Electric power density in either volume or weight is OK in a car, where once you've accelerated you're rolling on low resistance tires on a smooth surface and the weight is less important. But in a boat you're fighting the equivalent of the rocket equation where the force on your planing hull has to have more batteries to lift more batteries to go farther.
It's fine, though, for a lot of watersports uses, where you want to put out hundreds of kW for a few seconds to pop a skier up, do a set, talk to your skier without asphyxiating them in exhaust fumes and without shouting over engine noise, and then plug it back at the dock. Less fine for cruising, where you want a tank of fuel to last you whole day. You only need enough energy storage to outlast human quads and forearms, not more to outlast human bowels!
[1] http://www.ltsmarine.com/english/lts-water-ski-boat/
[2] https://nautique.com/models/super-air-nautique-gs22e/overvie...
People used to say that electric cars were "nowhere near being a practical solution" until suddenly they were.
FWIW they were the most common automobile type at the turn of the 20th century. The naysayers were just internal-combustion apologists.
The C-7 is a toy. I have no doubt that they can make a non-toy at some point in the future, but the future isn't here yet.
Here is what a 1.2 meter one looks like, just exiting an inlet: https://www.youtube.com/watch?v=rV41E9PQwiU
Worst case, couldn't you go slower and just act like a regular boat?
You now have two choices, steer to the left or right of it. Those split-second decisions have consequences, depending on which way the prevailing waves are headed. Say if you turn toward port, you might be parallel to the waves which you never want to do: they can capsize you. So you don't turn to port.
Heading toward starboard might be a better choice, but it's still not perfect - you might head toward a crest that will definitely slam into your bow violently. Or there is another boat wake you now have to deal with that wasn't a problem seconds ago. In either case, you need to alter your speed to either avoid it or cut through it more gently. You start dropping off plane, cut through a favorable section, and then angle yourself and go wide open throttles to miss a trough. Maybe you rise back on plane before you avoid it, or just slightly afterwards but on most V hull powerboats doing all this is not a problem. In normal traffic, this might happen a few times a minute.
But hydrofoils have two things going against them right there. They have massive drag at lower speeds and thus maneuver poorly in an emergency, taking more time to accelerate on plane or change direction. Then the weight ratio is a challenge: they need a lot of power for cruising speed, so they weigh more than you'd expect and then weight management is one of the primary design concerns. That all means the propeller is optimized for power over acceleration, and that means that these three congruent design choices have painted you into a corner, limiting collision maneuverability. Hydrofoils handle rough seas far worse than any conventional hull shape and more importantly, colliding with anything underway is far less forgiving than striking an object on the bow or outdrive.
Emergencies notwithstanding, being normally outside a propeller's designed operating range and you have cavitation and ventilation problems, which then erode the propellers. And that weight constraint means fewer seats, fewer people to carry, so less efficient per capita.
My general skepticism regarding hydrofoils comes from, quite frankly, not seeing that much damage with them because they don't exist 'in the wild' so much here to establish a conventional sense for most U.S. east coast watermen. Plenty of other issues that everyone I know is familiar with, most commonly groundings and striking objects, and occasionally catching cage lines on props. The petrol-era weight bias sticks around and this is where I hope you show us very wrong. I'd love to see some rough water videos of your C-7.
My original response from where everything else stemmed from is that right now this isn't a practical conversion for a common man with something like a 27' cabin cruiser, certainly not in the pocketbook. I do not know how much the C-7 costs, but was my sticker shock that far off?
The price stems from a high production cost, the whole boat is built like an aircraft, in carbon fiber (the designer used to work at Eurocopter) to be as light as possible. The weight of the hull and deck is about 240 kg. So our main goal for the future is to reduce the costs - by a lot. But think about C-7 as the Tesla Roadster. But our foiling ferry for the city of Stockholm will be launched next year, and then people using the Stockholm public transport system can go foiling for 2€.
Besides, it might not sink, but the collision will throw people around, and it's not like boats have seat belts or people accustomized to using them.
Hitting a rock might bend your prop and you might need to replace, many carry a spare prop on board as it can be replaced relatively easy even out on the water with an outboard.
That same situation with a foil boat would be disastrous, again just one very expensive and dangerous mistake.
I suspect their power declaration is marketing language claiming burst power, and that its nominal cruising range is well under it but regardless of any of that, the batteries costing $180,000 is still an issue.
Edit: forgot one important thing for that power matching, the weight. Replacing an engine and full gas tank -- the electric motor + mid-range battery pack overall adds about 2,500 lbs to the boat, so it'd be a 7,200 lb boat at that point and I'm not sure I could trailer it anymore.
- Evoy hurricane inboard, 600 kW: NOK 1.497.000
- Battery pack: medium range, 252 kWh: NOK 1.476.000
In US dollars, that's $360,000 and I'd have to ship it from Norway. Closest second option was about half that price using BMW i3 batteries, at half power. Meanwhile, a new marine engine was $10,000 by comparison, or to repair exhaust and rebuild to last another 17 years, $5,000.
Boating is a luxury and I can't excuse it in environmental terms, same as I can't excuse electing not to walk 2 hours to my local starbucks for a consistent grande pike I enjoy in favor of brewing my own. But marine electric is currently nowhere near being a practical solution, and you cannot hydrofoil in all boating conditions, especially in choppy waters, or where there are seasonal floating obstructions.
Boats do have notoriously small efficiencies (1-4 mpg) in both displacement and planing modes, but consider that for an automobile to have its 25 mpg average we pour approximately 124,000 gallons of crude oil per mile every 5-10 years for a two lane asphalt road, a coarse byproduct of refined petroleum.
My whole point here is not to equivocate in any terms, but that it's easy to overlook externalized costs. Moving an object anywhere is a difficult problem.