Extra floatation to avoid full capsize

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

  1. rxcomposite
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    rxcomposite Senior Member

    Found it. The English version. It was in my other computer. Thanks Dolfiman for the other link.
     

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  2. trip the light fandango
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    trip the light fandango Senior Member

    Both Dolfiman and Buzzman suggested ways to raise the centre of buoyancy in the amas, it is interesting that the oceanic trimarans take on 800kgs of water ballast to lower the centre of gravity to comply with safety rules , I assume this is in the centre hull/vaka.
     
  3. rxcomposite
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    rxcomposite Senior Member

    TTLF- I was going to answer your post but it vanished.

    The connecting beam/crossbeam/aka initial buoyancy may not be enough as it create a notch in the righting arm graph. The practice is to gradually increase the buoyancy of the ama/outer/side hull. The form suggest a wedge as shown in the picture. It serves two purpose. Smooth out the buoyancy curve and increase moment of inertia of the connecting beam. As the boat heels and the ama submerges, the moment arm changes from the centerline of the main hull to the center of buoyancy of the outer hull to the connecting point. That is the distance from the outer hull center of buoyancy to the outermost point of the connecting beam.
     

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  4. buzzman
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    buzzman Senior Member

    It might even make sense to add a similar 'wedge' to the outside of the amas, as this is the side that is pressed most deeply in the water due to the heeling effect.

    My feeling is that a narrow V-shaped lower ama hull, broading to a much wider, shallower 'V' would be ideal.

    Alternately, an upward curved aka, the outer end of which was higher than the inner end, would enable an ama of even larger volume - like 200% or more.

    I'd be interested to see how such an arrangement affected the righting arm. And I'd also be interested to learn how increasing the floatation of the amas affects design strength/thickness/size of rigging and spars.

    For example, if I were to increase the beam by 20% and increase the size of the amas by 30%, by what percentage might I need to increase the size of the "originally designed" rigging and spars...??
     
  5. trip the light fandango
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    trip the light fandango Senior Member

    I deleted the post because it was Doug's thread and it felt like a hijack, but Randy Smyths design is apparently strong enough to withstand capsize according to the test Brian referred to from Multihull magazine, for a 40 ft multi.
    Getting back to your point rxcomposite, one factor is that the extra drag created by submerging the outer hull so deep should have slowed the boat down , perhaps long enough to take some action such as releasing the mainsheet.
     
    Last edited: Apr 13, 2020
  6. buzzman
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    buzzman Senior Member

    The 'transparent foil' is actually physically part of the 'mast' (wing) and is designed so that the taper at the masthead enables the entire 'wing' structure to 'feather' about the pivot points top and bottom. So, no, it won't revolve like a VAWT, it would simply turn tail into the wind and remain there in that position, which was how it was built by Fast Forward Composites - initially for Randy's Sizzor and later for the 40' Caliente cat.
    The masthead features an offset each side to attach the runners/stays such that they clear the narrowed tapering wing as it swings.
    This is, of course, provided the 'soft-sail' component of the wing has first been reefed/lowered.
    The idea is that you don't have to manually lower the hard wing at anchor, as it can 'feather' to windward (in normal conditions).
     
  7. trip the light fandango
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    trip the light fandango Senior Member

    Buzzman , yes the requirements for a cruising trimaran call for a higher percentage of design criteria to be dedicated to safety and comfort, 200% plus floats may impinge on comfort,,? I don't know. Speed has a safety factor of course, part of the reason trimarans can be so good . Flat sections in a shallow V while still fighting leeway may be less prone to sharp sideways knocks from messy wave direction/ current forces, than a sharp V you'd think. The floats on sizzor are interesting, they look just a little like old wind surfers..ha. There is bound to be a fairly straight forward formula for required mast strength for those factors you mention. Those factors you are increasing a close to what I am doing/ plan to do, but more like 15% for the beam. I'll be going with if it starts bending back off, then add extra hounds from the spreaders, then start looking for a stronger mast. Rxcomposite possibly has a definitive answer.
     
    Last edited: Apr 13, 2020
  8. trip the light fandango
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    trip the light fandango Senior Member

    I was thinking really wild winds ,perhaps the only direction that would make it spin is directly down , I get the feeling in a cyclone or 50+ knots the wind is kind of in a frenzy. It could be a factor sailing near cliff faces etc. Thinking about it I'm getting mixed up with the soft wing vertical shape that Jim Brown was explaining. err sorry about that.
     
    Last edited: Apr 13, 2020
  9. redreuben
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    redreuben redreuben

    Keep in mind that any flotation device up the mast like gas systems, floats, or sail pockets or whatever rely on one important factor.
    And that is the mast survives the capsize, no mast, no leverage.
    Does anyone have any figures on that ?
    Or best guess ?
     
  10. brian eiland
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    brian eiland Senior Member

    Did you happen to see this recent posting,..
    Hybrid wing sail-used by Randy Smyth to win Everglades Challenge https://www.boatdesign.net/threads/hybrid-wing-sail-used-by-randy-smyth-to-win-everglades-challenge.59514/#post-873187

    PS: oops, I see it was mentioned already. Oh well, the article from ProBoat was not referenced
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    There is a recent interesting discussion on mast builders by Brian Eiland
    Just thinking aloud..virus musings.
    The mast is going to be loaded up with half the weight of the boat or at least the windward hull, presuming that the other half will be submerged, but then there's water trapped in the sail when righting.The mast has to be able to take the weight of the boat>>? it would be either very expensive or very heavy or both. Half the weight of the boat then..? If the whole mast has floatation like the hybrid wing that would help spread the load... Lock Crowther apparently built beams for 5 times the breaking load stress,..if I'm quoting correctly
    How about the weight of both amas/ floats combined,.. or one ama?
    The width of the beam increases the height/leverage of impact with the water so some extra would be nice, wider beams need stronger masts anyway.

    Floats/amas that have less buoyancy may lighten the load on the mast and slow down the action/re action, but then more water may be trapped for longer...
    I don't think I would have a hope of lifting 140kgs from one end of my Hobie 18 mast, 70kgs 155lbs easily if the load was spread out

    The weight of the outer hulls is the main factor, also the speed at which it flips , how much water is held by the sail/how quickly the main is released.

    That weighted below axis lever device that a clever old salt made[the case for multis,Jim Brown?] that releases the main is beginning to look like it's worth its space even in a small helm .
    In summary a well stayed mast built with a safety margin a little more than standard should be ok..ha, they cop a hiding from sail and waves.
     
  12. brian eiland
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    brian eiland Senior Member

    Additional text from that article on ProBoat,..
    Interestingly I do not note any mention of flooding or non-flooding of that fabric coated aft portion of the wingfoil? I would assume that any flooding (and retaining) of water in that portion would have made things much more difficult.

    On the smaller daysailing cats such as Hobie, Prindle, NACRA, etc it was always the procedure to seal up any fittings on the alum mast tubes, and particularly the mast head fittings and pop rivets to try make the mast a sealed tube against total capsize/turtle. Then depending on the weight of the different boats, one brand might be more easily righted by the skipper/crew.
     

  13. Doug Halsey
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    Doug Halsey Senior Member

    I can testify to the importance of this.

    I was able to recover from this capsize of my 15' tri (Broomstick), even though I was using a larger-than-normal rig with a heavy Hobie-14 mast, thanks to its being well-sealed. Usually, that was also true of my normal rig, with a much lighter cut-down 505 mast. However, eventually the seal around the fittings deteriorated, and the extra weight of the water inside the mast made recovery impossible.
    Capsized.jpg
     
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