Right on, Dan. I have a 12kw motor and also a couple of 5kw motors, and for a given RPM, the 12kw motor doesn't use a bit more juice than the 5kw motor to get things done. When limited to the same current, both motors get the same amount of work done. But I know that the 12kw motor will last a lot longer and be more dependable, and if I do ever need the extra power, it is there. In the real world, this is all so. Like you said, an engineer might raise issues and say no, not really... blah blah blah. but on an actual real physical boat instead of on paper, the real world data is what counts.
The bigger prop = better prop thing does have a mathematical explanation, though. Water pushed radially or laterally away from the prop varies with the circumference, but water pushed astern varies with the swept area of the prop. As the sideways wasted power increases in linear fashion with increased diameter, the useful thrust increases by the square of the diameter. But I like simply knowing that lots of guys have tried different prop sizes and they nearly all found that in the real world, on an actual boat, the bigger prop, properly matched to the motor with a reasonable reduction ratio, always worked better. That's why I didn't use apps and spreadsheets and formulae in selecting components for my repower... I just started with what other guys were using successfully, and went from there. And I am still experimenting.
When I try to think, sometimes nothing happens. When I try to do, usually something does.
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