The Perkins will have a BSFC of around 210 to 220g/kWh at that sort of power level, so (assuming that the consumption figure is in US gallons), it is burning about 3.785 litres, which, with a typical diesel density of 832g/lt means that the engine will be producing around (832 x 3.785) / 210 = 15kW to maintain that boat speed.
There will be some additional losses through the diesel engine transmission, I expect, plus I doubt that the propeller will be more than around 50% efficient, so you may be able to get away with an electric drive system that is a little lower in power.
Interestingly, using the "1000W per ton" rough rule of thumb gives a power requirement of 16kW, which is remarkably close to the actual power being produced by the Perkins at cruise.
Jeremy
--- 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?
>
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