My boat sounds kind of like what you're suggesting. However, the numbers are different and my experience may not scale as well as you need them to.
You'll find that your boat's displacement affects your propulsion needs more than length. My boat is a 30' ketch that displaces a little more than 10,000 lbs. My 5.5kW drive will push the the boat to about 6kts (93% of hull speed) at full power. At about 1400W (26% power), the boat runs at 4.3 kts (66% hull speed). With my 8kWh (160Ah @ 50V) Lithium battery pack, my boat has a usable range of 6.8nm at 6kts (5200W) and a range of 20nm at 4.3kts (1400W)
I've got a Yamaha ES2000is generator (2000W) that is rated for 1600W continuous. Running my Elcon PFC 2000+ charger off of the generator, it can push a solid 1500W of 54VDC into my drive system. The charger is actually pulling 14.5A of 115VAC (1700W, measured with a Kill-a-Watt meter) which at that point is exceeding the continuous rating for the generator, but the generator has run for an hour under that load.
So I believe that I can drive my boat with my generator at 4.3kts (66% hull speed), without pulling any power from my batteries, as long as my gasoline lasts. I'm able to do this because I was able to match the charger load closely to the generator output and the charger is about 90% efficient at converting 115VAC energy into the 48VDC (nominal) that the drive uses.
I know the power to speed conversion for my boat with my drive through detailed observation, it turned out that the drive exceeded my previously calculated performance. The rest of the process relied on carefully matching my chosen components; batteries, charger, generator, etc.
The question will be just how much power your boat will consume at various speeds and whether your chosen drive system and additional components can deliver to your stated expectations. I'm going to guess that your hull speed is around 7.5kts (it depends mostly on your waterline length) and therfore 80% would be about 6kts. I'm not familiar with the electric power requirements on a boat your size (there seems to be a lot more real world data for boats in the 27'-32', 5,000 to 12,000 lb range) but I would say that 6kts might be right around your target of 10kW. On my boat, each knot of speed costs me about double the energy. If that holds true for yours, 5kts would be a much easier to sustain 5000W.
If I had a boat your size, I would aim for the back-up ability to drive at 5kts under ICE generation and size the batteries to be able to sustain the same speed for 4 hours on batteries alone. That would allow you to extend your 6kt (10kW) range to 4 hours by running the generator (5000W) and pulling 5000W from the batteries. 4 hours later, your batteries would be depleted, and you would be relegated back to the 5kts that your generator could suport alone. Using the previously stated rule of thumb, 7kts would take 20kW, so if you wanted that kind of speed available, your drive would have to be sized accordingly. This turns into an exponentially growing system, the bigger motor needs more batteries and the extra batteries require a little more motor to push them around, etc. etc. I believe that one is better served by a moderate system and good seamanship.
I know that some parts of the PNW have some pretty significant currents. If you go electric, you will probably not be able to fight a full tidal current. However, if you dig back to your sailing roots, and plan your outings to take advantage of nature's helping hand rather than pushing against it, then you'll be much better off. Unlike most diesel systems, electric drives are usually best sized without excessive reserves, that is one attribute that factors into their increased efficiency.
I hope that this has given you some food for thought. I think that your idea is doable and if you're willing to accept some compomises, you could be very satisfied by going electric. But you need to be realistic about what an electric drive can and cannot do. You have rougher conditions to deal with than most of the members of this group, including me. Building a reliable system that is sized for your boat may be more expensive than you are thinking, but if done right, it should be well worth it.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "pepperwynn" <pepperwynn@...> wrote:
>
> Hello, E boaters. I'm fishing for opinions and technical info in my hopefully pending conversion of a 40ft sailboat. Its a big heavy cruising boat (with a dead engine), and I shudder at the thought of putting another diesel engine in the thing. The problem is where I live in Puget Sound the wind is fickle, so i'm trying to reconcile changing my cruising habits with available technology. Now that I can get a powerful enough AC motor, my issue is how to make a defacto diesel electric. I'm thinking lotsa 8D batteries and a 10kw genset would allow me to cruise 3 to 4 hours a time with a 10kw draw at 80% of hull speed.
> Has anyone out there tried this?
>
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