Trimaran with accomodation in the amas

Discussion in 'Multihulls' started by eiasu, Nov 23, 2012.

  1. cavalier mk2
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    cavalier mk2 Senior Member

    "A tightrope walker is more stable when carrying a pole PROVIDING he doesn't rock it. But once in motion it will be very hard to stop."

    This is more applicable to a powering craft as the sails are an effective damper on the pivot point.
     
  2. mcarling
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    mcarling Junior Member

    Is my understanding correct that 100% buoyancy, in the context of a trimaran, is defined as buoyancy equal to the weight of the whole boat (vaka, akas, and amas)?
     
  3. Richard Woods
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    Richard Woods Woods Designs

    Correct

    Richard Woods
     
  4. catsketcher
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    catsketcher Senior Member

    Carling - I have a lovely library that was in some part donated to me by a nice guy in Peakhurst NSW - John. In those mags there are few good articles about ama design.

    In one Derek Kelsall talks about his sterns. Kelsall's boats tended to have floats with real transoms. This enabled the float to have more volume in the ends and gave more resistance to pitching in flat water ( I'll come back to that) and higher prismatic so higher potential top speed.

    Crowther had an idea used on the Twiggy that you did not want extra bouyancy at the stern of a float. His reasoning was that if you had a float with the same bouyancy distribution as the main hull (less volume but same proportions) then the the bows could end up going under. The reasoning was that when you are pushing a float down you are pushing hard. Therefore there is a sizable downward force on the bows from the torque of the Ce to CR couple. So the bows will go down and you will lift the rudder out so you will lose control and lose the bows. So with many designs the stern was reduced so the ama stern could not "overpower" the bow. The reduction in float volume aft was meant to compensate for the torque from the rig / hull resistance couple.

    John Shuttleworth did some very nice articles (read them on his website) and worked out that volume in the sterns was bad in a seaway to windward. As a tri comes off a wave the last part of the boat to leave is the leeward ama float. Therefore the extra volume that Kelsall thought would reduce pitching would actually increase it - generating a bow down couple. Shuttleworth's tris (Livery Dole in particular) used this approach and he said it made a big difference.

    When Apricot and Paragon came in the moved tris into a different era and I don't think that they are useful for learning much about cruising tris for the layman. I have sailed an ORMA 60 only 400 miles but it was stressful, hard riding and incredible!

    One thing you should keep in mind as well is the downforce from the rig when heeled. When a tri heels the rig will develop a downforce that you can work out using trig. Shuttleworth talks about this in one article. I spent 20 years believing it till I read a book on the symmetry of sailing. Of course the downforce is reacted against by the upforce form the tilted centreboard -if you do not cant to board. However Shuttlworth is still correct in some situations like when the centreboard ventilates when falling off a wave and produces little lift. So you need the extra bouyancy then anyway - darn waves.

    In Australia after a couple of tri capsizes people started using floats of 160 to 200%. Even then the decks would get awash. One reason is that designers are notoriously optimistic when designing boats. They often come out way heavier than designed. Also in the nastiest of conditions a multi is subject to large vertical accelerations. These increase or decrease the weight of the boat dramatically. Add waves as well and any tri with 160% floats will get very wet decks (even awash) when pushed offshore in waves.

    cheers

    Phil
     
  5. warwick
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    warwick Senior Member

    Thanks Phil for your post about float volume distribution.
     
  6. Richard Woods
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    Richard Woods Woods Designs

    Some years ago I spent a very interesting afternoon walking round the docks before an OSTAR with Nigel Irens. We were looking at his trimaran designs and how his outrigger shapes, in particular, have changed over the years.

    So a useful exercise would be to study his designs from the 1980 Gordono Goose onwards and see how the shapes have changed. That's just one designer of course, and his boats tend to be for offshore racing, so are designed for speed and offshore safety. But the trends are obvious.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  7. cavalier mk2
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    cavalier mk2 Senior Member

    The Nicol ama stern variation was in response to the semi planing/ planing main hull. The stern was kept fine for waves but the keel profile run was flatter aft and lengthened to help keep the main hull stern from squatting too much at speed. The MK2 Vagabond was indeed faster than the MK1. These amas were also added to the Wanderer and used on the Cavalier for their extra buoyancy as well as the ride. Lots of variables in amas....
     
  8. rberrey
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    rberrey Senior Member

    A bit more on the subject from Ed Horstmans book. (What affects do you getfrom carrying the useful load in the amas?As far as the dynamics of the trimaranare concernedyou will increase the mass movement of inertia about the longitudinal axis but this is nothing you should worry about.) He uses light aircraft with wing fuel tanks as an example. ( The aircraft with wingtip tanks has a proportional higher mass moment of inertia than the trimaran , yet it must react against air when countering any rolling tendencies, while a trimaran ama counters any rolling tendencies by displacing water, which is over 800 times as dense as air. As the aircraft has no real difficulty because of this placement of weight at its wingtips , a trimaran should have even less from carrying weight in the amas when the amas are well designed and connected to the main hull. There is however one condition where you may want to move some weight aft to effect the trim. This is when running before a sea. If already excessively loaded , or just loaded for cruising, you may want to move some weight aft thereby adding more buoyancy to the bows so they will more easily carry over the waves if surfing faster than waves.) EDs view. Rick
     
  9. mcarling
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    mcarling Junior Member

    I would like to thank everyone for the abundance of good information. Most of the Shuttleworth articles I could find are from 25-30 years ago, so probably do not represent the latest theories or empirical results. From what I've read, current practice seems to be about 200-220% buoyancy in each ama. Is that right?

    When foils are attached to the amas (e.g. Hydroptere), are they hinged? What keeps the foils in the water when needed and out of the water when needed?

    It seems to me that the upforce from the centreboard would be exactly equal to the downforce from heeled rig IF both the centreboard were the only thing resisting slippage to leeward and the centreboard were perfectly effective in stopping all slippage to leeward. Of course, neither condition is ever satisfied, so I believe the upforce from the centreboard must be less than the downforce from the rig. Am I missing something?
     
  10. eiasu
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    eiasu Junior Member

    It's really great to receive so many information,
    thank you all.
    Here is the ideal Chris White mix: amas from Atlantic 77,
    central hull from Hammerhead 54, scaled to 78
    and rig from the innovative beautiful A47 MastFoil,
    do you thing is somehow possible something like that?
    Combining the room of Ed Hostman tristar80 with the
    superior performance and efficency of Chris White deisgns.
     

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  11. mcarling
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    mcarling Junior Member

    eiasu, how much beam do have in mind? How far would the amas move up and down during change of tack?
     
  12. eiasu
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    eiasu Junior Member

    the tristar80 has 14 meters beam,
    the kurt hughes tri 71 has 17 meters beam,
    i have in mind something between this two values.
    The question about the amas moving up and down is all about confort and performance, what makes more sense. From what i see from the pics the horstman tristar has constantly the 3 hulls in the water, so no moving up and down, I don't know if it possible to have only two wet hulls and still have confort in the living space of the ama during the tack.
     
  13. mcarling
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    mcarling Junior Member

    With a beam of 14-17 meters, I would not be worried so much about comfort as about necks and skulls fracturing as people are thrown violently against the overhead during snap rolls.
     
  14. eiasu
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    eiasu Junior Member

    do you mean is too little beam?
     

  15. warwick
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    warwick Senior Member

    Mcarling, may be referring to the float water line above the water line. Both floats immersed may be more stable.
     
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