Hi!
General discussions on full scale prop efficiency is probably not leading anywhere..!
The guy just asked about what prop would be best for his 15cc fourstroke.
In the model world every experienced flier knows that a 2-bladed prop is better than a 3 or 4 bladed one.
For a 15cc (.91) foustroke a 13x6, 14x4-5 or 13x8, preferable APC would be good to try. (don't think there is a 3-blade 14x5 APC though ...but there should be).
A 13x8, 14x5N, 14x6 or 15x4 APC 2-blade prop would still better though!

So why choose a 3-blade then...like the CL aerobatic guys sometimes use and pattern pilots did in the eighties...!?
The reason CL guys use 3 or 4 blade props is because lack of ground clearance on their models and also..in only a minor part, some more inertia (heavier prop deliver a more steady engine run). Remember these guys are not after high performance (speed and rapid climbing ability) but steady engine behavior.
The pattern guys in the eighties also favored 3-bladed props! Why? Because there was strict noise regulations in F3A competition....and 2 bladed props with their slightly larger diameter was to noisy!! Propeller tip speed noise was reduced with 3-bladed props.
I have seen one bladed props in action to but these are a bit more difficult to manufacture and only made in small numbers by CL speed competitors. As you well know the one bladed props are preferred by CL-speed fliers.
For the rest of us seeking best performance for our models, a 2 blade model prop is always going to out perform a multi bladed one. Provided you seek out the best one....or modify one

Regards!
Jan K
Sweden