Colin Girvan wrote:
> Good point Dennis, I hadn't thought about actually losing power at cruising speed. So I guess when people quote the advantage of an electric motors ability to put out 2 or 3 times its rated horsepower for short periods that it's actually a bit of a fallacy as this is not really usable power.
>
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Depends on how long you think you will need that peak power. The motor
I used (Perm132) has a continuous rating of 110A but a peak of 200A. But
that peak is only for 10 minutes or the motor will be getting too hot.
So going to 100% overload would be fine for manoeuvring out of trouble
or docking - you would only use bursts of that full power, but would be
no good for spending an hour fighting a strong wind back into port, the
motor would start to burn.
I don't have any information on the 30 minute and 1hr ratings for my motor.
When I did the final figures on my boat - at 'cruising speed', the
electric system is using near 1/2 the kW of the former petrol engine.
(Same prop) In my case since the petrol engine didn't appear to have a
lot of head-room, I've assumed the petrol engine had to be run at higher
power simply to get the torque to turn the prop. (And the engine and
prop were supposedly a 'matched set'). The electric motor's different
torque curve causes it to have enough grunt to turn the prop at lower
power levels.
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