Prop Shaft

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.

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.

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.

Those little set screws must take a lot of the stress off the key. I pulled a key out from a coupler that did'nt have the set screws in it and the key looked like it had been offset ground from all the load being transferred to it.

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Originally Posted by 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.

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Originally Posted by sangerdan

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.

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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?:bangmyhead::smackhead
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.

I hope I can make this understandable because I'm not good at posting pics. If you could make the I.D. of the collar a "D" shape where the flat of the "D" matched the flat on the shaft, metal to metal fit that would be better than the set screws. The set screws and flat on the shaft are as close as is economical to produce. Keys are not meant to transfer large power but are the easiest way to establish the relationship between the shaft and collar so everything mates square and in a specific location. Those set screws are less affective if they are not at 90* to the shaft flat. If that key splits and spins long enough, it will weld the whole works together, friction welding is viable process in the right place.

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Originally Posted by gn7

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?:bangmyhead::smackhead
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.

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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.

GN7, with a split collar in front of the strut, how does the bushing get water to lube and cool? I can't imagine water being able to pass through the split at high rpm.

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Originally Posted by HotWater

GN7, with a split collar in front of the strut, how does the bushing get water to lube and cool? I can't imagine water being able to pass through the split at high rpm.

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Hasn't proven to be a problem to date. The bushings last about the same amount of time as without the collar. We usually change the bushings during the spell between the Nov opener and the next race in March. Its our longest down time and that's when we do stuff like that and blast plates, steering cables, pulley bushings etc.

It larger than a split 2 bolt safety collar. Like I said, its 4 bolts, and it tapers. Its probably about 4X as long as a regular split color. You can just see it in front of the strut in this picture. Its actually rule in GN. Some are just stouter than others.


http://www.hotboat.com/frm/attachmen...chmentid=34204

I've seen some little sleeves that had a taper to the front that had holes drilled so water could supposedly pass through to the bushing. They only used two little set screws to hold them in place though. Can you get your shaft out without removing the rudder?