I love boating, but I hate the fact that powerboats guzzle loads of fossil fuel. I assuage my guilt by noting that the distance traveled is small — a couple of hours for each return trip to the cabin, and there are sadly less than twenty of those per year. Then I got into a discussion on /r/boating about whether electric boats are practical, so herewith some scratchpad calculations on that subject.
I’ve ridden on an electric boat, on the Daintree River in Queensland, on a small alligator-watching tour. This thing was flat and had room for maybe fifteen tourists under a canopy, necessary shelter from the brutal tropical sun; on top of the canopy were solar panels, which the pilot told me weren’t quite enough to run the boat up and down the river all day, he had to plug it in at night. The motor was a 70HP electric and our progress along the river was whisper-quiet; I loved it.
I should preface this by saying that I’m not a hull designer nor a battery technologist nor a marine propulsion engineer, so the calculations here have little more precision than Enrico Fermi’s famous atomic-bomb calculation. But possibly useful I think.
Narrowing the question · There are two classes of recreational motorboat: Those that go fast by planing, and those that cruise at hull speed, which is much slower and smoother. Typically, small motorboats plane and larger ones cruise. I’m going to consider my Jeanneau NC 795 as an example of a planing boat, and a Nordic Tug 32 as an example of a cruiser, because there’s one parked next to me and it’s a beautiful thing.
’Rithmetic · My car has a 90 kWh battery, of a size and weight that could be accommodated in either boat quite straightforwardly. A well-designed electric such as a Tesla typically burns 20 kWh/100km but you can’t use all the kWh in a battery, so you can reasonably expect a range of about 400km.
The Jeanneau gets about 1.1 km per liter of fuel while planing (it does two or three times better while cruising along at hull speed). Reviewers say that at 7-8 knots the Nordic burns about a gallon per hour, which my arithmetic says is 3.785 km/L.
A typical gas car gets about 10L / 100km, so 10 km/L. So the Nordic Tug is about 38% as efficient as turning fuel into km as a car, and the Jeanneau is only about 11% as efficient. (And of course both go much slower, but that’s not today’s issue.)
If the same is true for electric “fuel”, the battery that can take a Tesla 400km could take the Nordic tug about 150km and the Jeanneau a mere 44km.
Discussion · There are boats that get worse mileage than the Jeanneau, but they’re super-macho muscle boats or extravagant yachts the size of houses. So for recreational boats accessible to mere mortals, the Jeanneau, which is in a class called “Express Cruiser”, is kind of a worst-case, a comfy family carrier that can be made to go fast on the way to the cabin, but you pay for it.
So the tentative conclusion is that at the moment, batteries are not that attractive for express cruisers. But for tugboat-class craft designed for smoothness not speed, I’d say the time to start building them is now. Among other things, marine engine maintenance is a major pain in boaters’ butts, and electric engines need a whole lot less. On top of which they’re a lot smaller, and space is always at a premium out on the water.
Variations and Futures · The following are things likely to undermine the calculations above:
The Jeanneau could fit in quite a bit bigger battery than most cars; packing it into the hull so that the boat still performs well would be an interesting marine engineering problem.
The Nordic Tug could pretty easily fit in a battery two or three times that size and, at hull speed, I suspect it wouldn’t slow down much.
The torque curves of electric and gas engines are vastly different; which is to say, electrics don’t really have one. You have to get the Jeanneau’s engine up north of 4K RPM to really zoom along, which I suspect is not exactly fuel-optimized performance.
Related to the previous item, I wouldn’t be surprised if there were considerable gains you could pull out of the low-RPM/high-torque electric engine in terms of propeller design and perhaps hull shape.
Battery energy density is improving monotonically, but slowly.
Boats tend to spend their time out under the open sky and many have flat roofs you could put solar panels on to (slowly) recharge your batteries. In fact, many do already just for the 12V cabin batteries that run the fridge and lights.
I expect installation of Level 2 chargers on the dockside would require some innovation for safe operation in an exposed environment full of salt water. I doubt that it’d be practical to offer 50-100kW DC fast-charging gear at the gas barge.
I’ve long lusted after tugboat-style craft, but they’re expensive, bigger (thus moorage is hard to find), and go slower. Given a plausible electric offering, I think I could learn to live with that.