I think the 12 1/4" x 3.75 is too much prop for the O.S. 46fx. Doesn't let the engine turn up, where these engines like to run. I have seen this used on the newer O.S. .50 and I think it is a much better prop for that engine.

I have had good success with the following props.

Apc 11.5x4 (Probably my favorite)
Apc 11x5 (2nd choice)
Zinger 12x4 (3rd choice)