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Electric Multihulls

Multihulls like this Fountaine Pajot Isla 40 make great candidates for alternative drives

Multihulls like this Fountaine Pajot Isla 40 make great candidates for alternative drives

Well, as ever in the world of sailing, the electric revolution is taking a little bit longer to filter down. “We are so far behind Europe in this technology,” says Sally Reuther of Annapolis Hybrid Marine. “But we’ve seen a change in [recent] months. At boat shows, people come up to us with questions, and they’ve already done their research.”

Indeed, there is a growing crop of system suppliers, among whom the best known for leisure sailors are Torqeedo, Oceanvolt, Bell Marine and Elco. Volvo Penta is also making strides with a comprehensive electric range in the work, a project in which it has been working with catamaran builder Fountaine-Pajot.

Among the chief attractions of this kind of technology are its silent operation, instant torque, the absence of exhaust fumes and freedom from the tyranny of having to buy diesel. If it was that simple, though, boats would all have converted to electric long ago. The problem is that there are also two major problems with electric propulsion for marine. The first is cost: you can expect to pay a good deal more for a new boat when you spec electric motors. The second is that even if you go wild in terms of battery capacity there is still the problem range and what to do to keep from running out of power.

There are some savings, of course. There’s the diesel itself, for one. Electric motors are also more reliable than combustion engines, cutting maintenance costs. “Running costs were incomparable,” says Pierre Vanwinsberghe, who installed two 8.6kW Oceanvolt motors on his Aventura 33. “An oil change every year and one $220 seal kit every five years. Nothing else to do!”

Beyond that, it should come as no surprise that Volvo decided to work with a multihull builder, as catamaran and trimaran hulls are more easily driven than traditional monohulls, making them better suited to a hybrid or electrical propulsion system.

Multihulls also offer a good deal more surface area for fitting solar panels, courtesy of their broad beam. “A multihull gives a lot of possibilities in space for energy ‘scavenging’, think here about solar panels, solar collectors, wind generators,” says Bart Meenks of Dutch supplier Bell Marine.

Solar, I hear you snort in disbelief? Absolutely, because solar panels are increasingly seen as a key element in the design of any electric propulsion system for yachts. In fact, if you speak to Michael Köhler of electric catamaran brand Silent Yachts, solar is the only game in town. “When you hear the wind turbine making all that noise, you think it will be producing lots of power, but it’s not. When we did the measurements, solar produced much more.”

Solar panel technology is also improving all the time. Where once, a good panel offered 5 to 8 percent efficiency, they can now manage more than 20 percent. With its 49 square meters of solar panels, the Silent 55 can generate some 10kW when the sun is high in the sky. “The boat’s house load is 5-10kWh per day,” explains Jean-Marc Zanni, who designed the 55’s electrical system. “So you can go for weeks between recharging. The aim is to provide full comfort and silence at anchor or in port.”

François Bouffard, who specced his Slyder 47 with twin Oceanvolt 15kW motors, agrees. “Solar, solar, solar!” he says. His boat carries 880W of solar panels, but he says he would install three or four times this capacity if he were designing the system again. “With 3,000W of solar panels, there would be a slight surplus on the whole domestic load.”

Along these same lines, Jan-Dirk Lohmüller fitted his Fountaine-Pajot Saba 50 with an Oceanvolt system for a world tour and has enlarged his solar array at every opportunity. He began with 880W from the yard, then added three 320W panels in Panama. The boat is now in Tahiti, where it has just had a further four 320W panels fitted on a gantry over the davits. “Since then we enjoy complete energy independence from fossil fuels on medium sunny days,” he says. “Even with intensive induction cooking, a coffee machine for 12 or so coffees and one or two washing machines, we have no problem.”

The schematic of a Gunboat electric drive system

The schematic of a Gunboat electric drive system

And Solar is only part of the story when it comes to renewables. Nothing but the water turning the propeller under sail can also generate from 100W at 5 knots up to 5kW on a big yacht at speed. Here again, multihulls are far more effective than monohulls, due to the fact there faster cruising speeds deliver much more power from this hydrogeneration. Catamarans, in particular, enjoy a further because they have two motors.

As an example, Bouffard says his Slyder managed a full week of fossil-free sailing during a 2017 Atlantic crossing. “For our water and cooking needs, and everything else, we relied solely on regeneration.” He estimates his two Gori props generated a combined 300W to 500W over the course of the week—equivalent to around 9kWh per day.

That said, he also says he is cautious about the technology’s potential. “On my boat, which is a heavier prototype, I start to produce over 100W at 7 knots. But when the waves come from the back, regeneration is much less. Something to do with the motion of the boat.” Things improved when he removed the mini skegs that protected the saildrives. “They were creating turbulence and reducing the efficiency of the propellers,” he says.

Other installations have also had positive results. Lohmüller says that at 5-8 knots, each motor on his Saba 50 can generate up to 700W, while on fast passages above 10 knots they have seen 1kW from each unit. Similarly, Oceanvolt has now developed a proprietary regenerative system with a variable-pitch propeller with blades that sweep through a full 180 degrees. The electronically controlled ServoProp, as it’s called, can produce 1kW at 6-8 knots of boatspeed. Tests on a 38ft monohull produced 3kW at 12 knots.

Different mentality

Even with a decent solar array and a regenerating propeller, range still is going to be an issue for all but the lightest and fastest multihulls. At the end of the day, and even accounting for the much lower efficiency of a combustion engine, diesel still works out 10 times more energy dense than commercially sensible batteries. There More efficient battery types are in development, but none have yet been demonstrated affordably at scale.

What that means is that in order to get the range provided by a 100-liter diesel tank, you’d have to carry around a ton of batteries. The good news is that you can place them below the waterline where they act as ballast. The bad news is they’re expensive. In a sense, though, it is wrong to even think this way, i.e., to try and recreate the characteristics of a fossil fuel system onboard.

Rather, Annapolis Hybrid Marine’s Reuther says, sailors need to change their mentality, use their sails more and avoid just rushing from A to B. “Often sailors are closet power boaters,” she says. “The idea of being on a boat is to enjoy being out on the water, not looking at your watch the whole time. Don’t try and do it on a schedule!”

With this in mind, Wolf Boss has taken this principle to great lengths in the design of his Neel 51 trimaran. Not only has he installed a 50kW Deep Blue from Torqeedo with 60kWh of batteries, he has also introduced a rainwater harvesting system on board, a small farm and even composting for waste and gray water. The aim is to reduce reliance on fossil fuels to zero. “The boat is part of a wider project to promote another way of boating,” Boss says.

A more typical approach is to install a hybrid system that includes a dedicated diesel generator to recharge the batteries. It puts hydrocarbons back on board, but runs much more efficiently than a pure diesel system. In practice, onboard renewables can cover shorter bursts of motoring, the generator facilitating longer motoring distances.

“When you look at bluewater, of course, you will have a diesel,” Torqeedo’s founder and CEO Christoph Balinn says.

Bouffard agrees, noting his setup includes an 18kW Fischer Panda generator. “You cannot beat the energy density of fossil fuels. So then the concept of the boat is to sail with low wind. Using lots of engine was not my goal anyway—I prefer to sail slower.”

On passage, Bouffard’s Slyder 47 manages 5 knots in calm conditions with each of his 15kW motors running at just 4.5kW. The generator is at optimum efficiency churning out 14.5kW, where it consumes a gallon and a half of diesel per hour. Taking into account the surplus power going into the batteries, he estimates the generator propels the boat around 30 percent more efficiently than the engine.

In fact, the most efficient setup employs a DC generator. However, these are specialist pieces of equipment and therefore costly. Bouffard’s advice is to use a cheaper AC unit. During his 2017 cruise from Europe to the Caribbean and back, he ran his generator an average of about 30 minutes per day.

An example of a Gunboat control panel screen

An example of a Gunboat control panel screen

Installation issues

Power is the first issue to tackle when speccing a new system. Specifically, because an electric motor offers optimum torque at low and medium speeds, manufacturers say a 10kW motor, for example, can be 50 to 100 percent more effective than a 10kW combustion engine, especially when paired with the right prop.

Meenks at Bell Marine has a useful rule of thumb here. “The total installed power in kilowatts should be around 2.5 times the displacement in metric tons,” he says. “This gives enough power for maneuvering in hard situations.”

Following this rule of thumb, a fully loaded Lagoon 450 displacing 20 tons would need 50kW across the two hulls. Divide this by three to get the consumption at cruising speed, then multiply by the number of hours you want under power to get the necessary battery capacity. Finally, Meenks says, add the necessary safety margin and a margin for your other onboard systems.”

Note, as an added benefit, the inherent flexibility of an electrical system can also make it easier to install. For example, you can elect to install two entirely separate systems with their own batteries, to preserve the redundancy that twin-engined catamarans enjoy. Or you can pool resources with a single battery bank, charger, inverter and so on. If you choose a hybrid system, the generator can be positioned remotely from the motor and batteries—it just requires a hefty enough electrical cable to carry the power to where it’s needed.

Two views of an Oceanvolt electric saildrive unit

Two views of an Oceanvolt electric saildrive unit

The market already offers a wide choice of AC and DC motors with shaft drives up to 100kW of continuous power, saildrives up to 25kW and pod drives up to 10kW. There is a lot of noise about the benefits of AC versus DC, but in the final analysis, what matters most is quality.

“Claiming that an AC motor is more efficient than a DC motor, or vice-versa is similar to suggesting that all white wines are superior to all red wines,” says David DiQuinzio of Clean eMarine, which supplies Thoosa DC motors. Good motors on both sides will be capable of around 93 percent efficiency.

Finally, given the stakes, it only makes sense to take a good long look at why exactly it is you’re considering going electric. Does it make sense in purely financial terms? Emphatically not. Some even question the environmental benefits given the total carbon footprint resulting from the manufacturer of Lithium-ion batteries.

When it comes to quiet operation and the absence of onboard emissions and comfort, the benefits are clear, so much so that if and when the next big step comes in battery technology, the competition may be over.

“Theoretically, they’ve tested batteries in labs that are 10 times more efficient than lithium,” says Torqeedo’s Christoph Balinn. “And if that comes through, then gasoline is done.” 

MHS Summer 2021

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