I've been following the conversations above with interest. Here is my understanding.
Watts - Whilst we can assume a nominal battery voltage, the actual voltage varies due to load and state of charge, more so with lead acid technologies than Lithium ones, so it is useful to have actual battery voltage at the time a current reading is taken. I use a Victron 700 series battery monitor which does the instant multiplication to give watts making life simple. Usefully it can also give battery temperature or mid point bank voltages.
Regen - Oceanvolt use a folding prop which gives useful regen subject to LWL but more if fixed. http://oceanvolt.com/create-energy-while-sailing
Prop efficiency - My understanding is that a two blade prop is more efficient than a three blade where efficiency is torque at RPM at the shaft compared to the measured thrust ahead or astern that the propeller delivers. I base this in part on the understanding that a three blade will propwalk more than a two, for a comparable design of prop and prop thrust. That is less of a percentage of the input power is converted to thrust which moves the boat ahead or astern with a three blade when compared to two. Is this what we mean by prop efficiency which whilst related is quite different to slip?
DC v AC motor efficiency. I use a brushed DC Lynch D135 with peak efficiency of 92%. I understand the brushless AC Motoenergy motors many use are similar. I estimate at my cruise speed of 4 knots in calm waters my motor voltage/current combination gives around 88 to 90% motor efficiency. This without seeing an actual map plot but calculated using current graphs at 12, 24 & 48v motor voltages that use to be on the AGNI (now Saietta) site.
John
https://facebook.com/ElektraYachts/
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