Propulsive power

A brief encyclopaedia of power figures – what power figures really mean for boats

Different manufacturers of propulsion systems for boats use different power figures and comparisons are difficult. In our opinion, the most useful power figure for a propulsion system is the propulsion power. This indicates the power actually available for propelling the boat, taking into consideration all losses including propeller losses. In commercial shipbuilding, propulsion power has been used for almost 100 years, but other, less informative power values are used in the leisure boat sector.

To provide as much transparency as possible, we publish all the relevant data of our motors, from input power to propulsion power and static thrust.


			» Petrol outboards
		Performance indicators: propshaft power (in HP, PS or watts)
When petrol outboard manufacturers quote power figures, they mean the propshaft power, i.e. the power available at the propshaft. However, propshaft power does not take into account propeller losses. If these losses are considered, more than 60% or perhaps only 20% of the propshaft power may actually be available as propulsion power. It would be good to have a precise figure.
			» Conventional electric outboards
		Performance indicators: input power (in HP, PS or watts)
The input power indicates the energy consumption of a motor, i.e. it tells you what power a motor uses, rather than what it develops, as you might think. There is no indication of what percentage of the energy used is lost through inefficiency and what percentage is actually available for propulsion. These shares, also referred to as efficiency, may vary considerably, between 18% for trolling motors and 56% for Torqeedo motors. So the input power alone in fact tells you very little about a motor.
			» Trolling motors
		Performance indicators:static thrust (in lbs or lbf)
The static thrust is an indication of the ability of a motor to power a boat from standstill to a very, very low speed. However, it does not provide any information on the motor's ability to propel a boat at normal speeds and is therefore not a very useful performance figure.
			» Torqeedo electric outboards
		Performance indicators: propulsion power (in HP, PS or watts)
Like shipbuilders, we always quote the propulsion power of our outboard motors, i.e. the power actually available for propelling the boat taking into consideration all losses, including propeller losses. All outboard motors could be compared on the basis of propulsion power, but you will unfortunately not find these figures in other manufacturers' catalogues.