Here’s my idea. Paddle wheel on the waters edge near the dock. This paddle wheel spins up a very heavy flywheel on the dock. This flywheel is usually at max energy given it’s constant power source of the river. <br><br>The boats all have smaller flywheels in them that have enough power to get them across the river with a good deal of reserve, plus the flywheels can keep batteries charged on board for any electronics necessary for navigation/safety/entertainment. And since there will be batteries on board, you can have a small electric motor for backup if the boat’s flywheel fails somehow. <br><br>Came up with this because I thought of reaction wheels when I saw the title and so I figured this is what you meant.