[Retric]

Solar power being useful doesn’t require 100% of propulsion to come from solar panels.

You see solar panels added to a wide range of boats because even bunker fuel isn’t free and panels are light for the power they provide over even a few days. A current 399.9 * 61.3m container ship doesn’t need panels everywhere to benefit, but the potential savings is significant if they do.

[svnt]

This is unfortunately not true because of the dynamics of diesel engines: there is by design surplus energy relative to requirements from running them at efficient operating points. Otherwise the ship is not a good ship.

[Retric]

You can always scale design to fit reduced demand. Also, loss of efficiency is more than made up for with vastly lower energy demand.

“Lowering speed reduces fuel consumption because the force of drag imparted by a fluid increases quadratically with increase in speed. Thus traveling twice as fast requires four times as much energy and therefore fuel for a given distance.”

https://en.wikipedia.org/wiki/Slow_steaming

“Container ship Emma Mærsk in Aarhus, 5 September 2006 Mærsk Line's E-class container ships such as the Emma Mærsk can save 4 metric kilotons of fuel oil on a Europe-Singapore voyage by slow steaming.[5] At typical fuel prices of US$600-700 per tonne,[4] this works out to a saving of US$2.4-2.8 million on a typical one-way voyage. Maersk's Triple E-class container ships were designed for slow steaming and have less powerful engines than their predecessors.[5]”

[svnt]

Sure, but what does this have to do with what I said? You need design and operating margin, and the engine is always running.

[Retric]

Reducing the load is always going to save fuel. There’s no difference between energy used to move a boat and energy used to run the lights.

Put another way if there was excess torque being generated it would go somewhere such as increasing the engine RPM.

[svnt]

What you seem to be missing is that your understanding is not true because of the practical realities of operating large internal combustion engines.

For example, one tonne of fuel is about 11 MWh. So if you run the calculations, you will see that adding solar panels to a diesel boat, even if the energy they provide is free, essentially never ROIs, and makes the boat less reliable and useful as a boat.

These kinds of engines generate tens of megawatts when they are on, and they are always on when the ship is moving.

[Retric]

One tonne of fuel is more like 5.5 MWh. You did a mathematical calculation while ignoring engine efficiency.

The reality of large internal combustion engines is you still pay for every single kWh. These ships already have extremely complex electrical systems with multiple redundancies and load balancing etc.

The dealbreaker is R&D as unlike a house or sailboat you can’t just yolo where panels are placed and wires run etc, this is all bespoke engineering with few of any give design being manufactured and little available space.