Most sailors are accustomed to horsepower when talking about diesel engines, while the power of an electric motor is normally expressed in kilowatts. Horsepower can easily be converted to kilowatts, and vice versa, since both are units of power - 1 kilowatt is 1.34 horsepower.
Still, we claim that a seemingly less powerful electric motor can replace, and even outpace, a diesel. Let’s explore the differences between our electric motors and typical marine diesel engines. Let’s also figure out why our “conversion factor” from “electric kilowatts” to “diesel horsepower” is about twice as high as the real 1.34.
Thrust, the force created by the turning propeller, is what moves the boat. On displacement boats it is much more efficient to create thrust using a large slowly turning propeller than a small fast turning propeller.
That’s why when people ask us the horsepower ratings of our motors, we tell them instead to focus on the size of propeller their boat requires and the maximum rpm it needs to turn. Our propulsion systems are designed to work at moderate speeds at the propeller-shaft, so generally they take a larger prop than the prop turned by a standard-equipment diesel with a much higher horsepower rating.
Let’s consider a 25 kW (33.5 horsepower) electric motor designed to turn the propeller shaft at 800-950 rpm and compare it with a typical 80 horsepower marine diesel. The diesel would achieve its rated power at approximately 3000 rpm. A diesel saildrive has a reduction gear giving the propeller a maximum speed of 1,200-1,350 rpm. To run the diesel at a healthy load the propeller should allow the engine to reach its rated rpm. With the setups described above, the electric motor can be equipped with a considerably bigger propeller than the diesel.
The different characteristics of a diesel engine and an electric motor also play a role when considering how much power is required. The instant high torque of an electric motor in the low rpm range provides explosive acceleration which is a clear benefit when manoeuvring in tough conditions. Torque at low rpm provides the possibility to drive at very low speed which is nice when approaching the moorings.
It is difficult to quantify the advantages mentioned above that the electric motor provides. Nevertheless, they give a superior sense of control compared to a diesel engine in a wide variety of conditions.
Most boatbuilders use 4 kW per ton as a guideline. With Oceanvolt, the recommendation is between 1.25-2 kW per ton. Therefore, we calculate 2.5-3 hp/kW instead of the standard 1.34 hp/kW. This will provide a comparatively similar control when manoeuvring the boat even in harsh conditions.
In conclusion, while traditional diesel engines have long been the standard in marine propulsion, electric motors present a compelling alternative. The instant torque, enhanced maneuverability, and efficiency in power usage make electric motors particularly suited for a variety of marine conditions. By focusing on thrust and propeller efficiency rather than raw horsepower, electric propulsion systems can match or even exceed the performance of their diesel counterparts. As we move towards more sustainable and efficient boating solutions, the adoption of electric motors is set to revolutionize the industry, offering a cleaner and more responsive experience on the water.