That figure may not be too far off. 20kW is less than 30hp at 100% efficiency. Even an electric drive will deliver only about 10hp to each prop with that amount of energy. Another way to estimate the power needed is to evaluate the fuel consumption.
Stephan,
Do you know the fuel consumption of your boat in gallons per hour at your target speeds of 6 to 8kts?
Given that figure, I can back into the estimated hp that your boat needs to sustain that speed. From there, it's fairly easy to convert the ICE output to electric demand. While this process is not as accurate as a tow test, it should get you in the ballpark of what size drive would be appropriate. Of course, you'll probably want to add some reserve capacity to deal with adverse conditions.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "luv2bsailin" <luv2bsailin@...> wrote:
>
> Hans,
> You are assuming he will need 20KW to get 6KT, but is that realistic? Is there any available data on power requirements for that hull (or something similar operated well below hull speed? It seems obvious that something designed for displacement speeds will be more efficient, but since there are so many more "non-displacement" boats out there it might be worth some research. A simple tow-test could establish the drag numbers (thrust requirements) and a little number crunching will get you the HP requirements.
> Jim McMillan
>
>
> --- In electricboats@yahoogroups.com, Hans Rijnbout <jrijnb@> wrote:
> >
> > Stephan Hinni wrote:
> >
> > My idea is to use the yacht for scuba diving and pleasure trips. I'd
> > like to cruise at 6 to 8 knots and have a daily range from 40
> > nautical miles.
> > I want to use two 10 kW Mars Motors and let them run at 72 Volts.
> > Battery storage is no problem once the gas engines are out! The
> > battery charging is at the slip (110V) and while navigating 4 Solar
> > Panels bring in a bit of extra amperes. A gas generator is on board
> > too but it have to be fixed first.
> >
> > The yacht is currently powered by 2 Chrysler 250HP gas engines. The
> > length is 30.4 ft, breadth is 12.8 ft and she weights 16000 lbs. The
> > propellers are 16RH16 and 16LH16 and the gear ratio is 1.52:1.
> >
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