Stephan,
From your answer, I don't know if these figures are for each engine or for both, but you can adjust my answer accordingly.
Assuming a thermal efficiency of only 20% because of low throttle settings, a reading of 3.3gph works out to 29.2hp delivered at the engine crankshaft(s). So figuring for 3.3gph for the boat at 6kts, you'll need 15hp from each motor to achieve the same speed. 15hp is 11,250W (15hp * 750W/hp), so your projected 10kW motors may be a little undersized. These figures aren't too far off of the 20kW that was estimated earlier.
You can quickly see why large power boats run big engines, the hulls aren't very efficient, but hundreds of hp are pretty easy.
However, that doesn't bode well for electric conversions. Once you find a suitable drive motor, using batteries for storage gets crazy pretty quickly. Using your figures, a 40 mile journey @ 6kts will burn about 22 gallons of gas. One gallon of gasoline through an IC engine is roughly equivalent to 10kWh of electricity through an electric drive. So your round trip would require more than 200kWh of usable battery capacity. Going high tech, Lithium Iron batteries are about half the weight and size of lead acid batteries and this trip would require more than 6000 pounds of LiFePO4 batteries that cost over $100,000. Lead Acid batteries would be somewhat cheaper, but come close to the weight of your boat in batteries alone.
As you try to scale back to control costs and weight, you can see that $10,000/600lbs of LiFePO4 batteries will drive the boat at 6kts for only about 45 minutes before they're flat.
All of these numbers are broad estimates, and you results would vary, but these numbers are close enough to seriously re-evaluate the overall feasability of the project. Imagine that your results turned out twice as good as my calcs, I think that the answers are still prohibitive.
This shows again how electric is not a universal answer. It's is very well suited for some boats, but not very effective for others.
Fair winds,
Eric
Marina del Rey, CA
PS. before anyone questions the thermal efficiency value of only 20%, increasing that constant makes all of the numbers even worse...
--- In electricboats@yahoogroups.com, Stephan Hinni <hinni_mb@...> wrote:
>
> Hi Eric
>
> the gas flow meters would show me a reading of about 3.3 gal/hr at 6 kts and 3.8 gal/hr at 8 kts.
>
> Thanks for your support!
>
> Stephan
>
> --- On Mon, 9/27/10, Eric <ewdysar@...> wrote:
>
> From: Eric <ewdysar@...>
> Subject: Re: [Electric Boats] Power Boat Conversion
> To: electricboats@yahoogroups.com
> Date: Monday, September 27, 2010, 11:31 PM
>
>
> 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, 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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