The technical point was made later in the article.
[During the Wenatchee’s conversion to battery-powered propulsion, shipyard workers removed two locomotive-sized diesel engines. They kept two diesel-driven electric generators to supply backup power for when the batteries run low. The ultimate goal is to operate purely electric most of the time. But for the next few years, those diesel generators are the full-time source of power for all of the boat’s systems....
We anticipate saving 25% on the fuel just running in hybrid mode on this boat until we can start charging” from shore, Washington State Ferries chief Steve Nevey told reporters on board Thursday.]
25% fuel economy improvement is millions of dollars worth of diesel fuel per year they could skip the shore power and just run the ships and hybrids and laugh all the way to the bank.
Those smaller generators run more efficiently than the large EDM type diesels especially when ramping up or down or at idle. Those huge EDM locomotive sized engines also weight 20 tonnes each. A battery pack of that size in modern LFP cells would be 20,000 megawatts each double or triple the horsepower of the engines that replaced on a kilogram for kilogram basis.
There is a reason nearly every large ship is electric drive units on azipods or Z drives. All of our new Navy amphibious assault ships are.
This confuses me. A hybrid is an internal combustion engine (in this case diesel) powering a generator to drive electric motors. How can this be more efficient than direct drive from the diesel engines to the propellers. There is a large loss of efficiency in the generation of electric power in the generators and further loss of energy in the electric motors that power the propellers. Oddly hybrid power works in cars. The engine runs at its perfect efficient rpm and charges a battery and then on to the electric motors. It works because a car is driven in many different modes of speed and stop and go. A ferry is nominally a constant speed device until docking and departure.