This is my usual post, perhaps it should be a sticky? If you are breaking shafts, especially if the break is at the keyway, then at the coupler end, the coupler is probably loose on the shaft, and at the prop end the prop is probably not properly lap fitted. This is often the cause for shaft breakage, as the coupler or the prop bangs against the key and causes the shaft to crack at that stress point. When you replace your shaft, it is recommended that you also replace the coupler and you really must lap fit the prop.

Good luck.