Tested: Silent 55 Solar-Electric Power Catamaran

The Sound of Silence

Silent-Yachts looks to prove that a luxury motoryacht can run green and clean with their solar-powered catamaran.

Standing on the flybridge of the solar-electric Silent 55 in Ft. Lauderdale, I looked up from my notebook and realized we were already 15 feet from the dock. I hadn’t heard the captain, who was standing right in front of me, start the engines, put the boat in gear or pull away from shore. The whole exercise had been utterly, well, silent.

I’d been on boats with electric propulsion before, including a small, surrey-topped Duffy day boat that went about 7 knots flat out. But the Silent 55 was something new. Solar-powered and self-sustaining, it’s a 55-foot, ocean-going catamaran with four staterooms and heads.

Headquartered in Austria, Silent-Yachts is the brainchild of cruising couple Heike and Michael Köhler, who have voyaged more than 75,000 miles around the globe. Solar power pioneers, in 2009 they launched the SOLARWAVE 46, billed as “the first fully self-sufficient bluewater catamaran.” Following its success, the Köhlers developed a line of solar-electric production yachts ranging from 44 to 80 feet. While the line is built in different shipyards around the world, all Silent vessels utilize resin-infused,vacuum-bagged, carbon-fiber-reinforced construction.

In 2016, Silent-Yachts formed a collaboration with alternative marine propulsion specialist Jean-Marc Zanni, president of Searious Power, with the goal of increasing the yachts’ performance while maintaining a low environmental impact. “The 64 [a discontinued model] only had 30-kW e-motors and everyone was asking for more power,” Zanni said.

A product of this design collaboration, the Silent 55 launched in 2018, but the team has continued to improve and upgrade its drivetrain. The hull I sea trialed in late 2019 was powered by twin 250-kW e-motors, giving it a solid cruising speed of 10 to 12 knots. Under solar power alone, the yacht makes 5 to 6 knots.

Among the many things that make the Silent 55 unique are the 30 solar panels arrayed on its coach roof. The roof incorporates a hydraulic hardtop that “pops up” to provide access to the flybridge helm and seating area. Deploying the hardtop shades the remaining solar panels, however, so when the boat is not in operation, it’s more efficient to keep the top closed.

Solar energy charges the Silent 55’s lithium-ion batteries, which have a capacity of 210 kWh—the equivalent of three Teslas, according to Zanni, although the propulsion system is quite different from a car’s. The batteries are housed in an easily accessible hatch on the bridge deck. They provide power for the Silent 55’s twin 250-kW “synchronous permanent magnet” e-motors, which were built by Dana TM4 in Quebec. Zanni said The motors are designed for buses.

The Silent 55 also has a 100-kW Volvo Penta diesel generator housed in the port hull. “It engages when you exceed a preset amount of power or when the batteries are low; the operator can choose the setting,” he said. “Most of the time if you have full sun, you never even have to run the generator … If you use a lot of AC, you run the generator two or three times a week.” The yacht’s appliances and other house systems are powered via a 15-kVA inverter. Having a genset on board as a backup to the solar panels enables the boat to continue to voyage at night and in less-than-ideal conditions for solar collection.

The result is a silent-running yacht with low maintenance, low operating costs and a low carbon footprint. This combination is what convinced Charles Deyo of Key West to buy the Silent 55 we were aboard for our sea trial. He had learned about Silent-Yachts by Googling “solar boats,” then chartered a sister yacht in Mallorca. “From the time I Googled to the time I purchased the boat was less than three weeks,” he said. Clearly an early adopter of green propulsion, he also has ordered the all-electric Porsche Taycan and owns an electric bike. “I’m ‘E’ everything,” said Deyo, who has been named a “global ambassador” for Silent-Yachts.

Driving the Silent 55 from the flybridge in the Atlantic Ocean off Ft. Lauderdale in brisk, 15-plus-knot winds and 3-foot seas was a pleasure, particularly because the only sounds we heard came from the wind and the music softly playing through the speakers. It was a wind-in-your-hair experience since there is no windscreen due to the “pop-up” hardtop. The double helm seat faces a clean array of electronics, including a proprietary energy management monitor that shows the e-engines’ rpm, the battery charge level, the generator’s operating state (which was “off” on this sunny morning in Ft. Lauderdale) and the electrical consumption by the yacht’s utilities.

The yacht handled like the power cat she is, providing a smooth, stable ride through lumpy seas. We ran at a comfortable cruise speed of 10 knots at about 1000 rpm, but only hit a top end of 13.5 knots that day due to fouled props, according to Zanni. Silent-Yachts says that the Silent 55 is capable of speeds up to 20 knots with the right drivetrain and conditions.

While the yacht’s alternative propulsion and ultra-quiet ride—I recorded 60 dB(A), the level of normal conversation, in the VIP suite while we were underway—set it apart from “traditional” motoryachts, it is a luxury power catamaran in every other respect. Its large main deck, which holds the salon, galley on one level, with a step up to the dinette and lower helm, measures over 430 square feet despite wide walkaround side decks on either side.

A U-shaped galley is located aft, where it serves both the interior and aft-deck dining areas. In nice weather, you can open up the aft glass sliding doors to create one huge space for entertaining.

Silent-Yachts gives new-boat owners lots of flexibility when it comes to the Silent 55’s sleeping accommodations with five different layouts offering three to six staterooms and three or four heads. Our test vessel had four staterooms, including a large, full-beam master suite forward of the helm. The other three staterooms are tucked into the hulls, power-cat style. All of the lower-deck accommodations have large hullside windows that fill the rooms with light.

Storage is abundant throughout the Silent 55. Its exterior decks, which include a large foredeck with twin trampoline lounges, are free of utilitarian obstacles such as the tender, which hangs from a davit beneath the aft-deck sunpad. Naturally, having two hulls, the yacht does not offer one single large swim platform, but there’s a smaller platform on each transom, accessed by wide, ergonomic stairs.

Does the Silent 55 signal the start of a new era of solar-electric marine propulsion? “It’s more expensive than traditional propulsion. The price of the batteries is most of the cost, but that is coming down,” Zanni said, adding that over the past decade he has seen the cost-per-kilowatt hour of battery power go from $1,500 to $300, largely due to the growing popularity of electric cars.

“Ten years ago, we could not have dreamed of building this boat because we did not have the power,” he said. Today, as the Silent 55 has proven, you can run silent and green, day or night, without sacrificing comfort.

Silent 55 Specifications:

LOA: 54’10”
Beam: 27’8″
Draft: 3’11”
Displ.: 38,000 lbs.
Fuel: 132 gal.
Water: 160 gal.
Power: 2/250-kW Dana TM4 E-motors
1/10,000 kWp solar generator
1/100-kW Volvo Penta generator
Price: $1.55 million

This article originally appeared in the February 2020 issue of Power & Motoryacht magazine.