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Old 14-07-2013, 19:17   #46
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Location: still in a roll of fiberglass around Cape Town
Boat: Leopard 40 (new model)
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Re: Stopping the Prop.

Quote:
Originally Posted by HopCar View Post
I know that Guy is correct about airplane propellers creating more drag when free wheeling that when stopped.(snip) We would use the engine to get to altitude, shut the engine off and glide down. Our engines had centrifugal clutches that allowed the props to spin very easily. We fitted brakes to the shaft to stop the prop. Our gliders were equipped with very sensitive vertical speed meters. When we stopped the prop you could feel the glider accelerated and the rate of decent would go from about 700 feet per minute to less than 500 feet per minute.
I am afraid that it is too much of a generalization to say that airplane props will create more drag when freewheeling that when stopped, period. In the model airplanes (rubber powered!) I used to tinker with frewheeling beats locking by a lot. There is plenty of research that shows that for a given (fixed blade of course) propeller there will be a speed over which that statement is true, but below that speed the opposite will be the case.. That crossover speed will be a function of propeller pitch among other things. In your case I suspect that when coming down you were probably going faster than the down the crossover speed.

See the following links for data and theory:

Stationary and Windmilling Propeller Drag

which says "at lower pitches there is more drag on windmilling propellers and at higher pitches there is more drag on propellers held stationary. "


ASK DJ Aerotech Question

which says "There is one case with a free-wheeling prop where you would be better off to stop it. If the prop has very flat pitch, and the airplane's gliding speed is relatively fast in comparison to that pitch, it is possible that the natural unrestrained windmilling rpm of the prop is fast enough that the profile losses are greater than the flat plate drag of the stopped prop. It is probably not a good idea to just restrain it to a lower rpm through braking, since that introduces induced losses that will probably exceed the savings in profile losses. "

All this is about airplanes. For boats the evidence is easier. There is plenty of experimental research showing that a windmilling propeller beats a locked propeller, assuming fixed blades and setting aside the case of a two-blade prop that can be locked vertically behind the keel.. See some research from MIT, University of Strathclyde plus some interesting testsdone by Maine Sail (still live in the forum?)


Propeller Drag under sail, Article
http://www.catamaransite.com/files/propeller.pdf

Stationary and Windmilling Propeller Drag


I have done rough experiments in various boats; almost always got a small benefit from freewheeling and never got benefit (other than silence!) from locking. That makes me wonder why people insist in that locking the prop is faster...... Anyone has any data to make that point?

C
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Old 14-07-2013, 19:40   #47
Guy
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Location: So. Oregon, USA
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Re: Stopping the Prop.

Interesting. I have to admit that my experience is limited to props with a pitch of somewhere between 46" to 76". The aircraft with the 76" pitch prop was very sensitive to having the prop stationary. There is a big difference between my boats 13" and the planes 76".
Perhaps I will stand corrected again.
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