Trimaran Turning Radius: Ama Position

Discussion in 'Multihulls' started by Doug Lord, Jun 27, 2012.

  1. Doug Lord
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    Doug Lord Flight Ready

    What does it mean for manouverabilty(tacking specifically) if the ama falls inside or outside the approximate turning radius of the boat?
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    Rephrased question: What does it mean for manoverability(tacking specifically) if the ama falls inside or outside of a circle whose radius is equal to the distance between the cl of the rudder and cl of the daggerboard? What affect on turning does an increase or decrease of this diatance have on manouverablity?





    be sure to click on the plan view:

    Pictures(Kurt Hughes designs),L to R- 1 & 2-40' high performance, 3 & 4 40' cruising-
     

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    Last edited: Jun 28, 2012
  2. latestarter
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    latestarter Senior Member

    Based on geometry, if the ama on the side to which the trimaran is turning is inside the circle it will move forwards obviously slower than the main hull, if it is outside the circle it will move backwards and if on the circle it will not be moving through the water just rotating.

    Having never set foot on a trimaran I will leave it to others to explain the implications.
     
  3. Silver Raven
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    Silver Raven Senior Member

    Gooday Doug - Corley, Gary or Kurt may know - I sure don't. However I do see that pics #2 & #4 are not treating the 'rotation the same - they aren't taken at the same point of the main hull - so the relativity is not equal - so what's the point of the discussion if the aren't taken equally ??? I'm missing something - as usual - like a heavier wing is better than a light one - I don't get that either. ??? Ciao, james
     
  4. Doug Lord
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    Doug Lord Flight Ready

    turning radius

    =====
    Thats the point , James: 2 & 4 show the radius taken with the daggerboard trunk the center of the circle formed by the radius which is equal to the distance between the daggerboard and the rudder. It's quite different on both boats.
     
  5. Corley
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    Corley epoxy coated

    As you tack the leeward float has more drag initially but it matters less there as your still powered up. The displacement of the boat shifts to the main hull and away from the leeward float as you tack and the combination of the rocker on the main hull and the pivot point of the daggerboard helps the boat around as soon as the windward float touches the water (dihedral elevates it out of the water when sailing) it helps rotate the boat around onto the new tack (it's becomes a new pivot pont if you like) so there are a few processes at work that help a trimaran tack faster than an equivalent catamaran.

    Also keep in mind that in the case of Kurt's F40 trimaran the boat has a rearwards trim (it sits back towards the transom if you like) which unloads the bows when tacking.
     
  6. warwick
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    warwick Senior Member

    Is as if you have a moving pivot point, from one float to the other with the center board assisting?
     
  7. sean9c
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    sean9c Senior Member

    Probably wrong but aren't you pretty much pivoting the boat around the leeward ama, not the board?
     
  8. basil
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    basil Senior Member

    Or is that the other way around - pivoting the boat around the windward float?
     
  9. latestarter
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    latestarter Senior Member

    I think sketches 2 and 4 are using different conventions.
    Sketch 2 is the path the centre of the boat traces.
    Sketch 4 is the path traced by the bowsprit as the boat swings around.
     

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  10. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Wrong ,as I mentioned earlier: both use the exact same "convention": the radius is the distance from the rudder to the daggerboard on both boats! The
    circle described by the radius is very, very different on boat boats but, then again, so is the daggerboard position on the main hull.
    See post #4.....
     
  11. fng
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    fng Junior Member

    Doug if you know how foils work, or in this case a centerboard and rudder you will know that they wont turn in the circle drawn, then you should be able to work out the drag of the immersed arma, or armas, and work out the true turning circle.
    If you ever get the chance go for a flight in a glider and see how there motion and movement is through the air, Also ask the pilot if they can loop it for you, then think about that turning radius
     
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  12. Doug Lord
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    Doug Lord Flight Ready

    ====================================
    I used the(daggerboard/rudder) radius as an approximation of the turning radius to illustrate the large difference in daggerboard position between the two boats. And to bring up the different response of the amas of the two boats in a turn. The one thing I know for sure is that when a tri has the board as far back as #2 it is in much greater danger of stalling in a turn than the boat in #4.
     
  13. fng
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    fng Junior Member

    But what about the fact that pic 4 has the centerboard off set from the centerline
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Not significant. I think the fore and aft position of the board is far more important for manouverability.
     

  15. fng
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    fng Junior Member

    What are you really asking ? most people are primarily concerned with the vessel sailing balanced - hence the board position, turning is a compromise with most vessels, especially a multihull.
    Hey what about the hulls rocker....
     
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