I've not got solid data for 5kts, as it's over our speed limit and I've only (unofficially!) been that fast a couple of time. The power needed was around 250 watts, but these weren't controlled tests, so I couldn't swear to this figure being correct.
The prop efficiency measurement was a long drawn out series of experiments. As I had accurate data for the hull resistance at several speeds, all I had to do (!) was modify (or change) the prop, run another series of tests and see whether or not the power input to the motor increased or decreased. I already had a dyno plot for the motor and controller combination, so knew their efficiency under various load conditions; working back to get the prop efficiency was reasonably straightforward. I've used a lip seal, rather than stuffing box, to reduce shaft drag and the motor reduction drive uses a 15mm wide 3M pitch HTD belt, with losses of only around 4 watts. The prop is a 13 inch diameter, two blade stainless steel one, rather like an aeroplane prop. It folds to reduce the weed entanglement problem.
I'll admit this was a lot of amount of work, but I was very keen to try and make this boat independent of fossil fuelled or shore power, if at all possible, and this meant optimising the efficiency of every component. The solar panels are only 200 watt maximum (they are four 50 watt monocrystalline panels mounted on a canopy), and even with a high efficiency MPPT I can only get between 100 and 130 watts charge power at best, the average here in the UK summer is often lower than this. I want to be able to cruise all day, with short breaks for lunch etc, and keep the battery pack topped up, so have to keep the power consumption fairly close to the power I can get from the panels, ideally somewhat lower.
Jeremy
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Good info Jeremy,
>
> Since you quoted power predictions for your boat at 5 kts, I was wondering if you had collected any data for that speed.
>
> And you quoted for prop efficiency at about 86% (measured). How did you measure that? I'm curious to see if there is some way that I can figure that out for my prop while it is installed on my boat. The rest of my system sounds close to yours, Lithium batteries, controller, gear box, stuffing box, etc. though I'm operating my motor in the 80-85% efficiency range according to the manufacturer.
>
> Thanks,
> Eric
>
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