Multihull Capsize Prevention <split>

Discussion in 'Multihulls' started by MikeJohns, Jun 23, 2011.

  1. warwick
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    warwick Senior Member

    Out of curiosity how would a twiggy trimaran compare to other earlier Crowther Trimarans of the boats in terms of warning signs. As the Twiggy had relatively low volume float place forward, combined with a wide beam. From What I understand they had diagonal stability but possibly as much for and aft. At the time of their capsizes They were sailed dead down wind?

    One change now, is in the way multihulls are sailed down wind.

    May have that also been a part in the yumi maru capsize, as asymmetrical spinnakers were not in a lot of use then I may be wrong but I don't remember them on multihulls then.
     
  2. oldsailor7
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    oldsailor7 Senior Member

    How do you figure that one out :?:
     
  3. warwick
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    warwick Senior Member

    Would it be that part of it would be that the flat deck area be angle up towards the bow pushing down on the boat. If you were to anchor stern first. Has there been any aero dynamic testing been done to look into this or has it not been thought of. Would it be possible with computer simulation?

    From what I under stand of Automotive aerodynamics the problem would be to do with the air packing up underneath giving lift. The problem could be similar to catamarans.

    May be if you could put something over the deck to under water to stop the air getting underneath to create lift.
     
  4. cavalier mk2
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    cavalier mk2 Senior Member

    For many cruising multihulls the "apparent wind will be really far forward" is a urban myth. Many of these boats don't go fast enough for this to happen and need a fuller chute for offwind courses, be it asymmetrical or otherwise. In the PNW wind tends to be upwind or downwind but shipping lanes, fiords etc... often make it practical to steer more downwind. Performance is great but when cruising there is plenty to do to save the tacking for upwind.....
     
  5. Corley
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    Corley epoxy coated

    Yeah I tend to agree Cav for most cruising multis generating enough apparent wind to make tacking downwind worthwhile would be unlikely. Even with our club boats that are an assortment of racer and racer/cruiser style multi's sometimes sailing deep rather than tacking downwind can give better results (depends on the conditions).
     
  6. cavalier mk2
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    cavalier mk2 Senior Member

    There are tables that lay out the speeds required to equal the "on course" speeds. If nobody posts one I'll upload the one from Rob James book on Tuesday.
     
  7. Silver Raven
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    Silver Raven Senior Member

    Gooday 'Cav - nuck' I haven't got my copy anymore but I thought those were for monohulls ??? I can remember that we read or just knew that 60* was the off wind/course - from the top mark - over a true 'olympic course' sailing a wing-mast - una rig cat. That was 60* out until the return leg - to the bottom mark was at 60*. Then jibe & go for it. Nothing ever got to the bottom mark in front of us - not even the 2 fully imported 'Tornados' from England. I can remember we would use a lesser degree formula of some kind when we were ocean racing in a 34' mono - & mostly it seemed to work - however you really had to have a big commitment to hang in there - cause for quite a distance - the others seemed to be running away from you - big time & it wasn't until within a 1/2 mile from the mark that you actually knew that you'd made the right call or bumed-out real bad.

    I'd sure like to see the figures - when you dig them up. Thanks & ciao, james
     
  8. Autodafe
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    Autodafe Senior Member

    Interesting thought. The extra weight usually found in the stern would help too.

    As others have pointed out when the wind gets strong enough then pressure under the bridgedeck could cause a flip, whatever the shape.

    It all depends what the critical wind speeds are...
     
  9. warwick
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    warwick Senior Member

    Thanks corley and Cavalier mk2 for you comments as to spinnakers.

    Autodafe do you think there be way of bocking the wind from getting underneath, which may help with the situation as well. It is just another thought.
     
  10. Autodafe
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    Autodafe Senior Member

    If the rig is intact then it should be stable at ~100° of heel.
    With the rig completely gone and all hatches open on the pod then it will rest on one hull and the pod, at about 20° (160° from upright), assuming full load displacement. The pod is then about 2/3 submerged. The air space is quite large, but may not be "dry" in rough seas.

    The hulls are designed with a survival compartment, but access means cutting a hole in the hull, so remaining in the pod is the preferred option if I want to "self-rescue" when the weather clears up.

    Partly inspired by this thread I'm trying to fiddle my pod shape so I can get more floatation in the pod roof and keep the whole thing dry...

    Otherwise, not much progress worth telling - I've been working on the boring bits of the design, lighting, anchoring, cooking etc.
     
    Last edited: Jun 4, 2012
  11. catsketcher
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    catsketcher Senior Member

    A little on Yumi Maru

    Yumi Maru capsized in 1982 - a bad year for multi racing. IIRC she was under small kite (Not very small actually bloody big) and there was mis-communication between the helm and the hands on the kite.

    IIRC Phil (the owner) was on the helm and asked for sheet to be eased. Instead the brace was eased and this helped cause the broach. The boat then rounded up and laid on its ear. The roll over was very gentle.

    As Corley says she then had bigger floats put on. Also after the stack of capsizes in 1982 - Yumi, Sailmaker, Pumpkin Eater and the retirements from the Gladstone (D Flawless, Vixen) there were reductions in kite size in the IMORC rule.

    I have to agree with problems with inverted multis. Cats are very easy to design with inverted flotation yet my cat is the only cruising cat I have been on with this built in. It's sad that people don't realise they have to design to their boats weak points. The race boats are usually better prepared with lifelines under the bridgedeck, a clearly drawn space for cutting an access hatch and radios and safety gear put in a place above the inverted waterline. Not hard to do. As a child of the terrible 80s I had it drummed into me by my mono friends and people like Ian Johnston and Cathy Hawkins (Twiggy's and Verbatim's screw).

    On another level the capsize risk has to be weighed up against the user. Multis differ greatly from their mono counterparts in that almost all of them can be sailed over if built and rigged well. All they have to be is driven too hard - and that is almost the definition of racing. A mono designer knows that if his boat gets pushed too hard the crew will pull down the kite and put up a smaller one or reef down and go faster because they are in control. Broaching is slow. It did not take me long racing multis to come to a point where I felt the boat should be reefed but my competitors were still pushing. So the better sailor goes slower by being safer in a multi. So for me I prefer to race monos which reward skill over bravado and and cruise multis where I feel safer.

    One point that should be made is that more than a few capsizes have occurred in coastal waters. This is one area where a multihull can be much safer than their mono friends. We often have done a daylight run of 85 nautical miles from river bar to river bar and crossed each river bar easily whereas our mono compatriots have to stay outside or try to surf in to the bar with deep draft and a boat that is hard to steer at 15 knots. Kankama has surfed heaps of bars and with her high bows and moderate sterns (unlike the shape of the Lightwave 10.5) she does this with ease. Many times we have been well anchored or even tied to a tree (Percy Island comes to mind) when a vicious squall or southerly hits our cousins. Or the time we entered a little creek on Keppel with the wind about to blow 25 from the north and then have a 30 knot southerly. We had a great day tied to a tree when the monos had to either (and some did) anchor on a lee shore in the early evening or wait for the change and move.

    So there can be no objective risk assessment based on every boat being used in the same way. Kankama may be less (although I doubt it) seaworthy than a lightweight mono (that I could be happy sailing) in a big blow but she can get away from so many with her speed, bar crossing ability and creek access that she almost never gets hit. In 3 trips to the reef we once got hit by a bit of weather and that was only because we thought we should see how she handled it. So in effect she is safer than a deep draft and slower mono doing the same coastal cruise if she is sailed to her strengths and that is what I do. So for me she is safer.
     
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  12. Autodafe
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    Autodafe Senior Member

    I don't see why not, but I'm no expert. The loads on any "skirt" used for blocking could be huge (waves as well as wind) but a practical solution may still exist.

    A fabric skirt closing could maybe "give" to gusts and waves but still provide a adequate pressure drop between the front and the bottom of the bridgedeck.

    My personal solution is a pod cat, with lots of open net up both sides.
     
  13. warwick
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    warwick Senior Member

    Thanks for your comments Autodafe, I agree with the idea of a pod catamaran could be better. Mind you the public that like their gin palaces wont want to give their space. in terms of a skirt possibly on the rear cross beam to the transoms, depends on rear beam location.

    Thanks Catsketcher for the information as to the yumi maru capsize, I probably read it about 1982. in the early nineties I got interest Automotive aerodynamics and got rid of all my magazines, since mid 2000s I regained my interest in boats and buying magazines again. This year learning as much as I can.
    It can be a bit of a problem what to call I an and Cathy s second boat as it also went under the name of bull frog sun block as well when racing in NZ.
     
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    Curved surfaces tend to create lift, and as Autodafe said the weight distribution also. Bridge deck clearance is also lower in stern.
     

  15. Angélique
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    Angélique aka Angel (only by name)

    Which makes it dangerous for a bridgedeck cat to lie behind a sea anchor, better pull a drogue . . . . . .

    (see also the comments on anchoring from the stern)

    Cheers,
    Angel
     
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