Unfortunately the energy density of batteries is really bad (in the bigger picture) and, besides, we're supposed to be using weight for cargo. Tankers also burn

Unfortunately the energy density of batteries is really bad (in the bigger picture) and, besides, we're supposed to be using weight for cargo. Tankers also burn very nasty fuels that can't be used for anything else - so in practice they're very efficient. There have been cracks at putting "sails" of one form or another on ships but they've always been heavy, relatively ineffective and basically just not worth the hassle. The speed of a ship is actually quite easy to work out. You find the time/value curve for the cargo and the cost/time for the ship and see where they cross - essentially. The long and short of it is that the lighter and more valuable your cargo is, the faster the ship goes. This is why container ships are much faster than oil tankers.