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post #11 of Old 08-20-2013
SlowButSteady
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Re: Why the Bristol 32 backs so poorly

If the rudder is attached to the keel (as on a B32) you'll ALWAYS have little or no control while backing. The control surface (rudder) of the foil (keel) is on the wrong (leading) edge to have any real effect. A control surface changes the lift generated by a foil. When going forward, as you turn the rudder to port the starboard side of the keel develops more lift. This causes the aft end of the boat to swing to starboard, and the forward end to swing to port. However, when backing, at any but the very smallest rudder angle, the edge of the rudder will "trip" the boundary layer and cause turbulent flow all along one side of the keel. The more the rudder angle, the more turbulent the flow. Hence, the unpredictable results you experience.

Spade rudders are foils unto themselves. As they turn the "angle of attack" of the foil changes and differential lift is produced at all but the highest rudder angles. A rudder on a very narrow skeg will also work, but not as well as a completely unattached (spade) rudder.

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