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Old 10-31-2016, 10:33 PM   #1
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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.
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Old 10-31-2016, 10:42 PM   #2
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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.
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Old 10-31-2016, 11:35 PM   #3
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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.
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Old 11-01-2016, 12:11 AM   #4
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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.
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Old 11-01-2016, 01:19 AM   #5
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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.
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Old 11-01-2016, 01:40 AM   #6
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Quote:
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. 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.
No surprise!?

Really?
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Old 11-01-2016, 03:19 AM   #7
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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...
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Old 11-01-2016, 07:25 AM   #8
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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.
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Old 11-01-2016, 07:42 AM   #9
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Originally Posted by Pgitug View Post
How many folks carry a spare propeller shaft? Or a spare propeller?
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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Old 11-01-2016, 07:57 AM   #10
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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.
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.
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Old 11-01-2016, 08:14 AM   #11
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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.

The spare shaft storage sound ingenious. Never would have thought of that.
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Old 11-01-2016, 08:28 AM   #12
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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.
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Old 11-01-2016, 08:30 AM   #13
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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.
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Old 11-01-2016, 10:01 AM   #14
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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.
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Old 11-01-2016, 10:22 AM   #15
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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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Old 11-01-2016, 10:46 AM   #16
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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.
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Old 11-01-2016, 11:14 AM   #17
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No surprise!?

Really?
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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Old 11-01-2016, 11:27 AM   #18
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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.
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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Old 11-01-2016, 11:41 AM   #19
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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.
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Old 11-01-2016, 02:20 PM   #20
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It doesn't just happen to trawlers!

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