Overall beam of Tris vs Cats

Discussion in 'Boat Design' started by patrik111, Sep 25, 2003.

  1. patrik111
    Joined: Sep 2003
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    Location: Sweden

    patrik111 Junior Member

    Hello All,
    Apart from structural issues, why is the beam on catamarans generally about 1/2 LOA but on trimaran up to 1/1? What would be the difference of a tri sailing on one hull compared to a cat sailing on one hull?

    Or is the difference obvious in other conditions? ( tacking, gybing,high seas, a s o )

    Would a cat/tri be more prone to dive/get lee helm or something else?

    What is the main reason for this difference?

    ( I'm thinking of making a Cat/Tri with a very modest center hull ( really a central beam for mast support.) or a down-scaled ORMA 60 at about 6 m LOA)

    Please tell me what the catch is in this, because I suppose there might very well be a reason why this sort of design is not around.

    Would be most grateful for input in this matter.

  2. Tohbi
    Joined: Jul 2003
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    Location: arizona, usa

    Tohbi Senior Member

    i guess i just feel like writing something because i don't have the data on this, but i would speculate: the tri amas are much smaller and gain their stability from the length of the akas [arms].

    the cat hulls are just that, hulls, heavier with more flotation. their weight is a greater factor in stability than the amas of the tri.

    it just isn't necessary to have such big amas on the tri. distance from the main hull provides enough righting moment.
  3. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I assume you're comparing racing cats with tri's. The difference in beam is a combination of structural and performance differences. Structurally, it's hard to give a cat the same beam as a tri because the beam needs to be deep in the center to have adequate stiffness against the mast compression. You naturally get that with a tri's center hull.

    From a performance standpoint, tri's are designed so the amas are both out of the water when the boat is upright. The wetted area of the center hull is less than the wetted area of the cat's two equally loaded hulls. So for performance in lighter conditions, it's desireable for the cat to be able to fly a hull, and the beam is reduced accordingly. This leads to a crossover in performance between the two types when designed to similar requirements. The tri will be faster in the lightest airs, the cat will typically be faster in some medium wind range - especially on a reach - and the tri will again be faster in heavy air - especially upwind - because of the greater beam.

    John Shuttleworth discusses the tri-vs cat issues in http://www.steamradio.com/JSYD/Articles/BritFertalk.html .

    Maximum beam on both cats and tri's is limited by diagonal stability. If you divide the applied moments by the displacement of the vessel, you get the location of a virtual center of gravity that will produce the same heel and trim. This is a very useful way to visualize the stability as compared to the geometry of a multihull. Here ( http://www.tspeer.com/temp/stabilityindex.gif ) is the "multihull footprint" of a performance cruising trimaran design. The green lines show the aerodynamic moments implied by Shuttleworth's stability index criteria ( http://www.steamradio.com/JSYD/Articles/NESTalk.html ). The blue lines show the location of the center of buoyancy at 100% displacement for various combinations of heel and trim. In steady state conditions, the virutal c.g. will be over the center of buoyancy. The red line shows the location of the virtual c.g. at which the lee ama bow will become immersed.

    This plot also shows the importance of having adequate freeboard or sheer at the bow for a multihull. The sheer at the bow should be at least level when the boat is pitched down at the attitude corresponding to its limit of diagonal stability. Straight sheer, as is the style these days, gives up the most critical part of the waterplane area when you need it the most.

    Current racing trimaran amas typically displace 200% of the boat's weight when fully immersed, and are nearly as long as main hull. Modern cruising trimaran amas aren't all that much smaller - being more like 140% - 160% displacement. So the tri's amas aren't all that much smaller than a catamarn hull. The main difference is they don't have the freeboard. A modern racing tri will fly both hulls and ride on one ama - that's why they have rudders on the amas as well as the main hull.
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