How to choose the right electric boat engine

The sheer versatility of the modern electric motor has been harnessed to offer a bewildering choice of drives, but they essentially fall into three main categories: outboard, inboard and pod (or thruster).

There’s a bit of cross-over between concepts – saildrives, jet drives and hybrids, for example – but for any conversion project the propeller will be spun by one of the three.

Before you start, there’s a key question to ask yourself. Why choose an electric boat engine?

The engine hours of a modern leisure boat are usually low. The average season for a UK-based motorboat is only 50 engine hours and a sailing yacht is even less.

So, your electric conversion won’t be to save fuel.

For the cost of an installation you could probably buy enough diesel to see you out.

The reason for a switch to electric is more likely for the near silent running, total lack of exhaust, virtually no maintenance and the chance to ‘go green’ and replace your fuel from renewables.

You will also have access to restricted waters.

Norway, for example, is about to ban the combustion engine from some of its fjords.

For these reasons, the electric boat engine should be seen as a long-term investment.

“Our motors can last for more than 50,000 hours,” said Brandon Salls, CEO of US-based Elco. “We have one of our first electric inboards from 1890 still going strong today.”

Apart from the motor, which will usually cost more than a brand-new internal combustion equivalent, the outlay is then doubled by the batteries.

Unfortunately, the price of lithium-ion chemistries, currently the most efficient way to store traction energy, remains high.

That said, economies of scale from the burgeoning automotive sector are helping to drive down prices while cheaper battery chemistries are emerging all the time.

Once again, the battery should be seen as a long-term investment.

If properly cared for, it should give you around 2,000 cycles – or around 10 years use or more – and can also drive mains appliances on board via an inverter.

Which drive system?

Having decided the cost is worth it (which many already have) how do you choose the right drive? Your boat will give you a clue.

What is driving it already, and is there an electric equivalent? Chances are, absolutely.

The choice of e-outboards now is huge and across all powerbands, with several companies also making sail drives.

Meanwhile, the e-inboard market has already blossomed, with just about every output catered for.

Then there are the fixed or directional pods, and even units that can be deployed for manoeuvring only.

You can even stick a battery-powered e-jet into your surfboard or kayak.

Whatever currently drives your boat can either be directly swapped for an electric version or supplemented with one.

Alternatively, you can opt for a drive that frees up essential space in the accommodation.

Swapping a centrally located inboard diesel for a low profile electric saildrive has proved a useful and relatively simple upgrade.

How many kilowatts?

This is trickier. The standard conversion from horsepower to kilowatts is to multiply by 1.359, so a 10kW electric outboard is – theoretically – equivalent to 13.5hp. Except it isn’t.

The electric drive will often be more powerful, as Sander Dijk, chief technology officer of the Netherlands-based Propel, explained: “Engines are producing torque, the rotational equivalent of a linear force,” he said.

“So, one has to convert torque to horsepower to measure an engine’s hp. Add in the equations relating to torque and velocity, and you’ll find that horsepower always equals torque (in pounds-force feet) multiplied by the rotations per minute (rpm), divided by 5,252.”

Power at all speeds

Dijk says that as electric motors can produce thrust at just a few rpm, this allows a larger and more efficient propeller to be mounted.

Unlike combustion engines, the motors are also less prone to extremes of temperature, humidity, and fuel quality and have a smoother torque curve.

This makes them more effective across the entire rev range.

“In our tests, we have seen the 13kW (16.9hp) Propel S1 outboard motor with an optimized 12.8in propeller outperform a 25hp internal combustion engine, both in acceleration and top speed,” Dijk said.

“At the end of the day, it is more important to decide on the most suitable electric motor based on the characteristics of your boat and intended use, and not simply to compare kilowatts with horsepower.”

Which batteries?

After choosing your electric boat engine, the battery arrangement is the next big question.

For the smaller electric outboard engines, you may only have access to proprietary types, a bit like cordless power tools.

You can carry several battery packs as spares and just drop them into the outboard when needed.

Larger outboards allow for additional external batteries to be connected in parallel for more range.

Inboards and pods will need an internal battery and these are usually housed under the floorboards or bunks.

For displacement boats, especially on the inland waterways, traditional lead-acid traction batteries are the cheapest option, with the weight also useful as ballast.

Sailing yachts or high-performance motorboats need larger lithium-ion packs, although cheaper lead-carbon has also proved useful in some applications.

Motors up to about 20kW are usually 48V, this voltage deemed ‘safe to accidentally touch’ and so available to unqualified enthusiasts.

However, systems are now getting more powerful, with 96V being seen more often and some high-performance motors using automotive batteries of up to 800V.

Whichever motor you choose, you can make a calculation based on its peak output and the kilowatt hours your battery can provide to determine minimum range, remembering that the controller will shut the battery off at a specified voltage to increase its life.

Hydro-regen?

Another decision is how can the range be extended?

Many modern motors, including some outboards, now offer hydro-regeneration from a freewheeling propeller under sail, although four knots or more is needed to be effective.

Some propeller companies also make self-pitching propellers that maximise the regeneration possible.

Brunton’s Ecostar is a prime example.

Solar panels and wind turbines can top up your batteries during – and between – outings, and some panels can even be sewn into the sails.

Solar glazing and even solar paint are on the way to maximise future recharging opportunities afloat.

System wide approach

The final decision will be how the system is monitored and controlled.

Manufacturers are making the monitoring and control of their system as easy as possible, many with ‘plug and play’ installation.

There is also the option for remote monitoring via an app, so you can check the battery charge from home ahead of a trip.

Accurate monitoring reduces ‘range anxiety’ and many manufacturers design the entire system themselves, including the batteries, to ensure all components are optimised.

With brushless DC motors, loose external connections are usually all that goes wrong, to the annoyance of service engineers.

So, if you want to commit yourself to many years of near silent, smoke-free and low maintenance motoring, with all your fuel bought in advance (and often topped up for nothing) then think electric.

If you’re worried about range or speed and don’t want to invest in a big battery, then go down the hybrid route.

The choice for both is huge.

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