Extra floatation to avoid full capsize

Discussion in 'Multihulls' started by trip the light fandango, Oct 9, 2019 at 11:27 PM.

  1. trip the light fandango
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    trip the light fandango Senior Member

    I should really be cleaning or prepping something for glassing, but anyway, lets say your 25ft tri has just been overpowered by a gust and you are hanging on to the outer hull high in the air. The question is where would you prefer the extra bouyancy to be, they are sausage fenders tied securely,[100kg 220 lbs]float inboard immediately behind the fore and aft[ parallel to the main hull] seating back rest under the tramp or tied to the now submerged hull that has been threatening to bury completely, but the speed was just so much fun.
    I suppose it depends on how much positive bouyancy the outer hulls have, 100% or 200%, if there is greater bouyancy the tri will more than likely flip over without much hesitation, whereas the lower volume amas will submerge deep in the water. Agreed?
    Just before it's all over and you have to wait to be saved and the submerged hull passes under the centre axis on the main hull, time and the speed of the action tends to slow down for a few seconds, yet your pinned, hanging on, looking on dumbly from up high, amidst the shock realising the magnitude of what your frivolity and lack of caution just created. I'm thinking the fenders are better closer to the axis point, behind the seats, than tied off on top of the amas. What do you think ? Ha..
     
    Last edited: Oct 10, 2019 at 7:57 PM
  2. gonzo
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    gonzo Senior Member

    If you are sailing really fast, using dynamic lift to keep the leeward ama above water would be a better solution. Fenders or sausages of any type will generate a large amount of drag and probably round down the boat; a dangerous situation.
     
  3. aussiebushman
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    aussiebushman Innovator

    It will not prevent capsize, but filling every available empty space inside the hulls with ping-pong balls at least provides much buoyancy to keep them afloat. They weigh next to nothing and will fit into otherwise impossible places.
     
  4. kapnD
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    kapnD Senior Member

    To avoid full capsize, flotation is most effective at the top of the mast.
    It’s been done, but is unsightly and bulky, and puts weight/windage where you least need it to avoid capsize in the first place.
    Cats and tris have some unique problems in the self righting department, I will be watching this thread for ideas!
     
  5. trip the light fandango
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    trip the light fandango Senior Member

    I've been collecting foil bladders from water casks, with taps tied together,..nearly as good.
    With lee beam capsize ,tris designed with significant flare and reserve bouyancy above the centre midship, seem to be the best design to counter the last bit of the "oh no it's really happening " roll. but the outer hull has to submerge or there will be little warning...apart from the fact you'e flying along with two hulls out ..
     
  6. trip the light fandango
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    trip the light fandango Senior Member

    It always comes back to the Gougeon Brothers ,Strings, etc. There are systems that release the main when the angle reaches threshold, and never cleating the main when flying or gusts are likely.. The float on the mast is ugly but practical ,you can seal the mast, not sure about foam filling and corrosion though.. a gas canister air bag packed into the top of the mast , or using a halyard to send two life jacket type with long strings attached to their release valves...
     
  7. Corley
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    Corley epoxy coated

    Keep the mast relatively short to keep lever arm under control and the boat should right itself if it has low buoyancy floats. Lock hauled the Kraken 25 over to 80 degrees and it still self righted. Ian Farrier always said his early boats with small floats were the most resistant to capsize.
     
  8. Dolfiman
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    Dolfiman Senior Member

    The oceanic racing tris have such system, i.e. an hydraulic release of the main when a given heel angle, adjusted by the skipper depending conditions, is reached. Usually with two thresholds : a ring alarm for the first threshold, the automatic triggering of the release if the second threshold is reached. Order of magnitude : first alarm at 17deg on ultim Macif , dixit F. Gabard in one of his videos. But all is about the reliability, video here below is a famous example of a main ease which does not work, during a training sailing with media men in an helicopter and inside the boat. Another system in experimentation is a foil at the tip of a central daggerboard, which can be put instantly in negative incidence through a button and hydraulic transmission : advantage , to create an immediate extra RM, inconvenient , as long as the tip of the daggerboard is still in the water (relation with the heel angle threshold) and that it is sufficient to overcome the gust of wind (I approximately quote an interview of Vincent Lauriot on the subject that I can not find anymore).
    The fact to add buoyancy on top of the mast, for mast of big tris at least, could lead the mast to break because angle and efforts can be important , and anyway the capsize to finish upside down. So could be worst than better.
     
  9. Dolfiman
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    Dolfiman Senior Member

    To answer the initial question, a tentative proposition : from a float design with a volume of less than 100% Displacement in charge, one could imagine a curved volume extension in the upper part so that to recover an extra volume but for a higher heel angle, e.g. twice the one necessary to immerge the float itself, as sketched attached. That could give both an RM bonus and more time to react and to ease the sails, without adding significantly more drag which is also a capsize factor.
    IMG1721.jpg
     
  10. tspeer
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    tspeer Senior Member

    Masthead flotation doesn't have to be unsightly. The scow classes add flotation to the mainsail at the head. This can be done two ways. The top panel of the sail can be doubled so as to form a pocket, or a pocket on each side, into which foam pieces are inserted to provide the floatation. Or, the flotation can be in the form of a sleeve that fits over the head of the mainsail.

    Because of the leverage of the mast, it doesn't take a lot of buoyancy to prevent turning turtle.

    E scow:
    [​IMG]

    MC scow:
    [​IMG]
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    .My eye would like a little more angle in the outward splay, say 10 to12 degrees[harder to build though] to improve the release, but then it may increase the wind resistance of the flying hull at the submerged angle..hmm.. eek.. Would the extra float on top increase the chances of gybe, or as gonzo mentioned rounding,, and a forward roll... I suppose hull shape, mast position are factors there. It looks like minimal wetted surface,.. really interesting ,thanks Dolfiman. One point though, if the float overall has say 200% buoyancy then it still may flip the main hull partially submerged ...? That percentage is an important factor..
    I think buoyancy on the centre hull flange/ between and including centre hull beams at gunwhale height[or a bit higher] is a good spot . This could be sealed frames running parallel as part of the backrests ,joining the main beams. That general design makes the unsupported beam length shorter and good for folding points, but they often have 200% floats.
     
    Last edited: Oct 12, 2019 at 9:42 PM
  12. trip the light fandango
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    trip the light fandango Senior Member

     
  13. Marmoset
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    Marmoset Senior Member

    if all else fails just remember, zip up your pants first! that way your friends won't laugh at you at your funeral! lol assuming the "you just go for a swim" plan doesn't pan out




    Barry
     
  14. Dolfiman
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    Dolfiman Senior Member

    My proposition of course cannot work with float already over 100% in volume. The final 120% proposed is supposed to take also into account the vertical component of the sails force when heeled.
    As a complement, these orders of magnitude :
    *** from wind and gust forecasts like the ones given by Ventusky, an average gust is around +40% more or less 10% the average wind force.
    ** for a boat sailing upwind at a speed of ~ 0,4 to 0,5 times the average wind force, such gusts +40% give an increase of the apparent wind force of about +30 % (from elementary trigonometry)
    ** the heeling moment being in squared apparent wind force, such gusts lead to ~ +70 % of heeling moment
    ** starting from an equilibrium with float displacement of 60% , that means ~ 102 % peak volume to balance such heeling moment peak, and corresponds to the maximum righting moment that can be mobilized.
    *** so to sail at an equilibrium with a float displacement of more than 60% exposes to the beginning of a capsize in case of gust, times to be ready to ease the sails ! Could be useful to have a visual landmark on the float for such displacement (although not simple for small tris with variable payload).
     

  15. trip the light fandango
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    trip the light fandango Senior Member

    Sorry Dolfiman I misread your diagram showing 120%, I'll have to reread your last post a few times to grasp it I think,.. regards
    .Oh just another interesting aspect of percentages, the fast cruising tri carries provisions so if you match your weights so that the float is around what we're discussing ,some extra float will still submerge because of the extra weight in the main hull.. ..more gadgets ,food ,comfort and water, if you can find somewhere to stow them,.
     
    Last edited: Oct 13, 2019 at 9:44 PM
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