• Packing a Traditional Stuffing Box

    Many thanks to 25thmustang for his contributions to this article!

    Packing a traditional stuffing box is normally not hard to do once you have an understanding of how it works. The hardest part is usually the contortions you have to put your body through to get to it.

    Before we begin lets make sure we are clear on some of the components of the stuffing box. One component that is not shown is the shaft log. The shaft log is a tubular fiberglass passage through the bottom of the boat where the shaft passes on its way to the strut and cutlass bearing. The stuffing box assembly attaches to the shaft log with a length of hose and a pair of hose clamps.

    Attachment 928

    Many people use the term Stuffing Box to refer to the whole assembly and that is acceptable, but for the purposes of our discussion we will specifically refer to the threaded housing that the Lock Nut and Packing Nut thread onto. When you repack a stuffing box you are actually placing the packing into the Packing Nut. The Packing Nut is secured in place with the Lock Nut to prevent it from coming loose.

    Ideally to do this project you want the boat out of the water, but if you don't have the option the packing may be replaced with the boat in the water. If the boat is in the water, you mayl need to find a way to block the water flow while you pack the nut. The best way to do this is to use plumbers putty and go under the boat and seal the shaft log (where the propeller shaft enters the boat). This putty will have to be removed once the job is complete! Some people have reported success using plastic bags wrapped around the shaft, tied off and pushed snug up against the boat. I have replaced the packing on my boat while it was in the water without having to block the water source. The waterflow into the boat was not excessive and easily handled by the bilge pump. Water pouring into a boat can be disserting at first but you can easily check the amount of water flow by slowly removing the packing nut. If you are comfortable with the amount of water coming on board, gently thread the nut back on and prepare the packing as described below.

    Tools Required

    To begin you will need a few tools. You can invest in spanner wrenches and specially designed tools but pipe wrenches will work just as well. Personally I have two pipe wrenches that I use. I shortened the handles by appx. 2 inches to enable them to fit into the tight space.

    Razor knife or razor blades to cut the flax to the correct length.

    You will also need a device to remove the flax. A bent ice pick, a piece of stiff wire bent 90 degrees or many auto parts stores sell a package of hooks and picks with various bends that resemble screwdrivers that work well.

    The Process

    Essentially you are going to cut the new flax to the proper length, loosen the Lock Nut, unscrew the Packing Nut and remove the old flax with the pick, install the new flax into the Packing Nut, tighten the Packing Nut and then tighten the Lock Nut. Simple Enough!

    Most Lock Nuts and Packing Nuts are normal threads, however on twin-engine boats it is not uncommon to find either the port or starboard stuffing box to use reverse threads. So be careful when you are loosening the Lock Nut and not apply too much pressure until you determine how it is threaded.

    Cutting The Flax Packing

    Ensure you have the correct size flax packing for the stuffing box, if the old packing is relatively intact you can measure the thickness. Most boat manufacturers will readily provide you with the correct size information and all it takes is a phone call. As a last resort you can measure from the shaft to the inside of the Packing Nut. You don't want to get flax that is too thick, it will exert too much pressure on the propeller shaft and over time wear a groove that will ruin the shaft.

    Ideally you want at least 3 full wraps of flax around the shaft to fill the Packing Nut. Flax normally comes prepackaged with enough to complete one stuffing box along with a little extra.

    Now you need to cut the flax to the proper length to fit completely around the diameter of the shaft. To determine the length you can use a piece of pipe the same diameter as your propeller shaft. If you don't have a piece of pipe you can use the actual propeller shaft, however do this outside the boat in front of the propeller or an area that is not uder the packing nut (this avoids scoring the shaft at the stuffing box).

    To properly cut the flax wrap it around the shaft or pipe and cut it diagonally using a razor blade along the red line illustrated above (appx 45 degrees). This will allow the ends to mesh together once the Packing Nut pressure is applied. Once you have your flax cut to the appropriate length, it is time to install.


    Loosen the Lock Nut by placing one wrench on the Packing Nut and the other wrench on the Lock Nut. The stuffing box is normally clamped into a rubber hose that is attached to the shaft log going out the bottom of the boat. Using two wrenches helps prevent twisting and damaging the shaft log hose.

    Attachment 933

    In the photo below you can see that the Lock Nut has been loosened and the Packing Nut has been unscrewed from the Stuffing Box.
    Attachment 934

    Removing The Old Flax

    Once the Packing Nut is unscrewed from the Stuffing Box, use your pick to carefully remove all of the old packing from the Packing Nut. It helps to have a mirror to inspect the Packing Nut. Use caution during this process and avoid scoring the Packing Nut or the propeller shaft.

    Installing The Flax Packing

    Wrap the first piece of flax around the shaft snug up against the stuffing box, ensure the 45 degree ends fit flush. Now slide the Packing Nut up against the flax and carefully force the flax into the Packing Nut (do not allow it to twist). Avoid rotating the Packing Nut while doing this as it will fray the flax. Once the flax is as far as it will go into the Packing Nut, use a wooden dowel or other blunt object to carefully ensure the first wrap of flax is fully seated into the nut. Note where the 45 degree joint in the flax is located because you want to install the other wrap joints equally spaced out around the nut. It helps to mark the Packing Nut with a sharpie or similar pen. Now install the remainder of the flax as described above, remembering to space out the flax joints.


    When all of the flax wraps are installed in the Packing Nut, gently begin to thread the Packing Nut onto the Stuffing Box. If the Packing Nut is close to being full it may take some pressure to get the threads started but this should be done by hand to avoid cross threading (remember the threads may be reverse threads). Once the Packing Nut is on the threads then apply more hand force to begin to tighten the Packing Nut. When the Packing Nut will no longer turn using only hand pressure, slowly remove the nut and ensure that the flax is seating properly. Then re-install the Packing Nut and tighten snug using a wrench. Once it is snug then loosen it slightly and snug it up with your hand. This ensures the flax is seated and also ensures that it is not too tight when you first turn the propellers.

    Now thread the Lock Nut snug against the Packing Nut and tighten using your wrenches.


    Now with the boat in the water you want to adjust the Packing Nut to the proper pressure. If the boat was in the water, remove the plumbers putty or the item you used to block the water. Now check the Stuffing Box while the props aren't turning to check for leaks. Ideally you want no drips while the prop is stationary. If you notice any dripping loosen the Lock Nut and tighten the Packing Nut a bit more.

    Now check the drip rate while the propellers are turning at idle rpm. A drip rate of appx 1-3 drips per minute is optimal and should provide enough lubrication for the shaft. (if you are using the Gore GFTO packing you can tighten the nut until the leaking stops completely but you will need to check the temperature of the stuffing box after running the boat for a short time). The packing box should not be hot to the touch. You should check both drip rates after a few hours of use and expect to adjust them a few more times as the flax begins to seat itself.
    Ana and WindRiff like this.


    Comments 30 Comments
    1. BoatDog's Avatar
      BoatDog -
      thank you for a very helpful article, I read a few others I found on the web and this one filled in the missing pieces, job was a breeze. I will try to take pictures of the next project I do if you want.
    1. TimG's Avatar
      TimG -
      Thanks boatdog and certainly take lots of shots from all angles and maybe we can turn them into an article, all contributions are welcome!
    1. kw210's Avatar
      kw210 -
      Good info.Very helpful
    1. gab's Avatar
      gab -
      thanks a lot!
    1. greenghost39's Avatar
      greenghost39 -
      Likewise I have read many articles on the subject
      None like yours
      Thankyou verymuch m8 very clear calm and thorough descriptions
    1. Lee Catalina27's Avatar
      Lee Catalina27 -
      Does anyone have any information on PSS shaft seals in comparison to a stuffing box?
    1. Roger's Avatar
      Roger -
      Quote Originally Posted by Lee Catalina27 View Post
      Does anyone have any information on PSS shaft seals in comparison to a stuffing box?
      We changed our old stuffing boxes to PPS Shaft Seals two years ago.

      The starboard side has no issues. Installed easily and hasn't had a drop of water since. The port side had some issues. The first attempt leaked. It is better now but at planing speeds it does throw a fine mist. Not enough to add water in the bilge but the mist mixed with the carbon from the seal face throws a black ring on the hull around the seal. Over all I'm happy I made the change.

      Here is the post from 2011 when I installed ours.
      Haul Out for Service and Bottom Paint
      Scroll down to post #4 for the PPS install part.
    1. Lee Catalina27's Avatar
      Lee Catalina27 -
      Quote Originally Posted by Roger View Post
      We changed our old stuffing boxes to PPS Shaft Seals two years ago.

      The starboard side has no issues. Installed easily and hasn't had a drop of water since. The port side had some issues. The first attempt leaked. It is better now but at planing speeds it does throw a fine mist. Not enough to add water in the bilge but the mist mixed with the carbon from the seal face throws a black ring on the hull around the seal. Over all I'm happy I made the change.

      Here is the post from 2011 when I installed ours.
      Haul Out for Service and Bottom Paint
      Scroll down to post #4 for the PPS install part.
      Thanks Roger!
    1. edtella's Avatar
      edtella -
      Good article and GREAT pictures of the process! Anyone have thoughts on using a shaft lube like Syntef prior to installing the new packing material?
    1. TimG's Avatar
      TimG -
      Thanks Edtella, I haven't used Syntef myself, maybe some of the others have.
    1. captmick's Avatar
      captmick -
      I need to replace the stuffing in my stuffing box, and boat is in water. I'm curious about the "plumbers putty" comment, as haul out, just to replace 3 strands of flax is somewhat cost prohibitive (obviously when inspected when last out for bottom job, through hull replacements & new depth sounder x-ducer, it appeared fine...once back in the water, was dripping too much & tightened to stop dripcompletely. Would like to know the "Putty Trick" IF it'll actually hold back the water and not interfere with replacement process? It's a 15T displacement Sailboat & a 1" shaft behind a Perkins 4.108 & Walter V-Drive. Does the plumbers putty get applied to exterior hull at shaft exit point by a diver?
    1. onwhiskeycreek's Avatar
      onwhiskeycreek -
      Over the years we went from using using keystone style packing on everything to changing everything over to mechanical seals to reduce maintenance. Mechanical seals worked great on most things but not all things. In a perfect world mechanical seals are the best product and have improved greatly. What we found out was that the keystone style packing on some applications added features that mechanical seals couldn't provide. We progressed from dial to reverse dial and then to laser alignment, and used vibration analyzes to check the results. Keystone style packing had the ability to dampen vibration due to misalignment, and also helped to carry the load during a bearing failure. Bottom line is when the packing is shot it needs to be replaced, or you just may see the bottom. A lot of things came and went over the years, but there is no substitute for doing it right.

    1. TimG's Avatar
      TimG -
      Mick if you have packed the boxes previously then you may not necessarily have to use the plumbers putty. The last time I did mine I just had everything right there and ready to go and did it on the water without using anything.

      But if you decide to use the putty, yes it goes on the outside where the shaft exits the boat.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Don't forget to remove the putty and clean it up when you're done. For what it's worth, I helped a slip neighbor change packing on one shaft in his Trojan, we stuffed a trash bag backed by a fun-noodle into the shaft-well against the outside of the stuffing box. When we pulled the old packing out, there was barely a trickle of water to contend with while we put the new flax in. Worked very well. Still not a fun way to burn a day, but certainly part of involved boat ownership. Good luck!
    1. Mitch's Avatar
      Mitch -
      Great info!

      I can't figure out where the stuffing box is on this 1988 36 Foot Carver Mariner. Can you help me?
      Attachment 3574

      Reverse angle
      Attachment 3575
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Mitch... Your stuffing box is the metal portion attached to the rubber hose. You have the inner 'shaft tunnel' with the bolts coming through it (those hold the flange the rubber hose is connected to) the other end of the rubber hose is connected to the stuffing box. I'd have to see it from the end more to give any additional input though. Good luck!!
    1. Mitch's Avatar
      Mitch -
      Here are some more angles. Not sure what to disassemble. Is it the end of the metal casing opposite of the rubber boot?
      Photo 1
      Attachment 3576
      Photo 2
      Attachment 3577
      Photo 3
      Attachment 3578
      Photo 4
      Attachment 3580

      Photo 5
      Attachment 3581

      Photo 6
      Attachment 3579
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      It is. Though I've not seen one quite like you have there- hopefully someone else will recognize what you have. Mine were a pair of nuts threaded into the metal portion- one locked the other. Yours looks almost like a two piece clamp with maybe Allen screws securing it? I would ask around, or maybe see if you can find anything stamped on the casing for more info!
    1. TimG's Avatar
      TimG -
      That looks like a dripless (packless shaft seal) system. In other words it is not a traditional stuffing box it uses a stainless steel ring around the shaft that is slid up against a carbon ring to form the seal, see this https://www.youtube.com/watch?v=3UvyF-NRooQ
    1. Mitch's Avatar
      Mitch -
      Thanks TimG! But what is the metal cylinder between the robber and the ring? Looks like it has 2 screws going into it. There is residue of some type of fluid beneath it in the bilge. Guessing its coming from there.
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