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Kayak and Canoe Design Bulletin Board
The Rest of the Rudder Story
Posted By: Paul Jacob
Date: Tuesday, 4 May 2004, at 12:26 p.m.
My original question was really directed more towards surfskis than kayaks. The challenge with open ocean surfskis is that often the rudder comes out of the water when the surfski is surfing a wave face. I'm trying to find ways to deepen the rudder to maintain penetration into the wave while surfing while at the same time keeping drag to a minimum and also preventing eel grass and kelp from getting snagged on the rudder blade.
I have two basic options that I can see. First is to raid my surfboard fin drawer for a deep, high aspect ratio, nose riding surfboard fin and mount it to a rudder shaft and see how it goes. I have a perfect candidate except that would entail cannibalizing my best $50 surfboard fin. My second idea is to shape a custom rudder based on the basic 4 digit NACA airfoils developed in the 1930's and hope they work as well in water as they do in air. Shaping a NACA foil sounds more fun but getting the shape accurate is something I haven't figured out how to do. I'm no sculptor and I don't know how to translate a tracing of a foil shape into a swept back, variable width rudder. If I could figure out how to make a duplicate casting of the surfboard fin, that might be the best way to go, but really I'm still fishing around for ideas.
The other question I'm looking for the answer to is at what angle does a rudder stall and add far more drag than turning force. I believe a foil shaped rudder will be better at this than a flat plate but I don't have any hard data. If I knew, I could arrange the rudder pedals to hit a stop at the point of maximum turning force and prevent stalling. Now, when the rudder comes out of the water, I put the pedal down all the way in frustration, only to have the boat bog down when the rudder comes back down at full deflection.
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