| PV energy capability per unit surface area is low enough that it's impractical for large-scale propulsion. Marine PV has seen use on pleasure craft (particularly sailboats) for auxiliary power needs (electronics, nav, lighting, pumps, light cooking/heating), and for drone craft (there was a startup out of Alameda, CA, producing what seemed to be aimed as US Naval marine survey craft a few years ago, largely wind-powered but with electronics and back-up propulsion powered by solar arrays, see: <https://newatlas.com/marine/72-ft-autonomous-saildrone-map-s...>. Wind-capture remains the most viable alternative to fuel-based marine transport, and "windjammers" remained in commercial operation through the 1950s particularly on very-long-haul routes, though ultimately even that niche was captured by oil-powered propulsion. See: <https://omeka2.hrvh.org/exhibits/show/a-new-age-of-sail/last...>. Containerisation probably accelerated and/or solidified that decline as this permitted not only far more efficient loading and unloading, but requires clear decks and permits vastly larger ships and concommitantly greater crew efficiencies (crewing is almost entirely independent of ship size and capacity, so a larger and faster ship (factors which also trend together given hull speed and its relationship to length) is virtually always vastly more efficient than a smaller one, despite possibly greater total fuel consumption. Fuel use itself per unit cargo is also lower for larger shipping. Current ship-size constraints are largely determined by harbour and canal dimensions (hence ship dimensions defined by <location>max, e.g., Panamax, Suezmax, Chinamax, Batimax (Baltic Sea, not Baltimore Harbour), etc. I'm fond of the designation "Handy Size", which calls to mind "fun sized" though I'm told there's little relation. Maximum length capable of sustaining large seas (waves) without keel breakage is another marine engineering consideration which has limited ships to ~1,200 ft / 660m maximum length. At larger sizes ships are simply too susceptible to coming apart in heavy conditions. Ships in general are constrained by the smallest viable dimensions of a port of call or route, and larger ships have fewer viable available ports or routes, though the busiest of such tend also to favour larger hull dimensions. <https://en.wikipedia.org/wiki/Cargo_ship#Size_categories> <https://en.wikipedia.org/wiki/Handysize> |