If your boat has inboard engines, chances are it is equipped with a stuffing box to allow a watertight seal for the propeller shafts. Stuffing boxes are also used to seal rudder posts that penetrate the hull below the waterline too.
Stuffing boxes are actually the seals that allow prop shafts and rudder posts to come into the boat but yet keep the water out. Today's stuffing boxes are made a bit differently than those of just a few years back, thus earning the name "drip less" seals. But this article is about the older boxes found on older trawlers, motor yachts and sailboats.
Keep in mind that the older stuffing boxes were just a compression fitting and sleeve filled with a flax material; they were intended to "leak" a little with the water providing necessary lubrication. A drip or two a minute is about right. But if want dry bilges, you can make them drip less too. How? By substituting your present flax packing with a new material called GFO Packing, made by Gore-Tex. The new packing is made of Teflon thus permitting the packing to be constricted to a point where no water will leak from the stuffing box itself. We have used it on our trawler, the Patricia Ann for two years and have dry bilges.
Replacing your packing is easy too. Take off the four nuts that holds the stuffing box together and work the fitting apart. You may need to tap it with a mallet to loosen it. You will also need a packing removal tool obtainable at any yacht yard.
Take out the existing packing by using the extraction tool like a corkscrew and force the old packing out. You can do it while your yacht is in the water as you will only take on about half gallon of water. Over time, packing has been tightened some the packing may get compressed such that it becomes hard enough to actually wear a groove in the prop shaft == a condition you want to avoid. On power vessels, the shaft packing should be replaced at least every 2 years. Sailboats may not need to have the packing replaced for 5 years or more, but when the stuffing box starts requiring frequent adjustment, it leaks too much or if it begins to feel hot, it's time.
If the old packing comes out relatively intact, use it to determine what size packing you need. If it comes out as distorted wads of gunk, then measure the space between the shaft and the inside of the packing nut to define the correct packing size.
When all of the old packing has been removed, place 1 new segment of new packing back into the compression fitting and push the sleeve back into position by hand. The easy way to do this is to wrap the packing around the shaft in some accessible location and cut across the overlap with a razor knife on an angle making 4 individual rings of packing. Wrap one of your cut lengths into a ring around the shaft and push it into the stuffing box. Tamp it evenly with a little dowel pin or a square screwdriver to push it all the way to the bottom of the box. Push a second ring into the stuffing box on top of the first one, staggering the joint about 90 degrees. Add a third layer, then the fourth, each time staggering the joint. If it does not look like you have room for the fourth layer, hand tighten the adjusting nut to force the other rings deeper, then loosen it again to see if this made room for an additional ring of flax.
By the 4th section, any water flow will have about stopped. Now push the compression fitting into place as much as you can by hand. Next screw on the two compression nuts and tighten with a wrench to continue to apply pressure to the packing. Not to much however, just a firm application until the leak stops. You will need to probably adjust it a bit after you run the boat the next time. Finally, install the remaining 2 lock nuts to complete the job.
A common mistake is winding the new packing around the shaft as a continuous piece. Packing installed this way will not seal correctly. It must instead be installed as a series of stacked rings. This necessitates cutting the packing into lengths that just circle the shaft with ends meeting preferably with cuts on an angle.
Now you can brag to all of your buddies that you have drip less seals on your yacht too.