Don’t Be Cruel To Your Stuffing Box

Regular replacement of the packing around your engine shaft will keep your bilge dry and protect your drive train.

By Frank Lanier
Chesapeake Bay Magazine (2004)
Offshore Magazine (2004)

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With the possible exception of seacocks, no other critical system on your boat is likely to receive less attention than the stuffing box – until it fails. Since their introduction in the seventeenth century to seal the bearings on windmill pumps, stuffing boxes (also known as packing glands) have developed a proven record of safety and dependability, and they’re still used on most vessels to keep the water from entering the boat where propeller shafts and rudder posts penetrate the hull.

Stuffing boxes have become victims of their own reliability. They carry on so well with so little attention that many boaters don’t even realize they need regularly scheduled maintenance. Despite their relatively foolproof design, stuffing boxes do require care, not just to continue functioning, but also to prevent damage to drive train components. Here’s how they work and how to make sure they keep operating properly.

Regardless of the variations touted by competing manufacturers, the basic design for all stuffing boxes remains the same – the shaft is sealed by compressing packing against it, most often by using a hollow nut that screws onto the inboard side of the shaft tube or a tightening arrangement that uses a plate secured by nuts and studs on either side of the shaft. The size of your boat’s stuffing box depends on the boat’s size, the engine size and the thickness of the shaft. The more you tighten either type of gland, the more the packing material gets compressed against the shaft.

Most packing consists of a square plaited material and comes either as traditional greased (or waxed) flax, or a more modern version impregnated with Teflon. Some stuffing boxes have a grease fitting (either on the unit itself or connected by a hose from a more convenient location), but many rely solely on the lubricant in the packing itself. In the latter case, the packing will harden over time (the lubricant dries out) and get worn away by shaft rotation, allowing water to pass and enter the vessel. When this happens, most boaters simply tighten the packing nut(s) to reduce the leak and this works to

a point. But as the packing gets smaller it also gets harder, until it becomes so hard that if you keep compressing it, you’ll eventually begin to score the shaft. You can avoid excessive leaking and shaft damage by simply replacing the packing on a regular basis. This bit of routine maintenance should only take most boaters about 30 minutes a year, and it normally costs less for materials than you’d spend on a large coffee and doughnut.

When to Repack?

Let’s start with the most obvious question: How can you tell when it’s time to repack your stuffing box? If you have to scratch your head while trying to remember when you last replaced the packing, chances are it needs to be done now. In terms of frequency, power boaters would be wise to make it part of their annual maintenance, while most sailboat owners (due to typically lower engine hours) can normally get by with repacking every two years. If your sailboat racks up a lot of engine hours each year, however, you might want to do it annually.

Most folks will want to repack with the vessel hauled. However, it is possible to replace the packing while some vessels are in the water, provided the shaft doesn’t have to be removed. Once the old packing has been taken out, water will enter via the gland, although just how much depends on the closeness of the fit between shaft and gland. Taking water onboard the vessel is always a lively affair, so having all the necessary tools and material at hand for the job and doing it quickly are critical for obvious reasons. In most cases it’s a lot easier to repack the stuffing box during a vessel’s annual haulout, so for the purpose of this article we’ll assume that’s when you’ll be doing it.

Removing Old Packing

The first step in repacking the stuffing box involves accessing and removing the old packing, a fairly simple process that could be hampered by limited access. Stuffing boxes are notorious for being located in hard-to-reach areas with little working room, which is one reason boaters tend to neglect them. If you know what size packing is needed (it comes in various sizes), you can purchase it beforehand and get one step ahead of the game. Otherwise, you’ll need to remove the old packing first so you can measure the space between shaft and the inside of the stuffing box (more on this in a bit).

To remove the old packing, you’ll first have to open the stuffing box. If your boat has the type with a single, hollow nut, this involves first backing off the lock nut (which secures whatever adjustments are made) and then backing off the packing nut completely. For those boats that use glands with the two side bolts, loosen both primary and lock nuts and pry off the end cover with a pair of levers, such as two sturdy screwdrivers.

Now you have to pull out the old packing. The ideal tool for this job should be thin enough to slip between the shaft and stuffing box, and it should have a hook of sorts to grab the old flax packing. It should also be soft enough that it won’t maul the shaft while you’re fishing around. A special flax-removal tool does exist; it looks a bit like a

long corkscrew. However, most people adapt the tools at hand. A sharp screwdriver and a bent piece of sharpened wire will work well, as long as you’re careful not to score the shaft. Needle-nose pliers also come in handy for pulling out packing, once you’ve worked it near the end of the tube. Make sure you’ve removed all of the old packing, because any that's left behind could prevent the next packing from sealing properly (be careful, as the last layer in the gland may be so hard that it feels like part of the stuffing box itself.)

Once all the packing is removed, clean the shaft and stuffing box interior of any hardened grease (a rag slightly moistened with paraffin or engine de-greaser works well) and inspect the shaft. If it’s scratched or heavily oxidized, polish the portion of the shaft that comes in contact with the packing by wet sanding it with a piece of fine silicon carbide (wet/dry) sandpaper. Afterward, wash the shaft thoroughly with fresh water to remove any residual traces of abrasive and apply a light coat of grease to ease reassembly of the packing nut or plate.

Installing New Packing

Next up is installing the packing material. The packing itself is square in cross-section and comes in rolls or strips of various sizes, typically 1/8 to 5/8 of an inch. Determine which size you need by measuring the gap between the inside of the stuffing box and the shaft. Install the packing in rings rather than a continuous spiral, which means you have to purchase it in precut rings or buy single strips and cut the packing into individual rings that will fit around the shaft without overlapping.

The easiest way to do this is to wrap the new packing strip tightly around the shaft five or six times (making sure it sits flat on the shaft) and score it lightly by drawing a knife along the packing perpendicular to the wraps (in the same direction as the shaft), making sure that the knife blade doesn’t touch the shaft. Then take the packing off the shaft and cut it all the way through, thus avoiding damage to the shaft. Count the number of packing rings you took from the gland and install the same number, normally four or five, but at least three. The packing should fill the stuffing box, and you may even find that you can add an additional ring or two more than you removed after you pack them down properly.

Install the first ring flat and with no twists around the shaft and push it into the stuffing box (the flat edge of a large screwdriver works well for this). Continue by installing each additional ring, making sure that the split or joint in each ring is offset from the previous ring to avoid leaks. If your stuffing box contains a metal greasing spacer, place it between the second and third packing ring.

Once all the packing rings are installed, bed them by reinstalling the packing nut and turning it hand-tight, adding another half-turn with a wrench. If you have the other style gland, fit the cover and tighten the nuts slightly to ensure all rings are properly seated. In either case, make sure you don’t over tighten the packing.

Getting the Pressure Right

With all the packing rings properly seated, back off slightly on the packing nut, or nuts, and turn the propeller shaft five or six revolutions by hand. It should turn freely – if it doesn’t, the stuffing box is probably too tight and needs to be loosened a bit. Next, install the backing nut (or nuts) loosely. This will get tightened after you’ve made the final adjustments. Once the vessel is launched, make sure the stuffing box isn’t leaking excessively. If it is, use a wrench to tighten it, but just enough to stop the leaking. Then run the engine in gear for a few minutes to set the rings.

Tighten the nut or nuts slightly to finish setting the rings, but remember it’s better to have them slightly loose rather than over tightening. You may find that a couple of shots in the grease fitting at this point (if so equipped) will reduce leaking. The desired drip rate should be two or three drops every couple of minutes when the shaft is rotating. The gland should be virtually dripless with the shaft stationary. Use two wrenches when you tighten the locking nut(s), one to hold the packing nut still and the other to actually tighten the locking nut.

Fine Tuning

Over the next few trips, you’ll want to monitor the stuffing box for leaks and excessive heat. The packing gland may get warm during these initial runs, but it should never get hot. If it does, that’s a sure sign that the packing is compressed too tightly and you should loosen compression nuts a bit. A laser point-and-shoot thermometer is a great tool for checking the heat, and you can buy one for around $50 at most automotive stores.

Now that you’ve taken the time to repack the stuffing box, take a moment to inspect the hose connecting it to the shaft log. Look for chafe or deterioration and verify that it’s double-clamped at each end with marine-grade stainless-steel clamps. You might also want to consider adding a splash guard to prevent the shaft from slinging water around the engine compartment if a leak does occur. Rotating shafts can generate a fine, often invisible saltwater mist even with acceptable amounts of dripping. This will not only corrode the outside of your equipment, but it can also become ingested by your engine. You can make splash guards from a wide variety of materials, from sections of large-diameter hose to empty plastic soda bottles clamped to the stuffing box. The main requirement is that the guard provide 360 degrees of coverage around the shaft and that it extend at least four or five inches along the shaft beyond the opening of the stuffing box.

A few shots into the grease fitting every couple of months and visual inspection for leaks every now and again will help you attain inner peace and stuffing-box nirvana for the coming season. Now about those leaking chain plates . . .