Trimaran Center Hull Designs & Materials

Discussion in 'Multihulls' started by Samhill, Apr 10, 2013.

  1. Samhill
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    Samhill Junior Member

    What are the thoughts on using skin on frame construction for a Trimaran Center Hull?
    It is generally less expensive, quicker to build, and considerably lighter.
    But would it be too light?

    Say for example I wanted to build a scaled down Maxi Tri using Supercat 17 hulls and a skin on frame center hull.
     
  2. Doug Lord
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    Doug Lord Flight Ready

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    I wouldn't try to scale down from a "maxi"-at least as far as the hydrostatics go. Do the design(or have it done) for the length, load and level of excitement you're looking for. Scaling from very large high performance boats doesn't gain you anything and you may lose the advantages of a proper design. You can probably capture the "essence" of the appearance of the bigger boat without negative effects. Good Luck!
    PS- I have no experience with skin on frame construction so I can't help you there.
     
  3. El_Guero

    El_Guero Previous Member

    Skin on frame compared to other building methods would easily be compared using WW2 examples.

    The Zero (skin over frame) was fast, but came apart easily.

    The Mosquito (plywood) was significantly stronger, but still suffered in engaging stronger aircraft.

    The p47 (Jug, steel) and others like it, took a beating.

    Plywood can be built up to about the same strength as using a metal hull. Skin on frame would be difficult to obtain the same strengths with.

    And unless you are set up for skin over frame, I am not sure that would be faster. Several aircraft kit manufacturers seemed to move away from skin on frame, because they did not have a great success rate at being finished. IMHO.

    I would say go with marine ply, and go stitch and glue.
     
  4. Skyak
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    Skyak Senior Member

    If you are willing to give up the interior of the main hull you can build a space-frame in aluminum that will be light and sufficiently stiff. It won't be easy because you will need lots of diagonals. The bottom will not be as smooth as a solid wood or FRP skin due to the stringers showing through.

    Personally I think that a light simple stressed ply center hull is easier and will prove more durable and valuable. Use the 'skin on frame' to add soft light accommodations.
     
  5. Samhill
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    Samhill Junior Member

    My original idea was to use a 20' version of Dave Gentry's Shanendoah Whitehall with Supercat 17 hulls and rig.
    The reasons were to have dry, speedy boat with a centerhull that could be used independently of the trimaran.

    Also I was thinking that a skin on frame boat is extremely light and the increased wetted surface of the Whitehall could be negated because the rig could be light enough to fly the center hull.
     

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  6. Doug Lord
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    Doug Lord Flight Ready

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    I'm not sure how well that hull will work-the rear end seems to tuck up a lot-it's designed as a displacement hull, right? I guess it would depend on how quickly the boat flew the main hull. Structurally, if you designed most of the load to be on the connecting structure something like that might be able to handle it. If you used that same construction on a hull designed to exceed "hull speed" it could work I would think. Something that will be eye opening to you(probably) is how much RM(righting moment) a tri like that can have and how much power(SA) you need to fly the main hull. There is a reason you don't see many small tri's flying the main hull. Ideally, if you could fly the main hull in 5-7 knots of wind you'd have it made but to do that requires a lot of SA and that introduces a whole host of other problems. Think about it and do some preliminary calculations. You might gain something by reading the "Under 20 Tri thread."
     
  7. cavalier mk2
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    cavalier mk2 Senior Member

    I agree with Doug. A whitehall is really slippery at displacement speeds but will squat and drag a huge wave if trying to go faster immersed. You'd be better off with a skinny double ender like the modified 17' Herreshoff shown in John Gardner's book "More Building Classical Small Craft" Something along those lines could be adapted for skin on frame. Stretching it would help too.
     
  8. cyclone
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    cyclone Junior Member

  9. Samhill
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    Samhill Junior Member

    Thanks for all your advice.
    Doug you really make sense and have opened my eyes to other issues potentially involved with this.

    I am very interested to learn more about the calculations required to create such a rig.
    Short of signing up at Westlawn, is there a reference book you could recommend to get me started?

    Thanks!
     

  10. Doug Lord
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    Doug Lord Flight Ready

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    Sam, the problem is that there is a good deal of engineering involved to do this safely and lightly. The hydrostatics are fairly simple if you're already quite familiar with boats. An updated issue of Skenes elements of Yacht design can give you simple formulas for calculating displacement. Finding maximum righting moment on a tri is simpler than on a monohull in some respects especially if you plan on flying the main hull-its the weight at the CG times the distance to the center of buoyancy of the ama(or center of lift with foils) or a combination of both. Because there are so many interrelated things I'd suggest that you invest some money in a conversation with Eric Sponberg. He has helped me with engineering on my Dream Flyer(SRT/MPX) 18' trimaran for the crossbeams. He charges a reasonable hourly fee and that can allow you to call him with questions.
    If I were you I'd work out ,in as much detail as you can, exactly what you want- then talk to Eric. And study everything you can find on small trimarans but beware that there are very few light weight high performance* tris either designed or in production. The only potential example I am familiar with is Ted Warrens Ultralight 20-the inexpensive version is something like $15,000 and might be the best initial investment you could make-use it as a learning tool, study, talk to Eric and eventually you could come up with exactly the boat you want w/o shooting in the dark while getting sailing immediately. I'll tell Eric that I've recommended you to him.
    http://www.sponbergyachtdesign.com/

    Ultralight 20 trimaran- http://www.warrenlightcraft.com/specsul20.html

    * My definition of "small high performance tri" : one capable of beating a performance beachcat or other performance cat it's own length.
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    It's my belief, based on lots of experience designing, building and sailing small tris that ,properly designed, they can offer more comfort and more speed than any other small sailboat. Keep in mind there are lots of good small trimarans available as plans or ready to go that don't fit my definition of "high performance". So you'll have to decide what you want in a boat.
     
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