Since the year 2000, most new boats have come equipped with shaft seals that require no packing, but inboard powered boats built before then have traditional stuffing boxes with packing.

All stuffing boxes work on the same basic principle: Rings of special wax-impregnated flax packing are wrapped snugly around the prop shaft and stuffed into a hollow packing nut that is screwed to the outside of the stuffing box housing. Tightening this packing nut squeezes and compresses the packing material around the shaft to form a watertight seal, even when the shaft is rotating. And except for a slow drip, the seal is watertight. A lock nut screws around the housing and tightened against the packing nut to keep it from losing as a result of vibration.

Proper maintenance will preclude:

  • Excessive leaking.
  • Extend shaft life.
  • Less opportunity for crevice corrosion.
  • Reduced chance for trapping air and running the box dry and cooking it.

Stuffing Box Basics

When a stuffing box starts to leak excessively, you should tighten the packing nut until the leak stops. This may be all that is required, but each time the nut is tightened, it squeezes and compresses the packing further, forcing it tighter around the shaft. As the flax packing compresses and the fax wears away, the chance of shaft damage increases. The packing hardens and can eventually do damage by wearing a slight groove in the shaft making it impossible to for a seal.

Replacing the Packing

Once the flax packing wears down, it’s time to replace it. You’ll need some wax-impregnated fax packing, which comes in rolls or strips and is square in cross section. Its width measures 1/8 to 5/8 of an inch. The size you need depends on the space between the shaft and the inside of the hollow packing nut.

Before you get started, you will need a couple of wrenches to loosen and tighten the nuts. One wrench is a traditional pipe wrench, and the other is a spanner designed for a sink drain. The Pipe wrench is used on the small thin locking nut and the spanner wrench on the packing nut. When loosening the nuts, the two wrenches are moved towards each other. When tightening the nuts, the wrenches are moved away from each other.

Start by loosening the locknut. Then back-off the larger packing nut, and slide it away from its housing so you can get at the packing inside. Dig out all the old packing with a thin screwdriver or a bent piece of stiff wire, but work carefully to avoid scratching the shaft. Make sure you remove all the layers of packing.

If you are having trouble breaking the nuts apart, due to corrosion, use a little penetrating oil. Be very careful not to get near any engine or transmission seals. Penetrating oils will eat engine seals and may cause catastrophic failure for the seal.

Cut a piece of new packing just long enough to wrap around the shaft with no overlap. Stuff this ring of packing carefully into the hollow packing nut, then cut a second piece of packing the same length. Wrap the second length around the shaft, and stuff it in against the first layer, staggering the joints in each layer. Add enough additional layers to fill the packing nut completely, and then screw it onto the threaded sleeve of the stuffing box. Tighten firmly by hand, and then use a large wrench to give it an extra half turn.

Check the Packing

Start the engine and put it in gear. Let it run for two minutes to see if the box leaks. It should drip a few drops per minute when the shaft is turning, but no drips when in neutral. If it does, give the packing nut another half turn and check again. Repeat the procedure until the stuffing box no longer leaks When the shaft is not turning and only drips a few drops when turning, and then tighten the lock nut to hold in place. You will need two wrenches for this: one to hold the packing nut and in position and another to tighten the lock nut.

All traditional stuffing boxes require water for lubrication. If the box is allowed to drip, it allows for excellent cooling, longer shaft life, less opportunity for crevice corrosion and less opportunity for trapping air and running the box totally dry and cooking it.