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Old Mar 12, 2007, 09:19 PM
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kallend's Avatar
United States, IL, Chicago
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Rudder shape

On my (semi) scratch built 32" boat I chose an elliptical shape for the rudder, not unlike a Spitfire wing planform. Looking at pics of lots of other boats, I see a variety of chapes but nothing elliptical.

What factors affect the optimum shape for a rudder?
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Old Mar 12, 2007, 09:54 PM
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my guess is that no body knows...the reason? there are so many different shapes and designs.

when people do know what works best, those shapes seem very similar to each other.

with that, i recamend shaping it to what ever YOU think is right, test it, and go from there.

you may even discover the best shape that sets the standard for all.
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Old Mar 12, 2007, 10:38 PM
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and if it works... don't fix it !

Keven.
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Old Mar 12, 2007, 11:45 PM
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I think there are two reasonably good resons for deviating from the theoretically 'ideal' eliptical planform.

First, with many hull designs, there is a tendency for the stern to lift out as the angle of heel increses. A tapering planform increaes the ruddder loading per unit area with all the undesirable effects that entails.

Second (and mainly full-size) forward sweepback (in terms of area) makes it easier to achieve an acceptable compromise between easy manufacture of a metal stock and acceptable rudder (not helm) balance of a balanced spade rudder. This should be a dinosaur problem by now and is in any case only relevant if there is tactile feedback from the foil to the control device.

A.
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Old Mar 12, 2007, 11:57 PM
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another thing might be that is is easier to cut out a square than an ellipse. also, while that has more wetted surface=drag, it also has more surface area, more turning power at lower speeds... i have always liked the elliptical rudder, but in some cases, wouldn't a swept back, rudder be better? [i am thinking of high speed aircraft here, and the fact that the elliptical wings of WWII caused so much drag at high speeds that they fell off?]
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Old Mar 13, 2007, 12:37 AM
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Completely beside the point. I don't doubt that elliptical wings did have problems, but the shape didn't cause any of them to fall off. Has more to do with exceeding the design speed of the wing than it's shape.
One advantage of a 'swept-back' rudder is that if the wind fails while the boat is in the middle of the pond, you can wiggle the rudder and 'scull' the boat back to shore. I'm sure they have other advantages/disadvantages, but that's the one that I'm most familiar with.
- 'Doc
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Old Mar 14, 2007, 11:50 AM
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Plymouth, Minnesota
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Rudder Shape

Last night I ask my friend who owns Computer Keels at http://www.computerkeels.com/, a company that for years has designed rudders and keels for racing sailboats about rudder shape. He said that the shape of the rudder is determined by the structural needs and that the elliptical shape distributes the load better. He also said that other than turning the boat there is no other purpose for a rudder and that you should not concern yourself with shape.
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Old Mar 15, 2007, 02:52 AM
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It is purely a piece of self-justification, but ..

I agree that with a transom hung rudder, an eliptical planform distributes the load well. However, with a balanced spade mounted on a stock this is often not he case - or maybe my geometry just got screwed up or my engineering assumptions are too stiff. I find that there is odten difficulty in getting enough 'meat' in the rudder to allow a sufficiently large diameter stock at the lower bearing without recourse to exotic materials or, alternatively, to avoid an undesirably high level of rudder balance..
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