Thanks Jeremy,
Swapping my prop for testing purposes is cost prohibitive ($700 haul outs and $400 per prop). That and the lack of decent hull resistance numbers means that I won't be tackling that exercise any time soon. With the prop in an aperture and a thick rudder right behind it, I'm guessing that prop efficiency isn't my boat's strong suit. Of course, it's the boat that I already own and love, I wasn't thinking about electric when I bought it.
I went with a PSS dripless seal, the friction is low with a carbon block riding on a flat stainless ring forming a flat face seal. The gearbox manufacturer claims 98% efficiency in the helical cut gears and another 1% loss to the lip seals on the gearbox. So the driveline doesn't have much room for improvement.
Overall, my boat seems to perform well compared to similar sized electric conversions, but most are within a 10% range at most operating speeds. So I think that most of us are doing pretty well.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@...> wrote:
>
> 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
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