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Stuffing Boxes

2010 May 10
Adjust or repack the propeller shaft stuffing box to stop excessive dripping

My wife Liz and I had just set off from the marina aboard our 1976 Bristol 24 Elizabeth. The day was bright and sunny. The wind was gentle and warm. It wasn't quite enough to sail with, but it was sufficient to carry the fragrant scent of pine across Maine's Casco Bay as we motored toward Great Chebeague Island.

All was right with the world. Until I smelled smoke. And it wasn't diesel exhaust.

"I think we're on fire!" I said, instantly tense.

I dashed down the companionway, lifted the lid to the engine compartment, and was met with a cloud of acrid smoke coming from the just-repacked stuffing box. I was new to boat ownership (and stuffing boxes) at the time. I naturally found the situation alarming. I shouted for Liz to shut down the engine, which she did. I leaned way into the compartment and felt the prop shaft. It was warm to the touch. I felt the stuffing box. It was warm, too, but not overly so.

A call to the marina via VHF radio settled us down. "The shaft isn't hot?" the mechanic asked. "You're sure?"

I assured him it wasn't.

"You're OK then," he said. "Sometimes when you repack a stuffing box, you've got to burn the packing in. Just keep an eye on it."

The mechanic didn't need to tell me twice. The stuffing box was fine. After a few hours of running time, I did need to tighten the packing nut ever so slightly. Then I had no more problems with it.

Traditional stuffing boxes are the bane of many sailors, and many find them intimidating. I guess the potential of sinking the boat tends to give one pause. Yet, they are a fairly simple configuration designed to allow lubrication for the prop shaft without letting in too much water in the process. The main components are the shaft log (a fiberglass tube protruding from the hull into the engine compartment); a heavy-duty, wire-reinforced hose connecting the stuffing box to the shaft log; the stuffing box (a metal tube with male threads on the end); a thin locking nut; and a fat packing nut (the female end of the stuffing box). The shaft runs through the whole works to a Cutless bearing and out into the water.

Rings of flax impregnated with Teflon circle the shaft and are compressed inside the packing nut, which threads onto the male end of the stuffing box. This is what keeps water from pouring into the boat. The thin locking nut holds the packing nut in place. As the shaft spins, a small amount of water lubricates the shaft and drips into the bilge. When properly adjusted, you should see about four to eight drips per minute while the engine is in gear, and none when the boat is at rest, though some stuffing boxes may still drip a little.
Over time, the packing wears down and you have to tighten the packing nut. Finally, when the wear becomes excessive, the drip rate will increase when the engine is in gear and while the boat is at rest, requiring you to replace the packing. Some mechanics say you should replace the packing every other year, every two years, or every five years. Use your own best judgment.

Adjusting the packing nut
To do this job with the least amount of hassle, you need two wrenches that exactly fit your locking and packing nuts. Most sailors go for adjustable wrenches. My preference is a common pipe wrench to grab the thin locking nut, and a spanner wrench for the packing nut. West Marine sells adjustable packing nut wrenches.

Before you start the adjustment process, check the hose connecting the stuffing box to the shaft log. If it's mushy, it will require replacing. As with every other hose below the waterline, it should be double-clamped.

To loosen the two nuts, pull the wrenches toward each other. If the nuts aren't seized together, this will back them apart. Make sure you don't twist the stuffing box inside the hose. You can use WD-40, Liquid Wrench or a penetrating oil (don't get penetrating oil on engine or transmission seals) to help free up stubborn nuts. Once you have the nuts freed up, take an eighth-turn on the packing nut. That should stop or greatly reduce the annoying drip while the boat is at rest. You can go to a quarter-turn, but don't try more than that. If the packing nut is too tight, it'll rapidly wear the packing inside, and it can also score the shaft, which will start a leak you can't stop without replacing the shaft. Tighten the locking nut while holding the packing nut in place.

Run the engine in gear, and count the drips per minute. Feel the shaft and the stuffing box. Neither should be hot. Repeat the adjustment process as needed. If you go through this process and the stuffing box still leaks, your packing is probably worn out and needs replacing. A scored or bent shaft, an engine out of alignment, or a worn Cutless bearing causing too much play in the shaft can also cause leaks that won't quit after properly adjusting the packing nut.

Replacing the packing
Eventually, you're going to have to replace the packing. You can do it while the boat is in the water, but know what you're doing, have all necessary tools at hand before starting the job, pre-cut the new packing rings so they're ready to go, and make sure your bilge pump is capable of quickly pumping out a full bilge. You might want to try repacking your stuffing box for the first time when the boat is on the hard. When you're done, make sure the nuts are tight, and warn the yard crew before launching the boat that you've replaced the packing, especially if you're not there at splash time.

As for tools, you'll need your trusty wrenches and a packing extractor to remove the old packing. You can buy a packing extractor at any chandlery. You can also use an old sheetrock screw, a fishhook or a bent wire.
You will need the right size packing for the outer diameter of your particular shaft. If the boat is on the hard, pull one of the old rings out of the stuffing box and measure it to find out what size packing you need. Otherwise, you can measure the space between the shaft and the inside of the packing nut. Multiply the OD of your prop shaft by 14 to figure out about how many inches of packing you'll require to make three or four rings.

In most cases, you'll require from three to four rings of new packing for the shaft. Make a 45-degree diagonal cut in each ring so the ends properly seat against each other when you wrap it around the shaft. You are essentially creating a butt joint. Don't wind the packing too tightly around the shaft before cutting it or you could end up with a gap at each end. Be careful not to cut into the shaft, or, better yet, use some scrap tubing or a dowel that has the same OD of your prop shaft and pre-cut the rings.

Next, back off the locking and packing nuts, and clear all the old packing out of the packing nut with the extractor. If you don't get it all out, your new packing won't seat properly. Use a flashlight and a mirror if you can't see inside the packing nut.

Wind the first new packing ring around the shaft right at the end of the stuffing box, and gently push the packing nut over this first ring to seat it inside the nut, marking the nut with a Sharpie to locate the joint on the first ring. Don't turn the nut! It'll mess up the packing positioning. Do the same with the next two rings, making sure that the joints are each staggered roughly 120 degrees apart. You don't want all the joints lined up one on top of the other because the packing may leak. If necessary, you can use a blunt screwdriver or a slender dowel to poke the packing inside the packing nut.

When all the new packing is loaded into the packing nut, rethread the packing nut onto the stuffing box. Tighten the packing nut until you feel some resistance, but don't overdo it. You don't want the packing nut too tight. Then hold the packing nut in place while you tighten the locking nut. Traditional flax packing will tend to swell when first immersed in water. Most experts agree that waiting at least a day before starting the adjustment process is ideal to account for the natural swelling that will occur. Run the engine in gear for about two hours, and check the drip rate. If it's fine, you're all set. If it's not, continue adjusting the packing nut until you get it right.

After the first time you've repacked the stuffing box, you'll see that it's really pretty simple, though getting access to it probably isn't. You'll be good to go for at least another two years before you have to think about doing it again.