Extra floatation to avoid full capsize

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

  1. gonzo
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    gonzo Senior Member

    The problem with mast top flotation devices is that they do not prevent capsizes. They may prevent the boat from completely turning turtle. However, in the sea conditions a cruising cat capsizes, and the speed at which it does it, the flotation device would have to be very large and deploy very fast to be effective. Shortening sail and self-release sheet clutches are better ways of preventing capsizes. Of course there is nothing better than good seamanship.
     
  2. TANSL
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    TANSL Senior Member

    You are right to say that mast top flotation devices is that they do not prevent capsizes. But it is also true that some of the best navigators in the world have overturned with their ships. That is why to prevent capsizing, something else is needed.
     
  3. trip the light fandango
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    trip the light fandango Senior Member

    Thank you those that added to this thread, before it gets covered over by more current threads there's some points that it are probably worth making..low volume floats are slower but safer.. but by the time you've added cruising extras some added volume can be compensated .. somewhere around 95 to 120% is likely.,this is extra load on structure, fast trimarans like to be light. If a float was designed like Dolfimans proposal and the top of the float was designed to foil/drive to the surface when most of the lee hull is submerged, increasing the volume could be safe[r],.. as long as the trimaran doesn't lose too much forward momentum. No particular order,..just for the acronym..ha

    Floatation in the head of the sail.
    Extra floatation contained in hulls to counter swamping.
    A short strong mast,low volume floats
    Reserve buoyancy midship a gunwale height
    Extra diligence ,reef early
    Don't cleat main in gusty weather when pushing hard
     
    Last edited: Oct 16, 2019
  4. trip the light fandango
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    trip the light fandango Senior Member

    After reading oldmultis info on capsize I would go with the experts and say 150% or more in the amas /floats is safer. I didn't take into account being flipped by a wave and that lower reserve buoyancy halves the overall beam when the lee float is submerged. Also having extra reserve buoyancy forward in the amas forward of the centre of effort even protruding forward of the main hull may improve safety and sea keeping as I understand it . The other ideas stand though ,..so far. Employ a drogue when wave height passes beam width is a good one he mentions, so more beam that's well engineered is advantageous.
     
  5. jehardiman
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    jehardiman Senior Member

    On the other hand, even the dinghies at the Cal Berkley sailing club have milk jugs at the masthead to prevent loss of the rig when it hits the bottom in the shallow bay mud. FWIW, I wonder if the backpressure from the helo down draft caused that capsize.
     
  6. trip the light fandango
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    trip the light fandango Senior Member

    There's a lot to be said for mast floats and making an aluminium mast itself airtight. The aesthetics of those hobie style mast head floats I can't deal with, I'm wondering whether a wing mast style addition to the head [ on a rotating mast] could be made to look good and be effective.It 's not a great spot for adding resistance and the worst place to snag the halyard or sail, but it offers something a little like a square top. One shape that could work would be an upside down sail, but more likely something that tapered out to 200 to 250 mm would be manageable on a strong stout mast, perhaps 1 or 2 metres 3 to 6 ft. down from the top
    It would be much better to not capsize but if the semi capsize can be turned so that the deck is facing windward and wave action it may well right itself and/or enable more floatation to be run ,..well.. down.. the halyards.
    Another idea is that the float is raised at the same time as the mainsail perhaps with slides so the contour of the aft of the mast is followed, much easier to take down, then how do you drop the mainsail, this requires more thoughts from people with more experience .....cheers..
     
  7. oldmulti
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    oldmulti Senior Member

    TTLF Lets look at this in a slightly different way. The original Tremolino was 23 x 16.5 foot and weighed 800 lbs. I will assume a 300 lbs add on crew. Its total righting moment will be limited by the H16 outer hulls which in real terms can carry about 800 lbs / hull. Therefore the righting moment is 6000 foot/lbs. A Hobie 16 with 300 lbs of crew on trapeze has a righting moment of about 4800 foot/lbs.

    The Tremolino Mark 4 has floats that can support about 1250 lbs on a 18 foot beam giving a maximum righting moment of 11250 foot/lbs. The basic weight of a Mark 4 is 1000 lbs. With a 300 lbs crew the floats have about 100% buoyancy. You have 2 problems here if you have a standard H16 rig you are at the limits of the structural strength that it can safely take with a standard H16 rig. If you increase the buoyancy of the floats to 150% and your crew sits on the float not the main hull your H16 rig will be under great stress. Mast head buoyancy of any variety in a boat this size starts to get to a problem of strength of the mast tip above the stays. Mast head floats were popular once but as people tested them in reality on wider boats they did not always work as you could not get enough buoyancy up there to support the weight pushing it down in a capsize. EG a Trem Mk 4 capsized with the mast tip at the water will need to support about 200 lbs or 3 cubic feet of buoyancy, that is 3 ft long x 1 ft x 1 ft, at the mast tip.

    Now let’s look at the wind speed it will take to capsize a Mk 1 with 16.5 ft beam and H16 floats. 17 knots of wind speed if sheeted in hard. The Mk 4 with 18 ft beam and 1250 lbs floats. 23 knots of wind speed if sheeted in hard.

    Translation there are a lot of larger multi’s that have to reef at 25 knots. If you have Mk 4 floats and beam you will have a relatively safe boat that will (should) not need mast head buoyancy.
     

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    Last edited: Dec 24, 2019
  8. trip the light fandango
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    trip the light fandango Senior Member

    That's really useful information Oldmulti, thanks very much.
    "....the mast tip at the water will need to support about 200 lbs or 3 cubic feet of buoyancy, that is 3 ft long x 1 ft x 1 ft, at the mast tip".
    Then maybe a float 4inches wide tapering out to 1ft or a little more and 6ft long..that would overlap the stays ,combined with an airtight mast.. and be easier on the eye, it would submerge but be fighting a full capsize...It's just an idea for a backup when things are looking grim..
    It sounds like increasing the beam is the best bet, hopefully avoiding the problem completely,.. I will be doubling the beams on fandango, when I get closer to that stage I'll write about it and explain the idea. All food for thought, thanks again.
     
  9. BlueBell
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    BlueBell Ahhhhh...

    The Malcolm Tennant "Streaker" I sailed on ( a catamaran ) had a hydrostatic-release "bag" we'd hoist in rough conditions.
     
  10. redreuben
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    redreuben redreuben

    Lets just make something clear, you can't add flotation without adding volume, adding foam, ping pong balls or wine cask bladders doesn't add flotation to a sealed hull or float space indeed it reduces it.
    Yes in the case of hull damage it can mitigate flooding but it does not "add" flotation to a viable hull to prevent capsize.
     
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  11. trip the light fandango
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    trip the light fandango Senior Member

    No you can't,. although sealing/foam filling the mast so as it floats adds a little, and is a low volume small exception.,without adding volume to sail heads , mast, floats,gunwales etc . Gas filled float bags like canisters on life jackets built into the mast with an emergency cord leading down. somewhere around the spreaders..? The mast base would probably need strengthening too I'm guessing. I've read somewhere that there is a release mechanism for the mainsheet that works on mast angle that could be a possibility, ...that helps to avoid capsize. Avoiding sinking is what the balls and bags are about, not capsize.
     
  12. buzzman
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    buzzman Senior Member

    If you increase the beam, you increase the forces acting on the rig. Ditto if you increase the floatation (volume) of the amas, as they will resist overturning (depression/submersion) greater than the lower volume amas, thereby also increasing load on rig.
    If you increasre the beam and/or increase the ama volume, you also need to beef up the mast and rig to cope with those additional forces.
    Havign said that, if you want to increase ama volume, to help resist overturning due to wind forces, then you *could* add XPS (extruded polystyrene) foam to the upper part of the ama hulls longitudinally, shaping the foam (and perhaps using multiple layers, to *only* increase the volume of the upper part of the hulls, thereby the additional floatation would only ocme into play when the amas were already near-submerged, thus acting as a bit of a 'safety-valve' at that extreme.
    Shape the foam prior to glueing to hull, and glass it in.
    Additonal flatation would then be almost as aerodynamic as the original hull - forward to aft - and only the additional width/volume would impact the submersion factor.
    You might need to add 100mm of foam thickness and perhaps 200mm depth for the full length of the hulls. That's almost an extra cubic metre of floatation per ama.
    Would be "relatively" simply to add. BIt of foam, some e-glass and epoxy. Simples, yes?
     
  13. trip the light fandango
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    trip the light fandango Senior Member

    I think it's a good idea. If the risk of capsize is because a wave has hit the tri abeam then this idea would help the trimaran slide down the breaking face and possibly stop the craft from tripping over the amas/float in the trough of the wave, rounded hulls would slide best without daggerboards. If the aka/main hull is out of the water because wetted surface area has been reduced due to the load /power coming from the sail, nearly submerging the loaded ama/float should still be an advantage to submerging the ama completely except that the boat will be going much quicker from a smaller base so the maximum righting moment will come up faster. The flair seen on modern main hulls potentially adds buoyancy when the loaded hull is submerging[ if it's around 100%] and the combination would slow the boat and give time to the helm for correction. A cruising tri with lower volume floats may produce a more comfortable ride, so having volume as Buzzman suggested may not alter that much. It is more likely that a cruising tri will reef early negating the likelihood of either capsize, but for a freak wave hitting broadside 200%, [higher volume] floats are probably safer. Just some thoughts on the topic, regards .
     
  14. buzzman
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    buzzman Senior Member

    I doubt slipping sideways will be an issue. You'd have to drop almost vertically, side-on, off the face of a curling breaker to be too worried about a tri flipping sideways due to wave action.
    The notion of 'tripping' is usually associated with catamarans lying a-hull with the leeward board down, and thus 'tripping' over the board, which prevents the boat 'slipping' sideways when wind or wave driven.
    Tri's have the centre- or dagger-boards in the main hull, so it's less of an issue. Probably still want most of it up, if you were lying a-hull.
    From what I've seen on other's boats (yet to try this myself) [try this...sorry] stuffing the leeward ama into the back of a wave, or wind-and-wave-assisted burying of the bow of the leeward ama is usually the greatest cause for concern, as this tends to slew the tri, creating a pitchpole (base over apex). As the boat basically stops dead in the water, wind pressure on the sails (especially if the spinnie or main is cleated in) will then do the rest.
    But, having said that, people who race these smaller tris (i.e. not the big French machines - different league) report that it is, generally speaking, even when racing and pushing the limits of what's 'safe', REALLY hard to pitchpole a tri.
    SamNZ's Green Machine tried a few times, and no-one's ever accused that crew of not 'going for it' but they never (TMK) flipped or pitchpoled. Stood up vertically, and had people falling off everywhere, but never *quite* went all the way over.
    So on the basis of that 'lived experience', a cruising tri, heavier, slower and not 'trying so hard' would probably almost never be placed in such a situation.
    Probably best to avoid the Roaring 40s and hurricanes, though...just to be safe....
     

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

    Having just read Jim Browns excellent book 'the case for the cruising trimaran', it occurs to me that you wouldn't want your floats/amas to be too large and light, the windward float although held down by the leeward ama, could become a willing partner for capsize, like a kite.
    Admittedly the wind would need to be very strong, fine knit tramps are also not ideal particularly on really wide tri's.
    Jim talks about the importance of weight /ballast low down in the centre hull as an advantage trimarans have over cats. Central weight distribution is always mentioned in boat design and a cruising tri has to carry weight somewhere.
    It is an important factor for avoiding capsize that should be mentioned.
    For small trimarans the anchor and chain should be contained next to the daggerboard case.
     
    Last edited: Mar 30, 2020
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