Because of the totally different torque characteristics of electric motors, there is not a direct corelation between engine HP and motor kW. The nearest rule of thumb used is 1kW for every 3HP but this will vary depending on the hull design.
The following table is a guide to how much power will be needed:-
| Weight | Motorboat | Sailboat | Catamaran | |||
| kg | Length m | Power kW | Length m | Power kW | Length m | Power kW |
| 1000 | 5 | 2.5 | 6 | 2.0 | 6 | 2 x 1.0 |
| 2000 | 7 | 5.0 | 9 | 4.0 | 9 | 2 x 2.0 |
| 3000 | 9 | 7.5 | 11 | 6.0 | 11 | 2 x 3.0 |
| 4000 | 11 | 10 | 12 | 8.0 | 12 | 2 x 4.0 |
| 5000 | 12 | 12.5 | 13 | 10.0 | 13 | 2 x 5.0 |
| 6000 | 13 | 15.0 | 14 | 12.0 | 14 | 2 x 6.0 |
| 8000 | 14 | 20.0 | 15 | 5.0 | 15 | 2 x 7.5 |
For larger displacements 2.5kW per tonne is a very safe rule of thumb to achieve hull design speed in worst case conditions ie taking into account tide, current, high winds and manouvering power. If hull design speed is not a particular requirement, as is sometimes the case with sailboats & catamarans then you can use a factor as low as 1.0kW per tonne of displacement.
BTW, the recommendation of 1kW per ton of displacement has proven to deliver about 90-95% of theoretical hull speed on a number of different displacement hull shapes and sizes. That statement is accurate for my inflatable dinghy with a Torqeedo and for my 30' 5.1 ton ketch. A number of other conversions reported here fall into the same range. This does assume a drive (motor through prop) that is matched to the desired speed as discussed in my recent threads.
Eric
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Pamela,
>
> While that is true, the speeds that we're talking about (less than 95% of displacement hull speed) are considered the sweet spot. Specifically, the 6kt speed that we're discussing with Julian is 80% hull speed.
>
> Trying to get all the way to or past hull speed can take very large amounts of power in a displacement hull and that's where diminishing returns really start to take hold. To date, I've never recommended that anyone try to get to hull speed in displacement mode with an electric conversion.
>
> Fair winds,
> Eric
>> --- In electricboats@yahoogroups.com, Pamela Tilstra <pamlt@> wrote:
> >
> > While you're stressing about prop speed and hull speed, ermember the law of diminshing return. There is a point where you get little or no performance experience. only greater expense or less efficient power usage.
> >
> >
>
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