Green Machine

David Kampenga of Houston, Texas, asks:

“I’m thinking about converting my Hunter to an electric motor propulsion system, which would involve installing a 6kW 48VDC motor. I would like to keep the fixed two-blade prop that’s on the boat. I would also like to retain the existing gear reduction, which is 2.14:1. I don’t quite understand the reduction gearing and am curious how I should connect the electric motor to the propeller shaft.”

Nigel Calder replies:

If your present installation is a shaft drive, which I believe it is, you will likely want to connect the electric motor directly to the propeller shaft and not the reduction gear. If it is a saildrive, you will have to couple the input shaft to the saildrive. However, before following either course you will need to address an important issue, which is the operating speed of the electric motor.

To get you started, I’ll assume your current propeller is already well matched to your existing diesel engine, although my assumption could be wrong. If you look at the data sheet for your diesel engine, one of the graphs will show the power that is absorbed at different engine speeds by a “nominal” propeller. Find the 6kW (8hp) point on the propeller curve and read down that curve to find the engine’s rpm. If you leave the reduction gear in place, the maximum speed of the electric motor should be similar to that of the diesel engine when the propeller curve is at 6kW. If you decide to couple the electric motor directly to the propeller (i.e. remove the reduction gear), divide the engine rpm by 2.14 (your reduction gear) to find the speed at which the electric motor should deliver 6kW. In practice, for various reasons you will probably do best to have an electric motor that reaches the 6kW power level at a somewhat higher speed than that calculated.

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