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Old 07-04-2013
bobperry bobperry is offline
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Re: Spade rudders vs blue water passage making

Forces on the rudder when it's skeg or keel hung are twisting forces.
Forces on a spade rudder are both twisting and bending so the spade rudder will require a much stronger stock. Of course the greater the span of the spade rudder the greater the bending moment. Increase in chord length increases the twisting moment.

I favor carbon fiber rudders for my new designs. I have never had a failure with one. The build process of the carbon fiber rudder results in a monocoque structure with the blade and the stock being unified into essentially one piece. Given the strength of CF it is easy to apply a very conservative safety factor without getting a huge rudder stock. The modest size of the CF rudder stock, even when conservatively designed, means the designer has more freedom in choosing the foils used. The CF stock also has the advantage of being easily tapered to fit within the chosen foil with max stock size at the hull where the bending moment is greatest. With older, s.s. steel stocks the required stock diameter often drove the foil selection.
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