Hi , my interest was aroused in the bollard pull discussion because one important variable is, as you guys have pointed out, prop size and pitch. There are pretty clear guidelines for sizing props to engine HP , RPMs and trans. reduction. And pretty well known empirical means, i.e. black smoke and/or inability to reach rated RPM range on a diesel usually means to much prop. However, getting the best out of an electric motor is not so clear, at least to me. Any thoughts on sizing a prop to epower? I imagine , the more prop you have the more amps you'll consume for a given RPM, but where't the sweet spot? On the 30' Lion, I've currently got a 2 blade 16X13 that I put on for a 21HP universal w/ 2:1 that I was going to put in. Have yet to get a GPS aboard to get the speed, maybe 4 knots at about 60 amps. Best Regards,
Paul <paul136mc@yahoo.
Paul C. Smith 707 874 2112
PO Box 120
Camp meeker, CA 95419
____________
From: Paul Eaton <preaton@attglobal.
To: electricboats@
Sent: Monday, June 1, 2009 7:15:16 PM
Subject: [Electric Boats] Re: Electric power for over 30' [Question for Dave K]
What you guys are misunderstanding is that you are talking about different things.
Sure a "50HP" diesel will produce more power than a "8HP" electric motor. But bollard pull is measuring HP as applied to the water by the propellor. If the 50HP diesel needs to rotate the prop at 2000 - 3000 rpm to get it max power at that speed in a standing start the prop is just cavitating and in reality is providing very little effective thrust.
The slower and larger the prop (in general) the more the shaft HP is going to be turned into prop thrust. On top of that if the prop is in a nozzle efficient goes up another notch.
So Dave's experience is very likely to be correct. His friend's 50HP diesel was just making bubbles (and noise)
Cheers
Paul
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