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When you decide to get an electric motor, the choice of power is a very important element, much more important than if you have to choose a combustion engine. Indeed, with an internal combustion engine, one can afford to choose more power and not to exploit the engine to the maximum. With an electric motor, the power will have a huge impact on the capacity of the battery pack and therefore on the cost of the installation. This is why it is necessary to define the sufficient power, for safety reasons, but not too much for cost and weight reasons.
Matching hp with kW
To start our research, we can rely on the power of a thermal engine that would normally be adapted in place of our electric motor. From there, knowing that 1 kW is equivalent to about 1.4 hp, we could for example think that a diesel engine of 50 hp could be equivalent to an electric motor of 36 kW. But this mathematical calculation is far from the truth.
From the loss in line on thermals
On an internal combustion engine, the maximum power (50 hp in our example) is only given at maximum rpm. On a boat, the engine is rarely used at full speed. This is not the case with an electric motor, which can be used at full speed, even for a long time, without risk.
Moreover, the power of the combustion engine is indicated at the crankshaft output, but the engine is then connected to an inverter, not to mention the various accessories (pumps, alternator...) which also "consume" power. With 1 hp at the crankshaft, we will have much less at the propeller.
Good efficiency for the electric motor
With an electric motor, the efficiency is better. If the old electric motor technologies have lower efficiencies (around 80%), the new electric motors (permanent magnet synchronous type) have efficiencies above 90%. 1 kW is equal to more than 0.9 kW returned to the propeller, much more than on a combustion engine. In addition, an electric motor does not require a reversing gear, since in order to reverse, you only need to turn the motor in the other direction. It is generally considered that 1 KW electric is equivalent to 2 hp in thermal.
Rely on the boat's resistance curve
To calculate the necessary power, e-NAV Systems, a company specializing in electric motors, uses the boat's theoretical resistance curve. This is a mathematical calculation based on the boat, the shape of the hull and its exposure to the wind, which indicates the power required to move the boat forward at the desired speed. This data, which can be provided by the boat's architect or calculated by the e-NAV Systems design office, will determine the recommendation, the useful power in kW.
The importance of torque for a boat
Adjustments are then made according to the needs of use. Indeed, we realize that as long as the boat does not move out of the water, it is mainly torque that it needs rather than power. Torque is the ability to turn a propeller. For example, a fan has no torque, you can stop its propeller with a simple finger. On a boat, this should not happen to drive propellers with a large pitch and diameter. With an electric motor, all the torque is available immediately from the first revolutions (and does not vary until the maximum speed).
Change of reference for the boater
The price difference between a 20 HP and a 30 HP combustion engine is so small that one naturally chooses the 30 HP. But do we need it? For electric boats, it's different, an over-engine (which does not necessarily allow to go faster) generates a larger battery park and an unnecessary extra cost. The transition to electric power in the nautical industry induces a change of reference system as it is the case today in the automobile industry where we talk less about hp or kW than about performance: acceleration, speed, range. Electricity also leads us to question the needs of use: is it essential to move forward 1 knot faster if I consume 50% more?
