I am in a Boat. The boat does not float. At least not in the traditional sense. Manufacturers say “it will fly” with some enthusiasm. In effect, it hovers two feet above the water, supported on stilts attached to two horizontal carbon-fiber fins (hydrofoils), which cut through the waves and lift the hull out of the sea. Underwater, torpedo-shaped, battery-powered propellers mounted on the rear wings propel the entire 1.6-ton Candela C-8 forward.
The ship sails out of San Francisco Bay, speeding at the best spots between two iconic Bay Area destinations: the Golden Gate Bridge and Alcatraz. It was a sunny, warm and pleasant day, at odds with the region’s typically cold, Mediterranean climate of February.
The all-electric C-8 is a luxury vessel designed by Swedish maritime manufacturer Candela, but the batteries are made by electric car company Polestar, a subsidiary of Geely, the Chinese conglomerate that also owns Volvo. While Polestar is focusing most of its efforts on land vehicles, it partnered with Candela to supply the C-8 with a juice box. It’s the exact same thing — this boat uses the 69-kilowatt-hour battery pack found in the Polestar 2. Volvo has a long history of making marine engines, but this is the first foray by its EV sister brand.
The C-Pod, a dual-motor cigar-shaped drive with counter-rotating propellers, was redesigned from the previous C-7 for greater efficiency, launching the C-8 into a radically different boating experience, and is the key to the C-8’s ability.
The greater the power, the higher the efficiency
One of the main challenges of direct drive motors is that the motor spins at the same speed as the propeller. Motors are generally most efficient at high speeds, while propellers are most efficient at low speeds. So Candela designed a drive that, instead of having one big motor and one big propeller to provide the required torque, splits the torque between two motors and two propellers. This means that the size of motors and props can be reduced. One benefit of smaller propellers is reduced tip speed for the same RPM. This in turn means that the C-8’s twin propellers can stay below the cavitation limit (cause of performance degradation, blade damage, vibration and noise) even at high rpm.
Not only that, but Candela has created a passive cooling system for the C-Pod that uses only cold seawater from the outer surface of the case, so there are no rotating parts and no coolant required in the cooling system. Simpler, more efficient, and less error-prone.
This efficiency and innovative mechanisms naturally drive performance. When the boat reaches a speed of about 16 knots (18 mph for you landlubbers), it takes off with most of it above the water, supported by the hydrofoils below. The boat is level and you skim the waves leaving barely a wake. The more the boat is out of the water, the less resistance there is. The result feels more like a hovercraft than a traditional boat.
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