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post #37 of Old 07-18-2013
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Re: Backing up (or not)

Originally Posted by bobperry View Post
Hmmmmm,,,,,,I'll buy that. I need to ponder it a bit but it sounds good to me.
I think you may have part of the answer but I still think that the center of pressure on the keel plays a big roll. If it were static you would still have the same turning arm in reverse. I think having the control surface on the leading edge also may be the reason the C of P moves aft.
Another way to look at this is to think about what happens to the flow over the keel in the forward and reverse directions. Let's start with the boat moving forward; the flow is (or at least should be) laminar all along the keel and its attached rudder. Now, turn the rudder a bit to Port and the flow should stay pretty much laminar as long as it isn't cranked too far. There may be a few vortices being shed off of the trailing edge of the rudder, but these will only increase drag without having much of an impact on lift. So, turn the rudder to Port, lift increases on the Starboard side of the keel, the stern swings to Starboard and the bow turns to Port.

What happens when the flow is reversed (i.e., the boat is backing)? With the rudder centered the flow is again laminar. The maximum chord (the thickest part of the keel) is probably in the wrong place, but let's not worry about that for now. OK, turn the rudder to Port again and what happens to the flow? Now the edge of the rudder is the first thing the flow "sees" as it impacts the keel. Since this edge is relatively sharp (compared to the forward edge of the keel) the flow separates right at the leading edge (or what is the leading edge while backing). This causes vortices at slow speed and turbulence as the speed increases, in this case all along the Port side of the keel. This pretty much ruins any lift the keel might otherwise generate (i.e., the keel is "stalled"), and directional control is more or less nonexistent.

Never forget them. Do something to prevent it from happening again.
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Last edited by SlowButSteady; 07-18-2013 at 01:51 AM.
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