Do not use a 12 1/4 x 3 3/4 prop on a YS 63. You will over rev and ruin your engine. I use the Masteraircrew 13 x 6 scimitar. I have used many different props and this one works the best for slow high alpha maneuvers. Check out my web site you will find videos of this setup doing the stuff. I like the 16 X 4 on my 65" yak with my ys 110.