https://en.wikipedia.org/wiki/Steamboat#Sea-_and_Ocean-going
But if you like sails: my pet hypothetical technology is wind-driven hydrogen tankers (or tankers for some other e-fuel derived from hydrogen) that sail out empty, then cruise around wherever there is plenty of wind. They'd have a turbine/generator setup driven by the water passing by and use the energy harvested there for filling the tank. Cruise around as long as it takes to nearly fill the tank then return to port (and fill the rest on the way back). There's a lot of oceans where systems like that could cruise around on. (same concept could also be used for desalonation, there it would not only be about energy but also about avoiding local brine concentrations)
The largest Q-Max-class gas tanker is 345 meters long [1]. Let's say you manage to fit 3 giant Siemens wind turbines on it, with 100m long blades [2]. It's a bit cramped but let's say you have extenders on the side to make room for all 3 of them. And also let's say you found a way to prevent the ship from tipping over when the wind is strong. By deploying floaters on the side or whatever. Not unsurmountable.
Each of those wind turbine has a rated power of 14.7 MW [2]. Let's say that you found a place where the wind blows super strong (but not too strong) and steady all the time. It's possible, since you are a mobile ship, after all. Let's say that you have a way for the ship to keep in the same place despite the strong and steady wind pushing you constantly. Using engines is going to lower your efficiency, so let's say we found another way.
So, now your ship is generating 45MW constantly. According to ChatGPT, this is 32 kg of hydrogen per second, taking hydrolysis losses into account.
Tanker capacity is 18 620 000 kg of liquid hydrogen. It will take 581 000 seconds to fill up. 9697 minutes, 161 hours, or 6.7 days. Much shorter than I thought... Did I miss something?
[1] https://en.wikipedia.org/wiki/Q-Max [2] https://www.offshorewind.biz/2024/04/22/first-siemens-gamesa...
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.
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.
Must start some economics of it. Also marine environment is very unforgiving...
It wouldn't be floating exactly though. Moored and piled into the sea bed, sitting above the waves like rigs and wind turbines.
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)
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.
(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).
But yes, fuel-based powerplant, likely as part of a hybrid drive running electric motors powering propellers themselves.
Marine propulsion is already pretty optimised for efficiency (turn on engine, set to cruise power, maintain for 14 days, little acceleration, starts, stops, and/or hills), so a hybrid setup would work pretty effectively.
The far greater challenges are Q>1 and reliable fusion within a ship's structure.