Getting properly shafted

Diagram of stuffing box and stern tube

Inquiring minds will want to know how the engine inside the boat can turn a propeller outside the boat when the propeller is under water and we don't want the water to come into the boat. In the tidy little diagram at the right you will see the answer.

Stuff to cut up to make stuffing rings

In this diagram, the engine is to the right. The propeller shaft is hooked to the engine and passes through the stuffing box and the stern tube to the propeller at the left. It is basically a shaft passing through a tube stuck the hull of the boat. Now for the tricky part - making the arrangement leak proof. It is rather difficult to make the stern tube bearing snug enough to keep water out without having it so tight that it causes excess friction which causes heat which causes the whole thing to weld itself into a lump that will no longer make the propeller turn. The solution is to make the stern tube bearing loose enough to let some water get past it and help cool it.

The pieces labelled "Packing Gland" and "Packing Rings" are the secret to keeping water out of the boat. The rings are simply thick string - about 1/4" in diameter - often flax, heavily impregnated with a slippery lubricant. A ring is a length of string cut to just the right length to go around the propeller shaft. Stuff several layers of these into the shaft and then compress them using the stuffing gland and voila! we have an arrangement that doesn't leak - much. Normally, when the propeller shaft is turning, about one drop of water per minute will work its way past all the packing and drip into the boat. This helps keep the packing rings cool. One drip per minute is easily handled by any competent bilge pump. As time goes on, the packing rings get compressed and the boat maintenance guy has to adjust the packing gland to keep the proper amount of pressure on them to stem the flow of water.

The grim reality of the stuffing box environment

So much for the theory part of this post. The reality is pictured at the right. When you compare this picture to the diagram, please note that in the picture the engine is to the left and the propeller to the right.

When I bought Ayala, her stuffing box was dripping about once per second. A quick look at the stuffing gland told me that the gland was adjusted as far in as it could go. Either there were no packing rings or they had been compressed so much that they were no longer effective.

Now, it seemed to me that pulling the packing gland out of the stuffing box while the boat was in the water might be a dicey kind of proposition. Once the rings are pulled out, what is to keep the ocean from coming in? I consulted with my friendly local boatyard and was told that the water would not come in fast enough to sink my boat as long as I had my work well organized.

Before pulling the stuffing gland out of the stuffing box, I made several new stuffing rings and laid them where I could get to them easily. I elected not to dig around and remove what was left of the old rings. This being the first time doing this job, I didn't want to take the risk that I might do something wrong and end up with an embarrassing amount of water in the boat. I will repack the stuffing box next time I have the boat out of the water to have the bottom cleaned - in about a year.

Of course, since this whole assembly resides in a low part of the boat, it is somewhat greasy and dirty with some minor corrosion in evidence. In fact, it is so low, that it is below the floor boards in the compartment under the cockpit. Working on the stuffing box means crawling into a fairly small compartment, kneeling and putting my forehead on a big black tube for support and reaching down into the depths of the bilge with both hands.

Getting the nuts off the studs sticking out of the stuffing gland was a slow process. I could only turn the nuts about 1/6 of a turn before I had to reposition the wrench. Once the nuts were off, the packing gland refused to cooperate by sliding out of the stuffing box. Half an hour of applying PB Blaster (penetrating lubricant that also dissolves rust), tapping on the gland with a hammer and using a cold chisel as a wedge between the stuffing gland and the stuffing box persuaded the stuffing gland to let go and slide up the propeller shaft toward the engine.

I was gratified to see that the water was coming in at no more than a steady trickle. It was enough to cause the bilge pump to come on for a few seconds about every thirty seconds. Pushing the new stuffing rings into place was relatively easy. I slid the stuffing gland back into place and seated it with a few taps of a hammer before tightening down the nuts that held it in place.

Now the book says to start the engine and put it in gear so the propeller is turning. Observe the stuffing gland and see how fast the water is coming in. If it is more than one or two drips per minute, tighten the nuts so the packing gland compresses the stuffing rings a bit more. Then let the engine run for about five minutes and feel around the stuffing box. If it is hot, the stuffing is too tightly compressed.

At the end of the five minute test, all was well. Only two drops of water were observed and the stuffing box was only slightly warm to the touch. All that was left was to clean up.



Rat's nest of wiring dangling perilously close to the steering cables

Did I say "all that was left?" The observant reader will have noted from the second picture and the one to the right that this space, located under the cockpit, is pretty grubby looking. Also, the picture at the right shows a rat's nest of wiring with some of the wiring dangling close to the steering cables at the upper right. It is conceivable that when Ayala starts rolling and pitching in a rough sea, these wires could end up getting tangled in the steering mechanism. Not a pleasant thought.

It was another hour's work to scrub down the enclosure then reroute wires and use wire ties to secure them where they won't cause mischief.

With all that done, I have completed the last major task I need to do before I go out day sailing on the San Francisco Bay. Hopefully, the next post will contain a description of such an event!