Standardisation of power is nothing new. It goes back to James Watt, who defined horsepower in the 18th century to demonstrate the performance of his steam engine. Since then, it‘s been measured uniformly in HP or, in honour of its inventor, in watts. And with that, everything should be clear, shouldn‘t it?
Not completely. It depends on where and how you measure. The most meaningful performance indicator of a drive system is propulsive power, which indicates the performance actually delivered by the motor to move the boat, taking all losses, including propeller losses, into account. 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 shed some light on them.
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.
Our focus on optimising propulsive power and our use of the latest technologies means Torqeedo has the highest overall efficiency on the market. That is, every Torqeedo drive converts its available battery power to propulsive power better than any other outboard. This is very important for electric drives because it means more power and range from limited battery capacity.
Overall efficiency levels of various outboards
Comparing the Power of Electric and Petrol Outboards: Torqeedo’s HP Equivalent
Electric motors can achieve the same propulsive power as combustion engines with significantly lower shaft power. The reason lies in the different torque curves of electric motors and petrol engines. While the torque curve of combustion engines features a prominent peak, with maximum torque being available only over a limited engine speed range, electric motors feature a much flatter torque curve with ample torque available at any engine speed.
This characteristic allows them to run substantially higher efficiency propellers – even in lower performance classes – than combustion engines. Propeller efficiencies in the lower horsepower class can vary by a factor of 3 between petrol outboards and Torqeedo electric outboards.
To make the comparison easy for boaters used to shaft power ratings of petrol outboards, we always compare the actual propulsive power of our outboards versus petrol outboards. On the following pages, a Torqeedo outboard specified as a "3 HP equivalent" provides the same propulsive power as a 3 HP petrol outboard – even though its shaft power and input power may be substantially lower.