Water Stay on front beam only?

Discussion in 'Multihulls' started by Jetboy, Jul 13, 2015.

  1. Jetboy
    Joined: Feb 2012
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    Location: USA

    Jetboy Senior Member

    I'm almost done with my trimaran. It's a sliding tube design. I didn't built it with water stays and did the calculations such that my beams should be strong enough without them. However, I kinda like the idea of belt and suspenders... especially for the front beams where the majority of the loading occurs. Since I'm using a rotating mast from a Nacra 5.2 on a ball, I have a concern that flex will slacken the stay tension too much and could lead to the mast coming off the step.

    So I'm considering adding water stays to the front beams only. I think some of the Astus models use this method.

    Tell my why this is a foolish idea! I'd rather know now that I need both or none before I start adding the extra reinforcement to the hull.


  2. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    There is a difference between strong enough and stiff enough. Did you do a calculation for deflection. The equation is pretty simple

    Defection = Load x Length ^2/ 8EI

    You will also get deflection from the space in the telescopic beams. I think that excess deflection could cause you to worry way before the beam fails. I like the boat to feel like a single structure and not wobble about like three loosely joined kayaks. If the deflection is of the order of 10cm on a small tri I would probably put a preloaded waterstay on.


  3. Tom.151
    Joined: Jul 2009
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    Location: New England, USA

    Tom.151 Best boat so far? Crowther Twiggy (32')

    Is there some reason you expect the mast to jump the ball on yours? Does in not jump off on the NACRA donor boat?

    FWIW, you need considerable slack to insure the mast can fully rotate while sailing - so if for some reason you need to guard against it jumping the ball, then affix something to keep it on the ball -- as opposed to overly tightening the stays.
  4. Jetboy
    Joined: Feb 2012
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    Jetboy Senior Member

    I'm right in the borderline. The beams are 4" 6061 T6 .125 wall tube. There's very little play in the sleeve with .026" tolerance. Boat empty is ~ 600lbs, loaded more like 1200ish?

    I calculate about 1" of potential flex. Might be too much for the rig needs.

    It's easy on this one to add some waterstays on the front. The rear is a bit more tricky as it lacks a bulkhead where I'd want it.

  5. Jetboy
    Joined: Feb 2012
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    Location: USA

    Jetboy Senior Member

    I never sailed the nacra. It was my understanding that they are known for the possibility of mast jumping the ball. I have the newer model with the pin that will hopefully keep it in place. I was told by the seller that the rig tension is set much tighter than that on a hobie cat.

    I also have a mast from a Solcat 18 that I'd like to convert to down the road. It's a much heavier extrusion, and I picked up a set of spreaders, so at some point I'll swap over to the stronger mast, it doesn't have any mechanism for fixing the base on the pivot ball.

    If the Nacra 5.2 mast works well enough I guess I can just stick with it. We'll have to see how it goes.
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