Does electric drive make sense in a boat?

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.





I think that it makes sense, since electric will drive the boats at a much faster rate than any other means. Yeah, there may be other hazards associated with it but some trade offs are essential to make.
ship register

Faster? Nope.

Matthew's picture

"...electric will drive the boats at a much faster rate than any other means"

This myth keeps circulating; unfortunately it isn't true.

Assuming identical propellers running at the same RPM, and the same total weight in both cases, a boat will cruise at exactly the same speed with 50 kW of electric power as it will with 50 kW of diesel power. Or, for that matter, 50 kW of hamsters in wheels. The propeller only cares about the shaft RPM and the shaft torque; it has no idea what's on the other end of that shaft.

Electric drives do tend to have a slight acceleration advantage at low speeds, as many of them can reach peak torque without first having to increase RPM. They also tend to be a bit undersized and run at closer to 100% of rated power (while diesels are usually a bit oversized, and gas engines are usually way oversized, so that they don't have to run at wide-open throttle) which, looking at sales brochures, makes it look like they're faster for a given power. But this is not the same as actually being faster.

Sailboats vs. Powerboats

Hi Matt,

I was directed here by John Harries at AAC.

I think this is a good synopsis, but I think it de-emphasizes the trade-offs between power and sail-driven vessels.

For example, the "cruising speed" specification for sailboat is going to be much lower than a powerboat of similar size, and therefore a hybrid-electric system seems like it would make more sense. No?

Sailboats vs. Powerboats

Matthew's picture

Hi C.Dan,

In the analysis I've presented here, the choice of drive system is determined primarily by the ratio of house loads to propulsion loads.

Using sails as your primary propulsion source would affect the calculation as follows:

  • You have much less power available for propulsion, compared to a motor vessel, so you emphasize low-speed efficiency in the hull design. This also reduces the propulsion loads when under engine.
  • The boat spends very little time under engine, but the house loads are still on whenever the boat is in use. This skews the balance towards higher house loads and lower propulsion loads.

By adding sails and optimizing the boat design accordingly, we (1) can get by with a smaller main engine than would be needed in a powerboat, and (2) can run that engine less often. The house loads thus become a much larger fraction of the boat's total power demand. This is what drives us towards hybrid systems, and is why you are correct that hybrid-electric drive is much easier to justify in a sailing yacht with auxiliary power than in a pure motoryacht.

Fuell Cell Boats

Following the discussions above, as related to electric drive trains and house demand, it would seem that as you logically point out, the limiting factor with electricity powered boats is the stored capacity of batteries.

What do most boats have in common? They sit around basking in the sun till the owners decide its time to use them. That could be weekly, monthly but rarely do you see a private boat used every single day.

Off the shelf components available today can take DC power, created by PV or wind and convert it to hydrogen. Those same systems have storage capacity that can store the H2 under low pressure. Fuel Cell conversion back to DC is also available off the shelf, providing for relative unlimited supply of electricity.

A system that would use a bank of batteries as a buffer, H2 as the stored fuel, and renewable energy as the original source, IMHO is the next Hybrid type of drive for boats.

Take a marina that has the forward vision of the industry and it can provide the shade shelter to the boats, cover it with PV and create the fuel that would be sold to the hybrid H2 boaters.

Set your own private boat house up in a similar fashion and never buy gas/diesel again. Would you boat more often if the cost of fuel was not the limiting factor? Of course you would!

Please open this up for discussion, I'd welcome the feedback and the opportunity to build such a system for a forward thinking individual, or best yet....create a business model for the boating industry.

Fuel cells

Matthew's picture

Hi H2Fan,
There is certainly a lot of potential for hydrogen power in boats. While fuel cells have (so far) had very limited success in cars, the major technical problems they face there are less of an issue in boats. A car has very limited space available for hydrogen fuel tanks; a motoryacht can be designed with plenty of volume for them. A car has huge, fast swings in power demand; a motoryacht does not. The motoryacht application could be served by any of several different types of fuel cell; cars are, for technical reasons, basically stuck with proton-exchange membrane fuel cells.
This technology is still in its early stages and is therefore quite expensive, but that is changing. I suspect we will soon reach the point where a variety of drop-in fuel cell powerplants will be available, and we will start seeing them in some high-end motoryachts. The technology is not likely to find its way to more common boats until it becomes widespread in other industries, but that may happen within the next couple of decades.
I do plan to write a few more articles on this in the near future. Stay tuned.

Fuel Cell

We prefer to use diesel fuel because it is less volatile than gasoline. Hydrogen on a boat is scary - a leak could lead to a disaster!
In one regard solar panels and or wind generators reduce fuel consumption since neither the main engine nor a generator is needed to supply house needs. While not really a hybrid system, the result is less fuel consumption unless motoring.

Hydrogen safety

Matthew's picture

Hi Bob,
I'm actually a lot less scared of hydrogen than I am of, say, propane. It's hard to get hydrogen to collect anywhere on a boat; it rises out the vents and hatches where it can safely dissipate. The heavier-than-air fuels, like propane or gasoline vapour, that can pool in the bilge... those scare me.
Solar panels are rapidly becoming a more-or-less standard item aboard cruising boats, for exactly the reason you say- running an engine at idle speed to charge batteries really sucks.
A more detailed look at solar power for cruising boats is on my to-do list...

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