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I am trying to build a super-efficient electric boat. The dilemma I am coming across is this:

In normal long range running mode the boat will go at a slow 2.5 knots and only require 400 W power. This is the optimally efficient setting through the water. However for safety and practicality reasons like getting around in high winds etc I still need about 5kw max power available on the shaft. From my understanding any electric motor with a 5 kW capacity will be very inefficient at 400 W.

So my proposed solution is to put 2 motors on one shaft, a small one optimized for 400 W and a big one, the small one will be rigidly connected to the shaft and the big one via a clutch. I will then manually manually engage and disengage the large motor with a switch.

Can you please advise if this is a normal approach of if there is a better way to handle my situation? Am I misunderstanding something or reinventing the wheel?

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While the mechanism you described will produce greater motor efficiency than turning down a 5kW motor to less than 10%, you will lose overall system reliability IMHO with the clutch/2 motor system.

The clutch/2 motor system should work as described, but why not just use a VFD?

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