The issue is energy density, not motors. The ship burns 1,660 gal/hr = 6000 L/hr when at "economical speed". The energy density of fuel oil is about 10 kWhr/L. Googling tells me that a journey between China and California by ship takes 2-4 weeks. So for three weeks of economical speed, that is 30 MWhr of energy. That is the size of the largest battery installation in North America, the Tehachapi Energy Storage Project. The batteries will weigh about 25 times more than the diesel fuel would weight (assuming fuel oil at 13,000 Wh/kg and 500 Wh/kg for batteries).
You lost a factor of a thousand in there somewhere. 6000l/h * 10kWh/l is 60MW. So 30GWh total. Also lithium ion batteries are nowhere near 500Wh/kg.
So we might need about 150 tons of battery to match the energy, and that's the entire capacity of the ship gone. But batteries are more efficient, so let's say 50 tons.
It's possible, I guess, but not practical. Maybe if lithium air batteries live up to their promise it could be done.
That calculation is for a purely battery powered vessel. I think the person you replied to was suggesting much lower battery capacity, and running primarily while the sun shines, using non-stored electricity.
Still isn't enough, but it's a different calculation.
Let's do that calculation. It looks like if you covered the entire surface area of the ship in 100% efficient solar panels then at high noon near the equator you would get just about enough power to run at "economical speed".
Put on realistic cells and account for the motion of the sun and you won't have 1/10th the power you want.
Wow, I wouldn't have thought it could even get that close. But as long as we're spitballing....
If you're running on solar and you are only carrying enough batteries to last through the night, then your ship isn't carrying all that fuel, and the enormous engines to burn it (and, heck, alternators). So the boat will have a lot less mass, and a lot less draft - so you can significantly lower the power requirements. Add a SkySail, and relax your shipping deadlines, and maybe you could make it work?
Sail and relaxed shipping deadlines? Sure. But the engine being removed saves 2.3 kilotons and the fuel being removed saves 3 kilotons. On a ship that weighs about 50 kilotons and holds 150 kilotons of cargo. You can probably get the solar panels installed at less than 1 kiloton, and a battery that can hold a day's power is also less than 1 kiloton.
But even with an optimistic estimate of saving 10 kilotons, that's only a 5% reduction in ship mass.
You're also going to need a much bigger sail. The biggest skysail can save you 2MW right now. This ship uses something like 40MW or 60MW and solar panels might very roughly also give you 2MW.
So what you really need is a sail 10x the size. And once you have that, the question of whether your backup drive is diesel or solar becomes a lot less important.
