Cal,
That all makes interesting reading indeed, and even though your motor power you say doesn't particularly interest you, without it and without an indicative prop efficiency it's hard to see where the losses are or rather what is easiest/cheapest to improve. It still seems to me that at say 4 knots your energy consumption is close to twice what I would have hoped for, but then again you have done it on a budget and that is to be admired. Just the way I approached things initially actually, until I started optimising stuff to improve system efficiency and range.
You say, "One fact of interest is that for a thumbnail projection, Peukert effect can be conveniently ignored, at a discharge current equal to the 20 hour discharge capacity, in amp hours."
True subject to temperature. So if your 20 Hr rate is 11 Amps that means you have a 220 bank (210 x 0.05), but we wouldn't discharge that to 42V which would be full discharge resting voltage? To get a reasonable life cycle would you not discharge to 50% at 11 Amps and therefore get the mileage you suggest at about 2.4 Knots and keep your batteries longer?
For me I consider 3.5 to 4 knots to be my cruise speed but say you did that at 4 knots (40 Amps) to 50% SOC then your bank would be about 160 Ah corrected value, if Peukert 1.25 but might be more, so to 50% SOC you have 80 Ah useable as opposed to stored which is 2 hrs @ 4 Knots, so 8 miles range in calm conditions if a new battery and temperature 25 C or more. More by simply dropping speed. Reduce capacity further if less than 25C. All this based on your new 48V bank, so to 50% you have 5.28 kW of useable energy in optimum conditions which would help the life cycle of your batteries, less cycles obviously if deeper discharge. If less current and deeper obviously more range.
For me with my 2 x 12.8V 200 Ah Lithiums to 80% DOD I expect 16 to 20 miles range at 3.5 to 4 knots. So 4 kWh useable and 1kW @ 4 knots, or more range when dropping speed. Just demonstrating LFP. Add in 4 to 5 times the cycle life of lead acid and you began to see the value but capital risk is in the up to 5 times capital cost of AGM. Hence I understand using golf cart wet lead to play with. I stuck with AGM to start as I did not want wet batteries when sailing heeled and discharging or charging.
John R
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