Extra floatation to avoid full capsize

Discussion in 'Multihulls' started by trip the light fandango, Oct 9, 2019.

  1. tspeer
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    tspeer Senior Member

    A lot has been learned about multihull seaworthiness since Jim Brown wrote that book. Barry Deakin at the University of Southampton did a series of wind tunnel and wave tank tests to look at the capsize susceptibility of both cruising catamarans and trimarans. His study of casualty reports showed that trimarans were more likely to be pitchpoled or capsized by waves compared to catamarans, while catamarans were more likely to be capsized by wind gusts than trimarans.

    When it comes to wave-induced capsize, the most important factor is the beam compared to the size of the wave. In this regard, trimarans effectively have a smaller beam because the windward ama is in the air when the wave hits the main hull. The trimaran becomes a catamaran with just the main hull and leeward ama in the water. So it's important that the amas have adequate buoyancy to resist the wave.

    With regard to wind-induced capsize and pitchpoling, Shuttleworth has a stability index that can be used to calculate how much stability is needed for the operating conditions.

    I think modern cruising trimarans have ama volumes on the order of 160% of the displacement of the boat. This might seem excessive, as 100% is all that's necessary to fly the main hull. The question is, however, "Which 100%?" When a trimaran is hard pressed diagonally, the stern lifts out and the ama volume that is important is in the bow. Upwind, it is loaded more to the side, with the ama displacement distributed more evenly along the length of the ama. When you take the union of all the volumes needed to stabilize the boat under different conditions, you end up with much more than 100% of the boat's displacement.

    I know what it's like to capsize a trimaran at sea. And while I'm a Jim Brown devotee, too, I wouldn't scrimp on the reserve buoyancy of the amas, especially in the bows.
     
  2. buzzman
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    buzzman Senior Member

    Just out of curiousity, can anyone familiar with buoyancy principles etc comment on why many of the earlier trimarans had amas shorter than the vaka?

    Was there / is there any perceived advantage to having the amas shorter..??

    Perhpas, for example, the aft part of the ama not adding to the pitchpoling effect by being lifted by waves as they pass under the length of the boat.

    IOW, the *lack* of an aft section (extending to the same length as the vaka) helps to prevent such lift and reduce the notion to pitchpole..????

    No idea, myself ... I am totally hypothesising, but would welcome correction!
     
  3. trip the light fandango
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    trip the light fandango Senior Member

    I couldn't find anything on Jim's point about low central ballast in the centre hull and what improvements that made ,-particularly compared to cats. It is more likely to make a dodgy tri than a cat it seems, perhaps a third more, ha , ramping up decision making on the helm . The forward bouyancy in the amas and them creeping forward of the bow are modern advances.
    There is a lot of Jim Brown[and many others of course]in that Shuttleworth report.
    160% and amas shape/displacement distribution was really interesting, thanks for adding more credibility to the thread.
     
  4. buzzman
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    buzzman Senior Member

    I imagine the low down and central 'ballast' is simply that - ballast - and acts much the same way such mass does in monos, adds stability by resisting the overturning forces.
    It's why lightweight dinghies like the Hartley TS16 sometimes carry internal ballast along the centreboard baseline, as the lifting centeboards don't add much 'ballast' (resistance) to the heeling moment.
    Probably also why old-style clinker dingies with 'long keel' rather than centreboard, tended to also have ballast inside the boat along the keel line.
    Probably also explains why older long-keel sailboats tend to have big chunks of lead or iron fitted along the keel, to enahnce capsize resistance.
     
  5. trip the light fandango
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    trip the light fandango Senior Member


    Yes ,I agree, it is interesting because the maxim for trimarans is 'keep it light'.,and 'centre the load' for all boats,.. Jim's 'case for cruising tri's' shows a diagram that because of the low central ballast[being heavy equipment/stores etc] makes a tri less likely to be capsized by a wave than a cat, but no more modern studies consider this as a factor, or I could be wrong.. Modern studies just go with, 'you have to halve the beam of a trimaran because one half is in the air ' ..one of the floats/amas. The other in depth look may well have been Shuttleworths[or Barry Deakin] that old multi pointed out previously. I'll look it up tomorrow.
    Edit, it's tomorrow and I'm not going to try and find a specific reference, let common sense prevail[well all that this novice can muster], even better someone who actually knows could post.
     
    Last edited: Apr 1, 2020
  6. trip the light fandango
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    trip the light fandango Senior Member

    Old multi, page 44 Capsize analysis ..."A summary of all the above is wide beams, low centre of gravity of the multi, minimise windage,...."
     
  7. buzzman
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    buzzman Senior Member

    Shuttleworth notes that modern tris have a beam to length ratio from 1.5:1 to 1:1 - in other words, 66% to 100% B/L ratio.

    If, and I have no doubt, the 'half beam' concept is relevant, then should that not indicate tri beams should be increasing beyond 100% of length...???

    Or does that then have unintended consequences, such as increased likelihood of a pitchpole from yawing..??

    Can one of the designers comment..?
     
  8. trip the light fandango
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    trip the light fandango Senior Member

    Buzzman, I think the immediate problem to increasing beam is freeboard.
     
  9. buzzman
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    buzzman Senior Member

    Not sure I understand why? Are you referring to the greater likelihood of dipping the (wider) beams in wave crests..?
     
  10. trip the light fandango
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    trip the light fandango Senior Member

    Yes, and that being really wide has practical issues..unpopular in a channel or berth,dock,etc.
     
  11. buzzman
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    buzzman Senior Member

    True. But thinking of a *folding* trimaran..... like my TT720 Farrier. There was also his Command 10 33' version, the largest folder I'm aware of. I have a set of plans for that one also.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Interesting topic. Here are the principles involved. I have to search for the original article. What I have is an unfinished work on my Excel.
     

    Attached Files:

  13. trip the light fandango
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    trip the light fandango Senior Member

    Hypothesis 1, dealing with the dynamics involved in extreme wind with a stationary craft brings focus partly on how much the mast impacts on wind and then centre of gravity.
    It makes for a valid safety factor for being able to lower safely and secure the mast.
    The mast and stays probably add to the stability of the superstructure overall on some trimarans which then wouldn't benefit perhaps. I wonder if one day this could become a safety criteria for large racing trimarans, .Thanks for the info rxcomposite.
     
  14. rxcomposite
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    rxcomposite Senior Member

    You are welcome. I didn't get to the point to finish the math but the author was able to develop the safety margin for the couple provoking capsize. The hardest part is when one ama is in the through and the central hull is in the crest. This is a fully developed wave not unlike the standard sinusoidal wave where it is easy to plot. In non sailing boat, this will be aggravated by shifting of weights either cargo or passenger/crew.
     

  15. Dolfiman
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    Dolfiman Senior Member

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