Paul,
If your motor is rated at xx amps continuous you want a prop that causes the motor to draw xx amps with the throttle wide open. All you need is an ammeter to measure current to the motor. You can increase current with a larger diameter or larger pitch prop or change the gear ratio and speed up the existing prop. Any of these three things increase the load on the motor, causing it to draw more current (power).
I like a toothed belt reduction drive because you can easily change ratios by changing one of the pulleys and maybe the belt.
Propping a sailboat is a challenge because you want the smallest prop possible for sailing performance and the largest prop possible for motoring efficiency.
The propeller is by far the least efficient link in the power system. There is a measurable difference in efficiency between a 11" and 12" dia prop on my boat.
If you are going to do experiments it is critical to use a gps and be sure there is no wind or current affecting your speed readings. Do them both ways, too, just to be sure.
Denny
www.wolfEboats.
----- Original Message -----
From: Paul C. Smith
To: electricboats@
Sent: Tuesday, June 02, 2009 1:10 AM
Subject: Re: [Electric Boats] Re: Electric power for over 30' [Question for Dave K]
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
[Non-text portions of this message have been removed]
[Non-text portions of this message have been removed]
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