The Stuffing Box and Stern Tube - SailNet Community
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The Stuffing Box and Stern Tube

The prop shaft exiting the boat generally creates a large hole in the hull. Keeping the ocean from entering through this opening is a top priority. But probably because they are hidden behind the engine, the stern tube and stuffing box are a part of this critical opening, though they rarely receive attention they should get.

Inspect this important area by breaking it down into three parts, as I summarize here.

Shaft log
The shaft log is a tube allowing the prop shaft to pass and providing attachment for the packing gland. It can be made of machined bronze bolted to the hull or of a fiberglass pipe glassed into place. It should be firmly attached to the hull. Engine vibration and misalignment sometimes loosen the shaft log. Check for any movement.

Neglected Maintenance?

With a flashlight and a dental mirror, inspect the underside of the log. Stains indicate water has been weeping around the shaft log. If it is bronze, seepage indicates that re-bedding the fitting in necessary. If it's fiberglass, a major repair job may ensue.

Survey bronze logs, bolts and nuts for any reddish porosity which indicates electrolysis.

Some bronze shaft logs, especially on large cruising boats, have miniature water scoops port and starboard outside the hull just forward of the cutlass bearing. These funnel water through the bearing. Clear away old growth from the scoops.

On the outside of the hull, check around the shaft log for fiberglass strain cracks or corroded bolt heads.

Packing Glands

Connecting hose
Almost all shaft systems have a hose connecting the shaft log to a gland that rides on the shaft. Standard packing glands, often called stuffing boxes, use heavy-duty exhaust hose. The newer dripless shaft seals use a corrugated bellows with an accordion action. The integrity of this hose is critical. Replacement can only be done by partially dismantling the propeller shaft, which is not easy when a leak develops at sea.

Carefully check the hose for visible cuts or abrasions and, if in doubt, replace it.

Pinch, twist and feel the hose. It should be neither extremely hard nor squishy and should not bulge between the log and the packing gland.

The hoses should have four hose clamps. These clamps deteriorate rapidly, so watch them and replace as soon as any deterioration appears.

Seal or gland:

The seal or gland itself is where maintenance of the two types of shaft seals diverges.

Dripless shaft seals have a composite ring with hose barb clamped to the accordion hose described above. A second metal ring slides down the shaft and fastens to the shaft. The two rings ride against each other as the shaft turns with virtually no friction.

Dripless shaft seal

The metal ring must be pushed far enough down the shaft to put tension into the bellows. Too little pressure of the rings against each other allows leaking.

The metal ring has O-ring seals inside where it contacts the shaft. Before installation or subsequent movement, the shaft must be scrupulously cleaned. If it isn't the O-rings will be damaged when slid down the shaft. Do not use petroleum-based lubricants on O-rings or composites.

Traditional stuffing boxes are made of bronze or plastic and have four parts. The body has a hose barb on one end and threads on the other. A large retaining nut usually is threaded onto the body. Then the spud, or part that holds the packing, is threaded on. These three parts rarely fail unless they corrode together from lack of maintenance.

The fourth part, the packing itself, needs occasional adjustment or replacement. Flax packing is the old standby but the newer Teflon packing will produce less friction and shaft wear. Every boat should carry proper-sized packing for emergencies.

packing wrench

Re-stuffing a packing gland in the water can be very exciting because it usually causes a rush job due to water inflow. So obviously it's a lot easier-and less stressful-to re-pack before launching. Besides wrenches to fit the spud and body, the job requires a packing extractor, sharp knife and a tongue depressor. Here's how:

Wrap a circle of packing around the shaft with the second pass on top of the first. Cut through both layers with the knife held at a 45-degree angle. Don't score the shaft itself. Make four of these packing rings. Unscrew the spud, slide it up the shaft and pull the old packing out. The best extractor is an awl with inch of the tip bent 90 degrees.

Position a new packing ring at the mouth of the spud, align the ends and work it in evenly with the tongue depressor. Repeat with each packing ring, rotating the splice where the beveled ends meet so that it falls in a different place each time. If three rings fill the spud, save the fourth for the next time.

Slide the spud down and hand tighten it. Later, after the boat's been launched and you're under way, with wrenches you can adjust the spud until you get the advised amount of drip. You should have a drip every 10 to 15 seconds when under way. For a few weeks after a re-packing, check the drip rate often.

Look to see if a repeat performance is required at a rudder shaft packing gland. Is the packing the same size?

Photographs by Kathy Barron

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