Rudder Size vs Lee Helm

Discussion in 'Sailboats' started by ancient kayaker, Nov 7, 2013.

  1. Baltic Bandit

    Baltic Bandit Previous Member

    So for a sailboat normally sailing with say 15deg of heel (not unusual) the heel is inducing a 9% shift in CLR, so you need about 3 deg of rudder to compensate for the hull

    But the other factor is that the moment a sailboat heels, its CE also moves off axis. So the same 15 deg of heel has moved the CE of the sail 26% of the Mast height laterally (more complicated than that I know because its a triangular sail so its really half of that 13%) So you how have an force couple that gets mightly complicated mighty fast because you have also changed the immersed planform as well as shifted it to leeward.

    But I'd say 10% of mast height is probably not wrong. So on a 35' mast, that's a 3' force couple change. Seems to me that too would add to weather helm

    And to some extent that's why very high performance dinghies sail with 5 degrees of weather heel. it neutralizes the rudder and thus reduces drag and increases speed. Whereas on a keel boat it reduces the lift generated by the rudder, which is more important than the drag
     
  2. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    As I said the moment comes from the L_D vectors of the hull as a lifting body as well as the rigs relative vector contribution.
    The figure given of around 6% per 10 degrees already takes the rigs vector into account (the leeward shift of the driving force of the sails).


    Racing craft with low drag bucket rudder foil shapes are more interested in fine tuning helm balance, whether the rudder is at 7 degrees instead of keeping it under 5 makes a big difference in drag ( for the racer that is). In a seaway with often turbulent flow conditions that's a mute point.
     

  3. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    I'd like to add my thanks too. I think the issue has been thoroughly discussed and my questions answered, together with a number of interesting issues I hadn't thought of. I intend to unsubscribe, but you guys are welcome to continue any other topics that grew from this one. If you need my inputs please PM me . . .

    Terry Haines
     
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