Stephan,
Most of the larger conversions that I've seen are replacing engines up to the 20-30hp range. A couple of people have replaced 50hp engines with some success. But higher power systems require more storage and the numbers can quickly get out of hand.
You can see that replacing 500hp of IC power is a pretty tall order. Conversely, imagine mounting a 40hp outboard on the back of your boat. Do you think that it would still hit 6kts? Electric drives excel at slow and steady, high power generally means less range.
Good luck,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Stephan Hinni <hinni_mb@...> wrote:
>
> Hi Eric
>
> The figures I sent you are for one engine only, sorry I forgot to mention that.
>
> Thanks for for your calculations, you explained it good and they make sense to me. But cheese, the outcome doesn't look very good for me, does it?
> I know it's not going to be an easy task that conversion, that's why I am here and ask for support and ideas.
>
> Thanks for your time and help Eric.
>
> Regards,
>
> Stephan
>
> --- On Tue, 9/28/10, Eric <ewdysar@...> wrote:
>
> From: Eric <ewdysar@...>
> Subject: Re: [Electric Boats] Power Boat Conversion
> To: electricboats@yahoogroups.com
> Date: Tuesday, September 28, 2010, 5:00 PM
>
> 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...
>
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