Prop Shaft

[gn7]

So what about the key way cut on the coupler end??

Its not as susceptible because the shaft is clamped and the slot SHOULD be some distance from the back of the coupler. If you can keep the torsional twist away from the slot, outside the coupler, its not as much of problem.
In reality, the ouput shaft of the v drive is more prone to shearing off. Its the reason the smarter guys run a output 1/8 larger than the prop shaft.

Although it is possible that the SS80 shaft broke a prop ear first, and more than believable if you understand WHY, the shaft is also just looking for a reason to fail. A prop ear is more enough a reason.

If the shaft and the prop in the SS80 could finish a 20 lap main under my boat, I would be changing them both before I ran it again.