Prop Shaft

[gn7]

If there is any force applied to the key you have some issues. The square key is a locational item only. If your prop shaft is properly done there should be a milled flat where the set screws contact the shaft so they can be TIGHTENED to the shaft without leaving scars on the shaft that prevent it from coming apart. If you can tighten 4 set screws and 6 pinch bolts on the shaft AND are driving off the keyway you have problems that need to be investigated. 42 years as a tool and die maker /machinist and the round peg where a square slot should be is wrong on so many levels. The runout from a keyway cutter is of no consequence in the over all scheme of things and was the only way they were done in the good old days. Cutting with an end mill was what guys did when they only had one key to do or didn't have access to a key cutter. You have a square keyway on the output of the v-drive and on the prop shaft and prop, why would I worry about the one end on the prop and coupler. The discussion about have many variables exist in your round key would take another paragraph.

I think you are on the wrong end of the shaft here sangerdan, I have never, nor do I know anybody that has broken a shaft at the coupler.
My first boat was a 16ft Stevens with a warmed over 350HP 327 making maybe 375HP. The box side of the shaft had no slot in the Halibrand output shaft, and no slot in the prop shaft and no slot in the coupler. Just 2 set screws on each shaft side, and dimples in the shaft, plus the clamping of a 6 bolt coupler on the FULL diameter of the shaft. I never had a problem, UNTIL the shaft sheared on the prop end.

We are primarily discussing the prop end where shaft is taper and does have a key slot. The shaft is not at full diameter, and the length of the unclamped area is half as long as the couple, and the supported by another shaft affixed to the boat, and to top it off, the shaft is NOT under compression, the taper is literally trying to split the prop from the inside out. HUGE DIFFERENCE!
And before you get started on prop fit to the shaft, I will tell you have never seen a shaft yet that fail IN THE PROP. Its always just in front of the prop where the fit means next to ZERO. But has EVERTHING to with how the slot was cut.
I really don't much care how the slot is machined on the coupler side, so long as its not done by some yahoo in his garage thinking about how his income is made by the job turn over.