Standardisation of power is not a new concept. In the 18th Century, James Watt defined horsepower to demonstrate the performance of his steam engine. Ever since, it has been measured uniformly in HP or, in honour of its inventor, in Watts.
Still, there can be some confusion around the use of these terms. It depends on where and how you measure. The most meaningful performance indicator of a drive system is propulsive power. This indicates the performance that is actually delivered by the motor to move the boat, taking all losses in account, including propeller losses.
This method has been used in commercial shipping for nearly 100 years.
For petrol and conventional electrical outboard motors, the propulsive power is not normally revealed. Instead, less meaningful indicators are used, such as shaft power, input power, or even static thrust.
That wouldn’t be so bad if the differences between the various power ratings were minimal. But they aren’t; they’re very large. The propulsive power of a petrol outboard with 4 HP shaft power, for example, is just 1 HP. How can the differences in efficiency levels of different types of motor be measured? We’ll take a look at why this occurs.
Input Power is the drive’s power consumption. It is often used as a performance indicator for electric outboards (current x voltage) and is expressed in watts or HP. Input power is not usually specified for combustion motors such as petrol outboards or diesel inboards. However, it can also be determined (flow rate of petrol or diesel x calorific value of petrol or diesel) and is also expressed in watts or HP.
Shaft Power is the power rating of petrol outboards, comparable with cars (torque x angular velocity). The rating is expressed in HP or kW but does not take propeller loss into account, which can vary between 30% and 80%.
Performance indicator used by commercial shipping and Torqeedo (thrust x speed). The rating is expressed in HP or kW and takes all losses into account, including propeller loss, and clearly indicates the actual power delivered by the drive system for propulsion.
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