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Old 02-19-2019, 10:12 AM  
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 4,528
I'd suggest coming up with some use scenarios, then running the numbers on what you will need for batteries, solar panels and/or dock power to recharge, and see how it will all work before you go too far.

Reality is that there has been very little change is solar panels over the years, other than a big drop in price about 10 years ago to sub $1/watt.

Battery technology has improved, but generally not in ways that will help you. Power density hasn't changed much since Lithium Ion batteries started being used in consumer electronics around 20 years ago. Costs have come down, and management technology has improved to make them viable in cars, which is probably what makes you think there have been big improvements.

For your application, cost per usable kwh is probably all that matters, and in that respect the cost has about doubled from AGMs to LFP, and about quadrupled from FLA to LFP. I wouldn't say that's an improvement.

Here's an example to consider. Your use goals are modest, which is good and makes it at least plausible that what you want to do could work.

Let's assume you are happy pushing your live-aboard boat with a 3hp motor. It's just a guess, but it will probably go 2-4 kts. Then let's say you are happy with a max cruise time of 4 hrs. That's a total range of 8 to 16 nm. That will take you from the lock to Lake Union, a lap around the lake, then back to the lock. Or from the lock out into Lake Washington for a peek, then back again.

3 hp for 4 hrs is 9kwh. LFP batteries cost at best $1 per wh, and more likely $1.5 to $2 by the time you are done. So that's $9,000 to $18,000 for LFP, or about $4500 for AGMs. Probably more for AGMs since you will be draining them at a 4hr rate rather than their nominal 20hr rate.

Now let's look at recharge. With a 30A shore connection you have a max of 3.6kw available. Let's say you can allocate 3kw to charging. That will recharge LFP batteries in about 3 hrs. Recharging AGMs (or FLA) will take about 5-6hrs.

With solar, you get about 5hrs of full power equivalence per day, assuming it's a sunny day. And let's say you can fit 1200 Watts of solar on your top, which by the way would be a huge accomplishment. I fit 700W on a 60' boat, and will get about 1400W on a 68' boat. That will take 1.5 days to recharge LFP, and probably 2-3 days to recharge AGMs.

But the solar charging assumes no house loads. If you are also plugged into shore, then the solar power is all supplementary. But if you are anchored out, you have house loads to cover. If you have an average house load of 200W, that's 4800wh per day, and only leaves 1200wh per day to replenish the propulsion power you drew out. Now it will take 7.5 days to recharge. When was the last time you had 7.5 days of full sun where you are?

People love the idea of a solar boat and really, really want it to work, so they believe, as humans are so capable of doing. But even with your use case which is about as well matched for electric propulsion as you could get, it's borderline at best. And worst case you could find your self with a boat that doesn't meet your needs, and is completely unmarketable.

Now I probably sound like a solar hater, but I'm quite the opposite. I have an off grid house that is all solar except for about 100hrs per year of generator time from Nov through Jan when the sun hides a lot. But the house doesn't have propulsion which is an order of magnitude more load.

My last boat had solar, and the one I'm building now will have it too. But on the boats it's really just a supplement to offset house loads, and indirectly reduce generator run time. There is a very slight chance on the 68 that I'll be power neutral while at anchor, assuming no laundry or other heavy loads. But that's optimistic.
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