Multihull Structure Thoughts

I'm not a reverse stem fan. Around here it would be a real liability with logs and kelp.
Funny how different the last two boat in the list are. The funny thing is that I like them both a lot. I'd love a ride on Tallulah. John Harris is a gifted and very prolific designer in my opinion and I'm always curious to see what he comes up with next.

 

Let’s look at some detail of an Ocean 50 racing tri. ARKEMA 4 is part of the Ocean Fifty Pro-Sailing Series, formerly known as Multi50. The racing tri is 50 x 48 foot with a displacement of 8500 lbs. The 72 foot carbon wing mast carries 1,935 square foot of sail upwind and 3,065 square foot of sail downwind. The draft varies between 5.5 foot over the T foil rudders and 11.5 foot over the central daggerboard or depressed C foils on the floats. Length to beam on hulls unknown but 16 to 1 plus would be realistic.

Performance, incredible. One tester, who sailed on the tri, believes these words “These trimarans go fast even in the slightest of puffs. High speed racing with up to 30 knots SOG are possible, at one occasion, as skipper Vlamynck tells, they clocked in at whopping 39 knots upwind (!) leaving Toulon.” As for accommodation, find yourself a sail bag or a bucket for sleeping etc. There is a good navigation area and enjoy your freeze fried food.

This item is about how the designer, marine architect Romaric Neyhousser, and builders partially achieve this performance. The tri is built from foam glass with recyclable resins, new materials, 3D printing of technical parts and clean energy. Arkema 4 is skippered by Quentin Vlamynck was heavily involved in the construction of Arkema 4. Not quite what you expected? Arkema 4 trimaran is made primarily of a foam-glass-epoxy sandwich with carbon reinforcements that used resin infusion, is oven baked at 120 degrees. The cross arms are mainly carbon fibre. As for the floats and the arms, the structure and the bulkheads were assembled by structural bonding using methacrylate products from Bostik, an Arkema Group subsidiary. Elium® resin, a thermoplastic material formulated and developed by the Arkema Group has been used for all front fairings of the outriggers as well as the deckhouse and cockpit, which are areas where its impact resistance is invaluable.

Now on to the basics of the design. CENTRAL HULL Validated by many digital simulations (CFD), this hull is wide at the gunnel to increase the sheet effect on the front sails. Below the waterline, it is deep with V sections for less bumpy sailing. The clear forefoot allows for more efficient tacking. The C foils in the float are designed to lift the tri when sailing hard. The result is a float hull shape designed for speed with very full ends. The aft section of the float is full but left open to allow access to the rudder shafts and bearings etc.

The cockpit and cockpit roof are put as low as possible in the main hull to lower the crew weight. There is only 1 grinder winch in the cockpit with all headsail sheets etc lead to it to reduce weight. The lowered cockpit roof also allows the mainsail boom to be lowered creating more of an endplate for the mainsail. In one jpeg you will see a filler sail between the boom and the cockpit roof to enhance the end plate effect. The cockpit roof helps protect the crew when sailing but there is also light strong extensions that can be attached to protect the crew in strong winds and sea conditions. Remember these tri can create 60 knot apparent winds when sailing at full speed.

Because of the 60 knot apparent winds all the crossbeams have fairing fore and aft to create aerofoil shapes. The main central hull daggerboard is deep for upwind work but can be completely withdrawn for faster work where the C foils do both lifting and lateral resistance. The foils are all carbon fibre.

There are many more little features in this tri but this is getting to long, so I will only describe one. The sail slugs which are normally metal were considered to heavy so they made them from a combination of carbon fibre and special resins. That is being extreme in weight saving.

The jpegs give part of the story. The first set of Jpegs are the overall boat, the second set will be loaded later of the build.

 

The second set of jpegs for ARKEMA 4 are build jpegs.

 

The following is about an old day sailing trimaran that was designed by Arthur Berry and built by Aquadyne Boat Co. (USA) between 1974 and 1978. 700 were built of which about 50 active Sailbird owners on a Facebook group. The Sailbird 18 is 18 x 11.5 foot and can be folded to 8 foot wide for trailing. The weight is 405 lbs if they have the standard rig and side trampolines. The standard mast is 24 foot high with 125 square foot mainsail and a 60 square foot jib. Some Sailbirds had Hobie 16 rigs installed by owners and several had 15 mm plywood side panels installed instead of trampolines. Both options added weight. The draft of the kickup centreboard in the main hull was 0.7 foot to 2.7 foot down. The rudder is a kickup on the main hull stern. The floats had fine ends with an interesting flange at gunnel level which was for deck joint strength probably but would provide some lifting force at speed. The main hull is reality wide and the floats relatively small compared to modern designs so its top speed would have been limited.

The purpose of this tri is a fun day sailor and fleet racer in its day. The Sailbird 18 performance is unknown but for the 70’s it would have been reasonably good.

The structure is solid glass on the hulls. The decks would be reinforced solid glass probably with ribs. The cross beams are where the interest is. The approximately 75 mm aluminium tubes have a couple of special castings which allow the beams to fold. The castings have a pivot pin and a secondary pin to lock the beams into position. Good approach for the relatively light loads involved. But this idea has its limitations. Modern tris are wider, have more powerful rigs etc and would put more forces on the beam structure and joints, which is the reason for water stays and Farrier type arms to help strengthen the beam structure.

This is a fun tri for a day sail with a friend or 2. I am sure there would be quite a few in back yards waiting to be resurrected for some fun.

 

Regarding the reverse bow/stem topic, there have been a few threads about this and from what I’ve gathered there are designs that are more about a look, which is understandable in that it represent “modern” design, with the aesthetic being adopted from the development of a racing function that then trickles down into production just like it does in automotive design. Example: think of all the non-functional air scoops on production cars, etc..

But it seems that the other part of the equation for functionality is where the volume is placed in the hull.

From looking at the racing designs, it seems that the bows are also designed to flare out in volume at the mid to top section so that little wetted surface area is in the water, but then when pressed hard the flare and volume keep the bows from digging, and that perhaps is the reason for the secondary design characteristic of a reverse angle of the leading edge, so that it can divert and direct that water up and over the bows. Otherwise is seems the harder the hull is pressed, more volume enters the water displacing it, and that it’s easier to just direct some of it back up and over instead of straight up or around the flare.

Correct me if I’m wrong, but I guess what I’m saying is from a racing function view, there seems to be more to it than just the reverse angle of the bows, and that the “look” that has become a popular trend does not take full advantage of how the design is supposed to function, like the air scoop example.

I guess the big question is what’s next?

When will the reverse bow trend become dated and we look back at current designs and say, check out those hulls. Their lines are soooo 2020’s!

On that note, there are still a few classic Multihulls in the islands with the big Hawaiian style tomahawk axe blade bows designed for getting through surf and beaching -think classic Hotel Beach Cats from the heyday of tourism, and they are often coupled with low flat square transoms which is quite a juxtaposition. Seeing one and comparing it to modern designs kinda reminds me of looking at old trains versus new trains.

 

Well the reverse bow is an ancient concept. The ram from galley days! And it still does this well. Hydrodynamics are great for racing but when you hit something it does revert to it's ancient function. I've advocated replacement or inflatable bows for record setters to eliminate one of the major causes of aborted attempts or dropped out races

 

I've got a friend who wants to build a wing mast for his 45' cruising cat. I know there has been posts here about building masts (besides page 5), but I can't seem to find them. He'd be building from wood and Pultruded carbon. Can you give a page number?

 

Russell. Try page 16 an article by Eric Sponberg on masts, page 31 an article about PBB mag nbr 14 that talks about Goldcoast yachts masts and other things, page 51 carbon fibre home made masts. Also there is a long but informative article by AES NZ at rig design hints https://www.aes.net.nz/info.html which if you go down near the bottom gives CF details and fabric direct layups etc. Hope this helps.

 

John Perry has been mentioned here before as a gifted home designer, here is another example of his work. The tri is a day sailor camp cruiser that can be more easily righted after a partial capsize than most tris. The “Contest 1” is 19.2 x 13.1 foot and can be disassembled to 5.9 foot. The weight is 716 lbs with a displacement of 1,232 lbs. The 26 foot mast carries 193 square foot in the main and jib. An optional 193 square foot gennaker can be added. The main hull length to beam is 12 to 1. The draft is 0.9 foot to 4 foot over the kickup centre board. An outboard can be added. The float buoyancy is less than 100% of the displacement.

For trailing the hinges between outriggers and the main hull are dismantled and the outriggers stowed on their sides with the aft cross beam wings vertical. The low buoyancy floats are small and light enough to be handled ashore by 2 crew.

Now we get to the assisted righting. To quote John. “The design has outrigger buoyancy less than total weight so that when hard pressed the lee outrigger will be driven under. The boat will then be at a considerable heel reducing capsize risk by shedding wind loading from the rig in the same way as for a monohull sailing boat. If heel continues the craft will end up floating with the mast on the water and the lee float submerged by the depth of one cross beam. Although a slightly embarrassing position, there should be a better chance of recovery from this position than from the fully inverted position likely to be adopted by a capsized catamaran or trimaran with full buoyancy floats.” And “The method of capsize recovery chosen for this design is by use of adjustable tackles attached to the shrouds combined with a buoyant mast and a small masthead float. This can be a simple manually operated system, on a larger craft it would not be so easy to implement. 4:1 cascade tackles link each shroud to the chainplate and the tails for these are connected across the beam of the boat so that the shrouds only slightly slacken as the mast is inclined up to approximately 20 degrees.”

The outriggers are supported from the main hull by small decked wings aft and by tubular cross beams forward. Both the decked wings and the forward crossbeams are hinged from the main hull with hinges locked by stays which can be set to give two alternative outrigger heights. When sailing the outriggers would be in the high position so that in most conditions crew position can be adjusted to balance the boat mainly on the main hull minimising drag by reducing wetted surface and by making only a single main wave pattern. The lower outrigger position is for use when not underway and gives high initial stability to improve comfort for living aboard. A single large centreboard is fitted in a case at the forward end of the main cockpit. This is simpler than the alternative possibility of having a centreboard in each outrigger.

The construction is ply joined and sealed with epoxy. The joints between deck and topsides and between the main bulkheads and the hull skin are made by screwing and gluing onto wooden gunwales and framing, other ply joints are generally made with epoxy fillets. The structure is well reinforced at the outrigger mounting points. The structure is generally 5mm ply. All wood surfaces are epoxy coated and the external surfaces sheathed in glass cloth set in the epoxy coating. When the boat is capsized to 90 degrees the outrigger skin panels are subjected to hydrostatic loading of approximately 20kPa (3 psi). This loading is supported by ply bulkheads spaced 200mm apart inside the outriggers. The mainhull and the outriggers are vee bottomed and fitted with rubbing strakes amidships to protect the ply skin when dried out on the foreshore. These rubbing strakes are shod with strips of 16g stainless steel. The various hinges and stays which support the outriggers are fabricated from 4mm aluminium alloy plate and large section 4mm thickness aluminium alloy angle. Screw fasteners are mainly cut from 8mm stainless steel studding and hinge pins are large stainless clevis pins locked in place with snap fit spring clips. The stays below the outrigger cross beams are telescoping sections of thick wall aluminium tube with cross pins to lock at two alternative lengths.

An interesting design, good work John. The attached drawing is the only known item for this tri.

 

Thanks so much, Oldmulti. I'm hoping he finds the perfect aluminum stick, but hard to find a 3-wire rotating rig for a boat like that.

 

Jim Antrim is a very competent designer who has worked with naval architects Dick Carter, Britton Chance and Gary Mull before he founded Antrim Associates in 1979. He designs fast racing tris, cruising cats and fast monohulls. In 2010 he had a request to do a cruising version of his racing 40 foot tri. The result is “Angela” that is 40 x 34.1 foot with a “displacement” of 10,000 lbs. The 59 foot rotating MARSTROM carbon fibre wing mast carries a 742 square foot mainsail, 494 square foot jib on furler, 215 small jib, 968 square foot Code 0 and a 1,450 square foot Gennaker. The mainhull length to beam is approximately 8 to 1. The draft varies between 4 and 8.1 foot over the rudder and daggerboard. The engine is a Yanmar 20 HP Diesel with a sail drive and a folding propeller.

The accommodation has full standing headroom in the main cabin area. To port is an open galley, settee, toilet, sink, and shower. To starboard is a chart table with icebox under and a settee, which converts to single berth. The forward cabin features full standing headroom with access to starboard of the daggerboard trunk. Zippered padded doors are featured for reduced weight and noise. A queen berth is located under cockpit. Berth access is from main cabin to starboard with hinged companionway steps for increased access. This is a practical layout for a fit cruising couple. The power in this rig will require a lot of winch work if you wish to go 300 mile days.

The performance of this tri according to my simple calculator is fast. 10 to 12 knot averages with peaks over 20 knots. The power to weight ratio is high with a lot of stability due to its beam. I do not know actual performance numbers but judging by Jim Antrims other tris have high performance in all directions and conditions.

Angela was built by Haoyun Shipbuilder, Qingdao, China. The structure is composite sandwich foam glass epoxy construction with kevlar and carbon fiber reinforcements in selected areas. The cross beams are mainly carbon fibre.

The jpegs give the idea of an excellent design.

 

I remember a good thread about this on here that I think might have been in a different area that was posted in the last few years but couldn’t find it.

I did find the following which might be of interest:

https://www.ericwsponberg.com/wp-content/uploads/wing-masts.pdf

Rotating Wing Mast – theoretical discussion https://www.boatdesign.net/threads/rotating-wing-mast-–-theoretical-discussion.14714/#post-114367

Gougeon Wing Mast Plans https://www.boatdesign.net/threads/gougeon-wing-mast-plans.32195/page-3#post-905475

 

Yesterday we spoke of Jim Antrim 40 foot cruising trimaran, two 40 foot tris preceded “Angela”. “Aotea” built and raced in the 1990’s that won San Francisco to Japan, Singlehanded - 1992, 34 days 6 hours, Singlehanded Transpac Race Record, 1994 - 8 days, 20 hours, 16 minutes (record broken in 1998 by an ORMA 60 trimaran) and the Doublehanded Farallones elapsed course time of 3 hours 48 minutes, 1992. Aotea capsized in 1995 and was not recovered. In 2002 another lighter wider version of a 40 footer was designed and built.

Zephyr is 40 x 33 foot with a displacement of 6700 lbs. The rig size is unknown but I am assuming it would be close to yesterdays tri with a rotating Marstrom carbon fibre mast carrying a 742 square foot mainsail, 494 square foot jib on furler, 215 small jib, 968 square foot Code 0 and a 1,450 square foot Gennaker. The length to beam on the main hull is 10 to 1. The floats length to beam is 13.5 to 1. The draft is unknown but there is a daggerboard and kickup rudder. An interesting feature of this design is it is demountable and can be trailed as a 40 x 8.5 foot package on the road. I suggest a crane and a weekend to setup or breakdown the tri for use.

We need to understand Jim Antrim a little. Jim Antrim is a seriously capable Navel architect and engineer. He has designed and engineered roller coasters, monohull and multihull racing and cruising sailing boats, ocean crossing row boats, power boats etc. He wrote the PBJ sandwich-laminate engineering analysis program offered by Sandwich Software. Result when he does a design feature it is done for a reason. EG To maximize his amas’ performance whether they were being driven deeply into the water in heavy airs or prancing across it in light winds, Antrim carefully considered the bow and stern shapes separately. Zepher’s outer hulls exhibit significant rocker aft, so the after sections add no wetted surface in near calms. The increasingly elliptical sections oriented laterally give the floats unusually flat exits and high reserve buoyancy once pressed. Antrim says, “This is a crossover from different kinds of boats. The conventional thinking used to be that a trimaran’s amas always should have fine, skinny sterns in order not to generate considerable drag when the boat is pitching. But in a monohull, you would never make a skinny stern because the boat will then pitch a lot. I thought wide sterns would reduce high-speed resistance while also dampening pitching. It worked. Also, windage of a multihull is a huge percentage of total drag, in a multihull it’s as big as the wave-making drag. If you don’t need the volume aft on a float for any hydrodynamic purpose, then why have the weight, cost, and windage. Finally, with less volume aft but a full 200% of the boat’s displacement carried in ama volume, Zephyr amas are fuller toward the center of volume about a third of the way aft of the bow, which shifts slightly forward as the boat heels to keep the rudder in the water and lift the nose a touch. At speed, the big flattish sections forward encourage surfing and planning.

The structure of this try is basically foam carbon fibre in epoxy with some Kevlar. The crossbeam attachments on the main hull is interesting. There are link pins on the forward face of the crossbeams which are then “tied” onto the main hull similar to Wharram tie downs. The cross arm ends are pushed into the floats and bolted. Think about the logic and engineering, light effective and if the right materials are used strong.

The accommodation is similar to yesterdays 40 foot cruiser “Angela” but everything is narrower in the main hull. Zephyr is more racer less cruiser.

Again, no performance numbers but Aotea the previous 40 footer peaked at over 27 knots on occasions and Zephyr is lighter and wider. And finally, a small message. Zephyr cost over $600,000 to build and was sold a decade later for $60,000. There was nothing wrong with the boat but the financial conditions (about 2011) forced the sale. Space (accommodation) sells, but speed does not always sell.

The jpegs give the idea of a very fast, large transportable tri.

 

This was to be a simple report on inflatable wing sails. I looked for who invented the idea and found a legal battle between Fly, IWS and Michelin in Swiss and Italian over who owns the rights to the idea. The court cases are likely to run for years. The court fights are because each company have developed small boat versions but the real goal is to sell the idea to commercial shipping companies to reduce dependence on fossil fuels. Shipping produces about 13% of the total global emissions into the atmosphere.

We will just focus on the technologies and idea. The concept is an inflatable wing sail that is partially self supporting by air pressure. The sail can be blown up to full height or a variety of reef heights. The jpegs give more of an idea than my words.

Now there are several approaches to this technology. IWS uses a collapsible carbon fibre central mast and has fans on the leading edge of the sails at various heights that blows up a section of the sail to form a rigid wing sail shape. (Think about inflatable Stand Up Paddle Boards as to how rigid a shape can be formed by fabric and air). The sail is controlled by a lightly loaded single sheet as the wing sail is nearly balanced. If reefing is required you just deflate a section of the wing sail.

The Michelin system is fully automatic. To quote one test “As soon as he leaves the dock, our skipper presses a button on the Tablet PC in the cockpit, choosing full power, as the day's conditions are moderate. The 100 m2 of sail gradually inflate before even leaving the harbor. The process takes 5 minutes, with the help of 5 fans. The sail, which looks like a Michelin Man, and its internal telescopic mast, hoisted by electric winches hidden in the boom, reach a height of 16.56 meters. The sail mounted on a balestron, has no sheet. It is a motor that turns it around the mast to adjust the angle of incidence in relation to the wind, automatically. Depending on the power required and the wind strength, the control software developed by Michelin, tacks or reefs the sail and, if necessary, takes a reef without the crew even thinking about it. To do this, the system relies on three anemometers.”

So, the advantages of inflatable sails are shape control using the internal pressure, no battens and no local stress. The wing flies vertically and does not create local stress inside the membrane (light sail cloth used for both ribs and the fairing) with little heel angle upwind. Perfect for gigantic sails such as superyachts with free-standing, retractable and light mast hidden inside the wing. A refined deck space free of any hardware.

Another advantage is the sail is not fully sealed. The Wisamo version has fans that keep it pressurized at a small cost. This eliminates the need for costly sealing solutions and provides extra security in case of a tears as 1 sailor attests to this: "We didn't notice a tear of several inches until the sail was lowered on the way back to port, because the fans were keeping the sail inflated."

According to one companies statement inflatable wingsails on a cruising monohull about the same cost and produce similar speeds to a conventionally rigged monohull.

The jpegs give part of the idea. There will be a lot more to learn as the companies develop these sails.

 

We need to understand what the really rich, who have an understanding of sailing, want for a fast cruising catamaran. The Gunboat 80 is the largest series production Gunboat available. The Gunboat 80 is 78 x 32.8 foot with a weight of 65,000 lbs (standard version) and a maximum displacement of 77,160 lbs. The 94.5 foot carbon fibre mast carries a 2,314 square foot mainsail, number 2 jib 1647 square foot and a number 1 jib of 187 square foot. There are many spinnakers, Code 0 options etc. The hull length to beam is 13.5 to 1 with an underwing clearance is 3.3 foot. The draft over 5 foot over rudders to 15.1 foot over the C foil daggerboards.

The accommodation is semi custom within the limitations of the fixed cross beam bulkheads and hull structures. The jpeg below shows 3 vast double berth cabins with attached ensuites, several single berth or crew cabins with attached toilets in the hulls. The main saloon has the galley, seating, table etc plus at the forward end of the saloon a steering navigation position. The steering navigation position has access to a forward cockpit for access to the mast etc. Aft in the rear cockpit are 2 steering positions on the gunnels.

The structure is foam glass carbon in epoxy. This is an engineering achievement to get a cruising cat this large relatively light and structurally sound.

The performance is amazing according to my calculator. If you have the crew and courage to push the cat 15 knot averages are possible with peaks over 25 knots (calculator says 30 knots). The power to weight is very good and when combined with stability means electric winches are almost mandatory. To give an idea of this cat the first owner of an 80 sold the 80 and ordered a Gunboat 68 because it was “newer” and more practicable.

The jpegs give the idea of a very fast cruising dream.