Hi Dave,
Thanks for posting the numbers. That should be a big help for people that are trying to do something similar.
It looks like your range estimates were calculated by merely dividing your total Wh by the load giving you hours. This does not take the Peukert's Effect into account. We all know that batteries deliver less energy under high current, the cell's internal resistance wastes energy creating heat. Even lithium batteries are affected by this effect, albeit to a much lesser amount than lead acid batteries.
Using a Peukert's exponent of 1.04 (that's the figure for my Thundersky LiFePO4 cells, your cells might be different) your numbers look more like this:
3 knots/223 watts/67.5 miles
4 knots/396 watts/49.6 miles
5 knots/660 watts/36.4 miles
6 knots/1440 watts/19.4 miles
7 knots/2100 watts/15.3 miles
8 knots/2900 watts/12.5 miles
9 knots/3650 watts/11 miles
These are not big differences, but they do reduce your range by more than 10% at your higher speeds.
On another note, as we try to add more precision to the calculations, the accuracy of the original figures becomes more important. Can you describe the process and meters that you used to measure your watts to knots figures? I graphed your numbers and they're a little lumpy, which could easily be noise in the data introduced by the process.
Thanks again for posting your figures.
Fair winds,
Eric
1964 Bermuda 30, 5.5kW Propulsion Marine drive, 8 kWh Lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "davecanoehead" <elderton@...> wrote:
>
> Hi all, here is some preliminary speed VS. range data on sparky. She is based on a Hobie 16 platform, 5 KW outboard conversion, 5 KW/H lithium batteries. Range assumes 100% discharge.
>
> 1 knot/90 watts/55 miles
> 2 knots/127 watts/78 miles
> 3 knots/223 watts/67.2 miles
> 4 knots/396 watts/50.5 miles
> 5 knots/660 watts/37.87 miles
> 6 knots/1440 watts/20.8 miles
> 7 knots/2100 watts/16 miles
> 8 knots/2900 watts/13.8 miles
> 9 knots/3650 watts/12.3 miles
>
> She still needs more work around the prop and leg to minimize cavitation etc. so we may get slightly better numbers going forward.
>
> Best,
>
> Dave E
>
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