You'll be emitting nitrous oxides when burning that methane. Use wind to produce hydrogen, use that in fuel cells. Electric propulsion units are already in use, what remains to be solved is the megawatt-sized fuel cells needed for this application at a price which makes the whole scheme feasible.

NOx is addressable through diesel exhaust fluid (urea) and catalysts; maybe not elimination, but at least significant reduction. Methane is easier to store and transport than hydrogen, and there's potential for refitting existing engines to run from cng/lng (although I think purpose built cng powered devices usually start from a gasoline engine design and freight vehicles usually have a diesel engine).

How are you emitting nitrous oxides from a synthetically produced (ie very pure) fuel that contains no nitrogen?

You’re burning it in air which has lots of nitrogen.

Long term, I agree. But we already have methane and methanol powered huge container ships. Much better than burning bunker fuel.

It may be the easier option for electrically propelled ships, several of which are already in service. Mind I'm not talking about battery-electric ships but diesel-electric (hybrid, [1]) which have the range and load capacity needed for 'serious' applications. If - and that is a big if - the requisite fuel cells can be developed they could directly drive the existing electric propulsion units in those ships, replacing the diesel generators currently in use.

[1] https://www.wartsila.com/marine/products/ship-electrificatio...

How are you going to store all that hydrogen? You need extremely large and heavy high-pressure tanks, capable of holding 350–700 bar. Methane can be liquefied and stored at only 10 bar.

Hydrogen just doesn't have the density required here. Even if you want to use hydrogen-powered fuel cells, it might make more sense to convert it to ammonia for transport and storage.

how much hydrogen do you need to store though? it doesn't make sense in cars to me, but water-based vehicles seem like they wouldn't need to store problematic amounts given that they have access to h2o

That would imply the ship has the requisite power generation on board to produce hydrogen from water, in which case it would be more efficient to use that power to propel the ship.

So, yes, the ship would have to take all the hydrogen it needs with it.

oh. I thought we were talking about sails to make electricity to run electrolysis to make hydrogen to run engines. storing enough hydrogen for an entire voyage seems prohibitive to me, but I've not done the math

There's many ways to store hydrogen ranging from 'cryogenic' to 'dissolved in ... (acetone is a good candidate)' to 'converted to ... (ammonia et al)' to 'compressed'. Ships (can) have a lot of space so that is not much of a problem. Cryogenic is probably not feasible, compressed is probably also out but in between there are many options.