Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. redreuben
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    redreuben redreuben

    “Negative rocker aft” ?
    Sounds like a topic for further exploration, I’d like to see what that looks like and why it works.
     
  2. patzefran
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    patzefran patzefran

    Positive rocker induce succion at speed (negative pressure on the hull bottom aft maximum draft !), negative rocker act locally as a flap and provide positive lift aft.
    When you use foil assist (not full foiling !), negative rocker aft help to support the aft hull and avoid excessive pitch up.
    On classic A cat they use winglets on the rudders in addition.
     
  3. oldmulti
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    oldmulti Senior Member

    Redruben. I will address "negative" stern sections when I find some hull lines to show it. Patzefran described it reasonably well.

    I do not much about this catamaran design beyond a few basic dimensions. The cat is 32.6 x 24 foot displacing 2,600 lbs. Sail area is unknown. The hull length to beam is about 14 to 1.

    The cat is a very fast racer cruiser for coastal racing. Two have been built, 1 is in Australia and another was constructed in Thailand. The cat appears fast and if it is the displacement suggested with the rig in the jpeg it should be fast.

    The designer is interesting. Julian Spooner completed a BSc in Yacht and Small Craft Design in Southampton in 1993. He then worked in composites throughout his career including a 10 year period (2005-2015) at Princess Yachts. Whilst at Princess Yachts he headed a government funded project to study better composite building approaches. Julian says “Resin infusion improves both quality and performance.” and “We can now mold to the same strength using less material, so delivering a lighter product. Because we use less material, we can, and do, use higher-quality materials.” End result Class M motor yachts up to 130 foot that have 1 shot resin infusion hull components that use 6 tons (13,500 lbs) of resin in one run. Julian also opted for integrally heated tooling molds for final curing of the molded parts to avoid the need to erect large ovens with their associated expense and greater energy usage.

    You will not be surprised to find the 32.6 foot cat is epoxy resin infused carbon fibre honeycomb structure. There is also prepreg carbon fibre used in some structures on honeycomb. This results in a strong very light build weight which allows the all up displacement of 2,600 lbs.

    The jpegs give a guide.
     

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  4. oldmulti
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    oldmulti Senior Member

    Redreuben asked about “negative rocker aft” as specified in the Northwind 55 cat. Negative aft rocker is like all things in cat design, “it depends”.

    If it’s a power cat that has speed length ratio above 1.5 then it is likely to have a negative rocker or at least a flat stern line near the waterline to minimize squatting of the stern. The attached jpegs will give the idea. Malcolm Tennant was the master of this concept with his CS hull shape. As Tennant said if the power cat would do a maximum of 12 knots then a slightly modified sail cat hull would work well but at higher speeds there needs to be more flow control over the stern to prevent squatting. Planning hull power cats operate in a different mode and have a pretty straight hull rocker over the length of the hull.

    Sailing cat hulls have different issues. With length to beams of 8 to 1 or worse you basically design for minimum resistance, minimum wetted surface and weight carrying ability. This means round or U shape bottoms, full ends and smooth rocker from end to end. A 50,000 lbs displacement 45 foot cat may have a 90 foot mast but I assure you it is designed to sail at 8 to 10 knots as the hull resistance is relatively large.

    Next level of sail cat design starts at about 10 to 1 length to beam and moderate displacement. The hull resistance characteristics is a lot better allowing higher speeds with a moderate rig. Next problem arrives. Is this cat truly a sailing cat or has the designer taken the view that a cat is actually motoring 50% of the time as a lot of charter cats do. The full ended, U shaped hull shape can be good across a speed range under sail but may squat under power. A fatter flatter stern shape helps reduce squatting.

    Next level is 12 to 1 or above length to beam ratio lighter weight sailing cats. Hull shapes can vary markedly and depend on many factors. The displacement of the hull, the relative weight of the crew and the crews sailing position, the material the hull is built from, the position of the rig, the latest “fad” shape etc.

    The Tornado hull shape is fast but has a deep bow, a lot of forward rocker and along run to the stern. There is a lot of asymmetry with a wide stern and fine bow. The hull shape was partially dictated by the tortured ply construction.

    The Grainger racer type cats (Carbon Copy and Livewire 28) have flatter hull rocker forward with maximum rocker pushed right aft. The hull shape is partially dictated by the crew weight position (relative to the overall boat weight) when sailing. More displacement is required aft. Also, the rig position is pushed aft reducing the need for buoyancy in the bows. This is a fast hull shape that suits its purpose. As to how far you push the maximum depth of rocker aft requires really good VPP programs AND real world sailing experience.

    Really high performance light sailing cats like AC 45’s have minimal hull rocker and flat full end sections. Most of these cats are now designed to foil so the hull shapes are designed to help “take off” for foiling.

    Back to higher performance cruising cats. Grainger Raku 35 has 13 to 1 hull length to beam full ended, U shape hulls that have a smooth rocker shape end to end with its maximum rocker midships. The crew weight is a low proportion of the total boat displacement so you care more about where you place engines, water tanks etc. It is also assumed you will be sailing more than motoring.

    Back to the Northwind 55. The design team had a specialist VPP and hull shape guy who did a lot of testing to come to the negative aft rocker shape. What we don’t know is his design critera. Was it important to maintain an anti squat shape for foil stability to maintain the angle of attack of the foils in a controllable range or is there an expectation of the cat being powered 50% of the time?

    The Silent 60 solar powered cat which operates in a 6 to 8 knots cruise speed (can max at 20 knots with a short range) have a “sailing cat” type hull with a negative aft hull shape. Again, a very well researched hulls shape to suit the need. There are at least 10 on the water at $2 mill plus a piece.

    Conclusion. A designer considers many factors in a hull shape and adjusts them to suit a purpose. Sometimes the hull shape is comprised by build materials (think constant chamber, tortured ply, flat panel), the intended purpose of the cat (racer, cruiser, power etc) and what the designer is comfortable with. Some designers do not have access or trust in tank tests and VPP software, others think build simplicity is more important etc. In short a negative rocker may suit one cat but be inappropriate for another. There are also other considerations. EG More rocker generally allows faster tacking, if there is a low aspect ratio keel the hull may have less rocker as the volume in the fin allows a flatter hull shape etc.

    Jpegs give an idea. Power cats first, sailing hull shapes next and silent hull shape last.
     

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  5. oldmulti
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    oldmulti Senior Member

    The following is a 42 foot dive charter catamaran that is designed by Kurt Hughes, and is built in a rapid composite construction system using mainly flat panels. The cat is 42.1 x 18.6 foot with a weight of 8,650 lbs and a displacement of 14,800 lbs. The hull length to beam is 11.1 to 1. The operating draft is 2 foot with an underwing clearance of 2.4 foot. The boat is powered by two 150 HP 4 stroke outboards. No performance numbers but I suggest 20 knots is possible.

    The accommodation is simple with a deck house for steering, navigation, limited galley and toilets. Seating is available on deck as well as easy entrance and exit to the water by aft stairs for divers. Divers tend to need a lot of space to prepare for a dive. The layout of the cat will also allow a lot of room for fishing charters.

    The construction is based around flat panels of foam e-glass sandwich. The hull sides are 18 mm PVC (probably 80 kg) foam with triaxial e-glass inside and out. The underwing is 25 mm PVC foam 100 kg with triaxial e-glass on either side. There are 2 reinforcing stringers on the hull bottom panels with a reinforcing keel line. Not specified but I think the resin system would be vinylester. The structure jpeg gives an idea.

    The actual build is a series of shaped flat panel taped together with biaxial e-glass to form the Tennant CS type hull shape. Kurt designs good hull shapes and this flat panel version simplifies the build whilst appearing to be very efficient.

    The jpegs give the idea. This power cat could be converted to an effective, nice cruiser with a bigger deck house that could have a few cabins etc. Interesting design.
     

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  6. oldmulti
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    oldmulti Senior Member

    The VMV 53 is designed by VMV Yacht Design, an Italian design studio with a lot of design and actual sailing experience. The VMV 53 is aimed to be a very high performance cruising catamaran with C curve daggerboards and T foil rudders. The foils are to minimise wetted surface and assist in pitch control. The VMV 53 cat is 57 overall including prodder, 52 x 24.6 foot with a weight of 21,300 lbs and a displacement of 32,500 lbs. The optional rotating mast carries a 915 square foot mainsail and a 753 square foot genoa. The length to beam of the hulls are 12 to 1. The draft is 4 foot over the T rudders and 8.7 foot over the C foil daggerboards with an underwing clearance of 4 foot. There are 2 Yanmar 4JH, 40 or 57 hp, common rail diesels mounted midships with shaft drives to 3 blade folding props.

    VMV Yacht Design has opted for curved symmetrical fins and optional curved asymmetric foils which do not come out the deck level. The goal is to increase the anti-leeway attitude of the boat while releasing much of the pressure from the leeward hull. This way, the hull will have reduced wetted area and the bows are kept high with the foils assisting. The central mounting of the engines and liquid tanks also helps reduce pitching. The prodder allows multiple roller furling headsails for simpler sail handling for short handed cruising.

    The accommodation comes in several options with 3 or 4 cabins with attached toilets in the hulls. The main cabin has the galley, dinette, steering and navigation. The cockpit is accessible via a large door allowing an integrated entertainment area. The water tanks are 2000 litres and fuel tanks are 1200 litres. Now we get to an interesting problem. This is a light cruising cat that can carry a 11,200 lbs payload of which 7,200 lbs of it are liquids. This cat will need to be built very well with limited air conditioning, toys etc to be sailing at its displacement level.

    The construction of the first cruise version of the 53 is a 70% carbon fibre foam structure. A high performance version is virtually all carbon fibre foam construction. The foils are solid carbon fibre. The mast is optional a carbon fibre tube or a wing mast depending on the intended purpose.

    The jpegs give the idea of an interesting design.
     

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  7. Iridian
    Joined: Jan 2020
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    Iridian Junior Member

    I noticed that the VMV cat has a solid bridgedeck to the transom. Obviously not having a solid bridge deck at the bows is important, but how much does this matter at the back of the boat?
     
  8. oldmulti
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    oldmulti Senior Member

    This was to be a story about the VMV 63 foot cruise race cat, then it became a story about the Ascat 63, then it became a story about the Supercat 66 by ELEVA Yachts & Moana Shipyard. The connection is, they are all the same catamaran designed by VMV Yacht Design but being marketed by different organisations. The cat is 66 x 29.15 foot as a base hull unit the weight for the open bridgedeck “race” version is 31,350 lbs with a displacement of 40,300 lbs. The full bridgedeck cruise version weighs 42,500 lbs and displaces 51,500 lbs. The carbon fibre tube or wing mast is 80 foot high and carries a 1,325 square foot mainsail, 240 square foot staysail, a 1,215 square foot genoa and a 1,668 square foot code 0. The length to beam of the hulls is 10.75 to 1. The draft over the T foil rudders is 4.3 foot and 10 foot over the C dagger boards. The underwing clearance of the cruise version is 3.8 foot. The engines are 2 x Yanmar 57HP with shaft drives and folding props. The generator is a Wisper Power 5kw.

    As this cat is an enlarged version of the VMV 53 the foils again are intended to reduce wetted surface and improve pitch control. Notice the flatter bottom shape to help the windward hull to skim if the foils are providing enough lift. This approach of foiling is not intended to fully fly. The performance of this cat will be high due to its size, lower weight and reasonable rig. The builder claims peaks of 30 knots, my guesstimate is 12 to 15 knot averages in very good conditions.

    The accommodation options range from sleeping cabins and galley in the hulls for high performance models to full 3 or 4 double berths cabins with ensuites in the hulls and a main cabins that contains a large galley, dinette, seating, steering navigation and entrainment areas in the cruise versions. The equipment delivered as standard is a refrigerator, freezer, generator, watermaker, inverters, battery chargers etc. There is 2000 litres of water and 2000 litres of fuel.

    The Supercat 66 structure is carbon foam in epoxy. Other options are available if required but not recommended. The mast, boom and foils are carbon fibre.

    The jpegs give more of the idea. Interesting boat if you can afford the million plus Euro’s.
     

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  9. oldmulti
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    oldmulti Senior Member

    Iridian. Solid aft bridge deck sections are less important to the majority of sailing sailing cats in moderate conditions. But if you are sailing offshore in big seas, full aft bridge decks with large glass doors between the cockpit and main cabins are not just a risk but can be positively dangerous. A 37 foot cat had a wave land in its cockpit when running in a storm, leading to water in the bridge deck cabin and hulls. They deployed a bow drogue to allow them to sort the mess out.
     
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  10. oldmulti
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    oldmulti Senior Member

    The Astus 22.5 trimaran is a trailable sports tri that has limited accommodation. The tri is built to CE certification class (C) and can go along the coastline, on rivers and canals. The designers are VPLP design. This is an extension of the Astus trimaran family. The tri is 22.8 x 16 foot which can be folded to 8.2 foot. The weight is 1,460 lbs. The aluminium mast is 29.5 foot carries a 225 square foot mainsail, a 97 square foot jib and a 345 square foot gennaker. The draft ranges from1 foot to 4.6 foot over the main hull centreboard and rudder. There is optional float based C foils available for those who want higher performance. Another performance option is a carbon rig with textile shrouds.

    The accommodation is simple. The main cabin is lit by the side window of the roof and the deck hatch in front of the mast foot. The 4 berths are 6.5 foot long. The 2 quarter berths at the rear are 1.5 foot wide, while the forward double berth at the front is 4.6 foot wide at the head and 2.7 foot at the foot. A chemical toilet is located under the front bunk. The kitchen furniture is modular and removable.

    The performance is good. A test found “The boat accelerates quickly to reach 5 knots upwind, despite a light wind. With the wind stabilizing around 10 knots, without exceeding 12/13 knots, the Astus 22.5 maintains a speed upwind between 6.5 and 7 knots. As the wind remains light, we not reach double digit speeds and will not make long surfs. However, the meager 12-knot reefs allowed us to reach 9 knots with a crew of 3 adults.” Astus says the tri is capable of speeds up to 20 knots. A figure I could believe with C foils and an upgraded rig.

    The build is listed as “polyester resin infused fiberglass”. I suspect the resin infusion is correct. The resin system is likely to be vinylester. I can accept a single solid skin on the floats due to the curved surface. The main hull is probably has foam glass areas. The beams are aluminium and slide in fiberglass channels. The tri takes about an hour to assemble or disassemble for trailing.

    The jpegs tell the story. A good effective boat.
     

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  11. oldmulti
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    oldmulti Senior Member

    The global shipping industry is developing “sustainable ships”. The latest, unveiled in Sweden uses advanced technologies to harness the wind creating a sustainable category of shipping. The interest here is not the ship hull itself but the rig. The ship is a Swedish collaborative project between Wallenius Marine, KTH (Royal Institute of Technology), and SSPA. KTH is addressing the challenges in areas such as aerodynamics, sailing mechanics, and performance analysis. SSPA is contributing expertise for the development and validation of new testing methods, aerodynamic and hydrodynamic simulation methods, and risk simulation. According to the designers, this is the third iteration of the design as they have developed the hull shape and rig specifications.

    “Oceanbird” concept design is over 650 feet in length and would have a 32,000-ton displacement. The steel hull of a wind-driven vessel needs to be designed in an entirely different manner to a motor-driven one. The vessel is capable of carrying 7000 cars. The rig height over the waterline is nearly 350 feet. Primarily powered by the wind, the vessel will have an estimated speed of 10 knots permitting it to cross the Atlantic in approximately 12 days or 50 percent longer than today’s crossing that takes about eight days.

    The wing sails are like airplane wings using the same level of aerodynamic technology. They are also designed to telescope from a minimum height of 150 feet above the waterline to permit the ship to pass into harbors to 200 feet and a maximum of 262 feet permitting the ship to operate at optimal conditions or reef the sails in high wind conditions. At maximum height, the wing sails would be twice the height of today’s largest conventional sailing vessels. The rig is made of steel and composite materials and turns 360 degrees to catch the wind in an optimal way. To be able to get in and out of harbors, and to meet international safety requirements, Oceanbird would is equipped with an auxiliary engine.

    An interesting comment is: “The development project has come very far,” says Richard Jeppsson, VP Commercial & Partnership at Oceanbird. “We have started testing with 7-meter models in open water recently, and will continue this fall. The design will be ready for orders in 2021 and we are aiming for a possible launch in 2024.” The next big step for the model is to go sailing with four wings, when the model will be equipped with many sensors and an on-board computer to control the boat. KTH showed the 4 rig model in November 2020.

    Using a 1:30 scale model of Oceanbird, the tests in Stockholm are to measure flow conditions around the wings and optimized routing to meet ideal wind conditions, says Jakob Kuttenkeuler, professor at the Centre for Naval Architecture. The collaboration partners are confident in the success of scaling up the sailing vessel, although there are engineering matters to be ironed out along the way. “In scaling up, lots of things change,” he says. “Mass, stability and length are easy to increase while some aerodynamic and hydrodynamic effects are more difficult to scale correctly,” Kuttenkeuler says. Wave formation and wind speeds will make a difference, for example. “Also at a deeper, flow-mechanics level, a lot of changes apply to release around the wings and rudders,” he says.

    A single or dual wing version could be very interesting on a smaller boat. The jpegs give the idea.
     

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  12. oldmulti
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    oldmulti Senior Member

    The following proa is a home design and build by a man who was inspired by a photo of a Tevake's Tepuke trading canoe and Skip Smith “Umiak” book about skin on frame boats. Result is a 12 foot long proa as shown in the jpegs.

    The proa was built using store brought components. The main hull ribs, chines, gunnels are aluminium tent poles tied together with some nylon string. There are some red cedar dowels that fit into the aluminium tubes to form the full length gunnel and chine tubes. Over this frame is a nylon cloth painted with rubberized roof paint. After initial trials the main hull had a layer of closed cell (cheap foam) and another layer of nylon cloth painted with rubberized roof paint to form a sandwich shell. The cross beam is an 8 foot aluminium step ladder. The initial float was a foam filled PVC pipe but proved not to have enough buoyancy. The float was then a painted shaped foam block. After initial trials the aluminium ladder cross beam was replaced by 2 larger diameter aluminium tubes.

    The rig again is aluminium tent poles with a home made cloth sails. The rig failed after the first moderate wind sail and was subsequently upgraded.

    The guy then decided to upgrade the main hull. He found a Hobie hull which has some asymmetry. He again used aluminium tube cross beams and a simple rig for sailing. He found the Hobie hull could be used as a proa hull as it is as work well being sailed “backwards”. The float hull is a kayak. One builder warning: “If you build one the tilt the Hobie hull away from the outrigger 7.5 degrees as measured at the center of the deck. That makes it track right. The trampoline is a problem in high winds. Use a lattice of sticks instead. The sail is a couple of windsurfer sails sewed back-to-back.”

    You do not have to spend a lot of money or long building a fun day sailor. Reusing windsurfer rigs, broken Hobie components, a few simple tubes and house paint work OK. Fun can be cheap if you have an innovative mind. Jpegs give the idea. The last jpeg is the Hobie hull version.
     

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  13. oldmulti
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    oldmulti Senior Member

    The following is about 2 small boats and their experimental full wing sails. The first is a Hobie Bravo with a new mast step onto the deck, complete with blocks for handling lines to control the aft part of the wing and an extension to the mast brace tripod used on the conventional Hobie rig. The 2 part wing sail was too small for the heavy Hobie Bravo catamaran, but at least it answered the question about the center of effort of the wing and its controlability. It also fits easily in the bed of his truck, as shown in a jpeg.

    The ribs of these wings are made from styrofoam sandwiched between 6 mm layers of plywood at the leading and trailing edges. The leading edge is wrapped in aluminium and the "T" shaped control yoke on the aft wing section, along with other areas under more stress are built from 12 mm plywood. The mast is an old Sunfish mast. The clear plastic coating is strong. Small holes in the clear plastic can be fixed with the tape that is used to secure the plastic to the wing structure.

    Once the forward wing section in stood up in the Hobie the aft wing section is attached. The top brace extends from the top of the forward wing section and has a small, rounded off pin atop the forward wing section. That pin cannot be seen from the ground, so holding the aft wing section up in the air and guiding it into place was a bit tricky, not something to try in a strong wind. After the top brace is on the upper pin, the lower "T" shaped control for the aft wing section is bolted to the bottom of the forward wing section. It is possible to connect the two sections on the ground and then step the whole rig. The T control bar is then riged with sheets as is the forward wing section. This allows the 2 part wing to be controlled both in angle to the wind and chamber.

    The designer says: “Wing sails are a bit tricky to trim compared to soft sails. It seems the wing was more sensitive to improper angle of attack than a soft sail would be, and unlike a soft sail, it does not change shape or start to flutter at the edges to give the sailor clues about what is wrong. One way in which the two types are similar, though, is in rewarding proper trim. The wing sail Bravo could scoot right along at a good clip with the wing adjusted properly.” And “When I got the wing shaped and trimmed properly, it powered up the boat and I started moving faster, but that changed the apparent wind angle and my usual response of sheeting in a bit did not seem to be the right one. Changing the camber also changes the size of the slot between the sections.”

    The next guy, who is a pilot, did a full single part wing sail for his dinghy. The only common thing between this and the above wing sails is they used a Sunfish mast; a 2.25" diameter by 10' aluminium tube. This wing sail is 10 x 4 foot for a 40 square foot sail area which weighed 9 lbs in total. The materials are cheap store bought and cost about $50. The aerofoil section is a NACA 0012.

    The 25 mm styrofoam ribs were cut to size first. The top and bottom ribs were skinned with 1.5 mm plywood glued using TB-III. This allowed them to handle the loads of the mast bearings and sheets attached to the bottom. The 24 ribs had slots made in each for a pair of 25 x 6 mm pine stringers 10 foot long. The ribs were then strung 150 mm apart and bonded with Gorilla glue. The location of the stringers isn't critical. After experience, the ribs could have been spaced 300 mm or more apart, allowing for a slightly lighter structure, and a faster build. A 1.5 mm plywood "X" shaped stub mast (main spar) supports the section above the aluminium mast in one of the alternate sets of mast holes.

    The leading (LE) and trailing (TE) edges are simply Duct Tape. Try to keep the surfaces smooth. Three overlapping strips on the LE, and one on each side of the TE is enough. The covering material came from 2 large packages of 3M heat shrink window insulation film. It was attached to the four outer edges using 2" wide clear packing tape. I bought extra double sided tape to stick the film to every rib as well.

    The owner comments are: “Unfortunately for aerodynamic theory, the wing was at a negative angle of incidence when propelling the boat! The NACA 0012 generates lift between just 12 and 15 degrees to the apparent wind, not much margin. So as a rigid sailboat sail, it's the wrong airfoil without adding lift devices (flaps, slats, slots, jibs, etc.). Once we put on the usual Sunfish Mach II lateen sail, we rocketed up and down the lake at a blistering 2.5 to 5 knots (remember this is a monohull).

    The first set of jpegs are the Hobie Bravo 2 part wing sail, the second set of jpegs is the single part wing sail build.
     

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  14. Doug Halsey
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    Doug Halsey Senior Member

    It seems to me that the owner should consider that he might be doing something wrong before concluding that he's using the wrong airfoil section or that aerodynamic theory doesn't apply.
     

  15. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The Elfe 8 Aluminium catamaran is a French design from the late 1980’s. Several versions were built by amateurs. The cat is 26.2 x 13.2 foot with a “displacement” of 5600 lbs. The fixed aluminium 32 foot mast carries a 15/16 rig with approximately 345 square foot in the main and fore triangle. The hulls are flat bottom dory shape with low aspect ratio keels and kick up rudders that draw 2.3 foot. The underwing clearance is 1 foot at best. My thought is the displacement is not correct, I suspect this cat is heavier than 5600 lbs.

    The accommodation layout is a main cabin of dinette that converts a double berth, a galley in one hull, storage in the other hull. An aft single berth in each hull with a toilet forward in 1 hull. There is a berth forward in the other hull. The headroom in the main cabin is 4.5 foot, there is full headroom in the hulls. The cockpit has the engine cover to the 15 HP outboard.

    The construction is aluminium throughout. Most panels are flat with frame and stringers support. The boat mainly shown in the jpegs has insulation and lining materials throughout. The insulation and sealed compartments make the cat unsinkable. The advantage of aluminum construction is you can build almost all extra equipment yourself. EG the rig, rudders etc which lowers cost.

    I cannot find who the designer is or much further details about the design. Any help would be appreciated. The jpegs tell most of the story. An interesting minimum practical cruiser if it could be built lighter.
     

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