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Comment by simonebrunozzi | Hacker News Reader

I did a high-level study a while ago. It makes no sense for cars, or boats, to consider solar panels. They just don't produce enough energy to justify their presence there.

It might make sense if you incorporate solar panels into large sails, but the engineering required to make it work (main problem: too much weight; not enough elasticity) is essentially unsolved for now.

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We've got it working! (for an unmanned sailboat)

https://www.saildrone.com/

That looks awesome.

Yeah but generally if your sails are up you don't need panels that big!! My boat has 3 100w panels and that runs all the equipment plus a fridge and a freezer. I have to turn the engine on during long trips here and there but it isn't a big deal.

Couldn't it make sense if you only use the boat every day or two, and it's docked in the sun when not in use? I can see that it wouldn't make sense for cars since cars are typically parked (1) out of the sun, in covered/enclosed structures, and (2) near power outlets, which provide an alternate source of energy.

Some people might want to use a boat infrequently, and not near charging stations. But perhaps a speed boat isn't the type of boat someone would choose for this due to lack of sleeping quarters, fridge, etc.?

It definitely makes sense for some applications. For example, I believe Uma was running their electric motor purely using solar for charging for a few years: https://www.sailinguma.com/electro-beke Not a huge amount of solar either - 480W of panels on a 36-foot ocean-going sailing yacht, shared with all their other electrical needs. It worked for them because they were primarily sailing, with the motor used briefly for maneuvering in tight areas like harbours, so they had plenty of time on passage and at anchor to recharge the batteries between each use. (Their current setup has regen, so they can charge the batteries by sailing - basically a very indirect form of wind power. Some small sailing boats have actual wind generators but those are loud and take up space.)

For speed boats? Probably not. Those things are small and incredibly power hungry - going fast on the water is expensive in terms of power used.

If that's the use case, then it would probably be cheaper and easier to put solar panels on your boathouse, and still keep it plugged in. Then you wouldn't lose range to the weight of the panels, either.

Do you have any numbers to share? Is it 10x too slow, or 100x, 1kx etc? I don't know have any sense.

They are starting to exist, this is one thats already been produced and sold so has some real-world numbers: https://www.silent-yachts.com/silent55/

10kW of panels, 2x30kW motors (or 2x150kw for more $), 75kWh battery (150kWh for more $).

I forget where I saw it but iirc cruising at 5kts was less than 10kW out the screws. Meaning you can do 5kts for as long as the sun is up, and then for 7.5 hours more off the battery. If you want to do 9kts (on that short a hull) you're looking at like 50kW and therefore it would drain the bigger battery in 3 hours.

A rough rule of thumb for solar panels is they're worth four hours of full output a day, so that panel setup is good for about 40kWh/day. You can either use that to travel 200 miles, or use it to charge up the battery. It very neatly fits the 100 mile/day + 2 - 5 days at anchor travel lifestyle of most "cruising" catamarans (sailboats).

This guys youtube channel is cataloguing all the electric and solar boats he can find: https://www.youtube.com/c/JimWaltz/videos

Whats most interesting to me is all the indian/thai/phillipine solar ferry's that are getting built. Those are not eco projects, those are people finding niches where this really is the more economical choice.

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