That's generally true if the hull shape is the same and the displacement doesn't change very much.
--- In electricboats@
>
> Jason,
>
> Shorter hulls generally take more power to drive a similar speed than longer hulls due to the given speed being a larger percentage of hull speed. I know that multihulls can be different, but the general guideline usually holds true. My personal experience with trimarans indicates that an F-31 (31') will motor slightly faster than an F-27 (27') with identical engines. Your catamaran may take more power than Chris' to attain the same speed.
>
> In a traditional outboard, the gearbox does three things, reduces shaft speed through the gear ratio, allows for selecting forward and reverse and allows the vertical drive shaft to turn the horizontal prop shaft through a 90 deg gearset, all in an integrated package. The gearbox is located at the bottom of the drive leg and contributes to a loss of efficiency. Purpose built electric outboards like the Torqeedo and trolling motors can eliminate this efficiency loss by mounting the drive motor horizontally at the bottom of the drive leg and driving the prop directly.
>
> One of the difficulties in converting traditional outboards is that prop sizes are usually limited by the anti-ventilation plate part of the drive leg. There are also very few choices in pitch when you are dealing with the smaller (<30hp) outboards.
>
> Fair winds,
> Eric
> Marina del Rey
>
> --- In electricboats@
> >
> > Thanks for all of the comments.
> >
> > Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
> > I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?
> >
> >
> > Has anyone taken apart an old outboard? Is the gearing in the top section? Would electric benefit from the gearing or better without?
> >
> > My main lack of understanding is with prop sizes. With an electric motor, the best torque is in the low revs, so surely a large prop would be best? ...
> >
> > Thanks for all of the responses, (in adavance)
> >
> > Regards,
> >
> > Jason.
> >
>
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