GNHBus,
For my conversion I went with a very aggressive 4 blade prop to maximize slow speed prop efficiency. Because my prop is in an aperture I couldn't increase the diameter significantly. I know that I can mitigate the extra drag while sailing by "power-feathering" the prop with a little throttle.
But the comment that you made that mentioned hull speed got my attention. I don't think that any of the existing conversions spend any time at hull speed, the power requirements just aren't worth it. So, my drive system can't push my boat to hull speed (about 6.5kts) because I would rather optimize the system for the speeds that I actually power the boat at, the 3 to 5 knot range.
The recommendations of repowering a boat at 2.5-3hp ICE to 1hp electric take this into consideration. Speed is a hp thing and reducing the total available hp to 1/3 will probably reduce your top speed. There is an assumption in that conversion guideline that the ICE is actually over-spec'd and the boat would be just as usable with less hp. With ICE, the smaller engine might not have enough torque at slower speeds which justifies the larger installation. Electric doesn't have that problem, even motors with less hp have enough torque for operation at the lower end. So going with only enough hp to get near hull speed is generally good enough. The funny thing is that my old Yanmar SVE-12 diesel couldn't drive the boat to hull speed either.
So it is my opinion that hull speed isn't really important to electric conversion because the energy cost is way too high. I can't think of any emergency situation that 80-85% of hull speed wouldn't be sufficient. And my normal operation is closer is 40-75% of hull speed to preserve any semblence of range.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, GNHBus@... wrote:
>
> You hit the nail on the head with "watts - to - knots - to - rpm" It is very interesting when you start looking at the Power Curves between
> diesel vs. electric propulsion, as the interest steers toward the propeller for achieving hull speed.
>
>
> -----Original Message-----
> From: Eric <ewdysar@...>
> To: electricboats@yahoogroups.com
> Sent: Wed, Mar 9, 2011 6:24 pm
> Subject: Re: [Electric Boats] Thrust to speed numbers
>
> GNHBus,
>
> I don't know if you tried to attach something to your post...
>
> .... I plan on collecting detailed data (watts to knots to RPM) for my boat, but the weather hasn't been cooperating each time I've been at the boat for more than the last month.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
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