This sounds like a fun idea! I have a few ideas for you to consider. Here's my background to maybe lend some credence to what I'm saying:
I'm a marine engineer and have been on the water in about every imaginable type of craft since 2001. This includes 11 continuous months on a sailboat. I also owned an offgrid solar home for 3 years. So, here's my take:
- Yes, it would work, but whether it is practical, that's another question. You are going to have a challenge providing much propulsion for your vessel with the current battery banks, even a Powerwall. Boats are incredibly inefficient, especially a blue water sailboat. They are heavy and built for stability not speed (read massively inefficient). I think you may be able to propel the boat for a short period, but not enough to make the colossal expenditure worthwhile.
- Having a propulsion engine in a blue water sailboat is not just for docking. It is a big safety thing. Sure, they did it back in the day without motors, but those people were SAILORS, born on the deep blue sea. Today, we are lacking much of the skill to make it happen. It can be used to keep your bow to the waves in bad weather, help you make way when becalmed, and get you out of trouble (think a lee shore). Terribly choppy day and everyone is feeling under the weather? Put up your main sail and slow steam into the wind. It makes for a much nicer ride. So, my biggest beef is that while an electric motor is more reliable than a diesel, the entire system needs to be looked at. Reliability is an interesting beast in that it works like this: let's say your diesel engine is 90% reliable, your transmission is 99%, and your prop is 100%. You calculate the system reliability by multiplying. So, you end up with an 89% reliability for the system. When you look at your proposed system, you are introducing many more essential parts. Solar panels: 99%, wind generator: 90%, hydro (which would be a significant drag): 80%, charger: 95%, battery bank: 95% = overall system reliability: 68%. I can attest to this being reality from my experience with owning a solar home. You are constantly working on something. Add in the wild and unpredictable vagaries of blue water sailing and you would have some challenges on your hands. So, long story short, you want reliable back up propulsion. I think the jury is out on whether your proposed system is actually more reliable. Also consider that the power density of diesel is like infinity times more than it is for current battery technology.
- Marinization. This is a big deal. Salt water eats everything for breakfast. If it is not designed for use in a marine environment from the get go, you will have some significant challenges to get it there. I would bet you could count on doubling the cost of everything just to make it marine rated. And no, marine rating is not a crock. It is necessary. I've seen non-marine electronics used offshore and have them be useless in as little as a year.
- Your idea for having a small generator as backup to charge you batteries is clever, but could potentially be inefficient. One would have to do a close calculation to see if you actually come out ahead. I suspect not. You lose power as you transfer power made by the motor to electricity, then move the electricity around from generator to charger to batteries to propulsion. There are losses in all of that, especially so with DC. Even if you went with AC, you would have the added reliability concern of the inverters, and losses there as well. If you just took that same small gas motor and had it transfer that power directly to a prop, you would only be looking at the losses in the shafting and prop. Yes, the gas engine generator runs on its max efficiency curve when generating (if well designed), and gas propulsion is a bit more variable (especially offshore), but the same variability and inefficiency affects the electric propulsion. And I understand you gain a bunch of free energy from the sun, wind, and hydro, but I don't foresee that being a net positive.
- Charging. Like others mentioned, when you are using the propulsion, you will never be able to charge quickly enough for it to be available all the time. You may be able to use it in short burst, then wait a day or two for it to charge, but I think that would be best case scenario.
- Fire. Nothing is scarier and more deadly than fire on a boat. Lithium ion is touchy for that. Maybe see if anyone has been able to marinize a lithium ion battery. Lead acid is less of a concern, but they are heavy, huge, and a pain in the ass. If they are used for power (i.e. in a solar house or for propulsion) they HAVE to be recharged fully after every discharge or they are done in a month. They also HAVE to be equalized monthly or they are done in a year.
Now, I'm not trying to take a dump on your dream! I think its a super fun idea. I love it. I just know they have been looking at this type of thing for a while and it has not proven practical... yet. At least not with battery power. Now straight electric propulsion, with small batteries for auxiliaries, has been around for decades. Just search diesel-electric ships. I'm not sure if you are the mathy engineering type, but with a the right data, a bunch of time, and a solid spreadsheet, you could model this all out and see if it is indeed practical. I am just highly suspect, especially since I think it will be wildly expensive.
If you want to do a fuel free sailboat, then just do it like the old schoolers did. Go only sail power. When it comes to marine, I am a firm believer in the KISS principle. If you don't need the added headache of diesel or battery power, then scrap it. I know this goes counter to what I was saying up in #2, but with the right application of effort and education, you could make it a reality. People like
Bernard Montessier have done it. There are a several tallships in the US that go on sail power alone (some use a small skiff to assist in difficult situations) that you could volunteer on to gain the needed knowledge of how to do everything via sailpower. It would not only make you a legitimate badass, it would also cement your position in the Mustachian Hall of Fame!
If you do it either option, you had better make a post and let us know how it goes!
PS - Diesels are not hard to work on. Super easy. They need fuel, air, and compression. That's it! If she won't run, it's one of those.
