Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    If you have a few half finished projects lying around you may be able to build a proa in days. This guy had a 13.8 foot proa hull and a 22.3 foot proa hulls laying around from unfinished projects. He said I have some other stuff laying around so I will create a boat, let’s do a proa. The rough dimensions of the proa is 22.3 x 14 foot beam. The mast is about 29 foot high with a sail of about 150 square foot. All these figures are guesstimates scaled of the jpegs.

    Having some seasoned bamboo and discarded bicycle inner tubes has its advantages. The bamboo formed the cross beams which are partially tied together with old inner tubes as they provide semi flexible but strong joints. The other advantage is as there were no formal plans for this boat, if something didn’t work you could untie, reposition an item and retie it to its final structure. As you can see in the jpeg there are cross beams and diagonals to make a relatively rigid cross beam structure.

    The mast base sits on the centre of the diagonals. The mast is a bamboo as well but a windsurfer mast and sail could work just as well. Some stainless steel wire was used to make the mast stays and the luff/leech wires. The sail started out as a grey polytarp with some shaping on the edges. The shape was not that good, so the sail had a yard and a boom attached and turned into a crabclaw shape. The rig/sail worked better and the proa was more controllable.

    The hulls are plywood with some timber and ply bulkheads. The proa was painted with acrylic house paint which had a fast drying time, a water clean up but had poor adhesion on most surfaces and low water resistance.

    You will notice a lack of freeboard and the owner had to be very careful about centering weight over the hulls so to not submerge a hull or pair of bows. Due to the massive influence of shifting his weight, he didn't need to use a paddle to steer. The owner’s assessment was the proa was just too small and difficult to sail to be very good for anything much, but was a great learning experience and a lot of fun.

    The jpegs give the idea.
     

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  2. C. Dog
    Joined: May 2022
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    Location: Coffs Harbour NSW Australia

    C. Dog Senior Member

    @oldmulti is the main hull really 22'? It looks more like 16' from the photos, which can be deceptive of course.
     
  3. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    C.Dog I can only work of the written words from the owner. I agree the jpegs look shorter but there were only 2 hull length dimensions mentioned in the article.
     
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  4. C. Dog
    Joined: May 2022
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    Location: Coffs Harbour NSW Australia

    C. Dog Senior Member

    Understand mate, I'd say he's an amateur fisherperson used to stretching the arms.
     
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  5. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    You have kids or a group of teenagers who have sailed on dads lean over mono and say “I want to lay around, not get wet and be able to walk to shore”. Dad decided he wanted to design a boat and spent 110 hours designing a simple cat that the kids could sail on, escape for a weekend in a bay or river and not scare the dad. Result the Minicata. There have been at least 2 built, one by the designer and a second by a sailing club to teach there teenagers about boat building and provide a small sail training vessel.

    The Minicata is 14 x 11 foot with a weight of 320 lbs. The biplane rig carries two 7 foot high aluminum masts with a “spirit” type sails. The draft is 1 foot including the stern mounted rudder. The stern mounted rudders are claimed to be “3 meters apart”.

    The interesting part of this cat is accommodation, each hull has a single berth and some storage. A camp cruiser with some internal accommodation.

    The construction is plywood. The 5 mm ply sides are stich and glued along the keel and then the ply is semi tortured to the stem and stern posts. In this build there was 13 internal floors/bulkheads that were epoxy filleted in. The deck and cabins are ply also. The cross beam tubes look about 50 mm in diameter. The mast tubes look small in diameter but with such small sails they appear to work.

    I will quote the owner about performance: “It sails very fast on a run and very close to the wind. I have sailed her in all kinds of wind, and with numerous passengers aboard. The worst was with the wind on the nose and blowing 35 knots. The cat, NEVER, flew a hull with the biplane rig, and when I wanted to slow her down, I simply took one sail down and she went like a normal monohull.” I suspect 8 knots on a good day, 4 knots cruising around.

    A fun design for a day sail or for a few adventurous teenagers an overnight cruiser. The jpegs give the idea.
     

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  6. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    This Pacific proa is a design study that is meant to be transportable and can fold to 8 foot wide for trailering. The designer wanted a fun day sailor, camp cruiser for 2 people. He also wanted to reuse a Hobie 16 rig he had.

    The proa is 24 x 11.5 foot fully extended and weighs 500 lbs with a displacement is 1850 lbs. Float length 18 foot. The 24 foot mast carries a Hobie 16 (148 square foot main and 55 square foot jib) rig centrally mounted. Draft 2.5 foot when rudder board(s) down. Length to beam main hull is 15.5 to 1. Float length to beam 19 to 1.

    The main hull shape is an inverted bell shape to provide a reasonable cockpit. The righting moment is 5,000 foot lbs with a centre of effort at 12.75 foot. This means it can carry full sail to about 20 knots wind speed if 2 people (about 500 lbs) are sitting out on a wing seat. The bell shape hull provides a “buoyancy pod” if you are caught with the rig on the wrong side.

    The real issue in this design is how the rig will be held up and how the sails will change handle the change of direction. EG do you have 2 jibs and lower one raise the other or just unhank the jib and carry it to the other end. The mainsail should be able to swing “aft to forward” but how do you do the rigging wire to support the mast if the rig is caught with the wind on the wrong side. Runners maybe.

    The build is strip plank cedar with glass cloth and epoxy. Plywood cockpit floor, seats and bulkheads. The cross arms appear to be timber with pivot pins for folding.

    No indication if the design was built. If it is constructed as suggested this proa should have good performance with speeds above 15 knots. The jpegs give the idea.
     

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  7. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    This is a short one but a very interesting read. A man, Ginnie Jo Blue, from the Northwest Multihull Association recently did some work on his F32 Farrier and documented the fiberglassing journey. It turned into a good instruction manual on how to prepare surfaces, make templates, the materials required, how to glass, how to vacuum bag, how to finish the surface and how do it safely. It is one of the better 26 page documents (including jpegs) I have seen. It is a short read but a good one.
     

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  8. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    Nice job. Couple of things:
    • There's no need to wet the peel ply, the excess resin will do it.
    • If applying tacky tape to the bag rather than the table, lay a piece of packing tape where the tacky tape will be applied before you start laminating. Remove it immediately before sticking the bag down. Tacky tape on wet epoxy is a disaster.
    • It is easier to wet out the tabbing strips off the job, then apply them with a light brush to set them in place. The resin ratio is correct, there are no air bubbles, damage to the fillets or resin where it shouldn't be.
    • Rule of thumb for fibreglass tabs is the overlap needs to be 100 times the thickness. So, for 400 gsm glass (which is near enough 0.4mm thick) 40mm.
    • Unless there will be an upward load, there is no need to glass under the panels, a decent high density fillet will do the job.
    • Infusion is much easier, quicker, cheaper and less stressful than bagging wet laminate.
     
  9. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The Odisea ODC48 cruising catamaran was been designed by Markos Thiraios and is built in Vietnam. This is a real offshore cruiser that is intended to be taken to remote places. It is a modern conservative design with good performance but it can carry over 10,000 lbs of people, supplies and toys etc.

    The ODC48 47.9 x 24.93 foot with a weight of 18,400 lbs and a maximum displacement 29,762 lbs. The 65 foot aluminum mast carries a 827 square foot mainsail, a self tacking jib of 312 square foot, a Code zero of 1,540 square foot and a 2,220 square foot spinnaker. The draft ranges from 1.95 foot with the rudders kicked up to 5.25 foot when the daggerboards are down. The engines can be a diesel, hybrid or fully electric depending on your choice.

    The accommodation is 2 double berth cabins in each hull with a large toilet shower forward in each hull. The main saloon has a large galley and seating with table. There is also a navigation area and entertainment bench available. The cockpit is comfortable with raised helming areas. The davits can carry a 550 lbs dingy with outboard. The cockpit roof area can have solar cells to help power the cat. There is also an optional owners version of the layout with one hull devoted to the owner.

    The structure is mainly aluminum with honeycomb furniture and internal cabin door panels. The hulls are 4 mm aluminum and a 3 mm aluminum decks. A few jpegs will give the idea of the amount of stringers, frames and bulkheads required to construct a cat of this size.

    The ODC48 can be purchased in kit format for home building if you have the welding skills and patience to build the boat. All the aluminum panels and internal furniture are cnc cut ready for construction.

    The performance is aimed at high daily averages not outright speed. The polar diagram will show a speed range between 8 to 14 knots in reasonable reaching conditions.

    This cat is a modern solid long distance cruiser that will take its owners globally. The jpegs give the idea.
     

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  10. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    A short one to finalise the ODC48 story above, I am adding this series of jpegs which give you an excellent feel for the type of components and structure that is required to build a 48 foot aluminium sailing cat. Aluminium is slightly faster to build with than plywood for the same size of vessel, as aluminium may have more parts but plywood requires multiple glue joints and finishing that take time to set. Aluminium is a weld (if you can weld relatively cleanly).

    The jpegs tell the story better than I can.
     

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  11. rob denney
    Joined: Feb 2005
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    rob denney Senior Member

    Most build methods are quicker than plywood for anything but small boats.

    We've been doing some numbers for aluminium Harryproas. 4mm alloy weighs 11.1 kgs per sq m. Plus maybe 10% (?) for all the extra frames and stringers = 12.2 kgs per sq m. 3mm is 8.4 kgs per sq m, but is pretty thin for in water/impact/potential electrolysis use.
    600 gsm glass (2 x 0.6) either side of 20mm H80 foam (1.6 kgs) plus resin (2 x 0.3 for the glass + 2 x 0.2 for the foam) = 4 kgs per sq m, no stringers required. Cost ~$US50 per sq m, plus tax and shipping.
    2 outside, one inside x 600 gsm glass (3 x 0.6) 25mm H80 foam (2 kgs) plus resin (3 x 0.3 for the glass + 2 x 0.2 for the foam) = 5.1 kgs per sq m. Cost ~$US65 per sq m, plus tax and shipping.
    1200 gsm glass (2 x 0.1.2) either side of 30mm H80 foam (2.4 kgs) plus resin (2 x 0.4 for the glass + 2 x 0.2 for the foam) = 6 kgs per sq m, no stringers required. Cost ~$US80 per sq m, plus tax and shipping.

    These are ~average hull laminates for 50, 60 and 80' harryproas. More in the bottoms and between the rudders, less on the ww hull and ends.

    On the plus side, alloy does not need painting and has excellent resilience for bouncing on rocks. It is a bear to insulate and needs to be carefully isolated from stray electricity. 4mm sheet price in Aus is ~$US60 per sqm, probably less 10% for a boats worth, so $US54 per sq m.
     
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  12. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    This is about an aluminium power catamaran builder who produces a range of aluminum plate power cats from 22 to 29 foot. He can go bigger but for this item we will stay with the 22 to 29 foot versions.

    The designer/primary builder (Barry LeRay) has worked with work boats for decades and branched out to create the "Cape Fear Catamarans" brand in Wilmington, NC USA. He knows how to build a cat and from the owner comments understands the design requirements as the cats have high performance capability with a good ride and turning ability.

    The example model chosen is CFC 264 which is 26.4 x 9.5 foot with a weight of about 4,800 lbs. It has planning V shaped hulls. It can be powered by EG two Mercury 150 HP outboards which will provide over 40 knots of speed, bigger outboards the faster you go,

    The construction is mainly plate 5052 aluminium of 4.5 mm for the hull external shell and 6.4 mm aluminum for the transom. All the framing is 6062 aluminum (most tube sections are extruded in 6062 or 6082). 5052 is used as it is cheaper, can be brought in larger plate sizes and the slightly reduced stiffness can be compensated for by a thicker plate thickness which is wanted to reduce the amount of framing.

    The jpegs of various sizes of Cape Fear Cats will give the idea of the build and framing required for a cat. The same basic structure works for cats between 22 foot and 29 foot. The aluminium consoles are different in size but they add to the structure.

    The 29 foot version with a weight of 5,700 lbs can with twin Yamaha 200's has a top speed was 49 mph. With twin 300's it will get close to 60 mph. The 22 footer that weighs about 4,000 lbs using a single Mercury 150 will hit 50 mph and cruise at 35 mph.

    Interesting cats. The jpegs give the idea.
     

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  13. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Scott Juston of Juston Marine design began developing higher speed powered catamaran options based on the asymmetric planning hulls that dominant Class A offshore racing cats. Designing with both stepped and foil assist options these designs are now in production around the world and are successfully operating in both the commercial and private world. They are renowned for their fuel efficiency, sea keeping and class leading passenger comfort. The basic 34 foot cat design is now being used as tourist, fishing, private and coastguard patrol boats.

    Bering Marine Antalya Shipyard (Turkey) built an initial cat to test and explore the capabilities of the concept. Bering Marine now specializes in building fast, seaworthy and fuel-efficient HYDROFOIL supported boats between 23' and 42' in length.

    As an example, I will focus on the BM 34. The BM 34 is 34.1 x 10.9 foot with a displacement of 14.800 LBS. Power options are either diesel or outboards engines with a maximum of 2 × 400 HP. The hulls are asymmetric planning hulls and in cats that have enough engine power to travel at 30 knots an optional hydrofoil can be placed between the hulls that provides lift and reduce fuel consumption by up to 30%. Bering Marine uses HYSUCAT HYDROFOIL technology. The foil assist technology provides optimal self-correcting dynamics. The foils carry between 40% to 75% of the vessel's displacement. This reduces the hulls' volume which greatly decreases drag and improves seakeeping. Efficiency, sea-keeping and low wake wash. Reduced drag on the underwater sections of our vessel improves efficiency. This results in lower propulsion requirements, reduced fuel consumption, and a lighter vessel. The HYDROFOIL wings create an inherent dampening effect, giving the vessel a much smoother ride enabling the boat to operate at a higher average speed in rough seas.

    The accommodation layouts vary according to use from an open deck arrangement to a full deck cabin containing helming, galley, seating, toilet and forward a double bunk. The jpegs gives an example.

    Performance depends on power options but using say two 300 HP outboards you can peak at 42 knots, cruise at 30 knots and have a range of 275 miles. The are two 550 litre fuel tanks.

    The build is aluminium for the hull, deck and cabin structure. The interior is fully insulated marine plywood walls and ceilings with headliners are artificial leather and fabric finishes. Walls are painted and laminated finishes. Decks are slip-resistant vinyl and have acoustic liners.

    Bering Marine have a service company in North Carolina for their USA clients.

    The jpegs give the idea. The foil shape gives an idea of the hydrofoil between the hulls.
     

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  14. cando2
    Joined: Nov 2021
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    Location: washington state, USA

    cando2 Junior Member

    Did some googling and discovered his name is Jutson (not Juston) and there is a nice utube of him at the 2024 Seattle boat show, sitting behind the foil between the hulls.
     
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  15. jamez
    Joined: Feb 2007
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    Location: Auckland, New Zealand

    jamez Senior Member

    Possibly a stupid question, but I have wondered what advantage (if any) there is by having the front of the cabin inclined forward at the top like you see on a lot of commercial style powercraft.
     
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