Maybe one of you engineering types can answer this... how much loss is there in diesel/electric motors? Assuming that diesel fuel will still provide the bulk of stored energy, is it at all reasonable to consider going that route and then using wind and solar to supplement the diesel?
In the context of TF trawlers, I think it's extremely unlikely that a diesel-electric will bring any benefit, and is more likely to actually be less fuel efficient. And it will definitely cost more to build.
In a classic diesel-electric, the diesel turns a generator to generate electricity, and the electricity drives an electric motor to turn the prop. This is in contrast to the diesel directly turning the prop via a gear.
The conventional drive has single digit power losses in the gear. Say 5% for arguments sake. So of the power available at the engine flywheel, 95% of that is available to the prop.
For a diesel electric, the is about a 10% loss in the generator powered off the flywheel. That power gets rectified to DC, then runs through a VFD (a speed and power control) to drive the electric motor. That's about another 10%. Then there is the electric motor which is about another 10% loss.
So right out of the gate in the diesel electric, you have lost 30% between the engine and prop, where with a conventional drive you have lost 5%. That puts you 25% in the red. Now it may not be quite that bad, or it may actually be worse. But let's give diesel-electric the benefit of the doubt and say it's only 15-20% in the red. Now you need to make that up somehow.
The efficiency argument for DE is that you can run the diesel at a more efficient RPM/load and gain better fuel efficiency. With a gear drive for every prop load level, there is exactly one engine RPM/load point that you can use. The engine can actually generate the same HP for the prop at a wide range of rpm/load points. Lower rpm + higher torque gives you the same HP, and with DE you can pick any one of those operating points.
At different rpm/torque points, engines burn different amounts of fuel even though all points product the same net HP. Since you can pick any rpm/torque combination that produces the desired HP, you can pick the one that does so with the least fuel burn. That gains you back some efficiency.
Every engine's fuel efficiency is depicted in a "map" that looks much like a topo map. One axis is rpm and the other it torque. The contours in the map show the fuel burn to produce power at that particular rpm/torque point. Given the way the maps are done, the "high ground" in the topo is the most efficient, and the valleys are the worst. In a DE, you try to always run the engine on the "hill tops". With a gear drive, you can only operate at points along a single line that curves across the map.
The bottom line question with a DE is whether you can operate the engine on hill tops that are enough higher than the fixed gear line, and do that often enough to make up the 20% deficit that are starting out with, let along actually do better than a gear drive.
I guess that was a really long way of saying No.