That may all be true, but there are other benefits that could make it worth it. For example it could be, in theory, self-sufficient forever if something else breaks down making it unable to maneuver. Then you can at least sit in the middle of the sea and have your heating and cooking and desalination working until you repair the propulsion.

You already have MWh of batteries for that.

No you don’t because after a few days broken down, they are drained without solar.

I sailed around the world on a sailboat with solar. I know. It’s still better than none at all.

The energy is free. To capture it costs a little bit of money.

There’s something funny to me about taking your experience with solar on a small sailboat and extrapolating this to a commercial ferry that would need a very large solar installation that’s funny to me. Something tells me the experience isn’t transferable.

The point isn’t to power the main drive, the point is to preserve energy used elsewhere on the ship.

My experience sailing and dealing with vessels from 30ft to 180ft give me a perspective that you probably don’t.

Providing solar panels along the roof would give the ship a few KWh of power that would otherwise be drawing from the main batteries. This would extend the range of the ship by 5-10%.

Where are you getting your 5-10% numbers from?

The ship battery is 40,000 kwh and uses at least 10,000 kwh per crossing, with 10 minutes to recharge. A handful of kwh are negligible because this isn't a sailboat.

The electricity sector in Uruguay has 98% renewable power

For how much cost? The range of the ship is already handled well by the batteries. An extra 5-10% isn’t going to meaningfully add value nor reduce fuel costs. There’s no way to recapture the capital expenditure such solar panels would require.

The 5-10% number is completely invented. I doubt it's half as high as 5%, but until and unless someone does the maths, there's no point in speculating.

The math has been done many times for solar panels on the roof of cars, and it's not worthwhile. Ships are not the same though.

At any rate, it's inevitably far more sensible to put a larger solar panel + battery installation at a fixed place on land, and charge vehicles from that.

Adding range reduction turn around time. Ship is making money while it is moving, not while it’s charging. Also why roro batteries make most sense.

The journey it makes is 90 minutes and it can charge for that journey in 8 minutes. Offloading and onloading the thousands of passengers (and 220 cars!) takes much longer than the 8 minutes for the battery to charge.

I’m assuming that the boat gets charged fast enough for one way trip while passengers are loading. There’s no need for much more capacity beyond that.

Catamarans are perfect for scaling up solar like this. Even 40ft is enough to power it entirely off sol at hull speed.

I wouldn’t go that far. Not at hull speed. But a good fraction of it. The silent 60 for example.

Full throttle you’ll be out of juice in a week. Hull speed maybe a month. Depending on wave conditions. But going, stopping, having lunch, enjoying the day, going again, enjoying tomorrow, you can be out there as long as you have provisions.

It is big difference between mounting solar on your personal sailboat and installing them on a large commercial passenger ship. The regulations are totally different.