Rudder Angle And Effective Size Of Rudder...

Discussion in 'Sailboats' started by saildog, Mar 5, 2006.

  1. saildog
    Joined: Dec 2005
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    saildog Junior Member

    Hoping someone can assist me with some sailboat design issues with a shoal keel (heavy) type sailboat. Thanks in advance.

    a. Is there a minimum and maximum effective rudder angle from centerline?Anotherwords, is there any reason for the rudder to swing more than 45 degrees, port/starboard?
    b. As the weight of the boat goes up, does rudder size also increase? If so, is there a formula for rudder size to boat size/weight?
    Thanks again...
    Last edited: Mar 5, 2006
  2. Skippy
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    Skippy Senior Member

    Any foil will stall if its angle of attack is too high. I don't think the stall angle is ever much more than 16 degrees?

    From The Rules:
    "A rudder area of between 8 and 10 per cent of the total lateral plane or underwater profile is the desirable size for a sailboat." F S Kinney
  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Since the lateral plane is smaller now then then, the rudder may be more than 10 % of the total.

    When a boat with a short keel turns, it rotates, spins, around itself, then you may need a rudder angle close to 90 degrees.
  4. Skippy
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    Skippy Senior Member

    Also, if you're motoring slowly in a crowded area and need to take a very tight turn, 45 degrees or so may be necessary.
  5. DanishBagger
    Joined: Feb 2006
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    DanishBagger Never Again

    Not to mention, a 90 degrees swing of the rudder is the closest you get to a brake.

  6. bhnautika
    Joined: Feb 2006
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    bhnautika Senior Member

    Saildog the rudder angle doesn’t need to be over 35 degrees, as any more doesn’t add a lot to the turning moment. As stated in earlier posts, rudder area is usually calculated as a percentage of either lateral plane area or sail area. These percentages change with the type of boat (planing thru to displacement), for your boat as I imagine it, about 9 percent.
    For more “The nature of boats “ by Dave Gerr is a good read.
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