Bendik,
Another note on efficiency. Here's an image of a motor at 3 different constant voltages or speeds. I shaded in the area under the graph and from these voltages you can intuit for a given current what a particular voltage and current combination might be in terms of efficiency. Note this would be at a steady state motor temperature too, and core motor temperatures at homeostasis varies with ambient temperature which again affects efficiency.
As has been said before most systems without considering the prop are in the 80 to 85% efficiency region and most off the shelf props will be around 60% efficient, so you simply multiply that .8 or . 85 by your prop efficiency at a given shaft speed to get the overall efficiency but we also need to consider how/where the prop is located to find thrust. At the end of the day efficiency is no big deal if you've loads of batteries. It just means you consume more Wh/mile and a more efficient system less.
The bottom line is a shaft doesn't care what is driving it. For the same boat with the same speed and the same prop it could be hamsters, humans, warp drive, petrol or diesel. Thrust is thrust as a result of torque and shaft RPM. The only reason we focus on efficiency a lot with electric is we are trying to maximise what little we have in the battery fuel tank as it were.
PS As Hannu says elsewhere if you want to understand efficiency and boat speed and boat design giving economy then Steve Dashew of FPB fame is the one that gets it the most as it were.
John R
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