⚠️ This article is for general informational purposes only. Boat systems vary by vessel, and improper servicing of a stuffing box or shaft seal can result in water intrusion, damage, or injury. Always consult your vessel’s manufacturer guidelines or a qualified marine technician before performing underwater or propulsion system maintenance. Paradise Yachts assumes no responsibility for work performed based on this information.
This article was originally written in 2010 and has been updated for reference purposes. Paradise Yachts is no longer in operation and is mentioned for historical context only.
If your boat has inboard engines, it is equipped with a shaft seal that allows the propeller shafts to pass through the hull while keeping water out. Traditional boats use stuffing boxes for this purpose. Stuffing boxes are also commonly used to seal rudder posts that penetrate the hull below the waterline.
A stuffing box is a simple mechanical seal that allows a rotating shaft to enter the vessel while maintaining a watertight barrier. Modern boats often use mechanical “dripless” shaft seals, but many older trawlers, motor yachts, and sailboats are still fitted with traditional stuffing boxes.
Older stuffing boxes use a compression nut and sleeve filled with braided packing material, historically flax. These were designed to allow a small amount of water to pass through for lubrication and cooling. A slow drip while the shaft is turning is normal. Today, newer packing materials make it possible for these same stuffing boxes to operate with very little or no visible leakage, helping keep bilges dry.
One popular upgrade is GFO® packing, manufactured by W. L. Gore & Associates. This packing is made from expanded PTFE (polytetrafluoroethylene) blended with graphite. Compared to traditional flax, it runs cooler, lasts longer, and provides improved lubrication. When properly installed and adjusted, it can allow a stuffing box to operate with little to no visible dripping.
We have used this type of packing on our trawler, Patricia Ann , for two years with excellent results and consistently dry bilges.
Replacing shaft packing is a straightforward maintenance task. Begin by loosening the locking nuts and backing off the packing nut. The assembly may need a light tap with a mallet to free it. A packing removal tool, available at most marine supply stores or boatyards, is used to extract the old packing.
Remove all of the existing packing rings using the tool. This process can usually be done with the boat in the water, as only a small, manageable amount of water will enter. Over time, packing that has been overtightened can harden and may wear a groove into the shaft, which is something to avoid.
On powerboats, packing is commonly replaced every two to three years. Sailboats may go longer, depending on use, but replacement is recommended if the stuffing box leaks excessively, requires frequent adjustment, or becomes warm to the touch.
If the old packing comes out in solid rings, use it to determine the correct size. If it comes out in fragments, measure the gap between the shaft and the inside of the packing nut to determine the required packing size.
Cut the new packing into individual rings. Wrap the packing once around the shaft and cut it so the ends meet, preferably with a slight angle. Do not install packing as one continuous spiral.
Install one ring at a time. Place the first ring into the stuffing box and tamp it evenly to the bottom using a small dowel or screwdriver handle. Install the second ring on top, staggering the joint about 90 degrees from the first. Repeat this process for the third and fourth rings, staggering each joint. Most stuffing boxes accept three or four rings.
If space becomes tight, lightly thread the packing nut on and tighten it by hand to compress the rings slightly, then loosen it again to check whether there is room for another ring.
Once all rings are installed, slide the packing nut into place and tighten it gradually. Apply only enough pressure to reduce leakage to a minimal level. After operating the boat, the packing should be rechecked and adjusted if necessary. The stuffing box should be cool or only slightly warm during operation.
A common mistake is wrapping the packing around the shaft as one continuous piece. Packing must be installed as individual stacked rings with staggered joints in order to seal properly.
With modern packing materials, traditional stuffing boxes can perform as cleanly and efficiently as many newer sealing systems while retaining their simplicity and reliability.
⚓ Paradise Yachts was a yacht brokerage offering used yachts to customers worldwide.
This article was original published on 22 August 2010