So, taking your recommendation for reducing speed and duration :
for cruising at 3-4 knots for 2-3 h a day we will need a bank of about 10-20 kwh ?
Very very roughly, and I don't know my boat stuff that well, I've only ever had this kind of chat with a boat engineer once...
Cruising at 3.5 knots instead of 7 knots should cut your power requirements down to 25%, and your energy requirements down to 12.5%.
So, cutting speed in half an instantly you only need 15kwh.
If you're only travelling 2 hours a day instead of 8 hours a day, linearly cut that to 25% again. So 3.75 kwh. There's E-bike batteries that big.
But also keep in mind you've cut your speed in half and travel duration to a quarter, so you're only getting 1/8th the distance in a day.
As for recharging, We will likely stay off grid most of the time, and would like to charge with solar panels + a small generator. we have about 150 sq feet of roof.
I'd say there's no point in running a generator if you already have a diesel engine. You might as well directly push the boat with the engine instead of run a genny to charge a battery to feed a speed controller to run an engine.
With 150 square feet of roof you can fit about 2250 watts of solar. With 4.5 peak hours a day, you'll have 10kwh of energy a day to play with. That's still 3x your energy needs for the day (meaning you could travel 3x as many hours [6], or, at efficiency loss, travel slightly faster). If you choose to travel for more time, you've taken your range from 12.5% of your goal up to 37.5% of your goal.
Another option is just directly driving the motors from whatever the panels are generating, not storing it or having batteries at all. (Or, indeed, having only 3.75kwh, so you can kick it a bit when you need to).
Perhaps it would be good inspiration for you to watch a million videos on Jamie Manzel's youtube channel. He's built 4 solar boats now, himself, generally from materials and garbage, with very few tools, in the middle of Panama. He is foolishly and deliberately ignorant about electronics and batteries (and boat design, and propellers), half due to arrogance that he knows better than the rest of the world, and half due to him just wanting to discover things for himself, so do not copy anything he does or take advice from him. He's often just flat out factually wrong, he refuses to listen to anyone's advice on why things don't work. Like, he refuses to use a charger (because electronics just break), he only wants to direct charge batteries from solar panels, and then applies lead-acid knowledge to lithium batteries, only ends up charging them 20%, and then spends a year building a whole new boat because he's not satisfied with his battery arrangement. Instead of buying a $40 charger.
But, just in terms of capability, you can see what he's doing. He has sworn off gasoline and be pedal-powered boats a trip to "town" is like 8 hours round-trip, and not too much faster with solar help.
https://www.youtube.com/user/JMEMantzel/videos
For help searching, his 3 boats are:
1 - "
Shark Slicer", a heavy-ish barge-like boat to haul gravel and concrete for his projects (which he figured would be paid for in a year with how much cheaper it was to not pay for delivery). It's not even all that slow, but it is a bit clumsy.
2 - "
Zombie Chopper", the smallest, a light boat that's supposed to be the fastest, to get to town and back as fast as possible instead of taking the previous one. It holds two people, but barely, and
doesn't have much left over for cargo afterwards.
3 - "Solar Snailer/Sailer/Boat Chips", a rebuilt version of his first solar boat with a swivelling roof that didn't really work. Now has a trifold solar roof with a paddlewheel meant as a fast-ish 2-4 person craft for moderate cargo, since the previous boat wasn't practical once he started having other families settle near him. He's
almost done this one.
Just for casual mention, he also built a mini-bulldozer, an island castle, a 6-legged walking 12' tall walking robot, a videogame, a 4 story geodesic dome with the third floor being a giant trampoline, tons of interesting stuff.