Quote Originally Posted by sangerdan View Post
Sorry GN I guess I got lost in the drawings, but this is probably worse. Like you said the round key might try to roll if the locking taper wasn't perfect and would have a wedge action. This area would be more critical on the fit and the round key would be a pain in butt to get metal to metal fit. His pic showed a half round key , which would have a +or-and the keyway + or-. These materials are more "gummy" to machine and would be very hard for someone less than a perfectionist to get even a good half-round on a tapered shaft. How would Joe average check fit and location? Keys are keys and dowels are dowels. They mate as well as lions and Bambi. Thanks for clarifying for me GN. Always appreciate your comments and knowledge.
Don't get me wrong, the who and the how of cutting a keyway is important on either side. Just that topic has some previous history regarding shaft failing, and failing pre-maturely on the prop end.
I agree the round key would require an extremely well fit key, and that's more than can me expected of the average boater, or even racer in the field changing props, and I don't know how much it adds to the strength of the shaft.
Ideally, there would be no key. But it seems that no matter how well you lap the prop to the shaft, there is a need for the key. I am not sold that the fit if the key itself is the cause of the failures because every shaft I have seen that failed, it did so well ahead of where the key itself ended, and just ahead of where the slot stopped out side the prop barrel. I have yet to see a shaft fail in the barrel.
When the shaft fails very pre-maturely like has happened a few times lately, I tend to think it either the material itself, or as Westernearo mentions, improper machining changing the heat treat locally at the end of the taper, or the keyway.