Prop Shaft

[sangerdan]

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.

[gn7]

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.

[HotWater]

Very good info here, even if we have gotten a little mixed up on which end of the shaft. The pictures I posted are the coupler side. GN7 and I mentioned shaft's breaking at the prop end between the strut and hub. I originally posted the picture to get opinions and knowledge about the type of slots cut. We did'nt have a 5/16 saw cutter so we used a regular end mill bit in a vertical mill. I was told it would be fine but the saw cut slot done in the horizontal mill was less likely to lead to a crack forming at the end of the slot, the ramped up area. Westernaero has a different opinion. I won't try the full rounded bottom slot because I don't have a way to shape a key to match. Other than a bench grinder, which would not create good results. I like his advice about the fillet slot. I can sacrifice a Craftsman screwdriver for key material that will match that slot. Sangerdan mentions milling a flat surface for the set screws to seat against. Is that better than just dimpling the spot where the screws hit? Flat milled area would probably be better if you experiment with spacers and the screws don't always hit the same spot.

[sangerdan]

If I were going to do anything to improve the prop shaft, it would be to cryo-treat. There is no dimensional change and it re-arranges on a molecular level. In todays world you don't have any idea where the raw material comes from or where it is centerless ground, so the quality is always suspect. If you can find this material made in the U.S. (or maybe Germany or England) and ground here I would pay the premium. Some of the photos of keyways appeared to be the coupler end, have them done by someone you trust your life to. Machining these materials sucks and not every machinist has the skill, tooling, or patience to do it correctly.

[gn7]

Very good info here, even if we have gotten a little mixed up on which end of the shaft. The pictures I posted are the coupler side. GN7 and I mentioned shaft's breaking at the prop end between the strut and hub. I originally posted the picture to get opinions and knowledge about the type of slots cut. We did'nt have a 5/16 saw cutter so we used a regular end mill bit in a vertical mill. I was told it would be fine but the saw cut slot done in the horizontal mill was less likely to lead to a crack forming at the end of the slot, the ramped up area. Westernaero has a different opinion. I won't try the full rounded bottom slot because I don't have a way to shape a key to match. Other than a bench grinder, which would not create good results. I like his advice about the fillet slot. I can sacrifice a Craftsman screwdriver for key material that will match that slot. Sangerdan mentions milling a flat surface for the set screws to seat against. Is that better than just dimpling the spot where the screws hit? Flat milled area would probably be better if you experiment with spacers and the screws don't always hit the same spot.

Some of the photos of keyways appeared to be the coupler end, have them done by someone you trust your life to. Machining these materials sucks and not every machinist has the skill, tooling, or patience to do it correctly.

After I posted that first response if dawned on me that Hotwater pic was of a shaft re-cut on the coupler end. Who cuts shortens a shaft from the taper end?
So I kind of owe sangerdan an apology. I was kind of one tracked because we started this discussion about premature failures all occurring on the prop end,

I will say this though Sean, Westernaero is right in that that shaft saw some stress in the keyway, and it was looking to fail. This doesn't have to be the due to a improper machining methods, or bad material, it can be due to a keyway that is too long for the coupler. I see it all the time. 4 or 5 bolt coupler with a shaft that has the slot machined for a longer coupler, and the keyway either extends outside the coupler, or is right at the edge. I am devote believer in 6 bolt couplers and no more keyway length that absolutely necessary.

But in the idea of re-cutting a shaft from the prop end. If I had to shorten a shaft more than a couple inches, because of a strut being moved or it came from a different boat, I would cut it on the prop end, if for no other reason than to get rid of what is probably the most stressed area on the shaft.

When Gary H, Teague and the 911 boat broke shafts at the endure this year, and I knew they were all near new shafts, with very little time on them. I considered approaching them about buying them knowing they were useless to them, but more than long enough for my boat. Then I thought better of it. the WHY kept bugging me, and I am not satisfied as to the WHY they broke.

sangerdan is right about one thing. Although shafts seldom break on the coupler end, it is possible. I'd rather break one on the prop end and lose a prop, than break one on the coupler end and have the shaft make an exit passed the rudder. Seen a coupler fail, and the end result was tragic. Its why there is a two piece 4 thru bolt safety collar on my shaft in front of the strut, and two 2 bolt steel safety collars between the coupler and log. If the shaft or coupler fail, at least the shaft stays in the boat.

[djunkie]

After I posted that first response if dawned on me that Hotwater pic was of a shaft re-cut on the coupler end. Who cuts shortens a shaft from the taper end?
So I kind of owe sangerdan an apology. I was kind of one tracked because we started this discussion about premature failures all occurring on the prop end,

I will say this though Sean, Westernaero is right in that that shaft saw some stress in the keyway, and it was looking to fail. This doesn't have to be the due to a improper machining methods, or bad material, it can be due to a keyway that is too long for the coupler. I see it all the time. 4 or 5 bolt coupler with a shaft that has the slot machined for a longer coupler, and the keyway either extends outside the coupler, or is right at the edge. I am devote believer in 6 bolt couplers and no more keyway length that absolutely necessary.

But in the idea of re-cutting a shaft from the prop end. If I had to shorten a shaft more than a couple inches, because of a strut being moved or it came from a different boat, I would cut it on the prop end, if for no other reason than to get rid of what is probably the most stressed area on the shaft.

When Gary H, Teague and the 911 boat broke shafts at the endure this year, and I knew they were all near new shafts, with very little time on them. I considered approaching them about buying them knowing they were useless to them, but more than long enough for my boat. Then I thought better of it. the WHY kept bugging me, and I am not satisfied as to the WHY they broke.

sangerdan is right about one thing. Although shafts seldom break on the coupler end, it is possible. I'd rather break one on the prop end and lose a prop, than break one on the coupler end and have the shaft make an exit passed the rudder. Seen a coupler fail, and the end result was tragic. Its why there is a two piece 4 thru bolt safety collar on my shaft in front of the strut, and two 2 bolt steel safety collars between the coupler and log. If the shaft or coupler fail, at least the shaft stays in the boat.

So you've got an extra safety collar under the boat in front of the strut? Interesting. I understand the reason for safety just seems like it might be overkill. Probably not in a race boat though. Would think the extra drag from it would make a racer not want that. Please don't call me an idiot. I'm just trying to understand is all.

[sangerdan]

A spot face would work for the set screw if you hit the same spot every time and it didn't get to mushrooming out to raise a spot on the shaft. The easiest way is a flat slightly wider than the face of the set screw. No need to mess with end on the set screw i.e. flattening or pointing. When your doing your pre-boating checkup, check these set screws they along with the clamp screws pinching the collar keep the drive off the key. Driving that key will create more stress than a keyway radius.