Balanced rudders

Discussion in 'Multihulls' started by Cat Nap, Feb 5, 2015.

  1. Cat Nap
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    Cat Nap Junior Member

    I have a 37ft Pajot "lucaine"cat with unbalanced transum hung rudders. when the boat gets to 8 to 10 knots the steering gets heavy (sail or power). I would like to "balance" the rudders. Is there a "rule of thumb" reguarding the area that should be added to the front of the rudder in relationship to the main rudder?
     
  2. hump101
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    hump101 Senior Member

    For high aspect ratio foils of typical section, the centre of lift will be aft of a point 10% of chord forward of the fattest point, for most angles of attack, so this is a reasonably safe maximum aft position for the pivot. For low aspect ratio foils the centre of lift will move around significantly, so is much less predictable.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    H, I haven't seen it put that way, but I have read that lead should be 15-19% of the chord length aft of the leading edge of the rudder which would probably work out right where you suggested. I personally have used 17% on several multihull designs.
     
  4. Richard Woods
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    Richard Woods Woods Designs

    Not sure which design you really have???

    Up to about 20% balance is usually OK. The CofE moves at speed so it can overbalance suddenly if you go fast, not usually a problem under 10 knots though. So if you are close to 20% then actual balance depends on many variables, including AR and chord shape.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  5. Cat Nap
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    Cat Nap Junior Member

    Gentlemen, evidently my question was not worded correctly. This is an old boat with existing stern hung rudders. I don't want to build new rudders. What I would like to do is add to the existing rudders to create a balanced effect. So the question is, about how much should I add to the leading edge of the existing rudder ? Percent of existing or ?
     
  6. Cat Nap
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    Cat Nap Junior Member

    Mr Woods. Thanks for the reply. I'll try 15% for a starter.
     
  7. Richard Woods
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    Richard Woods Woods Designs

  8. PAR
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    PAR Yacht Designer/Builder

    I think I know what these blades look like, but can you post a picture? If bent on some balance, simply add 20% to the leading edge. Some foam and duct tape will do and go for a sail. Whittle down the foam, until you can live with the results, then make an addition to the blade or a new blade in something more durable than tape and foam.
     
  9. WindRaf
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    WindRaf Senior Member

    do not forget that the balance of the rudder increases efforts on the axis; I think 10% is right for your boat; 20% is used on windvane sailboats
     
  10. PAR
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    PAR Yacht Designer/Builder

    It's probable 15% or very near it will be the number. It depends on several variables as to how effective this will be.
     

  11. Cat Nap
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    Cat Nap Junior Member

    Mr Woods yes that is the boat. Raking the rudders would be possible, except that re bending the 2" dia alum tiller arms would be beyond my capabilities. They are a welded part of the rudder head. If not re bent the tiller connecting rod would be considerably elevated...Thank you.
     
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