Rudder area question

Discussion in 'Boat Design' started by Scuff, May 25, 2022.

  1. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Does reducing the chord of a rudder but increasing depth to keep the original area improve it? Assuming the same profile like a NACA 0012.
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Or in other words, increasing the aspect ratio of the rudder.
    All else being equal, a rudder with a higher aspect ratio (of Span / chord) should be 'better' (as in more efficient) than one with a lower aspect ratio, within reason (you don't want it to be excessive).
    One reason why you do not see aeroplanes with short fat wings instead of long slim wings.
    But short fat rudders are probably 'better' if you have a draft limitation to contend with?
     
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  3. jehardiman
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    jehardiman Senior Member

    Excessive being the operative word here. An AR>~5 starts to have stall issues. While high AR rudders work well at speed, for low speed maneuvering you want a lower aspect ratio.
     
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  4. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I was increasing the depth by 6" so not a big increase. That would make the draft 36" so not too deep I don't think. I didn't reduce the frames when I built the hulls so I'm thinking the draft will reduce by an inch or more.
     
  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    @Scuff can you remind us please of the boat you are building, and fill in the 'gaps' re your post above?
    So your rudder depth has been increased by 6" - what is the total depth (span) of it now?
    And what is the chord width?
    Is this a catamaran or trimaran that you are building?
     

  6. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    Sorry a Horstman 27 trimaran. The dimensions are 18" at the root and 13.5" at the tip. 32" length. I posted the drawing. kickup_1.jpg
     
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