A 4 to 6 kWh flywheel is either charged with shore power or functions as peak shaver in a diesel electric driveline. Flywheels can easily be charged in the time between crossings, and discharge during the crossing.
Lifetime is 30 years or 5 mln cycles, and all components are 100% recyclable. Storage of 1 kWh by application on a ferry costs 3 ct, and this is 5 times cheaper than storing in a Li-Ion battery.
Due to their highly fluctuating power demand, most short distance ferries are big polluters. (In Amsterdam the emissions of 9 ferries are comparable with 200 busses.) Applying a flywheel as peak shaver could reduce 40% of emissions, while a shore power charged flywheel could reduce 100%.