Sorry Michael, but it doesn't work like this. The propeller is absorbing the engine power, so that is what determines the rpm needed for any given power, all the engine has to do is provide what has been demanded of it. All propellers follow a cube law power vs rpm curve, so doubling the rpm will multiply the power needed by 8.
The easiest (and most accurate) way to estimate the actual power an engine is delivering under a particular set of conditions is to use the fuel consumption figure, as it is relatively straightforward to work back from that to get an approximate figure for power, as I showed in the last post on this topic.
Jeremy
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> okay, here is some additional info.... see if i am doing this right
>
> the 4-236 is rated at 2250 rpm or 65 hp continuous
>
> therefore at 1500 it would be producing about 43hp right?
>
> if so then it takes about 43 hp to move the boat at 6 knots?
>
>
>
> ________________________________
> From: Michael Mccomb <mccomb.michael@...>
> To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
> Sent: Monday, September 19, 2011 8:48 PM
> Subject: [Electric Boats] please take a stab at sizing a motor....
>
>
> Â
> 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:Â 6 knots via the following,
> Â Â Â Perkins 4-236 running at 1500 rpm turning a 24x9 prop
> Â Â Â 2.1 reduction drive
> Â Â Â 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?
>
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