It’s true that electric motors provide all of their torque all of the time, unlike IC engines, which make peak torque within a narrow band of operation. And torque is the force that turns a prop or a wheel. That’s why humongous vehicles like ships and trains use diesel electric. But both diesel electric and its kissing cousin, the serial hybrid, have a flaw. If the electric motor goes, you are without propulsion. Sure, you could rig a power takeoff from one of the generators to accept a chain or belt that could drive the shaft in the event of electric motor failure. But aboard today’s already-crammed-to-the-gunwales recreational boats, that’s generally not feasible.
A second problem is the losses between the IC engines and the prop. There is no free lunch. Suppose you need 400 hp — that’s about 290 kilowatts — at the prop to drive your boat. Well, because of losses in the system, you’d need an engine closer to 500 hp to deliver that 290 kW to the props. That negates the efficiency of a hybrid. Ships and trains benefit from economies of scale — and they need all that torque, all the time. The typical recreational boat with a planing hull does not.
Besides redundancy, the nice thing about parallel hybrids is that you can use the type of power needed for the task at hand. Full planing power via the IC engines that we are all used to lets us run as fast as conditions dictate. At lower speeds, we can save a bit of fuel by “boosting” with the electric motor. Around the dock, we get massive torque for increased control at slower speed maneuvers. And in no-wake zones, or during those “putting around looking at the waterfront” trips, using the electric motor keeps things quiet and uses no fuel, except that needed later to recharge the batteries.
Suppose we added solar panels to a parallel hybrid system...
Takeaway One: Most boaters use the terms interchangeably, but properly speaking, there are internal combustion engines and electric motors.
Takeaway Two: While gasoline hybrids can work, diesel has the advantage of delivering more-efficient combustion, more torque to run a generator with less loss of power, and the ability to burn biodiesel as fuel.
Boating has been covering hybrid boats longer than anybody. As proof, check out this article by David Seidman.
Look for our head-to-head test of two hybrid boats, Pearson 34 G and Greenline 33, coming soon only at boatingmag.com!