I have a battery powered trolling motor assisting a small sailboat. Its prop remains in the water at all times. Let's say with no friction (i.e., in air) it runs at 1500 rpm. In the water (now hard at work), it runs let's say at 750 rpm??...and moves the boat at 3 knots. Now the wind begins to blow gently at first. The combination of prop and wind moves the boat at a somewhat faster speed. Presumably the prop will also be spinning somewhat faster because it is being assisted by the wind. First of all, is there a formula for what that combined boat speed might be....roughly? Secondly, at some point the wind induced speed outruns the above combined speed (when the prop reaches 1500 rpm??) and we now have drag. I understand (or not) the arguments as to which ...a free spinning prop or a fixed prop.....produces the most drag. That is not my question. My question is whether a POWERED prop (as opposed to a free spinning prop) even though it is being outrun by the above mentioned combined speed of motor plus wind is producing less drag than were it stopped?? And finally, with a 12 sq. in. prop surface area and a 4" pitch, what IS that drag at various speeds from when the drag phenomenon begins on up to perhaps 12-15 knots (it is a catamaran) vs the drag with the prop stopped??
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