T&T: Propellers for that 50' trawler

[Bob Frenier]

Larry Z wrote: I hope the weather gets better so you won't have to put up
with any more of this drivel.
 
Reply: Hardly drivel, sir! 
 
Larry, I noticed that your calculations of the efficiency of various prop
sizes for your own boat were exactly proportional to the prop diameter. In
other words, when you (theoretically) changed your current 18" prop to a 23"
prop, that 27.7% increase in prop diameter resulted in a jump of "28%" in
prop efficiency. The same directly proportionate relationship existed
between prop diameter and prop efficiency in your other examples, too.
 
Is that direct relationship to be expected in other propeller calculations
in other boats? Is it that simple?
 
As for controllable pitch propellers ("CPP"), do you care to speculate on
the impact on efficiency of increasing the diameter of those props? I am
especially curious about your thoughts on how a CPP would affect horsepower
choice. 
 
And finally, should you be in the mood to really dazzle us, I also invite
you to tie in that discussion with our previous discussion on the impact of
low rpm operations on diesel health. In other words, if a CPP can increase
load at low rpms, would we escape the engine damage of low rpm use that
accompanies fixed pitch propellers carrying a low load?
 
Then again, if the weather is sunny...
 
Regards,
 
Bob Frenier
Chelsea, VT