Michael:
A large diameter prop (with pitch about = to dia) turning relatively slowly is the most efficient for displacement ( 5 - 7 kts for 20 - 30' boats) speeds. The real world constraint is that a large dia prop creates a large amount of drag when sailing, many boats have a prop aperture which limits size and a large prop on a power boat increases draft and may make trailering a bit harder. Even if you are building from scratch these other constraints will dictate prop dia.
A skilled boat designer will spec a prop dia, pitch, blade configuration and RPM and power for the targeted cruising speed. Doesn't mater if is ICE, electric, steam, whatever.
Now you have a power and RPM spec for the motor / transmission unit. Lets say you want 5 hp and 800 shaft RPM. A motor that can produce 5hp at only 800 RPM will be relatively large and will require relatively heavy gage wiring, contactors, switches, controller, etc. A smaller motor running at half the amps and twice the volts and twice the RPM will produce the same power with smaller components. Or with the same heavy duty components will have less electrical losses due to resistance heating dropping by a factor of 4. Now you need a transmission though. A cogged belt and a pair of pulleys giving a 2:1 reduction are inexpensive, quiet and reliable. No extra bearings are required as you need a thrust bearing anyway for the prop thrust and it can take the radial belt load just fine.
Pick a design and use the designers power and rpm spec to design two power systems, a direct drive one where the power and RPM are constraints and another where just the power is a constraint. You will find the second system will be lighter and probably a good bit cheaper. It will be more electrically efficient due to the reduced amps. I wouldn't be surprised if the slight losses in the belt drive were more than offset by the electrical efficiency gains.
Denny Wolfe
www.wolfEboats.com
On 9/6/2011 10:40 PM, Michael Mccomb wrote:
i am building up from scratch so i can provide whatever prop would work best.... don't bldc and pmac motors both develop full torque at any rpm.... if so and neglecting cooling for the moment, wouldn't even a 1% friction based loss in efficiency be unnecessary when using these motors as direct connects... using a belt also produces a side pressure load on the bearings that would seemingly be unnecessary with a direct coupling.....
i really do not know and ask because i wish to understand what i may be fundamentally misunderstanding
From: danbollinger <no_reply@yahoogroups.com>
To: electricboats@yahoogroups.com
Sent: Tuesday, September 6, 2011 9:31 PM
Subject: [Electric Boats] Re: belt drives vs directly coupled motor shafts and torque
Cogged belts are very efficient. And, when you realize that they can be used to match the motor's and prop's most efficient rpm, they can contribute to a drive system that is MORE efficient than a direct drive between the same components.
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