Originally Posted by mcarthur
@twisted, can you provide some evidence that "running a generator (say 30 kW) for a couple hours" "consumes MORE fuel and generates MORE byproducts vs running 8 hrs on a diesel"?
While there is an increasing use of DC diesel gensets in the increasing hybrid market, and their marketing is for "eco", it may not be the case so I'm keeping eyes and ears open to evidence one way or the other...
I posted about this in either this thread or another similar one a few days ago. Just look at an "energy equation" for the power system.
You want to run the boat for a certain distance at a certain speed, and that will require some amount of prop shaft energy. Let's say it's 200kwh. I'll do everything in kwh, just for simplification.
With a conventional drive you will put fuel into a diesel engine to produce 200kwh of shaft output power to make that run. To continue to keep the numbers round, let's say the diesel is 25% efficient, so it will take 800kwh of fuel.
Now use a hybrid drive as you described. You still need the same 200kwh of prop shaft energy, and that will be driven by an electric motor drawing from batteries. Let's assume very efficient motor drive electronics and a very efficient motor, so 90% efficient combined. That's probably unrealistic, but we will give it the benefit of doubt. That means you need 220kwh of energy coming out of the battery to power the drive motor.
Now the battery and charge electronics have losses too, but again we will assume the very best chargers and highly efficient LFP batteries, so another 90%. Now we need 250 kwh (rounding a bit) of energy powering the chargers.
We then power those chargers with a diesel engine driving a generator head. Again, being generous, let's say the generator head is 90% efficient. That means we now need 275 kwh of shaft power from the diesel to produce that energy. That diesel engine is also 25% efficient, so we need 1100 kwh of fuel to move the boat the same distance distance and speed.
The result is a much more complicated, much more expensive system that consumes 37% more fuel to do the same work. It's a lose-lose-lose.
Now someone has already mentioned that you might be able to run the generator engine at a more efficient power point vs the propulsion engine, and regain some efficiency. Maybe, but the available gains are very limited. Any reasonable diesel operated within it's normal power range, say 20% to 80% load, will have a VERY flat energy/liter of fuel prower production curve. It's standardized as the Brake Specific Fuel Consumption, or BSFC. My engine has at most a 10% variation, so let's say that instead of 25% efficiency, the generator is 10% better so operates at 27.5% efficiency. That brings the generator fuel consumption down to 1000 kwh, which is still 25% more than a direct drive.
Key to remember is that fuel consumption is very little about the size of the engine or how long you run it, and almost entirely about how much power output you actually draw out of the engine.