> This is not nothing. It's roughly as much power as a redlined sports car engine.
And a sports car is what, 3,000 pounds? A container ship is something like 200,000 tons. I guess they can power a redlining sports car engine when out to sea but not a whole lot else.
It doesn't take an enormous amount of power to move a big ship very very slowly; Froude's law, the first approximation of the relationship, scales power requirements with velocity squared.
A Maersk Triple E has two up-to-30MW-each engines and a design speed of 19 knots. You'd scale down speed by a factor of 12 to achieve 1.6 knots (0.8m/s), and out of that you would get to scale down power by a factor of 144 to meet your ~400kw solar power endowment.
The question is whether you can cross the most powerful currents, and at 1.6 knots you would certainly need to be cognizant of them to stand a chance, but you could ford them in the manner of fording a river: Get pushed downstream a bit, but cross the highest velocity area perpendicular.
The Maersk Triple E and huge bulk carriers are practically worst-case-scenarios for solar, because there's so much ship to displace water under every square meter of solar. A smaller lighter catamaran is likely to find itself with dramatically higher speeds, albeit not linearly higher with hull depth, since a longer waterline is more efficient.
I'm not suggesting this is practical. I think massive nuclear powered cargo ships capable of high speed is a hell of a lot more practical in a post-carbon world. But again: it's not nothing.
And a sports car is what, 3,000 pounds? A container ship is something like 200,000 tons. I guess they can power a redlining sports car engine when out to sea but not a whole lot else.