Review: Silent 55, the extraordinary solar powered yacht

Silent Yachts is tapping into the solar zeitgeist and creating a new meaning for the term ‘powercat’. Sam Fortescue reports

There is a slow, silent revolution under way in the yachting world. It is a revolution that is introducing tonnes of lithium and a sprinkling of silicon to the spec list of new boats. Holding out the promise of silent mobility, plus limitless domestic power on board, it made a big splash at the last Cannes Festival of Yachting – not least thanks to the new Silent 55 catamaran which debuted there.

From the pontoon side, the Silent 55 looks like a typical modern catamaran, with a big coachroof studded with windows and a flybridge helm. Except there’s no mast. Now, bear with me here. I realise that this is a sailing magazine, but we will shortly get back to more familiar territory. The unique qualities of this catamaran only become apparent from up top, where an expanse of solar panels stretches away fore and aft, embedded into the coachroof. The hard top itself carries yet more panels, and can be folded down flush to give an unshaded solar array of 49m2. During the heat of a summer day in the Med, this is capable of generating 10kW of power and up to around 60kWh in the course of the day.

“The boat’s house load is 5-10kWh per day,” explains Jean-Marc Zanni, who designed the 55’s electrical system using 140kWh of Panasonic batteries and the 370W Sunpower panels. “So you can go for weeks between recharging. The aim is to provide full comfort and silence at anchor or in port.”
It’s an intriguing possibility, and one that founder Michael Köhler is betting will appeal to a broad constituency of sailors. He has spent five years honing the concept of the solar-powered boat after he and his wife Heike finally sold the monohull in which they had sailed tens of thousands of miles. “We were annoyed by the fact that on a conventional sailing boat, you are usually motoring 50% of the time,” he explains. “And the whole energy system can’t supply all the boat’s consumption without running the generator.”
With insight gathered from years at sea, Köhler set about researching the best ways of harnessing renewable power aboard a cruising boat to keep the generator off. He weighed up wind, solar and hydrogenation (where the speed of the water flowing over the prop when the boat is sailing turns an electric motor to generate power).
“When you hear the wind turbine making all that noise, you think it will be producing lots of power,” he says. “But it’s not. When we did the measurements, solar produced much more.” The same is true of hydrogeneration using the main prop. “Regeneration starts making sense at 10 knots of boat speed. Before that, it is not as good as it seems.” In fact, Silent Yachts doesn’t include the contribution of regeneration at all in its power equation. “The simple truth is that solar is by far the most effective.”

But to make a solar system work in reality, Köhler had to go back to the drawing board on yacht design. The saloon and hulls have extra thermal insulation to keep air-con losses down, and the use of carbon and aramid in key areas helps reduce the overall weight to a decent 17 tonnes (a Lagoon 52 weighs 22.5 tonnes). He has tried to keep windows out of the direct sun with long overhangs and in contrast to the Lagoon’s 12 deck hatches, the Silent 55 has just two.

Holistic design

On the other hand, it has lots of opening windows, to allow a natural draught to do its job. “It’s a holistic approach – you can’t take the batteries and the drivetrain and drop it into another boat.”

Of course, using the propulsion system quickly takes its toll of the boat’s 140kW battery bank. The model on display at Cannes had two 135kW motors, giving you just half an hour of silent motoring flat-out, albeit at a top speed of over 20 knots. More reasonable 30kW engines and a single-digit speed give you greater range.
Nonetheless, the electric drive alone isn’t going to allow you to outrun a storm, or race home after a day at anchor, so the boat is designed to work with a generator hidden in the heavily insulated transom of its starboard hull. At cruising speed of around 5-6 knots, Köhler says there is rarely any need to use the generator, citing an owner who has just emailed him triumphantly about a second year totally generator-free. “In the end, you have to compare it to the performance of a sailing boat,” Köhler says. “It is as fast as a sailing boat in similar conditions – after all, there is no wind without sun.”
He went so far as to tell me during the sea trial in Palma, Mallorca, that he believed the majority of sailors would happily dispense with the hassle of sails and a rig if only they could enjoy silent motoring and anchoring. “As soon as people realise the incredible concept of this boat, they won’t understand why they ever did anything else.”

The market does not seem to agree with him – yet.
Sales of the boat have been good – they have already sold six, five of which are already in the water. But of those, four customers have taken the sail option, which means planting a 19.7m tall mast complete with boom and rigging slap bang in the middle of the coachroof solar array. “I was a bit amazed,” Köhler admits. “The shade from the rig reduces the energy generated by the solar area, while it costs more and is heavier, so consumes more fuel. Maybe it is for optical reasons.”
In fact, the shade of the rig slashes the average yield of the solar panels in half. In the Med, that means around 30kWh per day. But perhaps it figures. The typical profile of buyers is an environmentalist who has a Tesla electric car and is “an early adopter who likes to have things before others”. And at low speeds, with modest use of the air-con, the reduced energy generation should still cover daily consumption.

Under sail

The performance under sail should be reasonable because of the lightweight build of the boat, its broad 8.47m beam and stub keels added to each hull. Control lines are led back via conduits in the coachroof to the flybridge helm station, to make single-handing under sail a possibility.

More interesting, I think, is a sort of halfway-house option using a kite rig. This optimises the performance of the solar panels and gives plenty of propulsion. On the smaller 55 and the 64, Silent Yachts currently recommends a 19m2 kite that costs around €25,000 – a fraction of the cost of a new mast, boom, shrouds and sails. “The sail automatically makes a figure of eight above the boat, and you can steer it with a joystick or an app on an android phone,” Köhler explains. “It can propel the 55 at up to 6 knots, even in light winds.” Perfect for an Atlantic crossing, then.

For the bigger Silent 79, which will hit the water in the summer, a commercial grade Sky Sail system needs to be used – a smaller version of the ones used on cargo ships. This kite can propel the boat at ten knots, but it costs more than ten times as much as its smaller cousin. Both are capable of pulling the boat upwind.
So far, so new. But outside the novel energy and propulsion system, the Silent 55 aims to do what many other cruising catamarans are trying to achieve. “Most of our clients order for circumnavigation and long-term cruising,” Köhler says. So the boat is aimed to be as comfortable and capable as possible with watermakers, TVs and an induction hob that all capitalise on the boat’s abundant energy.
A flexible configuration allows owners the choice  of between three and six cabins – the latter designed for charter. The owner’s cabin lies forward of the saloon, under the windows of the coachroof, which provide magnificent views and abundant natural light. There’s a walk-around bed and steps down into the starboard hull give access to an en-suite shower room and heads.

In my view, the best cabin lies aft of this, accessed in the traditional manner down steps out of the saloon. The king-sized bed lies athwartships and the shower is larger than that of the master cabin. There’s more space down here, better headroom and still plenty of light courtesy of the many hull lights.

The finish is good rather than spectacular, with a range of choices around woods and fabrics. The intention is to keep weight down by using laminate where possible, but owners can choose glass or porcelain fittings wherever they want.
The 37m2 saloon is the star attraction on this boat, offering copious amounts of space for a well-equipped galley, a comfy dining or lounging area and a fully functional interior nav station.

It connects through sliding doors to the broad, uncluttered cockpit, which offers seating and a dining table with room for eight. A 4.5m tender can be slung from the underside of the bathing platform here, which can be raised and lowered hydraulically.There is more lounging space at the bow, where two little trampolines between the nacelle and the hull make comfy, if eccentric, nests. There are also cushions under the overhang of the coachroof.

On Trial

When I had the chance to sea trial the Silent 55, albeit in motorboat format, I jumped at it. It was a contrary autumn day on Mallorca with 15 knots breeze – just a shame, then, that this wasn’t one of the sailing configured versions.

To start with, getting on board is made really easy courtesy of deep boarding platforms on the skirts. She feels rather square because of that vast, glazed saloon with its deep overhang, and perhaps because of the utilitarian nature of the hard top, which is really about supporting more solar panels. Nevertheless, the side decks are broad and uncluttered.
The space up top is designed to concertina down flat, hence the hydraulic rams, fold-down seat back and lowering console. It makes a great sailing position, though, with all round visibility, and is also perfect for sundowners at anchor. When the rain comes down, this feels quite exposed, but there is a fully sheltered helm at the front of the saloon, and it is also possible to drive the boat from anywhere using a tablet thanks to smart electronics.
Under power, the handling is superb. The quietness of the motors is astonishing, and I gather they’ll be inaudible on the next boat, which will do away with the gearbox. Even in the aft cabins, directly above the motors, there is no more than a distant hum. The boat responds instantly to the power and the wind seemed to have no impact at all. As with any propulsion system, the power consumption jumps as you pile on the speed – it was sobering to see. At 6 knots, both motors drew 10kW but at 8 knots it was closer to 30kW.
I liked the huge saloon with its raised table for 360º views. And the sliding door and window gives great access aft, connecting the saloon and cockpit in fine conditions. The finish was smart and in muted tones, feeling more Scandinavian than German.

The future

Intriguingly, at least it seems to me, Köhler has tapped into something with the concept behind Silent Yachts – but not entirely for the reasons that he expected. Buyers are opting for the sail or kite versions of the boat because they want a comfortable wind-powered craft with abundant, quiet energy on tap.
It brings a whole new meaning to the  term ‘powercat’