> The researchers analyzed US-flagged ships less than 1,000 gross tonnage, which includes primarily passenger ships and three types of tugboats.

This is the buried lede. They are excluding basically all cargo shipping.

- Very little of the shipping industry is US-flagged. Most commercial ships sail under flags of convenience such as Panama and Libera, because of their reduced regulations and costs.

- Nobody carries cargo any distance in vessels of less than 1000 gross tons, because that scale would be uneconomical to operate. Modern seagoing cargo ships have about one crew member per 8000 tons of cargo.

Wow, this is a really good paper. Supplementary info is really great too- they get into details down to floating charging port stations as part of the infrastructure. Surprising how much demand is from tugboats. I have questions about how you'd safely hook up 5 MW connections, but it's definitely solvable.

A Lithium fire battery in a cruise ship in the middle of the Pacific would be a truly unique experience.

For marine applications, I think propane fuel cells will work better than batteries. Unlike batteries, energy density of liquified propane is good. Unlike hydrogen, logistics of propane is solved by now as it’s already widely used to fuel vehicles.

Currently people usually burn propane in combustion engines, instead of oxidizing in fuel cells. In theory, fuel cells are more efficient than combustion engines.

There’re commercially available fuel cells used for marine applications https://wattfuelcell.com/ however it seems low powered, not enough power for propulsion.

And there’re different ways being researched to synthesize propane: some biotech bacteria, traditional industrial chemistry with new catalyzers like trimolybdenum phosphide nanoparticles, etc.

The future of shipping is ammonia. It has, pretty much, all the attributes of fossil fuels but being NH4 it doesn't produce an earth warming byproduct.