used on the Drifter 17 trimaran. The cat has a lifting centreboard and central rudder. Mark designed and built the cat in one year. It is plywood with light e-glass cloth outside. It uses mahogany wood on the interior. I do not know any structural dimensions but similar cats (Jarcat 20) are 6 mm plywood throughout and the Drifter 17 tri has 6 mm plywood decks. The cat has aft sugar scoops with steps.

The cat is trailable on a 3500 lbs capable trailer (the trailer weighs 700 lbs). The engine is a 6 HP long shaft outboard. The custom mast raising rig, uses trailer winch. The cat has two single speed winches mounted on the cabin top. The cockpit is 5.5 x 6.5 foot and the cat has a micro galley with a 2 burner propane cook top and small sink. The cat has a custom private head with door on port side and a thetford porcelain marine head. The cat has a double berth with 2 child single berths. The front mini tramp can have 1 adult or 2 children on it is very comfy and cool to be on while sailing or at anchor. The cat is a coastal cruiser that is claimed to perform well.

 

The next e book for possible downloading is again a classic about modern multihulls. Chris Whites the cruising multihull. You will need to do a little work though. The attached address needs to be pasted into a new web page address bar. When the page comes up it will have a green bar with the word download in it. Press down load. The page will then do a 30 second count down and then put on screen a box saying “I am not a robot” click on that box then press the download green box below. The PDF will download to your computer. Now important click on the downloaded PDF and it will show the start pages of the full book. Now press “save as” and file the book on your computer before you explore another item. This site immediately overwrites the download with another download if you try and get another e book. Some of the future books I will suggest are up to 70 megs in size so check your space on your device. Can someone who has downloaded the E book please tell the tread to confirm the instructions are correct or advise of an update to the instructions. The web address is: The Cruising Multihull - PDF Free Download https://epdf.pub/the-cruising-multihull.html
The download is 42 meg. Also be warned the search bar on this site brings up some very different books on "cruising" and it is not about sailing!

 

VPLP are a very forward thinking design group that not only have designed global race winning yachts and some of the world’s largest cruising catamarans but they also have a very active research and development department. They have developed with Benateau the WOW a single reefable wing sail rig and now they have developed a 2 part reefable wing sail rig.

Drawing on extensive experience in the field of rigid sail development, VPLP Design developed a two-element wing sail which is furlable, reefable and entirely automated. “We’re offering a wind propulsion system that is reliable, simple and automated,” says Marc Van Peteghem. “We have developed a reefable and furlable rigging concept which exploits the aerodynamic qualities of multi-elements profiles.” Entirely automated, self-supporting and rotating 360°, Oceanwings® adapts its angle of incidence to the vessel’s point of sail to ensure optimal propulsion. Power is managed by trimming camber and twist. “Not only does it provide exceptional control, it’s also efficient to the point of halving the surface area required to propel a vessel under conventional sail.” VPLP Design is developing several concepts which use this patented technology. 360° rotating unstayed mast ensures optimum trimming regardless of wind direction. Camber is adjustable and reversible. Twist can be implemented to follow wind gradient and thus insure optimal angle of attack along span.

The jpegs give the idea. The mast elements are carbon fibre. The tri is a Tricat 23.5 which is 23.5 x 14.75 foot with a weight of 1300 lbs. The standard sloop rig has a 36 foot mast with a 215 square foot main and a 110 square foot jib. The final jpeg is the standard boat. Tricat no longer produce this model.

 

Carbon3 is a carbon fiber trimaran with the following specifications: 40 x 32.2 foot weighing 4000 lbs and displacing 4600 lbs. This tri may be 40 foot but it has no internal accommodation, it is a pure racer. The rotating canting carbon fibre wing mast has carbon diamond shrouds and aramid cap shrouds. The mainsail: 860 square foot, Jib: 380 square foot, Code 0: 1050 square foot, Genakker: 1500 square foot. The Bruce number is 2.74, seriously fast. The top speed recorded is 32 knots.

Built Denmark (now sold to Hong Kong). Material: Resin infused Carbon Sandwich. Designed by Nigel Iren. I do not know any specs for this tri but a light weight skin would be the easy part. The cross beam and associated bulkheads would be the real design challenge. Also the foils and the mast structure would be an interesting challenge with a lot of carbon being infused or autoclave.

The builder comment “It became a state-of-the-art project,” he says, “and we chose only to use the best. We’re also very familiar with vacuum infusion as we work on a lot of carbon fibre boats, and we wanted to use that technology, because if we didn’t we would have been concerned about losing flexibility.”

The result was Carbon 3: a lightweight, strong but flexible craft with the speed to spare. The jpegs give an idea.

 

Charles de Bodinat designed and built a 29.5 foot “trimaran”. But it was unusual, with the central hull being thinner than the side ones. The purpose was to combine the habitability of a cat hulls with a smaller centre pod for a central board, the weights of anchors and motor. I also wanted to have central rigidity for the front stay. It was called “Tricat” because of its mix between trimaran and catamaran.

This boat initially had a normal central mast and rigging. The mainsail was a Lanteen but although the sails were efficient, they were difficult to handle with the traditional rigging with a single mast because you have to change the yard and the sail from one side to the other of the mast each time you tack. So, he had the idea to combine a bipod mast and a lateen sail with a boom, so that the sail can turn alone. That combination created a very stable mast and flexibility of the yard. The bipole mast is in aluminium. The top of the yard is made of a windsurf mast.

You can sail only with the main sail if the wind is strong and take reefs on it (the yard is in two sliding parts). A jib can be added up to 130 square foot or any spinnaker on a pole. With any sail area the boat is well balanced because you can adapt the center board back or forward. With three on board, having a 55 square foot jib, with force 2 to 3, we sailed about 6 knots average, up to 10 knots by force 4. We have not tried yet with a bigger jib or spinnaker.

The boat is constructed in plywood /epoxy with fiberglass coat at underwater parts. The beams are in aluminium. The platform is rigid and you can sit on the deck sides or in the middle with the feet lowered in the central hull.

The boat is assembled and taken apart directly on the trailer. The process takes about one hour for the hulls and the platform, then one hour for the rigging. If you count the time to put the boat in the water or to lift it and fix it well on the trailer, it needs a total of three hours for two people. The major point about this post is the rig not the hull design or structure. The rig idea could be applied to other cats if you wanted to experiment. Jpegs give an idea.

 

Mair 47 cat designed as an aluminium kit cat from Western Australia. The cat is 47 x 24 foot, built weight is 19400 lbs with an average displacement is 22400 lbs. The sloop rig mainsail is 775 square foot, 421 square foot jib and 1290 square foot genoa. The aluminium tube mast is 56 foot off the deck. The length to beam is 8.7:1. The cat has low aspect ratio keels and skeg mounted rudders. Underwing clearance is 3.2 foot. The forward freeboard is 6.1 foot. Its basic engines are two 50 horsepower desils.

The structure is all aluminium. All plate is 5083 and 6082 extruded sections. The hull bottoms are 5 mm, hull sides are 4 mm, decks 4 mm, cabin superstructure 4 mm, main cabin roof 3 mm. Hull frames are spaced every 700 mm. The hulls and decks have fore aft stringers. Internal fitout is your choice but probably plywood and timber. The real advantage of building in aluminium is the builder will have the skill to do their own aluminium mast, integrated chain plates etc which can be a real cost saving. The other advantage, if you can build the external shell smoothly enough, is you don’t have to paint the cat ever. Unfortunately, most people want the cat to reflect a colourful aspect of their lives. Goodbye to $30,000 and a lot of hard work. The only downside of aluminium is corrosion control.

This aluminium “kit” to build the boat is delivered to your location as a 30 x 7.6 foot box that weighs 15,900 lbs. All you have to do is weld the pre-cut labelled parts together in the right sequence, fair the resulting structure, paint it, fit out the internal furniture, run the electric, do the mechanicals, fit out the boat handling gear, do the rig and after 10,000 hours of work and you are sailing. This is a solid cruising cat that will go anywhere.

 

Chris White designed the EXPLORER 44 which is 44 x 28 foot weighing 10800 lbs with a 60 foot mast and 1021 square foot sail area. The EXPLORER 44 trimaran was to design a fast and comfortable boat that could be built and maintained economically. We all know that multihulls, with their many hulls, complex crossbeams and need for high strength at low weight are costly labour intensive structures to build. Chris’s answer is simple; go LONG and SIMPLIFY. By extending the hull length out to 44' an excellent interior plan can be achieved that will accommodate up to six. Simplification must occur on many fronts in order to achieve a meaningful cost reduction. Another area particularly important to cost control in a trimaran design is the configuration of the crossbeams. The EXPLORER 44 crossbeams are fabricated from modern composites to achieve very high strength and stiffness at low weight but the shape of the crossbeams and their attachment to the hulls is designed to be easy to build.

Chris uses a low aspect ratio fin keel on the 44 because after 18 years of first hand experience with my own trimaran Juniper, which goes to windward just fine with the same fixed fin configuration used in the EXPLORER 44. Chris says a deep daggerboard will allow 2 or 3 degrees better pointing, but at what cost and longer build time. A lightweight trimaran with slender hulls, streamlined decks and crossbeams, a powerful sail plan and well made sails will go to windward exceptionally well with a shoal draft fixed fin.

Construction is in modern epoxy composite. Hulls are designed for Core-Cell foam core with epoxy/glass skins. Decks are foam cored epoxy/ glass and crossbeams are built from uni-glass and carbon fiber/epoxy with streamlined fairings. Accommodation plan provides a separate aft cabin which can be configured with either two single bunks or a very large double. The main cabin contains a functional galley with generous standing headroom and a large saloon that has seating for 6 and a panoramic view through large eye level windows. There is a loo and a forward double cabin.

Chris’s comment on performance are: “The rig is a 7/8 th sloop with a powerful fully battened mainsail and roller reefing jib. All sail handling and reefing can be done easily from the cockpit. Her wide beam and powerful floats will turn sailpower into forward motion. Driven hard, speeds of 15 to 20 knots under working sail will be common. Average cruising speeds over distance of 10 to 12 knots will be typical with a fair breeze.” This is an offshore capable tri that is a simple fast cruiser.

 

The Race to Alaska is a fun race that’s main rule is the “vessel” you choose must be sailed and/or rowed from Port Townsend, Washington to Ketchikan Alaska (total distance of 750 miles) via the inside passage or outside Vancouver Island. There can be no support for the boats along the way. The boats range from kayaks to 32 foot carbon sailing cats and anything in between. The majority of the boats are trailable for the trip home. Russell Brown won the single handed category in a G32 trailable cat recently. A fun race then this entry turned up.

A race organiser thought someone made a mistake but it said: Tritium Racing a 72 x 60 foot trimaran displacing 15,700 lbs carrying 3000 square foot plus of sail upwind and over 5000 square foot of sail downwind. He then looked at the crew list on the boat, an all pro crew of America’s Cuppers, Olympians, world champs, national champs, one of the runners up for the Rolex Sailor of the year, and R2AK veteran Tripp Burdwho was half of the Team Freeburd that finished 4th in R2AK 2015 onboard an Arc-22 beach cat. He asked if they had entered the right race, Tritium said yes and they had organised a pizza sponsor for the race. The betting on the race winner was futile after that.

So, what is Tritium racing? Some sailors who wanted to have fun and occasionally found sponsors to help them have fun. The tri started out as a VPLP designed in 2000 a 60-foot trimaran from the ORMA class. It was very beamy, pre-impregnated carbon structure, raised beams and high volume floats with a cloud of sail. Ex-Bonduelle then became GITANA 12 in 2005 and upgraded with a revised deck layout, larger cockpit, revamped interior (electricity, electronics, hydraulics…), altered daggerboard casings and rig and a new set of rudders for a period before being brought by Tritium racing. The boat was then modified - with floats lengthened to 72 feet - and cross beams reinforced, for the new loads of a bigger rig. The tri was then used by Artemis Racing for testing of AC wing and dagger foils for its AC 72 campaign.

From 2012 Tritium 72 foot racing tri was used in local races, breaking records etc. In 2013 it raced to Hawaii doing the Transpac, missing the record by 2 hours. This annoyed Tritium racing who borrowed a 105 foot tri in 2015 and broke the Transpac record by 24 hours doing it in 3 days 8 hours (25 knot average). The 72 footer then “became” Lending Club doing charter and racing from San Francisco breaking more records. Then came the entry into the Race to Alaska. A 40 knot boat racing against people in kayaks, interesting. The jpegs give you some idea of some of the history. The final jpeg gives an idea of what Gitana thinks a tri should sail like today in 14,000 miles dual handed races.

 

Turanor is a Swiss solar powered catamaran that circled the globe in 2011/12. It did the entire trip on solar power and batteries. No fossil fuels involved. Why is this cat of interest? It is about the structure but we will get to that later. Turanor is 100 x 49 foot (solar panels folded out make it 30 foot wider) displaces 190,000 lbs and is powered by 4 electric motors that draw about 20 kilowatts at its 5 knot cruise speed. The 4 motors can produce 120 kilowatts of power if needed and can drive the cat at 14 knots at maximum speed but the power drain is substantial. The 5400 square foot of solar panels provide 94 kilowatts of power under idea conditions which is feed to banks of lithium iron batteries for overnight or cloudy day running. Turanor can run 3 days in overcast conditions. The cost of the panels alone even at today’s prices would cost over $100,000, the rest of the power setup would cost another $300,000 plus. This isn’t a cheap way to cross oceans.

But back to the structure. The reason its of interest is that it can be a facsimile of cruising cat hull of similar size. The “legs” between the demi hulls and the above water main hull do not replicate a catamaran or tri cross beam but the rest of the structure has some relevance. The whole vessel is a carbon fibre (CF) epoxy Airex PVC structure. It was vacuum bagged and used prepregs in many areas. There are 3 basic layups: the main central hull shell, demi hulls and decks which are 2.2 mm (.09 inch) CF outside 50 mm Airex PVC 2 mm CF inside. The cloth orientation is 60% is 0/90 degrees, 40% is at 45/45 degrees biaxal. The 11 bulkheads in the main hull are 1.2 mm CF 40 mm Airex PVC 1.2 mm CF. The legs connecting the demi hulls to the main central hull are tapered boxes with 1.2 mm CF 40 mm Airex PVC 1.2 mm CF walls. The layups connecting the legs to the various hulls are doubled and reinforced with 50 mm CF tapes.

The 4 crew did not lack comfort as the jpegs show. 6 double berth cabins and a main saloon that can house 40 people without any crowding. But when you are crossing oceans at 5 knots you probably need some private space.

 

I used to drool over tris but am so happy I didn't build one, I was months away from starting on a design I had done based on a Searunner 40 meets a Spoon Bay 10.5 before a friend (who I was actually building a tri for) said to build a cat. He was right.

When I first went on a Spoon Bay in the 80s it seemed huge, but then everything is bigger than a Twiggy. I went aboard one a year ago and, man, what a hard life to be had inside! Tris are just really hard work compared to a cat. I had two tris I loved and lived on but I wouldn't go back.

The idea of long and thin is problematic for so many reasons. It is like living in a tunnel with a thin floor space and little room to get out of any one else's head space. You can't move without saying excuse me as you squeeze by. My wife and I were luckily very much in love when living on a 30ft tri but tripping over someone you love is fine, but what happens when you have to invite Ron along for a delivery cruise, or you get older, or have kids, or just want some alone time?

So I don't get the long thin tri idea anymore. A performance cat will be easier to build and have so much more usable room that these boats will only suit the single hander or a couple who love tris.

 

Long and thin is a bear to live on, but is faster, safer and more comfortable.
The trick is to keep the hulls long and thin and put the accommodation on top . Problem with doing this on most cats is the bows are too high, so the weight goes up, exacerbated by filling the extra space with 'stuff'. This then requires a bigger rig and the spiral goes the wrong way. Bob Oram is one of the few cat designers who understands this.
The other way of achieving it is the harryproa. One long, low, narrow hull to take the rig and rudders which is always to leeward and a narrow, higher, shorter hull (still with wl beam to length ratio finer than 1:11) with the accommodation. There are some compromises inherent in this (looks and shunting), but it is the cheapest way to go fast with reasonable room.

 

I've been away from boating subjects for quite awhile,..that market crash of 2008 was a stunner. I just found this VERY INTERESTING subject thread that I am skimming thru this morning. I am thoroughly AMAZED at your degree of knowledge about construction techniques, materials, layups utilized, etc,...fascinating. I've only reached page 27 thus far, and just skimmed the posting very quickly to get a general idea. I will have some questions for you in the future.

I was the original importer of the Fountain/Pajot cats,..their first production cat the Louisiane 37 ,...put into the Annapolis Boat Show in 1997-1998 I believe it was. John Conser saw it there, and later commented to me how it inspired him to do the Conser 47,....you can see a lot of similarities.