Shaft broke off at the cutlass bearing

A friend of mine just lost power while running at cruise speed.
The problem turns out that his propeller shaft fractured and broke off. There was no strut. The shaft extended less than an inch out from the cutlass bearing to the propeller. Both the propeller and shaft were lost.
30 foot trawler with Cummins 220 hp.
Anyone else have this happen?
How many folks carry a spare propeller shaft?
Or a spare propeller?
I am considering getting a back up.

My boat came with a spare prop, but as you say..not too many boats running around with a spare shaft! I have not heard of that happening before, unless it hit something.

This can be caused by a bronze shaft turning pink. They become brittle and if there was an engine alignment issue it could cause vibration to help along a crack.
Ed

With SS shafts the devil is crevice corrosion, and it typically occurs at places like the cutlass bearing where there is a low oxygen environment. During my refit we attempted to assess my old shafts, but it is a very difficult thing to do. You put a solution of some kind on any suspect areas, but it is really hard to determine if its merely surface wear/imperfection or something going much deeper.

The guys in the yard showed a failed shaft to me. Fairly scary, but I guess if you have twins you should still have one prop even if you are unlucky enough to have one shaft failure. Assessment of my old shafts was inconclusive, so I put in new shafts.

I do carry spare props and a prop puller on board but I would really hate to try and fit a 30 x 26 prop while the boat was in the water.

My current boat....we took it to the yard for the survey. The boat came out of the water and the survey was over. The props were eaten up. The shafts were eaten up...and the rudders were almost gone. We even determined that the boat was not even seaworthy enough for us to ride home on it so we got a ride home and left the boat at the yard.

The captain that was hired for the gig ended up taking the boat back home by herself(one of the best boat handlers I have seen and a super cool girl...and has become a friend since). When she went into reverse on one engine to make the sharp turn into the fairway, the propeller broke off right at the cutlass bearing. It was no surprise. I have the pictures somewhere. The crevice corrosion at that point was beyond belief. I would estimate about 1/4 inch all the way around on a 1 3/4 inch shaft. So with enough neglect, it can happen.

As an aside, I ended up with the boat because I low balled the guy and fixed everything. It was a gamble since we were never really able to sea trial the boat/engines. They only had 300 hours on them at the time....I just turned 1000 hours on them in the last week or two. MOSTLY trouble free operation...haha.

Baker said:

. When she went into reverse on one engine to make the sharp turn into the fairway, the propeller broke off right at the cutlass bearing. It was no surprise.

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No surprise!?

Really?

I keep a pair of spare zincs on the inside of my boat just forward of the cutlass bearing. It won't prevent an episode such as your pal experienced but will keep what's left of the shaft attached if the point of failure had been at the coupler. A shaft can let A LOT of water in when it gets removed.

On our 40'er we did have a spare shaft and two extra props. Her name should have been Redundancy. Still we went to remote areas where finding stuff would be impossible so having a spare was a prudent measure. (This was in the days before the internet -- circa 1950's and 60's)

I am well aware that zincs on the inside do no good whatsoever regarding stray current and electrolysis. Their sole purpose is to prevent a shaft from falling out (I've sheered keys, both the one at the prop and the one in my coupler/transmission) and to store my spares for the diver. I know where mine are when it's time for zinc replacement.

On Seaweed I have the old prop on a bulkhead in the bilge. It's the wrong size/pitch but would work in an emergency. I would not have gone out to buy one. With the internet I could order and have a replacement in a jiffy. Times have changed and supplies are readily available.

The spares I have aboard are more for convenience than necessity.

I wonder if a could have recognized this before it failed. My diver is pretty vigorous when he cleans the metalwork down below...

It happened to a 46' Markley charter fishing boat I crewed on years ago. We turned to line up with our slip before backing in and as soon as the captain shifted to reverse, the shaft sheared off. There was no vibration or visual sign of corrosion to the shaft prior to the failure. Single screw boat, with no way to stop we rammed the dock on the other side of the creek . It was a pretty powerless feeling but he was able to avoid the boats at the dock we hit and fortunately no one was hurt.

Pgitug said:

How many folks carry a spare propeller shaft? Or a spare propeller?

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It was not uncommon to find both on Bertram sportfishers up to the mid/late 90's. Can't speak for later models. The props were in the engine room on smaller boats and aft under the cockpit floor on larger ones. The shafts lived in lengths of PVC pipe glassed-in by the factory. Access was via a bronze screwed plug in the transom.

Insequent said:

With SS shafts the devil is crevice corrosion, and it typically occurs at places like the cutlass bearing where there is a low oxygen environment. er.

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Assuming that cc is real, does not a packing setup utilizng a good water back flush negate that issue. I've heard rumors/truths/bs that dry GFO systems can lead to shaft issues, whether cc or simulated generator corrosive effects at work.

Shoalwaters said:

It was not uncommon to find both on Bertram sportfishers up to the mid/late 90's. Can't speak for later models. The props were in the engine room on smaller boats and aft under the cockpit floor on larger ones. The shafts lived in lengths of PVC pipe glassed-in by the factory. Access was via a bronze screwed plug in the transom.

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The spare shaft storage sound ingenious. Never would have thought of that.

Something I have seen on many of the trawlers we looked at (including one we bought and sold shortly after) is a shaft log sealed up with no way for water to get into it ahead of the Cutlass. Very little water works its way in backwards through the Cutlass and the oxygen in the shaft log gets depleted. Stainless steel in the absence of a supply of oxygen to replenish the invisible protective corrosion layer that makes it "stainless" wastes away quickly. That is what crevice corrosion is, the oxygen in the crevice depletes and the crevice keeps getting deeper.

Every sealed shaft log should have at least scoops to force water into it or better, as on most commercial boats, a line from the RW system to a stuffing box with a water service connection or the shaft log close to the stuffing box.

A boat which sits for a long time with little water flow from current, can easily develop crevice corrosion inside the cutlass or just inside the hub.

Have a spare 28" prop on Bay Pelican. Have switched props once in 12 years since having a spare. But then like my hull insurance I really don't want to use it.

Pgitug

Not all stainless steel shafts are of same metallurgy. Does your friend know what shafting material he had. Some SS alloys are largely immune to presumed ravages of CC.

I have one spare shaft and two spare props. At haul out I pulled both operating props and shafts and sent them to a machine shop for checking. The shafts were set in the lathe and checked for truth along their length, and then sprayed with NDT powder. Under a black light any crevice corrosion, no matter how small would show up. When I re-installed, I added PSS dripless seals. What I like about them is they have water pumped through so no stagnant water or dry seals, therefore they are always in an oxygen rich environment.


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Not that uncommon when the safety factor for the shaft is below 5, the sharp milled angle for the key-way is a stress consideration, hence why in the old days they spooned out the end of the key way. Many other factors but without more (any) detail its throwing darts.

Capt.Bill11 said:

No surprise!?

Really?

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Not sure if I am following you Cap'n??? I don't know if you read my entire post. But the shafts were almost gone....hence the "no surprise" when they finally did fail....if that is what you were getting at.

Marlinmike said:

Not that uncommon when the safety factor for the shaft is below 5, the sharp milled angle for the key-way is a stress consideration, hence why in the old days they spooned out the end of the key way. Many other factors but without more (any) detail its throwing darts.

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Sometimes it's useful to have contacts in large corporations with lots of technical expertise.

Here's the summary of a metallurgical analysis done on a failed SS shaft.
I would have guessed crevice corrosion was the culprit but the metallurgist ruled fatigue.

Snapping in the described location is most often due to an incorrectly fitted prop or key binding when the prop was installed.

Shafting should ideally be AQ-22 for boats with big HP. I use 100% AQ-22 regardless of HP. Run of the mill SS shafting should not be used yet many unscrupulous machine or shafting shops will use inferior SS stock to make a better margin.

Props should always be properly lap fitted to the shaft taper using Prussian Blue to inspect & measure fit % and then valve grinding compound to arrive at the proper fit. Anything less than a solid 90% + fit to the taper is really quite unacceptable, especially in a high HP application. The taper is what handles the load they key is there as back up.

If the prop is not fit properly to the shaft taper then the key begins to take the load and the shaft usually snaps close to or at the end of the keyway.


Improper machining can also create and lead to stress risers at the end of the keyway. The keyway should ideally be spooned at the in-board end for a transition with now sharp sharp edges.


Prop nuts should also be installed and torqued correctly. With an improperly fitted prop the nuts will have even more tendency to become loose and load the keyway.

Small nut goes on first then big nut.

It doesn't just happen to trawlers!




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Pgitug - I don't think a spare shaft and prop is really necessary on a Nordic Tug. First, your prop is very protected, so little worry of hitting something. Second, I don't think you could change the shaft in the water. If your boat is like ours, you would need to drop the rudder or cut a hole in it in order to change out the shaft. Third, if you have a PSS seal like we do, there is a hose that pushes raw water through the cutlass bearings that keeps everything cool, and also prevents stagnant water. Finally, I have no idea where you would be able to store an extra shaft. I think ours is at least 10 feet, and yours would be a little longer. Finally, I've scoured the SENTOA owners forum archives and never saw a single instance of a shaft breaking on a NT. Possible, yes, probable on a well maintained boat, very unlikely. Just keep your towing insurance paid up.

I carry spare props and tailshafts because I have them. Shafts are monel. Because my boat was built 70 years ago, any new shaft would be custom built. Tailshafts make good ballast and weigh enough that two men can barely lift one.

Bacchus said:

Sometimes it's useful to have contacts in large corporations with lots of technical expertise.

Here's the summary of a metallurgical analysis done on a failed SS shaft.
I would have guessed crevice corrosion was the culprit but the metallurgist ruled fatigue.

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The shaft in the picture is made of 304 SS, not the recommended grade of SS for a higher speed vessel like the SeaRay. CMS summarized potential shafting issues succinctly. Not all shaft failures are due to CC, possibly very few.

Carolena said:

Pgitug - I don't think a spare shaft and prop is really necessary on a Nordic Tug. First, your prop is very protected, so little worry of hitting something. Second, I don't think you could change the shaft in the water. If your boat is like ours, you would need to drop the rudder or cut a hole in it in order to change out the shaft. Third, if you have a PSS seal like we do, there is a hose that pushes raw water through the cutlass bearings that keeps everything cool, and also prevents stagnant water. Finally, I have no idea where you would be able to store an extra shaft. I think ours is at least 10 feet, and yours would be a little longer. Finally, I've scoured the SENTOA owners forum archives and never saw a single instance of a shaft breaking on a NT. Possible, yes, probable on a well maintained boat, very unlikely. Just keep your towing insurance paid up.

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Valid points.
My friend had a 30' Mainship. I am guessing some kind of corrosion on the shaft. He doesn't think he hit anything in the water at the time of the failure. Rudder looked fine except for a ding caused by the loose prop.

Shoalwaters.
PVC tube with end screw caps can be cut to length and filled with old shot blast, sealed and glassed into position to correct listing or as ballast.
Insequent.
A handy wee tip.
If you use 2 hammers, give the prop base a sharp tap with both hammers simultaneously from opposite sides, rotate the prop and repeat, it will just pop loose. No need for prop pullers.
Just helpin..

A friend has 2 spare props for his sportfish. He used them to create a really good looking base for a round salon table. One is on top of the other with a stained dowel through them and a nice top supported by the dowel. Looks great.

Irish Rambler said:

Shoalwaters.
PVC tube with end screw caps can be cut to length and filled with old shot blast, sealed and glassed into position to correct listing or as ballast.
Insequent.
A handy wee tip.
If you use 2 hammers, give the prop base a sharp tap with both hammers simultaneously from opposite sides, rotate the prop and repeat, it will just pop loose. No need for prop pullers.
Just helpin..

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I just hate the idea of banging hard into the transmission gears. [emoji15]

Greetings,
Mr. P. I think Mr. IR is suggesting you simultaneously bang at right angles to the prop shaft on either side of the prop hub thus applying a percussive force that really should not be transmitted into the drive train. Whether or not one can get a good "swing" on a 3 blade prop is another thing. A 4 blade should afford a bit more room.
Now, I banging on the front or back of the prop hub will most certainly transmit forces into the transmission gears.

Bang on Firefly, pun intended.
Obviously at right angles.
Were talking sharp taps here, rotate the prop, couple of sharp taps, rotate again, etc. and it'll just pop loose. No need for Redneck wallops..
After all it's your girlfriend, so treat her gently.

Baker said:

Not sure if I am following you Cap'n??? I don't know if you read my entire post. But the shafts were almost gone....hence the "no surprise" when they finally did fail....if that is what you were getting at.

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I was just thinking that it had to be at least a bit of a surprise to the person operating the boat at the time.