Quote from a sailing forum:
As for efficiency, I would have thought the direct drive electric pod would be far more efficient vs. the gearing in the typical sail drive.
I think that people may be overestimating the losses in the diesel power train. In his Propeller Handbook, Dave Gerr states that the gearbox loss is about 3%, and about 1.5% per propshaft bearing. This seems compatible with the performance curves that Yanmar gives for their 4JH57 57 hp engine:
And in that chart, do take a look at the fuel consumption vs the propeller power curve (which shows the power actually delivered to the propeller at any given RPM). It looks like the fuel consumption is around 0.3 liters / hour / hp, over a very broad range of RPM. In short, there is not that much extra loss when running a diesel engine lightly loaded at low RPM.
We definitely need to define exactly what losses, or efficiencies, we are considering. But start from the propeller and work backwards. How fast do you have to spin the prop for a given speed? That takes a certain amount of horsepower at the propshaft, and it doesn’t matter if the power source is a diesel engine or an electric motor.
Then look at the losses in the transmission and shaft bearings. In a diesel, that may be 5%. Electric with only bearing losses, perhaps 1 or 2% These differences aren’t enough to magically turn one hp into three. Also, you generally don’t want to directly drive a propshaft from the electric motor without using a “thrust bearing” of some sort. Electric motors are not typically designed to take the pushing force that is developed by a spinning propeller (nor is a diesel engine).
Electric motors can deliver huge torque at very low RPMs (or when stalled). Diesels can’t. That makes a big difference in a railroad engine, but not very much in a pleasure boat. That’s why your internal combustion car has a gearshift, but your boat doesn’t. Take a look at the Yanmar propeller power curve and see how little power is required to spin the prop at low RPM — the diesel can do it with no problem.
Electric motors are very efficient in turning stored electrical power into work — 90% is fairly common. Diesel engines waste about half the power available in their fuel. But that type of efficiency is vastly overshadowed by the incredible amount of power stored in a liter of diesel fuel. Liter for liter, diesel fuel contains at least 30 times the power of even Lithium batteries, and pound for pound it’s about 100x. Even after dividing that by two for the diesel system losses you can still see why electric systems really only compete in a small subset of boat applications.