Trimarans: Angle of Heel at Main Hull Takeoff

Discussion in 'Multihulls' started by Doug Lord, Sep 1, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

    The question

    James, there are plenty of ways to look at how to sail a tri fast but every high performance tri will be faster flying the main hull in flat water IF it is designed to do so, IF the angle is small enough, and so on. Different conditions will call for different boat speed strategy-you know all that. My interest lies in trying to collect as much of the info as possible and primarily info on the smallest tris. Unfortunately, there are hardly any small tri's (under 20') designed to fly the main hull so I'll have to forget that. But the more info the better on any size tri.
    This thread was not conceived of to advocate anything-just to record as much factual info as possible from actual boats or projected info from new designs and the question is: what is the angle of heel of the trimaran one nano second after the MAIN hull lifts off? (in flat water)
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Siam TC 627

    Just measured the approximate angle of heel at main hull take off of this new design with the main hull just kissing and it's 13.8 degrees. According to "Vaka" the boat is designed to fly the main hull.
     

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  3. idkfa
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    idkfa Senior Member

    What percentage volume for ama did you use in your estimation? ie. waterline at vaka lift.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Angle at takeoff

    ==================
    Of course, I eduguessed at it but I used slightly over 50% of the ama section.
    If that is too much then the angle will be less. 13 degrees is excellent considering that some tri's that claim to be designed to fly the main hull are over 20 degrees. And these guys aren't using ama foils-if they were then the angle could probably be 10 degrees.

    See Clissold tri sections below:
     

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  5. bruceb
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    bruceb Senior Member

    weight?

    Doug, how much are you estimating that boat weighs? (or actual)?
    Also ;) Since you are already gathering info- how about trying to estimate or measure the actual dimensions of the foils on the boats that have them. I realize that is not easy, but seems like the next step. I certainly am interested in how much of the displacement various designers are expecting the foils to carry, and how they go about it. Size does matter:rolleyes: B
     
  6. Doug Lord
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    Doug Lord Flight Ready

    =================================
    Bruce, If I can figure a good way to do that I will-sometime. It's known that the ORMA tris and MOD 70's carry up to 70% of the boats weight on the ama foil.
    substantiated here, post 9: http://www.boatdesign.net/forums/multihulls/multihull-one-design-orma-70-a-32514.html

    and here, with additional info on the ORMA 60's, paragraph 2: http://www.wired.com/wired/archive/14.05/sail.html?pg=2&topic=sail&topic_set=
     
  7. oldsailor7
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    oldsailor7 Senior Member

    Well I've run out of popcorn. :rolleyes:
    It seems to me that any Tri which is standing still, or moving very slowly and then gets hit by a gust, will rely on the Ama to maintain righting moment----because the lifting foil is not moving fast enough to provide immediate RM.
    The inertia of the whole boat will ensure that a finite amount of time will pass before the boat will reach a speed,(Newton), where the lifting foil will produce any meaningful RM.
    During that time and especially in rough water, the boat may capsize. :eek:
     
  8. bruceb
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    bruceb Senior Member

    Lift

    OS, yes, that is what I have observed- (without the capsize :cool:)
    You sure can get close on my 24 ;). Leeward mark roundings, gusts, hitting the breeze coming out from behind an island can all be scary. I think learning to drive the boat with the foils is just part of the game, sort of like keeping a beach cat from burying the bows. Small boats like mine respond a lot more quickly than a 50+ footer, so they don't relate directly either.
    I am still not sure which configuration works best. A boat with lower volume floats "might" just bury the float and round up, where a high volume float will give you a higher limit, but capsize when pushed too far. It seems the pros are still "experimenting" also, and still going swimming too :rolleyes:
    B
     
  9. redreuben
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    redreuben redreuben

    Bruce, how does burying a float cause rounding up ?
    I would have thought the extra drag might bring you beam on ?
    Equally vulnerable though.
     
  10. oldsailor7
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    oldsailor7 Senior Member

    RR.
    When a boat heels the horizontal component of the sail force will be to the lee of the boats vertical CB. If he sail force is greater than the drag of a submerged float (ama) then the boat will turn up to windward.
    This force can be enough to stall the boats rudder, resulting in a wild broach.
    I have experienced this in a couple of different Trimarans, and frequently in a monohull. :eek:
     
  11. redreuben
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    redreuben redreuben

    Aah yes, rounding up under spinnaker when too high (mono)

    Surprises me it overpowers the drag of the float.
     
  12. tspeer
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    tspeer Senior Member

    Dave Keiper's Williwaw is a good example. I believe he capsized it three times, and all three were when he was sailing slowly and got hit by a gust. In his book, "Hydrofoil Voyager", the pictures of the boat show that as time went on, the amas got longer and longer!

    I believe a foiling trimaran still needs to be a competent boat when hullborne, especially in heavy conditions. When the going gets too knarly to fly, the boat has to slow down and float. And when the conditions are that rough, it's probably too rough to retract the foils. Keiper found his foils added damping when hullborne, and improved the seaworthiness of the boat. So foils and hullborne operation are not mutually exclusive.
     
  13. bruceb
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    bruceb Senior Member

    more boards and roundups

    I have very limited time with my a-boards, and none in heavy/extreme conditions, so I can't say what would happen.
    I have "had the pleasure" of a couple of roundups with the main and genoa where the float did bury some. My floats are about 90-95% of my sailing weight, so they will bury if pushed hard enough, and yes, the boat rounds up. The Buc is heeled past 25 degrees if the float goes under, so the COE is pretty far out by then. My rudder doesn't seem to let go so I still had some control, and of course, I had the helm down, as well as the main out some. Blowing the jib sheet let the boat come back up. One time was just momentary, the other time it went in pretty deep and scared me pretty well, but I learned to have more faith in the boat. If I had had an a-board down, I would have been going faster, but I don't know that it would have effected the outcome.
    B
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    In all the years I had Flash Harry, which had (past tense you will notice) very small volume floats/asymmetric foils, completely burying the float only occurred once under full main when sailing relatively slowly and hit by a savage gust; there was no slewing, boat just slowed and reached a somewhat alarming angle, a new experience, dumped mainsheet then sheeted in so boat accelerated and the foil safely took over - which always felt so solid at speed.
    Sid is the fastest accelerating boat I've ever been on and so far has never pressed float under; the harder the gust, the faster it goes and the foils feel as if you're leaning on concrete. But the floats, also short like Harry's are of proportionately greater volume because Harry's were really V's in cross section, whereas Sid's are more like a spread U - but the whole thing about foilers is to keep them sailing at near maximum speed ... and then the leeward float is airborne or near airborne anyway. But yes, in a lull, williwaw arrives, better hand that un-cleated mainsheet.
     

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  15. Samnz
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    Samnz Senior Member

    Just kissing. Fastest she goes upwind.
     

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