Another Trim Tab Question

Discussion in 'Sailboats' started by phum, Aug 26, 2010.

  1. phum
    Joined: Apr 2009
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    Location: Sunny QLD. in the great South Land

    phum Junior Member

    Hello all,
    Conventional Trim Tab design has the tab at the trailing edge of the keel.
    What are the pros and cons of reverseing the thinking and having the leading edge moveable?
  2. phum
    Joined: Apr 2009
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    Location: Sunny QLD. in the great South Land

    phum Junior Member

    Come on, this is a genuine enquiry, fire away.
    Tom Speer where are you?
  3. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    On a practical point of view its probably better to make the thicker part solid and the thinner part movable rtaher than VV.
    But think most of all about the angle of incidence the hull will have to the water and the consequent effect on jib setting.

  4. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    It depends on what your objectives are. Deflecting the trailing edge has a significant effect on the leeway angle of the boat. That has a in impact on the hull drag, profile drag of the keel, and the sail trim.

    Deflecting the leading edge may reduce the profile drag of the keel, but won't have a significant effect on leeway angle. It would be of value if you had a thin section that was subject to leading edge stall.

    One way to think about it is, in a very approximate sense, the trailing edge is going to go through the water at about the same angle in either case (especially if the tab is a large proportion of the chord). So the question becomes, do you want the hull to rotate with the forward part of the keel, or not?

    (If it seems strange to think of rotating the hull when you deflect a trailing edge tab, it's because the tab rotates the keel's zero lift line to weather - it will take negative leeway to get to zero lift, compared to zero leeway to get to zero lift with a symmetrical keel. The lift on the keel will be the same with the tab deflected, because the lift is set by what is required to oppose the side force from the sail trim. The angle of attack at which that lift is obtained will be basically the same when measured from the zero lift line. So in the steady state, the whole boat ends up being pointed more to leeward by the amount the tab moves the zero lift line to windward.)
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