25 Millimeters 

24 September 2000, by Tom Waid

We were doing the usual thing. Last June after sailing into Road Bay, Anguilla we were hot and sticky so we grabbed the snorkeling gear and jumped in the water to cool off. One look under the boat and I was immediately aware that something was wrong. The propeller shaft was extending out about one inch beyond its normal position and immediately a story unfolded in my mind that made it clear to me what had happened. This was something I should have anticipated but, in spite of all my efforts, I let it slip through the cracks and it wasnít just a little thing. Presently the problem wasnít serious but it could be much worse. The shaft could have backed completely out leaving a hole in the bottom of the boat.

As it played in my mind the story starts almost exactly two years previous when I was exchanging the propeller shaft seal for a conventional stuffing box. The shaft seal was a Volvo lip seal and had performed flawlessly for more than eight years but I was worried that the seal was beyond its useful life and that if it wore out during the voyage the boat would have to be hauled out of the water to replace it. Itís a messy job but a conventional stuffing box can be repacked while the boat is in the water, which is the reason why I chose this option.

There were delays. When I first ordered the stuffing box I specified that it was for a 25mm shaft but the man behind the counter assured me that a 1-inch stuffing box would do just fine since the dimensions were so close. The problem was that I incorrectly measured the diameter of the stern tube. This became obvious when I tried to install it and when I returned it to the supplier to make an exchange there was a different man behind the counter who insisted that I order a stuffing box exactly for a 25mm shaft, this time with the right size stern tube. "Itíll have to be shipped from the Netherlands," he said.

Weeks passed waiting for my metric stuffing box and when I finally lost patience I went back to the supplier and reopened the debate about 25mm or 1-inch. This time there were several people behind the counter debating this and the final conclusion was that the 1-inch stuffing box would do just fine since the packing is flexible and a quarter turn more would tighten it enough to make up the difference. (1-inch is about 25.4mm.) Since they had it in stock I went with this option.

Finally I had the part and was ready to actually tackle the installation, which started with the removal of the hub from the end of the shaft where it meets the back of the engine. This has always been an annoying process. The shaft is held in place on the hub by two bolts that extend across either side of a split in the hub to clamp the shaft tightly on. Additionally there was a setscrew that extended into a hole in the shaft. At the time I have to admit that I saw the setscrew as redundant since the clamping bolts gripped the shaft so tightly that I had to hammer a wedge into the split to loosen it. This was accompanied by quite a lot of sweat and bad language so when the threads in the hole for the setscrew distorted and jammed I decided to leave the setscrew out and deal with it later. To me the clamping bolts were doing a good enough job to make the setscrew unnecessary. I recognized at the time that it was important to eventually reinstall it but for now it could wait.

Time marched on and with more pressing things occupying my mind the setscrew submerged in my sub-conscientious. If youíve been following this site you may remember that immediately after setting out last April we had a problem with a leaking rudder seal that required us to haul the boat in order to repair it (The Leak, Part Two.) At the same time we experienced another problem that, while it was a little alarming at first, turned out to be minor. It happened when we were underway from the Little Choptank River to Solomons in the Chesapeake Bay. After motoring out of the anchorage we raised the sails and shut down the engine. It was then that we noticed a loud clunking sound coming from somewhere along the drive train. My first suspicion was that it was coming from the transmission and this did not make me happy. In addition to a haul-out bill we would have to pay for transmission repair. However after I settled down and took a good look at things I concluded that there was something caught around the propeller shaft and that was causing the commotion. The next day I donned a heavy wetsuit with mask, fins, and snorkel and swam under the boat to take a look. The zinc ball that was mounted on the shaft to control galvanic corrosion had come loose. It was an easy fix so when so a few days later when we hauled the boat to repair the rudder seal we fitted a new zinc to the shaft and made sure that it was tight enough to not come loose.

And then there was the "one drip per minute" ideal that continued to elude me. A stuffing box must leak a little water while the propeller shaft is turning in order to lubricate the packing material. I packed our new stuffing box with expensive gore-tex packing that required a minimal amount of dripping. In fact when I called the manufacturer I was told that only one drip per minute would be satisfactory, however no matter how hard I tried I could not adjust the stuffing box to drip only once a minute. The best I could do was once every 13-15 seconds. If I tightened it more than this the flow would completely cease and the stuffing box would begin to heat up. I began to wonder if maybe I shouldíve waited for the metric stuffing box from the Netherlands. This was, of course, all academic. Whether it should be 25 millimeters or 1 inch I was stuck with my decision. There were many things, big and small, that occupied my mind in those days. I fretted over a myriad of things but never once did the missing setscrew cloud my mood.

Until, of course, that bright and sunny day in Anguilla when snorkeling under the boat I discovered that the propeller shaft had backed out of the hub and was prevented from completely backing out solely by a zinc ball that, in my zest to keep it from loosening, I had over tightened. It was firmly wedged against the propeller strut.  Sometimes events conspire with each other to make things worse and sometimes they conspire against each other to mitigate a possible disaster. On a cold day in April I was alarmed by a loose zinc rattling around our propeller shaft and I over reacted by applying excessive torque to the mountings of the replacement zinc. Itís a good thing. On our arrival in Anguilla but for that over tightened zinc we couldíve had a 1-inch (excuse me, 25mm) hole in the bottom of our boat. I re-tapped the threads and fitted a new setscrew.


Happily at Anchor

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