Electric cars are trendy these days. Hybrid electric systems are perhaps a little less exciting, but still get a large share of marketing attention. Boatbuilders have been getting in on the act too, but does it make sense to consider electric drive for a boat?

In today's article, I'll summarize three common powertrain architectures (battery electric, hybrid IC/electric and conventional IC engine) and outline the logic that might lead you to choose an electric drive system for your boat.

TL;DR: Electric propulsion can make sense in a boat with high house loads, short range and minimal propulsion requirements, but is unlikely to pay off in faster, longer range vessels or in boats with low house loads.

Battery Electric

A large battery bank is used to drive an electric motor for main propulsion. The batteries might be charged from shorepower, by solar panels, or by a small onboard generator.

Consider using battery electric drive if:

You prefer short, slow trips

Battery electric systems are nearly silent and tend to give excellent thrust for low speed manoeuvring, and you don't have to deal with fuel and oil. But range and speed are limited; batteries are still very heavy and bulky compared to liquid fuels with comparable energy content. For a canal boat that has shorepower every night, rarely goes very far in a day, and has to obey speed limits, these limitations don't matter, and battery electric can be a convenient, economical option.

You use the motor only for short periods, but use electricity all the time

In this case, your main power requirement is the house loads- the fridge, lights, etcetera. The motor's used only to get in and out of the marina; by the time it warms up, you're ready to shut it off. A big battery bank with an electric auxiliary drive might work well on such a boat. Longer range, and a bit of emergency backup, can be achieved by adding a small generator. You do need to be able to charge this large, complex battery bank, so while it might work for boats that have regular shorepower access, a big-battery boat that lies at anchor or on a mooring would become very dependent on its generator.

You want to rely purely on solar power

This is a complicated topic, and one that leads to highly specialized, very weird-looking designs. Generally speaking, it is not feasible to retrofit an existing boat to be purely solar powered; the boat should be designed from the start with this in mind. I will discuss this further in a future post.

Hybrid IC/Electric

A diesel or gas generator produces electricity, which is used to drive an electric motor for main propulsion.

Consider using hybrid electric drive if:

House loads are comparable to (or larger than) propulsion loads

Consider the power your boat will use at anchor, or when alongside a dock: there might be a fridge, air conditioners, lights, television, and so forth. If all of this stuff- the house loads- leads you to consider running a generator constantly, and the generator would be of comparable size to the main engine, hybrid drive might make sense.

Example: Your boat normally comes with a 50 kW (67 hp) main engine, and you're going to deck it out with luxury gear so that the house loads will vary from 10 kW to 30 kW. It might make sense to use two generators (perhaps 20 kW and 40 kW), wired so that either or both can power both the house loads and a 40-50 kW electric motor for main propulsion. Many large cruise ships are rigged this way; they'll run a few of their engines to handle the house loads while at anchor, and fire up additional engines to add more power when the ship is underway.

Power demand is highly variable

Tugboats and some fishing boats have huge variations in power demand as they go about their duties. Electric drives offer the ability to decouple engine RPM from propeller RPM, so you can keep the engine at its most efficient operating point for the power required, regardless of boat speed and load conditions. If this is the case for your boat, it is worth doing the calculations to see if there would be any advantage to using hybrid drive instead of (or in addition to) the usual controllable-pitch propeller.

You want low-speed silent running in an otherwise conventional powerboat

Several manufacturers now offer electric motors that can be coupled to conventional diesel engines. At very low speeds, the electric drive can work alone, running from the batteries. For cruising, the diesel takes over, and when high thrust is needed (while docking, for example) both parts can work together. I'm hard pressed to come up with a sound economic case for these systems (they're expensive, and there's no reduction in fuel use at cruising speeds), but if you need that low-speed silent running ability and have money to spare, I wouldn't rule them out.

Conventional IC engine

A diesel or gas engine turns the propeller shaft through gearboxes.

Consider sticking with a conventional drivetrain if:

Main propulsion is the boat's biggest load by far

Example: If your house loads add up to 5 kW and main propulsion requires 50 kW, it makes no sense to convert most of that power from mechanical rotation, to electricity, then back to movement again. Just send it straight to the propeller through an appropriate gearbox.

None of the arguments in favour of battery or hybrid electric apply to you

If you're not OK with being limited to short, slow trips, you use the engine frequently, and you can't justify a hybrid system based on house load or variable power demand, the odds are you'll save money, effort and fuel by coupling the engine to the propeller in the simplest way possible: shafts and a gearbox. The main arguments for electric drive in a car are the regenerative braking ability, the high starting torque, and the ability to combine low-load efficiency with short high-power bursts; none of these arguments apply to boats. Where constant power at constant speed is required, as is the case in the vast majority of boats, the simplest drivetrain usually ends up being the cheapest and the most efficient.