Jake Kavanagh looks at how the burgeoning electric market for electric outboard motors is paving the way for more widespread electric propulsion

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The market for electric outboard motors has been growing fast over the past few years. Boat owners love the idea of silent, fume-free cruising using lightweight motors that will have a long service life. Standard propeller-driven outboards are still the biggest sellers but there are new designs that can convert electric power into thrust with up to 96% efficiency. The initial purchase price is still high, but the motor will gradually pay back over the years in saved fuel, service costs and consumables.

“With an electric outboard, there’s not much to go wrong,” says Nathaniel Evans, a sales engineer with Golden Arrow Marine. As an agent for the German-made Torqeedo range (now part of Duetz) Evans also looks after customer service but admits it’s a quiet desk.

“The motors are sealed for life, and the batteries can last for over 2,000 charge cycles. The only time we have to strip an outboard down is when it has been damaged, usually by hitting a submerged object. If the impact is hard enough, it can break a seal and let water in. But apart from that, the brushless motors require almost no servicing at all. If the dedicated battery packs are properly looked after, especially when it comes to long-term storage, they give faultless service too.”

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According to Evans, portability appears to be a major factor for customers buying a Torqeedo outboard. “Boaters love them,” he said. “There are just three components – the motor, battery and controller, all of which are relatively lightweight. This appeals to the older generation of boaters who want something dependable and highly portable.”

While smaller electric outboards can have a higher purchase price than an equivalent 4-stroke, Evans points out that they also have none of the ongoing costs. “The fuel can come from renewables, and there is no servicing or winterisation needed. Also, no storage issues with sump oil. This is all helping to drive sales.”

Another big advantage is longevity, a subject that US-based Stealth Outboards is keen to promote. “There is little or no obsolescence with the outboard,” says MD Scott Masterson.

electric-outboard-motors-charging

Venice has been pioneering ‘charging posts’ allowing visitors to plug in their electric boat during shore excursions

“Many of us wish our old petrol 40hp outboard still ran as great as it did on the day it was new, like electric does. Electric motors are rated in thousands of hours of operation, not hundreds. Resale values for electric outboards are excellent and used motors are well sought after. As batteries become lighter, more powerful and affordable, this industry will explode.”

Even quite powerful e-outboards are still easy to lift on and off a boat, especially when the battery is a detachable unit. As most proprietary batteries are lithium-ion, they are usually around three times lighter than a lead-acid equivalent anyway.

As a result of more dependable and ‘smarter’ motor technology and steadily improving battery chemistry, yacht owners who have converted to electric-only propulsion – even if just for the tender – usually endorse their decision. The experience with the cleanliness and silent running of the outboard often encourages them to consider electric propulsion for the yacht itself.

As TEMO’s Alexandre Seux explains: “Few boaters will invest €10,000 for an electric propulsion system for their yacht until they are fully confident with the technology. Instead, they will start with dinghy propulsion, but only once they are convinced by its advantages will they invest in higher power for their main boat. I believe the dinghy market will drive the growth of the entire e-propulsion market in the yachting industry. That’s why at TEMO we chose to start small and every year we will unveil a more powerful system.”

Pros and cons of electric outboard motors

Pros

  • Reliability
  • Longevity
  • Portability
  • Low running costs
  • Low maintenance
  • Low noise levels
  • No fluids
  • Renewable fuel
  • Good resale value

Cons

  • High initial cost
  • Range limits
  • Long recharge times

Electric outboard motor battery power problems

Battery range, battery price and recharge times are the main stumbling blocks for electric power in the general marine market. Per kilogram, battery chemistry is lagging a long way behind the energy density of fossil fuels but is steadily catching up. This drawback is being offset by much smarter control systems to eke out the amps.

The price of lithium ion, which is lightweight and can be repeatedly deep cycled without negative effects (unlike ordinary lead-acid), remains high, however lithium itself is becoming cheaper due to economies of scale.

In the automotive sector, leading companies such as Tesla, Honda and Volvo are pushing ahead with research into chemistries that can accept rapid charging. No-one wants to sit on a garage forecourt for more than a few minutes while refuelling.

Meanwhile, shipping is exploring long-range alternatives as the world’s merchant fleets attempt to become emission free by 2060, the hydrogen fuel cell being a clear leader.

Don’t write off the 4-stroke outboard just yet. They have far greater range in terms of energy density, and manufacturers are working hard to make them lighter and easier to maintain

How much power do electric outboard motors produce?

The output of the internal combustion engine is usually rated in horsepower. How that horsepower is harnessed depends on the gearing and the prop. Some petrol outboards are deliberately configured to drive a small yacht, with the prop pitch and gearing differing from a similar horsepower model intended to drive a tender.

A lot of that changes when it comes to electric outboard motors. The industry will still talk in terms of horsepower equivalents, but electric motor output is always given in kilowatts (kW). The basic conversion is 1kW = 1.34 horsepower.

Submersible motor-type electric outboards will operate without a gearbox, which is a key advantage. The power is direct and totally controllable, with no minimum revs needed to prevent a stall. The drive can also go from full ahead to full astern almost instantaneously, which can be extremely useful.

Battery capacity is the main factor to be considered when choosing the right outboard. The more power you have, the faster you can go (hull shape permitting) but the shorter the range. When it comes to electric outboard motors, they’ll usually be powering a relatively small craft with limited space to store the batteries. On a tender, the battery pack may even be a detachable part of the motor itself.

But, as Aimid Lazaaoui of Dutch-based Elva remarks, a limiting factor in high-speed, larger horsepower electric motors will be the ability to fast charge. His company produces outboards up to 3.5kW, with inboards of up to 9kW, but still sees most growth in the lower-powered cruising sector.

“The bigger the motor, the more charging power you need,” he says. “The average residential socket provides 16A at 230V, which can recharge a system of up to 3.3kW. But for larger capacities you’ll need specialised fast chargers.”

Calculating an electric outboard motor’s horsepower

The real output of any engine is the actual thrust delivered at the propeller. As such, care has to be taken when comparing the output of electric outboard motors against petrol equivalents. Beware of systems rated at ‘10kW’ when this actually refers to the kilowatt hours (kWh) of the batteries.

“We use a basic formula to determine real power on the shaft,” says e-Tech’s Norbert Fryca. “This is calculated by using input power from batteries multiplied by the efficiency percentage of the motor. Let’s assume a motor delivers 56% efficiency and has a maximum 10kW of battery power. So, 10kW x 56% max efficiency = 5.6kW on the shaft.”

This formula would vary according to the motor’s efficiency rating, and some of them are very efficient indeed. Vetus, for example, claims an efficiency of 96% on its direct drive E-Pod.

Hydro-regeneration is becoming more efficient for cruising yachts

Hydro regeneration

The ability for sailing boats to recover energy from a free-wheeling propeller has met with variable success over the years, but companies such as Torqeedo, ePropulsion and Seadrive have made this hydro-regen more efficient. The main issue has been the comparatively low rpm from a freewheeling propeller, so even the most efficient regenerators still require the yacht to be sailing quite briskly.

Advances in propellers designed specifically for electric boats, such as with models by Gori, Darglow and Bruntons have made the ‘reverse’ pitch needed more effective.

Torqeedo has long offered hydro-regen on even its smaller models, while ePropulsion provides regeneration models in its portfolio of new Spirit and Navy electric outboard motors. The propellers have been modified to be more efficient when freewheeling, while the controllers have been designed to accept even small amounts of returning current and feed it into the batteries.

Regeneration is usually not worth the prop drag below about 4 knots, but the power curves – as seen from this ePropulsion graphic – rise sharply once the system is energised

Regeneration is arguably more advanced in some of the pod drives, mainly due to their bigger motors and larger propellers.

Specially-configured props can greatly reduce the drag created, but some hydro-regen systems can still take up to a knot off the sailing speed of a yacht.

Consequently, some smart controllers take data directly from the GPS and don’t bother to engage regeneration capability until the extra drag will barely be noticed.

What voltage?

The higher the voltage in a system, the more energy can be passed through it. When it comes to electric outboard motors for tenders and small day-sailers, as opposed to the models for much faster speeds, the maximum is usually 48V, mainly because if an open circuit is touched, the shock is non-lethal. Smaller ‘trolling’ type motors are very often 12V.

For motors whose battery is built in, or a proprietary unit, 48V is most common. Some outboard manufacturers can even customise a motor to the voltage available on your boat. US-based Stealth, for example, can offer voltages from 24V-150V.

Electric outboard motors don’t need mechanical throttle linkages

Smart controller

Electric outboard motor controls can be fitted with more than just ‘on/off and speed’ functions. Many now include options for GPS and advanced diagnostics and can be connected into a boat’s entire electrical system to monitor and control the ebb and flow of energy.

The built-in controllers also protect the batteries by preventing overcharging or under-voltage. In addition, remote controls often only need a data cable – instead of mechanical cables – so the electronic throttle and screen can be mounted almost anywhere and duplicated with ease.

Electric outboard motor battery considerations

Several outboards have their own proprietary batteries, key to their successful operation. The manufacturers have deliberately paired their motors with a battery that can ‘talk’ to the controller. Others can be linked to any kind of battery, so long as the power available is the right voltage.


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‘Electric outboard motors: How new tech is weaning boat owners off fossil fuels’ was first published in the July 21 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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