Rudders that break

Discussion in 'Sailboats' started by rader, Apr 7, 2013.

  1. rader
    Joined: Dec 2012
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    Location: Chicago

    rader Junior Member

    The recent fatality during an offshore (California) race owing to loss of rudder raises the question of loads on the rudder. My boat has a NACA 0009 foil and I want to calculate loads in relation to angle of attack but data as to Lift Coefficient (CL) and Drag Coefficient (CD) are not easy to find. Some generic plots for NACA 00xx foils seem to show a nearly linear increase in CL up to the stall angle. A plot for CD appears more exponential (rather than linear) even though the formula calculating CD is similar to that for CL and does not look like an exponential type of equation. It also seems that the CL does not fall off to zero after stall, but dips a bit and then increases to a level beyond that of the initial peak -- a sort of 'dromedary' up to an AOA of about 40-50 deg before decreasing. CD seems to surpass the CL at about the stall angle and negate whatever further lift there is beyond that angle. I am wondering why CD appears exponential from AOA of zero up to stall angle?

  2. JRD
    Joined: May 2010
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    Location: New Zealand

    JRD Senior Member

    Principles of Yacht Design by Larsson and Eliasson has a worked example of this exact calculation and senario.
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