I had a 12x5 on a Venus 40 w/55AX and it was too slow. I think it has a 12x6 now. I was hoping for a 12x7 but the engine just didn't have the grunt. You might with the 61, so that's my recommendation. A 12" prop will pull the plane vertically just fine. Other choice would be an 11x8. It really is nice to have the pitch so you can cruise at low-mid throttle and have plenty of power in reserve for verticals.