Changing Catamaran Beams from Wood to Composite.

Discussion in 'Multihulls' started by Mark O Hara, Aug 4, 2020.

  1. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    You can design and build a really good beam in composites, most cats do.

    The great thing about a composite beam is that is can be deeper and have more bulk at the top and bottom flange, where it is needed - this is the archetypal box beam. So although some mechanical properties can be less, tensile strength and stiffness, second moment of area goes up fast - really fast. An oval (a typical mast section) is not really the shape you want in a beam. A thin (not too thin) and deep box is good.

    The second moment of area of a beam is related to the depth cubed. So if you want to replace your alloy beam with a composite one you can increase the depth slightly to cater for the reduced stiffness. There is more to it but that is the start.

    I would recommend you read "Stress without tears" by Tom Rhodes, a primer on aircraft design which goes right into designing a box beam and all of the intricacies. It is a really good read and you should be fine to redesign an equivalent beam (with updates for composite data). https://www.amazon.com/Stress-Without-Tears-aircraft-stress-mathematics/dp/1439207178

    Ply, glass and epoxy composites are nice for the amateur builder and designer - because a typical beam will start to show signs of stress with cracks that develop slowly. My 38ft cat had an aft beam that developed a small crack where it intersected with the hull bulkhead. It took months to show and then I could easily reinforce it with glass. 19 years later and all is good. I have friends that have designed and built their own composite cats, with lots of head scratching, building and careful looking at previous designs. One design is a super fast and successful racer that is lived aboard, the other circumnavigated - no naval architects to be found. Beams are not that hard, if you start with what has been done before as a guide.
     
  2. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Gday Wayne

    I reckon it would make a big difference. On the Bladerunner I sailed, when the anti-twist rope broke the hulls wracked an extra few degrees. Have a look at John Hitch's Wired. It used a 4 stay pyramid rig and was light fast and super stiff. Make a model and it is amazing to see how the rig resists hull wracking.

    The attached pic is not great but its all I can find. This is Wired after she was renamed but you can see she doesn't even have normal beams. Her beams did not resist bow down torque well but with her rig up she became very stiff. Great concept.
     

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  3. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Interesting subject thread, but above my head at this late period of my life.

    I did find this old posting of mine back when I was thinking about the wracking situation that Warta and Team Adventure attempted to solve,..

     

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  4. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Or this boat with NO HULL
    [​IMG]


    The 2009 version of the Mirabaud LX, Thomas Jundt’s foiler has a new hull that favours speed and stability by light wind. Its mast has also been reinforced in order to improve the upwind sailing angle, as well as the reactivity of the boat. Finally, the sensors used to trim the foiler’s flaps have been improved, allowing a better precision whilst sailing. Introduced in 2008 as “the boat with no hull”, the Mirabaud LX has recently proved that this wording could be translated into reality by sailing on its hydrofoils with no buoyancy at all. Thomas Jundt and his team have accomplished this challenge and validated a different and creative way to consider the sport of sailing.

    ...and check out this YouTube Presentation
    http://www.youtube.com/watch?v=Lx6K1PzqnG0

     
    fallguy likes this.
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