You have it reverse wind turbines need mooring, stay upright and so on. That's highly impractical. No, you build a fast-going sail vessel (using big traction kites, because it's not the 19th century anymore) and power the generator from much smaller turbine blades in the water. Hydrogenerator is the term established in recreational boating.

I sure would not expect any returns in days, more like months or years. But if we (humanity) could just solve the purely man-made problem of piracy (or would it technically be salvage?), I believe that a robotic fleet of cruising hydrogenerators could be a huge contribution to our energy needs.

The main issue is that prevailing winds have a direction, and there's not a continuous open ocean path other than the Southern Ocean which is harsh even by the standards of oceans. Sure you can steer along trade winds in the Atlantic or Pacific but there's quite some efficiency loss.

(Give climate change another decade or so and the Arctic Ocean maybe becomes an option, although by then we'll have bigger fish to fry, or perhaps poach).

Sailing technology has gone beyond "we can go downwind, yay!" for quite a while now...

Certainly of course, but not while delivering maximum continuous energy from a kite to a turbine.

Boats go considerably faster, as in overcoming more water drag, going crosswind than going downwind. I suppose that adding extra drag with a hydrogenerator will change the maths of that relationship, but certainly not so much that it would be prohibitively wasteful to not specialize a hydrogenerator carrier to downwind-only. (if the downwind-only setup can be competitive at all, not sure I'd take that as a given)

It strikes me that having the turbine and hydrolysis plant fixed in place, and having ships visit those sites to refuel, is probably an easier setup than mobile turbines.

But I think your maths is wrong somewhere. Hydrogen supplies 33MWh/tonne, and you've stated the ship capacity as 18620 tonnes. 18620/(33*24) gives a generation time of 23 days, even before we allow for hydrolysis overheads.

Marine hydrogen isn't a terrible idea though. Tank weight and bulk is prohibitive for aviation, but less so for shipping.

There are only so many windy places to affix turbines to. There's a lot of ocean to cruise on.

The mobile hydrolyser (not a boat to solve shipping in a quasi perpetuum mobile away, but an energy harvester that focuses on just that) would solve mooring: the "lateral lift" of the boat would take care of that, just how your plain old America's Cup boat isn't just slowly dragged downwind. It would solve linkup: a serious cost component in not all too conveniently located off shore wind installations is the grid connection. And it would solve intermittency: hydrogen is inconvenient compared to hydrocarbons, but it's super convenient compared to getting even more electricity at a time demand on your grid is already satisfied to saturation.

(GP's math is likely wrong, but the assumption that you could somehow cram multiple turbines from the bigger end of market offerings on a boat and call it a day seems so far off to me that I never really looked at the numbers)

Ugh, turns out I suck at math too. At least attempting it before coffee.

The ship's capacity in MWh is 18620t x 33MWh/t = 614460MWh.

At 45MW generating capacity, an electric hydrolyser at 80% efficiency delivers hydrogen at a rate of 36MW. That will unfortunately take about 70 years to fill the ship to its maximum capacity.

On a more positive note, 36MW is still a heck of a lot of power, plenty enough to run a mid-sized cruise liner or warship. So a marine generating station with three of these turbines could, for example, refuel a liner once a month, and then that liner have enough fuel to cruise for a month, and so on.

This would require a fuel tank with a more reasonable 750 tonne capacity. That's still several times more than the Shuttle, but not beyond the realms of feasibility - and a stronger, heavier tank allows higher pressures / smaller volumes.

I am totally mesmerized by the idea of a floating hydrolizing platform , where ships can dock and load hydrogen fuel ( not sure what form suits best ).

Must start some economics of it. Also marine environment is very unforgiving...

Platforms are does-not-scale hard though: they require mooring and every bit of ocean floor is different. Drilling platforms come in multiple "species", as different as spiders, fish and birds. Roaming hydrolysers on the other hand would be one design fits any ocean. You'd want to build lots of them, more Model T than Death Star. And where a stationary platform would have to be able to brace a hurricane, the roaming unit would just go to a neighboring sea with a friendlier forecast. Or ride along on the edge, if power throughput has enough headroom.

It is. But if we can handle maritime oil and gas rigs, which load/unload to tankers, hydrogen isn't MUCH worse.

It wouldn't be floating exactly though. Moored and piled into the sea bed, sitting above the waves like rigs and wind turbines.