T-Pain Is On A Boat: Re: [Electric Boats] Re: please take a stab at sizing a motor....

Monday, September 19, 2011

Re: [Electric Boats] Re: please take a stab at sizing a motor....

Michael,

As far as sizing the motor I don't think you want to go with the 10hp motor. It would most likely do the 6 knots you want but you would be driving the motor at the top of its continuos range. The two most quoted rules of thumb both give the same result for your listed stats. 1KW for each ton of boat = 16000 watts = 21hp, And 1 electric hp can normally replace 2 to 3 combustion hp. 65hp divided by 3 = 21hp electric.

So given that the rules of thumb have worked pretty well for a lot of people, with few or any complaints. I would shoot for around 20 shaft hp of electric motor for that boat.

On Sep 19, 2011, at 10:36 PM, Eric wrote:

Hi Michael,

For gasoline, the conversion is GPH = 0.0226 * HP / TE. Diesel contains 115% the BTUs of gasoline.

If we assume a 20% Thermal Efficiency for your engine at low RPM (just a guess), we get 1 * 0.20(TE) / 0.0226 = 8.8hp * 1.15 for diesel = 10.1hp

That's 7.6kW. If you look back at post #19718, my answer was that you could expect a 16kW drive to push your boat at 7kts. Half the power and slow down 1 knot means 6kts = 8kW. Two completely unrelated calculations that come up with the same answer +/- 5%. Weird, isn't it.

You can keep coming up with different ways to ask the question, but the answer is going to keep coming out the same.

At least this answer is better than the 43hp that you guessed on your next post...

Fair winds,
Eric
Marina del Rey, CA

--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> Alright, here is some info about a particular boat and from that I would like to determine what e-motor setup that would produce the same cruise speed.
>
> Speed: 6 knots via the following,
>     Perkins 4-236 running at 1500 rpm turning a 24x9 prop
>     2.1 reduction drive
>     16 ton vessel
>
> the thing uses about 1 gal/hr diesel doing this
>
> the engine is rated at 65hp continuous but of course it is not running at that output level
>
> i've been searching around for some 4-236 output curves but not been able to find any to this point
>
> anyone care to take some guesses?
>