Multihull Structure Thoughts

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

  1. Phlames
    Joined: May 2017
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    Location: Melbourne, Australia

    Phlames Junior Member

    The Spyder design had mast section main and rear beams with a 'dolphin striker' on a strut which passed through the main beam and took mast loadings directly. These beams slid into sockets in the hulls which were made of 8mm Western Red Cedar/epoxy/280g(?) uni. There were two other beams - a 100mm forebeam with 'seagull striker' and an intermediate beam between main and rear beams to support a central pod for outboard/battery, etc.
    I extended this pod forward of the main beam to support the forebeam because I used spinnaker/drifter sails from a prod. The empty boat weighed about 600 Kg. (930 Kg rated displacement), had about 450 sq. ft. upwind sail area and up to 1400 sq. ft. downwind. Top speed recorded was 28 knots (a bit scary).The hull shapes were fullish forward for the times but allowed the boat to be pushed hard, especially offwind although I have to admit a couple of nosedives 'in extremis'.
    Being a tube boat, there was some 'walking' of the hulls (floppy) but, apart from two masts breaking (my fault) nothing significant broke. Being a new design to the limits of the then MOCRA rule the original design was 7.6m LOA with a beam of 4.8m and a mast length of 11m. My Spyder was 8.3m LOA, beam 5.5m and a 12m mast. Launched in 1989, trailed thousands of miles, raced mainly in Victoria, Australia and winner of many 'fastest boat' trophies the boat was substantially altered in 1997 to become a 'mini-cruiser' of 8.9m LOA and a small bridge deck cabin with composite beams (see photo). I can supply information about the conversion if wanted but suffice to say that the boat became a lot stiffer, went just as fast upwind but weighed 1500 Kg. It was sold to a Queensland buyer in 2006 and afaik is still sailing. V54beat.jpg spinnakerup copy.jpg
     
    Last edited: Jun 18, 2019
  2. Phlames
    Joined: May 2017
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    Location: Melbourne, Australia

    Phlames Junior Member

    On further reflection about lightweight construction re the Spyder design I can say that the hulls were terrific in design but the beam design was possibly not so good. The mast sections used for the main beam were probably good enough but I recollect seeing about 25mm cyclic deflection where the mast support/dolphin striker passed through the beam. Fatigue was not evident during its life but I sometimes wonder if/when such fatigue might show itself via catastrophic failure if the lifetime was longer before I modified the boat. Another aspect of the design was the 'cantilever' daggerboard design which called for a horizontal web installed in the hulls about 400mm above the bottom of the hull. This meant that a shorter daggerboard could be made that only protruded through the top of the hull when the boards were fully up. When deployed down, the loads were taken by the hull bottom and the horizontal web. A pole was attached to the top of the boards to lift them up/push them down. I broke two daggerboards that were copies of the GBE boards using this system but eventually made full length WRC daggerboards and had no further problems. Finally, the Spyder design called for vertical transom hung dagger style rudders. These were very heavy on the helm and got heavier as speed increased, and the boat was very speedy at times. At 15+ knots, two hands were required to steer the boat and was tiring. Sometimes, due to the terrific acceleration the boat was capable of the helm could be wrenched from your hands as speed increased suddenly.
    I sometimes ponder what a modern re-incarnation of the Spyder design would be like to sail. I imagine a lighter, stiffer boat would be a real weapon in brave hands!
     
  3. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Phlames Thank you for the reply. The rudder problem is interesting. Were the dagger board rudders vertical or was the bottom of the rudder angled forward to try and supply some balance effect? If the rudders were vertical or angled aft I can understand the increased tiller pressure as you went faster (28 knots is fast!). Also I have seen many designs try and use shorter boards which requires a lot of additional strengthening in the hull if there is not a "natural" deck or cockpit floor to act as the top support point. It good to here that WRC boards worked for you. Were the original boards the fiberglass skin foam filled versions of the GBE boards built in a metal mold? The NZ 8.5 meter boats EG Attitude could substitute for Spyder. Only for the brave.

    The subject of the day is small home made wing masts or wing sails. This is tricky subject as a wing mast is meant to smooth the wind flow onto a mainsail and provide more drive if well designed. Problem is wing masts have to be angled correctly to the wind to gain the advantage. If your cruising or not watching your wing mast it may be incorrectly angled and slowing your boat down. The reason why some racers still prefer a conventional masts. Home built wing masts are possible on any boat but are probable best left to professionals on boats above 30 feet. Design concepts and materials vary a lot but there is one requirement. No cheap materials allowed. If the design uses timber and plywood its clear grain timber and marine grade or aircraft grade ply. The gumpeat mast is for a boat between 18 to 22 foot and is some solid timber and ply web and back skin. The easy mast is solid good quality timber but would be heavier. The wingmast 1 shows a frame structure that depends on a good ply skin to handle most of the forces and the wood epoxy wing mast is mostly timber but has a carbon fibre skin. I will go into the actual structural materials of a couple wing masts in a later posts. But be warned this is not a best guess game, please understand the forces you are playing with and test the results before going away from land. Even a 60 lbs mast breaking can really hurt people. The alternative approach to having a good airflow onto a sail is to use a technique Wharram popularized. Wingsail 3 gives you the concept and the Tiki 21 photo shows you the result. This is a good way of doing a cheap efficient rig but has one downside. Freedom 40 monohulls had basically the same thing and changed it after a few years, why? Because when sailing under load and the sails were wet it was very hard to pull the sails down due to friction of the sails on the mast. Even when pressure was taken of the main it still required a lot of force.
     

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  4. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Wing mast construction requires good materials. I will give 2 mast specifications. The first is for a 23 x 13.5 foot catamaran of maximum displacement of 2000 lbs. The rig is a fractional rig with the mast being 30 foot long with a diamond spreader on it supported by 3 wires. The central spar is a 75 x 25 clear spruce spar and a 25 x 20 mm nose spar and 25 x 20 mm triangle shaped aft spar. There are a few 3 mm webs up the spar. The skin is 1.5 mm birch aircraft ply wood. The plywood panels are joined by a 75 mm wide external 1.5 mm plywood strip around the mast. The entire mast is covered with 84 gsm glass. The sail bolt rope track is an aluminum tube that has a saw cut down its back. Reinforcements blocks are done at tang take off points, in the heel of the mast for the supporting "ball" and at the mast head. The Gumprecht wing diagram in the previous post is a different to the mast just described but is 30 foot mast 210 x 75 mm mast. It has a different more complex structure but again is for a 22 foot cat. Now finally this and a following post will give you a very good idea of how to build a wing mast for 25 x 18 trimaran of about 2500 lbs displacement. Again I warn you good materials only.
     

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  5. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Final post on small wing masts.
     

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  6. Phlames
    Joined: May 2017
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    Location: Melbourne, Australia

    Phlames Junior Member

    Oldmulti, the rudders on my Spyder design were indeed vertical. I made a metal mold ala Tennant and produced two daggerboards as you described - both broke eventually.
    I bought production daggerboard moldings from the then GBE maker in Brisbane and broke one of them before I made the full length cedar boards.
    Your wing mast discourse is interesting and reminds me of a mast I once built from 4mm strip cedar/ply/200g glass epoxy. There was a ply box section in the centre of the mast
    to accomodate the halyards and to provide a strong point and structure for attaching the three stays. It had a curved trailing edge with a max. chord
    at the halfway point along the mast (catenary? curve). Chord there was about 600 tapering to about 400 at each end. Nice shape but heavy at 85 Kg bare for 11m length.
    A stay came adrift at an overnight mooring and the mast broke when it fell - it was only used three times! I replaced the mast with an aluminium one (not a wing) designed for a
    IOR 1 ton monohull but I broke that after about 6 races (compression failure -mast had one set of spreaders and designed by a professional sparmaker who, like many of his peers in
    those days did not fully comprehend catamaran loads).
     
  7. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Banana Split was a Cat flotteur 42 owned by Antoine a french singer. He sailed it around the world owning it for decades. The bridgedeck catamaran is 42 x 21 foot weighting 21,000 lbs and displacing 26,000 lbs with 1100 square foot of sail. It is built of aluminium. Thick aluminum. The dory hull bottom is 12 mm thick, the sides are 10 mm. There is minimal framing in the hulls with the monocoque hull shell carrying most of the loads. The underwing is also thick with some stringers. The decks and cabin structure are more conventional with thinner plates and more framing. There have been several catamarans built the same way. One designer said that as a general rule, 1 mm of thickness for each meter of length for hull bottoms. I would be careful with this statement. An interesting approach if you want a very strong cruising catamaran. The company that built these cats Prometa also built a series of "Strongall" monohulls of about 45 feet. The monohull bottoms were 15 mm, chine and sides 12 mm, decks and cabins 8 or 10 mm aluminum with no stringers or frames but 5 BH's at strategic points eg under the mast.
     

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  8. Angélique
    Joined: Feb 2009
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    Location: Belgium ⇄ The Netherlands

    Angélique aka Angel (only by name)

    It seems only Antoine's ( Sailor) hair colour has changed a bit, thanks for all the posted info Oldmulti !

    [​IMG]
    — Antoine in Quebec, Oct 7, 2011 —

    - some biography pics - 1989 - 2001 - 2005 - 2010 -
    [​IMG] [​IMG] [​IMG] [​IMG]
     
    Last edited: Jun 20, 2019
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  9. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Many people like to build the perfect boat but don't realize the time it takes unless you are very good or have molds etc to help you. The result is designers try design practical simple hull shapes that use available materials such as plywood or flat fiberglass panels that are relatively simply to put together. The result is chine hull shapes. Chine hull shapes have are not the "best" hull shapes but how much do they effect the speed of a boat. According to my readings if you design a chine line correctly it will have minimal effect on the water flow minimizing any turbulence. To minimize turbulence you need to minimize any changes of angle to less than 15 degrees over a chine. Now the fun, water flows along a hull, so you have to look at angle changes from the top down and see if you can minimize flow changes. BUT Prout did tank tests and found either they did fine sterns for "seakeeping" or round forward and mid sections with chine sterns for faster speeds but a rougher ride. Prout choose canoe sterns to maximize seakeeping. Prout then maximized accommodation which had bad effects on seakeeping and speed. So what is the real effects of chines? To quote Richard Woods. Chines have far less effect on a boats speed than EG having good daggerboards instead of a low aspect keel. Thomas Firth Jones did 2 variations of his Dandy 26 foot cruising cat design, 1 with a chine hull the other with a round bilge hull said that when cruising for a day they would probably finish within minutes of each other. Many fast designs have chine hulls. If the boat is well designed, its more about the hull shape, chine layout, boat weight, rig design and keel type that produces a fast boat. Its not just about chines. Dory hull shapes like Banana Slip cat above are the most extreme of chine shapes but appear to work reasonably well. Richard Woods smaller designs of have chines and are very good boats that are relatively easy to build. The Woods Sango hull shape variations give an idea of the difference in design of round bilge versus hard chine shapes. Both boats in real world cruising would get to the same point in similar times. In racing there would be a small difference in light airs. The price you pay for that slight improvement would be a longer build time and probably a lower resale value. Take your choice.
     

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  10. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Has anyone built very simple shapes using glass covered thin ply with a little shape tortured the into the hull sides , similar to the woods design above, but with a flat bottom,with some expensive corecell foam laid then shaped then glassed to dress up the look and create the round?, presumably making boat more valuable and easier to sell, eventually paying for the water proof foam. this adds sealed flotation and some protection to structural integrity for a nasty grounding. It also makes glassing easier , removing the hard chine of the bottom or extra chines . Also PVC weldable tarp is so functional it's surprising that more isn't in use for long lasting strong biminis on cats and tri's particularly for owner builder designs. Enjoying the posts, regards
     
  11. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    TTLF Early Kelsall wide bodied tube cats used to have ply sides and bottoms with strip plank around the turn of the bilge. Dudley Dix larger cat designs uses the same approach and some used double diagonal ply bilge turns. There have been some other people who have done as you suggested and glued foam onto a flat bilge then shaped it. I suspect the lack of resale is more to do with a basic ply structure as well as a chine not just the chines. From my experience resale more about reasonable design to suit the need first, some form of fiberglass (foam or strip plank) construction second, condition third then price fourth. If the first 3 conditions are compromised then the price has to be cheap. Thanks for your support.
     
  12. trip the light fandango
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    It would make a practical bilge,... if timber or ply framing is avoided or shaped to look like fibreglass it may increase the value perhaps.. quality ply is such a good material, as the longevity and speed of some of the craft you've looked at proves.
     
  13. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    One small boat designer that takes advantage of materials is Ray Kendrick of Team Scarab Several of his designs can be built in foam or plywood. Result is a chine design that can be built from flat panels of foam fiberglass. If foam fiberglass panels are made on a flat table they can be full length and have a smooth external surface which will only need a minimal amount of fairing (around connecting taping joints) before painting. The real problem of male mold (build a shaping frame, then lay foam, fair foam, lay glass, fill and fair glass) foam fiberglass boat building is you continuously have to fair each layer. Then you take the finished external hull and have to lay glass inside and fair that. A LOT of work. If the design is well thought out and made of flat panels you can minimize the filling and fairing work enormously. Flat panels made on a table and glassed both sides of the foam over EG an 18 foot length still have some flexibility and can be curved around some bulkheads to form a chine hull shape. Te advantage is you end up with a foam glass boat with a minimal amount of work. This technique can be hand laid without vacuum bagging or you can vacuum bag the panels to get a slightly stronger and lighter layup. I will talk about different layup techniques in a later post. Attached is an example of an 18 foot tri build.
     

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  14. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Many people would love a carbon fibre mast but how much advantage is it. If you are a cruiser not much, yes it will be lighter and having a lower weight will minimize pitching but it will cost a bit if the mast is accidentally damaged probably have to be replaced. Carbon tubes often fracture if pushed out of column by EG a broken spreader. For an aluminum mast if its pushed out of column it is often be elastic enough to bounce back or still work with a slight dent. A carbon tube can have a hidden fracture that will break (often explode) when its fully loaded up in a strong wind. If you are a racer then a carbon mast can give you a real advantage. Do not buy second hand carbon masts unless you know there history, a quick visual inspection is not enough. Carbon masts can be home built but unless you understand vacuum bagging or can fillement wind carbon tows onto a manderal you may not get the strength to weight ratio's you want in the mast. As I have said before early carbon masts failed because people did not understand the design forces and construction of carbon fibre materials. Below is a diagram of a Farrier F 22 carbon mast that is 31 foot high. According to the builder it could go to 40 foot high for a boat of similar righting moment. If you want more understanding of carbon fibre masts and masts in general look at rig design hints https://www.aes.net.nz/info.html And finally the last diagram is from a company that sells carbon fibre wing masts by the meter at $860/ meter. Therefore 10 meters = $8600 plus shipping. Aluminum is starting to look good.
     

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  15. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    One thing that always causes problems with home built boats is details of constructions and short cuts done by a builder. The reason Ian Farrier tris if built to plan have good resale value is he was very detailed in his plans and if you followed them you will get a good result. One area that was missed by many designers and badly done by many builders was putting bulkheads into hulls. Plywood bulkheads have timber edging for 2 reasons, one to reinforce the plywood edge, second to spread the edge load over a wider hull surface area. If the plywood edge had no timber reinforcing, it will act like a knife edge over time. For foam glass construction the situation is worse. The inner skin of a lot of modern foam glass boats is one layer of glass of less than 1 mm thick. A plywood or foam glass bulkhead laying directly against that inner skin would cut though the inner glass over time. This is the reason than it is recommended that a fillet of bog is placed under the edge of the bulkhead to spread the edge load. This fillet is covered with a layer of glass a minimum of 150 mm wide at least 25% heavier than the inner skin that connects the hull side to the bulkhead. But Lock Crowther still found problems with some builders who did this approach. Reason why? Builders build the hull leave them for a while, build the bulkheads etc, then go back to the hull a month later and sand the hulls at the point where they are going to put the bulkheads in so the glass surface is rough enough to take the glass tapes to connect the bulkhead to the hull. Some builders were grinding though 30% of the single inner skin weaking the hull structure. Lock then started specifying an additional 300 mm wide layer of glass (same glass as inner skin) at the points were main bulkheads would touch the inner skin. These bands of glass were laid up at the same time as the inner skin of the hull was completed. A builder would then only sand these bands prior to glassing the bulkhead in and the loads would be distributed more effectively. Details are important.
     
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