They’ve been a long time coming, but marine hybrid propulsion systems are finally a working reality, as Sam Fortescue reports

Every sailor is familiar with the wet cough of the diesel engine, and the acrid smell of its exhaust. For some it’s the sign that an adventure is starting, for others it is the reassurance that all is well on board the boat. The traditional engine is perhaps your boat’s most important safety feature, but its days may be numbered.

The electric sailing revolution is coming – and though adoption in the marine sector is proving much slower than in the automotive world ashore, progress is being made.

The market is still relatively small. Clear market leader Torqeedo had sales of €25m last year, most of which was in ferries and compact outboards. It also offers a range of saildrive and pod drive motors for yachts displacing from 2 to 50 tonnes, or roughly 20-60ft LOA.

But sailors have been slow on the uptake, and for one good reason: if you’re planning to cross an ocean or take on tough conditions offshore, you rely on your engine to help you outrun danger or motor through the doldrums – sometimes for days at a time.


Oceanvolt AXC series is a modular shaft drive system (10kW to 40kW) that will fit in place of a tradition diesel engine

Even with the current crop of advanced lithium-ion boat batteries, the range of an electric system is measured in tens of miles, not hundreds. So a 35ft monohull with 10kWh of lithium battery (four units weighing 96kg in total) would have a range of just 24 nautical miles at 3.8 knots, or less than 16 nautical miles at full throttle.

Taking into account the incredible wastage of combustion engines, which dissipate more energy as heat and noise than they provide in propulsion, diesel is still ten times more energy dense than batteries.


Full-carbon luxury daysailer Yamila uses an Oceanvolt SD8 8kW electric saildrive system. Photo: Tobias Stoerkle

“When you look at bluewater cruisers, of course you will have a diesel,” says Torqeedo’s founder and CEO, Dr Christoph Ballin. “And it’s right that not many coastal sailors opt for pure electric.”

But that doesn’t mean that electric has no interest for cruising sailors – far from it. The more common route for ‘normal’ sailors will be to combine diesel and electric in a hybrid sailing system.

Under this model, the engine is replaced by an electric motor, hooked up to a bank of lithium batteries. This can be charged via hydrogeneration – when the speed under sail turns the propeller and puts charge back into the batteries – and solar or wind. But when extended periods under power are required a standalone DC generator, which can be installed anywhere on board, supplies the electricity.

This is the set-up recommended by Finland’s Oceanvolt, which has focused on the cruising sailing market with a range of shaft and sail drive motors from 3.7kW to 15kW (roughly 10hp to 45hp in diesel engine terms).

“In the case of the round-the-world cruiser, we recommend a hybrid system with a backup genset to support continuous drive when/if needed,” says Oceanvolt CEO Markus Mustelin. “A regenerating prop, which spins while sailing and recharges the batteries (sacrificing 0.2-0.4 of a knot, depending on the boat and conditions) makes it possible to be almost independent of the genset and use it only for backup.”

This system has the advantage that the generator is only needed on longer passages, so the boat still manoeuvres silently in and out of ports and anchorages.

And a well-designed, correctly sized generator is much more efficient at turning diesel into electricity than an engine not originally designed for the job. Some sailors opt for an in-line hybrid system, like those offered by Hybrid-Marine, which bolts onto the existing diesel.

These are easier to retrofit, with many of the same characteristics as the full hybrid system, but there’s the disadvantage of still having an engine boxed away somewhere near the middle of the boat.


Electro magnetism

Until now, most business has been done through retrofitting existing yachts. But an increasing number of yacht builders are looking to include electric propulsion as original equipment. The world’s third largest boatbuilder, Hanse Yachts, is perhaps the most advanced – offering its entry-level Hanse 315 with an electric rudder-drive option.

The system takes up less space than the standard diesel, is much quieter and vibration- and emissions-free. But Hanse admits take up has been disappointing.

The technology has found more interest among lake sailors. Innovative young German brand Bente has been fitting Torqeedo motors to its successful 24ft model, originally designed for Germany’s ‘Green Lakes’.

Closer to home, dinghy specialist RS Sailing has decided to fit a retractable electric drive to its new RS21 keelboat. Already christened the ‘invisible gennaker’, the system is based on Torqeedo’s Travel 1003 outboard motor.

Bigger race boats have also been attracted by the lure of low-weight propulsion. Just look at Malizia, an IMOCA 60 being prepared for the 2020 Vendée Globe with a lightweight Torqeedo system.

“Emissions-free round the world under race conditions, while simultaneously producing your own energy, is a thoroughly inspirational concept,” said Malizia skipper Boris Herrmann.

Electric has also been successful at the luxury end of the market, where lithium-ion batteries account for a smaller share of the boat’s overall cost. A 50ft Privilege 5 catamaran and a carbon fibre Gunboat 60 have both been retrofitted with Torqeedo kit, while Oceanvolt appears on a Swan 57 and an all-carbon Agile 42.


Overview of the Torqeedo Deep Blue propulsion system installed in the Gunboat Moonwave

The Gunboat Moonwave has two 25kW Deep Blue saildrives both capable of regenerating under sail. There is still a generator on board to extend battery range offshore, but “they no longer use the generator – it’s just for emergency,” says Torqeedo’s Ballin.

Spirit Yachts is also designing electric propulsion into its Spirit 111 flagship, due for launch this summer. With four big 40kW lithium batteries aboard and a 100kW motor, the yacht will be able to operate silently for hours, although it also has 100kW of diesel generator capacity.

“The real focus is not the propulsion,” explains Spirit director Nigel Stuart, “but that everything works in harmony, from galley equipment and hot water to heating, air conditioning, hydraulics etc.” The British yard is also building a 65-footer using Oceanvolt hybrid technology and a new 44-footer that is pure electric.

With racing on one hand and high-end cruisers on the other, there is something of a gap in the middle. By Torqeedo’s own admission, the cruising sailor hasn’t been a big focus of the electric revolution, but all that is about to change. “We started a bit late with sailing,” Ballin admits, “but in the next five to eight years it will be addressed big time.”


Fully integrated electric drive system will power the new 111ft Spirit Yachts flagship

What does that really mean? Well, in the first instance, it means system integration. If that doesn’t sound revolutionary, then imagine a set-up on board where solar panels, hydrogenerators, batteries, generators and motors all worked seamlessly together to keep the yacht supplied with ample power around the clock. “That’s what people are willing to pay for: plenty of energy with heating or air-con through the night,” says Ballin.

The future of hybrid sailing

In the near future, Torqeedo is planning a new range-extending DC generator specifically for hybrid sailing boats. Its existing unit is built by WhisperPower and provides 25kW, which is too much power for boats using the pod drive system.

The genset will be designed to operate at optimum revolutions, while clever DC to DC conversion decouples the battery voltage from the charging voltage, for much greater efficiency.

With boats, just as with cars, the breakthrough that will make all the difference is around battery capacity. Until range under electric power can match that of diesel, there will be many sceptics. And that isn’t likely to happen for a decade or more, according to Ballin.

“Theoretically, they’ve tested batteries in labs that are ten times more efficient than lithium,” he explains. “And if that comes through, then gasoline is done. But we are trying to combine long-term vision with short-term mindset.”

In the meantime, the prevalent technology is based on lithium-manganese-cobalt, and a process of steady development is making this 5-8% better each year. For example, BMW has just announced its next generation i3 battery, used by Torqeedo’s Deep Blue system, will be able to hold 40kWh of power – an increase of 33% for the same size, weight and nearly the same cost.


Torqeedo Cruise 2.0 FP Pod Drive is suitable for small yachts up to 4 tonnes – a folding prop can also be fitted

The other area of development is around the propeller. Most cruising systems use a folding or feathering prop designed for diesel engines. But Torqeedo’s own research shows that the consistently high torque of an electric motor is best utilised by props with variable pitch.

And yet it is Oceanvolt that has addressed this issue specifically for electric motors with its Servo Prop system, which it claims to be 30% more efficient ahead, 100% better astern and 300% more efficient in regeneration mode.

Oceanvolt says that this prop can pump around 500W into the batteries at just 5 knots – the average pace of a 30ft monohull. At 6 knots that rises to around 800W, and at a very manageable 7 knots for a larger ocean cruiser you get 1.2kW.

“A new technology can rarely compete in price with an established one in its initial growth phase,” says Mustelin. “However, we have passed this and today electric systems are offered at a quite competitive price. When you add to that the fact the electric system is almost service free, the total cost of ownership is turning in favour of electric.”

So, you may not hear them approach, but expect to see more and more electric-powered boats on the water as the revolution continues.

A question of torque

A key part of the viability of electric propulsion rests on the notion that a smaller motor can achieve the same work as a bigger diesel. There are two elements to this. First, a diesel engine is not an efficient converter of chemical energy into thrust, creating a lot of heat and noise in the process. Second, the torque characteristics of electric are much better than diesel.

Mustelin says that Oceanvolt’s 10kW motor “easily outperforms” a 30hp diesel. “Typically, maximum boat speed will be somewhat lower (0.5kt-1.0kt) than with a comparable diesel engine, but at the same time the boat will maintain the speed better in heavy seas and headwind due to higher torque. Manoeuvrability is much better in confined marina spaces.”

That’s because combustion engines only reach peak power (and maximum torque) over a small range of speeds. Torque is a measure of turning power – at the propeller in the case of a boat.

A diesel engine develops optimum torque between 1,800-2,000rpm, while electric motors deliver it from 0 to around 2,000rpm. This allows electric motors to use higher efficiency propellers that are slimmer and more steeply pitched.


Engine-driven: The ‘alternator on steroids’

It has taken years of development and over $10m of funding, but renowned boat systems expert Nigel Calder has helped design an alternator so powerful that it eliminates the need for a generator on board.

Mounted on the engine, on the second alternator position, the Integrel can produce five to ten times more power. Sitting behind the system is at least 10kWh of lead acid batteries (lithium is also an option), and Victron chargers and inverters.

“If you crank the engine it’ll charge the batteries; if you’re running with the engine in neutral, it’ll know it’s in standalone generator mode and switch to that algorithm,” explains Calder. “It will likely be cheaper than a generator installation, and eliminates the issue of the through-hulls, the cooling circuits, the long running hours, the maintenance.”

The system allows you to run all sorts of creature comforts on board that would normally require a generator: from hot water on-demand to coffee makers and freezers. “We honestly believe that this system is going to supplant generators on almost all boats that currently have, or would like to have, a generator,”
adds Calder.

With the engine in gear and at low revs, tests show how the Integrel can produce some 2kW of power without increasing fuel consumption or reducing speed – simply utilising the engine’s wasted capacity. This means it will work with the yacht’s existing engine – no need to overspec – and it has already been successfully installed on a new Southerly 480, a Malo 46 and a similar-sized Hallberg-Rassy.


Case study: Dufour 382 Alcyone

Built by Dufour in 2016, Alcyone was immediately retrofitted professionally with Oceanvolt’s SD15 saildrive motor, supplied by a 14kWh lithium battery bank. Owners Michael Melling and Diana Kolpak also specced an 8kWh DC generator for range extension. The fit out cost €30,600 for the motor and battery system, plus an additional €13,744 for the generator, and installation costs were around €8,000.

They charter the boat out near Vancouver, for exploring Desolation Sound and the surrounding area where silent, clean propulsion is a selling point. “Nothing spoils the joy of sailing – or a secluded anchorage – more than the noise and smell of diesel engines,” they explained. “Installing an Oceanvolt system in our new boat has freed us from that. It’s the way of the future.”

Charter manager Merion Martin said the conversion has also been popular with charter customers, adding: “The main advantage of the system is that it consistently uses around 40% less fuel than a standard diesel engine over the course of a week’s charter. But understanding the power management system takes a bit of getting used to, and the many components involved in the system can make troubleshooting a challenge.”

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