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Old 02-09-2010, 05:27   #1
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Transmission and Propeller Shaft Decoupled Out in the Open

Earlier this season, when sailing on the Baltic Sea, we suddenly found our transmission and propeller axle decoupled. The axle was not completely gone - it had just decoupled and moved 6 inches/15 cm backwards and its end was still inside spinning fast.

The reason for this trouble was fairly obvious. The axle is coupled to transmission with a flanged joint with four bolts. On the end of the axle, there is a flange fitting with straight 10mm holes (no threads) through the rim. The flange on the end of the transmission has the holes threaded to hold the bolts. There had originally been split washers to secure the bolts, but I just guess there is a time/mile limit for them as well. There has never been nuts nor would there be space for them.

It all had worked well for 30.000 miles/14 years. Then, one by one, the bolts had just got loose and ended up to the bottom of the deep bilge. When the last was gone, the oncoming sea did the rest and took the axle out. I thought just how lucky we were not lot loose the bolts that secured the flange fitting to the axle. Should those bolts have gone loose, there would have been just a straight axle which would have slipped out. Maybe the propeller would have hit the skeg and would have stopped there, however, it would have been impossible to get it in without diving or lifting the boat up.

For us there was an easy recovery. As the sea state did encourage for repairs while on the way, I just jammed the axle with a 2x4 blank for its flange fixture not to spin wildly against the through-hull fixture (the proper name for which I do not know in English). Then we just utilized the force 6 / 12m/s wind available and beated hard for the night and another day to the first island on our route (Gotska Sando), anchored to shelter, digged the bolts from the bilge (the hard part, took a couple of hours, only found two), pulled the axle back (took 3 secons), fitted the bolts (5 minutes), and continued our journey.

I hope I am the only one that has never paid enough attention to that coupling. However, should there be another careless sailor with a straight axle and just bolts with no locking screws: tighten them well and check them time to time. I sure do it now every time I open the engine compartment. I also nowadays carry a much wider selection of spare bolts including also very odd sizes I am very unlikely to ever need.
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Old 02-09-2010, 07:00   #2
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wow..younow know that it's a good idea to check the tightness of the coupling bolts (and the set screw if you have one) every so often (not every 30,000 miles)... if this would have happened while you were running in reverse, you might have lost it all ...a little loc-tight on the bolts is a good thing too..
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Old 02-09-2010, 07:35   #3
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wow..younow know that it's a good idea to check the tightness of the coupling bolts (and the set screw if you have one) every so often (not every 30,000 miles)... if this would have happened while you were running in reverse, you might have lost it all ...a little loc-tight on the bolts is a good thing too..
Yeah, that GREEN locktite. And maybe some drilled and safety-wired bolts.
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Old 02-09-2010, 07:42   #4
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http://www.nord-lock.com/

I would use these on critical components. And check on an annual basis.
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Old 02-09-2010, 07:52   #5
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........ if this would have happened while you were running in reverse, you might have lost it all ............
Wise and true advice here! Back in '72 on our first cruising boat we shifted into reverse when anchoring at Sapelo Island, Georgia and our prop and shaft shot out of the boat leaving a hole that fortunately matched a wooden brush handle that I quickly shoved into place. I'm glad to hear this warning again, 'seems I haven't checked my shaft coupling in a while. Take care and joy, Aythya crew
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Old 02-09-2010, 07:57   #6
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For bolts/set screws that are in sensitive, high vibration areas such as transmission flange to propeller shaft it is common the "safety wire" the bolt heads. This is done by drilling a small hold horizontally through the bolt head large enough to allow a stainless steel or monel metal wire to pass through the hole in the bolt head. Most boat supply stores sell the little rolls of the "safety wire."
- - You tighten down the bolts/set screws firmly and then thread a long piece of the safety wire halfway through the bolt head and start twisting the two strands until you get to the next bolt/set screw then pass on of the wire strands through that bolt head. Continue until all the bolts/set screws are "wired" together.
- - For bolt/lockwasher/nut combinations such as used when fastening the two halves of a transmission/prop shaft flange together you really need to use the "fine" thread version of of machine bolts rather than the standard coarser threads normally found on bolts. Proper torque wrench measurement or just tighten until "3 grunts" will keep everything together. It is possible to drill through both nut and bolt shaft and insert a cotter pin but then you will need to identify which bolt/nut went with which hole as the thickness of the flange combination can vary just enough to throw off the nut/bolt cotter pin hole.
- - But the key to safe operation of the whole engine/transmission/prop shaft system is frequent examination and testing of bolts subject to vibration to make sure they are still tight. Painting over the bolt/nut arrangement with spray paint can be used to help identify bolt/nuts that loosen as the paint will be cracked.
- - An easy way to prevent prop shaft lose is to buy one of those circular zincs like a donut and install it on the prop shaft between the stuffing box and transmission flange. If the flange breaks or comes loose the shaft can only drift backwards until the donut zinc is up against the stuffing box.
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Old 02-09-2010, 09:33   #7
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Yeah, that GREEN locktite. And maybe some drilled and safety-wired bolts.
Ding-ding...

I wouldn't trust split washers or loctite in this application.

Drilled and safety wired bolts would be the ticket.
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Old 10-09-2010, 14:10   #8
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I drilled and safety wired mine also. In the Air Force on F-16 fighter jets; ALL srews and bolts are safety wired. If it works at Mach speeds under the vibration of a jet engine, it should be sufficient for a marine shaft.
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Old 10-09-2010, 15:49   #9
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Here's a primer on how to safety wire: Safety Wire Techniques

The key is to twist tight enough, but not too tight (7-8 twists per inch is good), and with the wire pulling in the tightening direction for the bolt or nut (depending on what you're wiring). The anchor can't move, or if it does it needs to be pulling in the tight direction too.

The use of spinner pliers is optional (but faster). Standard aerospace grade stainless (corrosion resistant) safety wire is MS20995C32 - the C is corrosion resistant, the 32 is the diameter (0.032 inch diameter). Cheap safety wire pliers are around $15, pro quality are around $75. If you don't use the pliers, you (cough, cough) should use duckbill pliers (cough). At least if you're taking the FAA Airframe or Powerplant practical exam.

Safety wire is also available in Inconel (N)and Monel (NC), and aluminum (blue anodized, code AB) and copper (yellow, CY,) for safety seals in sizes from 0.020 to 0.091 inch diameters.

A pound spool (325 feet or so) of C32 stainless costs around $10 and is probably more than a boaters lifetime supply. Inconel or Monel is around $50 a pound.

As far as locktite, I use it (on boats, airplanes, cars, and around the house). Once the green locktite has set I generally need a heat gun or propane torch to get the bolt to release. But if you're a belt, suspenders and helium balloon kind of guy like me use both green locktite and safety wire. You can drill your own bolt heads if you have access to a drill press and drilling fixture (jig), but buying them is a LOT faster.
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Old 26-09-2010, 07:37   #10
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Oh life is a lesson. I had problems with my coupling bolts falling out regularly. Well I had heard of the safety wire fix so fix it I did. Drilled the holes in the bolt heads and put them in. Ran and twisted the safety wire up nice and all was well for about a year. We had just finished crossing from Florida to Abaco when all of a sudden we lost all power from our engine. Since I had experienced the loss of these bolts before, I had a good idea what was going on.
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Old 26-09-2010, 07:41   #11
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Dropped down in the bilge for a look and lo and behold, the bolts had all BROKEN OFF IN THE FLANGE. I was in a nine line bind. It took hours of "on my belly" dealings to finally get 5 of the 6 bolt pieces out. I have since re aligned the engine and prop shaft. I also contacted the company that makes the V_Drive unit and learned there are special coupling bolts for this set up. I think I would rather have the bolts fall out than break off in the flange.
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Old 26-09-2010, 07:57   #12
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. . . I think I would rather have the bolts fall out than break off in the flange.
I would suggest you mean that you would really rather to not have the bolts break off nor fall off the flange. Which means using the correct bolts as specified by the manufacturer. Separation of the transmission/prop shaft flange is invariably the result of improper installation and lack of owner periodic inspection and maintenance.
- - The biggest problem arises when the previous owner has replaced something using the wrong or improper parts because of laziness. Finding out that was done usually occurs when the part fails.
- - It is good that you took the trouble to find out from the manufacturer what the real problem was and are going to correct it.
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Old 26-09-2010, 08:12   #13
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Has the yearly checking of the engine alignment with the prop shaft went by the wayside? Embracing this annual ritual would probably have prevented all of the above problems. But then we wouldn't get the adrenaline rush , that ignoring it provides.
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Old 27-09-2010, 05:49   #14
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Thank you for all the replies. What I have now learned both from my experience and this discussion is pretty much the following:
- use original bolts that are meant for the task
- carry a plenty of spare bolts
- inspect them regularly
- also inspect the alignment of the entire driveline
- should one want more security, secure the bolts with a wire. Install the wire properly

On my case, I have located the reason for the bolts falling out to be currently between my keyboard and the chair where I sit: I had totally forgotten to check the bolts even though I have been advised to do so. I have a very rigid steel boat, so the alignment is unlikely to be the problem - at least it is currently perfect. The bolts were also original and meant for the task. This time it was just me.

Even though securing bolts in a flange with a wire makes a perfect sense, I have personally never heard about it before. I now have to try to locate a professional store where I could buy the proper wire and pliers - they do not seem to be available from every store selling bolts and nuts even though the store would specialize on them. On the installation, the link provided by Healer52 will sure be valuable.
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Old 27-09-2010, 06:19   #15
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Safety wire is known as "seizing wire" in the marine world. It is available in many stores and even West Marine. See:
LOOS & COMPANY Stainless-Steel Seizing Wire at West Marine

- - You can use any type of normal pliers to twist the wire, the specific-task pliers are used by aviation mechanics who are installing a lot of safety wire during their work. The proper technique is to align the wire so that it is pulling on the bolt head in a tightening direction.
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