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post #1 of 6 Old 04-11-2007 Thread Starter
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Question on fixed prop drag / hull speed

I am new to inboards, and now have a fixed 3 blade prop, and have read that they can cause a loss of speed, of a 1/2 knot, for example. My question: Is this only a concern when winds are light, and that drag will slow you down. I am thinking that, if there is lots of wind, the power from that wind would overpower the drag, and allow you to reach hull speed regardless. Am I wrong on this?
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post #2 of 6 Old 04-11-2007
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Generally, it will only affect you in lighter winds. In heavier winds, the force of the wind overcomes the drag generated by the prop. I haven't heard of a boat that couldn't reach hull speed because of the drag caused by a prop.

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post #3 of 6 Old 04-11-2007 Thread Starter
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We usually have good wind, so you have saved me several hundred bucks, as I may have hastily bought a folding or feathering prop, with the promise of less drag. I am sure they are nice, but not on my required list now.
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just watch out for the prop walk... because of it, the boat will tend to turn tighter in one direction than the other in forward. Also the boat will tend to turn to one direction in reverse.

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post #5 of 6 Old 04-12-2007
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Quote:
Originally Posted by sailingdog
Generally, it will only affect you in lighter winds. In heavier winds, the force of the wind overcomes the drag generated by the prop. I haven't heard of a boat that couldn't reach hull speed because of the drag caused by a prop.
I think this is a generalization that in reality, is dependant on a lot of factors including wind speed, sea state, hull design & weight, prop size etc. It seems a little ludicrous to say that you will achieve the same speeds sailing with a fixed-blade prop as without. Will enough wind overcome the drag inherent in a fixed-blade prop? Maybe. But unless you sail in that wind 100% of the time (and who's to say how much wind that is?) you will be significantly slower under sail than an indentical boat with a folding or feathering prop.
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I think it is more dependent on the hull design...than the other factors, aside from wind speed. A full-keeled design is obviously going to be affected far less than a fin-keeled racing boat design. In most full keeled designs, the prop is in an aperture between the keel and rudder. This reduces the drag that the prop is going to create quite a bit. In a fin-keeled boat, it is much more of a problem as the prop is usually further from the fin, and will create much more drag due to its far more exposed position.

However, I don't see any point in changing what was obviously a workable prop until he has used it and decided whether he can live with the limitations it imposes.

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