Many people who are new to the world of electric outboard motors are confused as to why their power is measured in kilowatts, but with an “equivalent horsepower rating”. For instance, the Spirit 1.0 Plus is a 1kW electric outboard that is equivalent to a 3hp gas motor.
Why aren’t electric outboards measured only by horsepower like the outboard industry has been doing for decades?
Both horsepower and kilowatts are a measure of power output. If an electric outboard is rated at 6kW, then 6kW is its maximum power output at full throttle. If an internal combustion outboard is rated at 9.9hp, then 9.9hp is its maximum power output at full throttle.
A straightforward comparison of kW to hp is that 1 kilowatt is equal to 1.34 horsepower. Conversely, 0.75 of a kilowatt equals about 1 horsepower. So how does a 1 kilowatt motor get to be 3 horsepower instead of 1.5?
ePropulsion and other electric outboard manufacturers use the term “equivalent propulsive power”. Propulsive power is a term that has been used in commercial shipping for nearly 100 years. It indicates the performance actually delivered by the boat‘s motor, taking all drive losses into account.
Without getting too technical, gas outboards measure shaft power, while ePropulsion measures the actual propulsive power of the propeller driving the boat through the water.
When combustion engines burn gasoline or diesel, they primarily use the stored energy to produce heat: 5-15% of the supplied energy is used to propel the boat and the rest is lost due to inefficiencies. With a gas motor, the power loss from the motor, through the gears to the propeller can be 85% to 95%. In contrast, the “shaft” on an electric motor is essentially just the wires running from the battery to the motor (which is actually under water, running inline with the prop).
A gasoline outboard with an advertised shaft power of 6 HP actually typically provides a mere 1.6hp of “propulsive power”.
Electric motors also deliver a lot of torque, which is available at any rotational speed. This characteristic allows them to turn large, efficient, high-pitch propellers that would cause an equivalent combustion engine to stall at startup. And gas motors only reach their peak hp output at specific RPMs.
There’s a lot more to it, but in order to stay out of the technical weeds, we can wrap it up here by concluding that what you need to pay attention to if you are familiar with gas motors is the “equivalent horsepower” rating to set your expectations about how an electric outboard motor will perform. But don’t forget that it will also perform without any sound, fumes, oily discharge and will continue to do so year after year with virtually no maintenance.
