Hi Myles,
Great reply thanks. I didn't know anyone actually looked at my sheet;) I agree with all your comments and I am aware efficiency isn't a fixed thing. I guess I just wanted to show the kind of things we need to consider. I've come to the conclusion that all I'm actually interested in is how much I consume at my constant cruise speed in calm water and what is my overall average in Wh/nautical mile taking in to account varying conditions and getting in and out of port.
I measured RPM at my shaft using a meter and it was pretty constant (apart from really low rpm) around the 40.5 RPM/Volt average so it suited my calcs for the area I run at most often. The 89% was an average efficiency too around my cruise speed. I took that data from a Lynch motor driving the same motor type as a genny and some graphs Cedric Lynch did at various steady state voltages and differing loads. Like you I've come to the conclusion that if one pays attention, if boat power requirements are low, then the main thing is the prop, a clean hull and knocking back the throttle a bit rather than fussing about a percent or two here and there - or much as IC engine folk do who just use more fuel, I could simply buy more batteries, if I could afford it! I'm totally sold on Lithiums, they'll likely outlast me!
John
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