Agreed Martin, one thing at a time!
BUT:
Stuart; I believe we are discussing your 24" Commander here.
In which case your results mirror mine with my 24" Sea Scout. Except that my prop shaft did not bind!!๐
It was also reluctant to plane with a 2S LiPo (7.4V nominal) but went very well and planed easily with a 3S (11.1V nominal). I also was using 4000mAH batteries.
I published my results using various batteries here in the Media - Video section.
Go to the Media Gallery and search for Sea Scout.
So, I would leave the motor and prop alone and (after fixing the prop shaft!!!) try a 3S battery.
I'm sure you will be satisfied ๐
Prop technology is complex as Martin said, our old friend Bernoulli raises his head for instance๐ค But, essentially bigger is better for more speed or same speed with lower shaft revolutions when applied to real size ships.
BUT 2: with our electric powered models bigger props often simply increase the load on the motor causing it to draw more current for a fractional increase in boat speed if any.
In essence props with fewer
blades
are inherently more efficient as you hinted. Minimum being 2 of course ๐ Two bladers are popular with the Fast Electric guys for that reason.
I don't do speedboats, I'm a scale guy and my maxim is; If the real vessel had an XYZ screw then so will my model!
Summary; first fix the propshaft; lubrication, alignment with motor shaft, ensure a few thou of clearance between prop hub / locknut and the end of the shaft tube. Preferably with a thrust washer in between.
Second, fit a 3S battery preferably crosswise above the C of G of the boat. As you can see in the pic of my Sea Scout 'engine room' in your Commander thread.
For those with a mathematical mind, undying curiosity and an afternoon to spare I attach a paper on the Principles of Ship Propulsion from MAN, the supplier of small to huge diesels to probably a majority of ships (commercial and naval) built today.
You should find Chapter 2: Propeller Propulsion particularly illuminating! Happy reading ๐
Just remember the paper deals with real ships and real water, we have scale ships but not scale water๐ค Basic dynamics still apply though.
Cheers, Doug ๐