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post #34 of Old 07-17-2013
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Re: Backing up (or not)

I've always looked at it like this:

When moving forward a keel with an attached rudder acts like an airplane wing with an aileron. The net effect on the lift of the wing is in the opposite direction of the movement of the aileron; the aileron goes up the wing goes down, and visa-versa. Now, turn the plane 90˚ so that the wing is pointing straight down and you essentially have a keel with an attached rudder. The rudder goes to the right the lift is increased to the left, and visa-versa.

BUT, reverse the flow and what do you have? The "aileron" is now acting more like a leading edge flap (and there's a good reason they put the ailerons on the trailing edge of the foil). It might reduce the stall speed a bit (that's why planes have leading edge flaps), and increase the drag of the foil, but it won't do much for the lift of the foil (wing or keel; OK, it may increase the lift a bit, but not as much as an aileron would). So, when backing a boat with a rudder attached to the keel the "control surface" is basically on the wrong edge of the foil.

A spade rudder works because moving it from side to side has the same effect as changing the "angle of attack" of an airplane wing. Increase the angle of attack and lift is increased, at least until the stall angle is reached. With a spade rudder such lift swings the stern right of left, and Voila! The distance from the center of pressure may also come into play, but the important dynamic is the effect of the control surface on lift (in my humble biomechanical opinion).

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