Good thoughts but the bottom line is that I don't have the capacity to determine hole centers at home. The manufacturer should QA some props and get it right instead of sticking to thier story that their props are balanced. I've never had this problem with the smaller APC props and balance doesn't really concern me as much on my .46 engine but when swinging the heavy 16 and 17 in props on a gas engine you want your prop balanced and not just close. Should be simple thing to do but again I'm not a prop MFG so I don't really know how difficult it would be for then to get it right I have used the little pylon props also. It's a whole different world and those props balance easily.
ORIGINAL: Ed Smith
First of all it is pretty dificult to produce balanced, injected molded props. There are too many variables such as the mix consistancy of the plastic, the maybe unequal pressure pushing it into the mold and probably many others.
I have used APC props at high rpm for many years of Pylon Racing. We must have them balanced. I have had to balance every one. I have not used the larger props but I think the general construction is the same. The centre hole that slips onto the crankshaft is a drilled hole it is not molded. The counterbore at the back of the hub is molded. This hole is the only hole that is true to the blades and square with the back of the hub. Now I am sure that when the shaft hole is drilled the prop must be in some sort of fixturing for accuracy. However it is still a drilled hole. It could be that the drilled hole is not dead centre in the body of the prop. If it was then the prop would probably balance. Maybe this is why APC claims its' props are balanced.
This is what I do. I drill the centre hole so that it is clear of the crankshaft. Usually the next standard size up from the shaft size. I then make up some sleeves from say K&S tubing that are a slide fit on the shaft and a tight fit in the c/bore.
I have noticed that on some of the electric props APC supplies plastic bushings for this purpose.
Ed S