Motor Sizing

Motor Sizing and Diesel/Gasoline Engine Equivalency

The first step in designing an electric propulsion system is choosing the right motor size. Below is a chart that will provide general guidelines. Contact Electric Yacht, or one of our dealers, for more specific information and assistance in making a selection.

Diesel “Equivalency”

Some suppliers of electric propulsion have claimed an electric propulsion motor is “equivalent” to a diesel engine of 2-3 times higher horsepower rating. On the surface, these claims seem confusing and untrue. In fact, it takes a certain amount of horsepower applied to the propeller shaft to propel a boat at a given speed, regardless of the source of the power. However, there are several factors that lend legitimacy to these claims

Rating Systems

First of all, diesel engines are usually rated in brake horsepower (BHP). This rating does not subtract losses in the transmission, alternator load or water pump load. The net shaft horsepower (SHP) the same engine actually produces, can be anywhere from 10% less on a larger engine to nearly 50% less on a small one cylinder engine.

The Propeller Curve

The power required to push a displacement hull with a waterline length of 25’ – 40’ will increase 1.5- 2 times for each knot of speed. Since many diesel boats are overpowered to achieve “hull speed” (usually defined as 1.34 * sqrt(LWL)), an electric motor of lower rating will often result in little or no loss in motoring speed.

Maneuvering Thrust

Finally, an electric motor for maneuvering will provide superior performance to a diesel or gasoline engine of higher rating. The ability to slow the electric motor well below an ICE’s idle speed, combined with full torque at any speed, makes for superior maneuvering performance. In addition, no starting, warm up and quiet operation of an electric motor make for superior overall performance.

Add all these factors together and it may be appropriate to replace an ICE of greater horsepower rating of the electric motor. Contact Electric Yacht or one of our dealers for assistance in sizing an electric motor for you.

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