Their brief description states that a generator powering an electromotor is more
efficient than a direct diesel engine propulsion so already off on the wrong foot...
Still, that boat's power system design is going where few others are daring as yet.
I think their statement is lacking in details. But in a properly designed diesel electric system, especially with newer battery technology, can probably come close to breaking even and in some cases exceed certain examples.
Having the ability to run a particular engine at a specific RPM allows you to target the most efficient operating range. BSFC of
existing engines varies greatly from idle to redline. When an engine has to operate across an rpm range it suffers in efficiency. Especially when including idling out of gear, which boat diesels have to do frequently.
So If you take a Cummins 6BTA that has to use an operating range from idle-out of gear all the way to max operating RPM to propel a vessel and compare it to the same Cummins 6BTA that will spin a generator at peak BSFC you are already gaining a tremendous amount of efficiency back.
But that is not where the huge gains can be had. The 6BTA above was designed to operate smoothly over a broad range of RPM and the engineers had to make many compromises to achieve that. Camshaft design specifics , valve size, port size, exhaust design, compression ratio, cooling capacity, and on and on was all a compromise in order to operate over a wide range of rpm.
Designing an engine with a very narrow range of operating RPM to specifically drive a matched generator can greatly increase average BSFC. In addition there are no idle times that result in 0 useful output since no propulsion operating time is charging large batteries.
Also factor transmission losses in a traditional configuration VS some direct drive electric systems.
I still doubt their claim is true but I have no doubt you could design a diesel electric/battery propulsion system that IS in fact more efficient than many old mechanical diesel. So I doubt its as simple as some might think. Both sides of the balance sheet need to be added.