If a ship's in port, there is no need for using the on-ship engines at all. You could just connect it to the land based grid, which, optimally, uses renewables.

If a ship's in port, there is no need for using the on-ship engines at all. You could just connect it to the land based grid, which, optimally, uses renewables. Hamburg harbour has project to do this precisely, and it seems Cuxhaven has one too.