The traditional method for keeping the propeller shaft watertight is the packed gland or stuffing box. Rings of a soft packing material are placed around the shaft inside the gland nut. Three or four rings are normal for small and medium yachts with auxiliary engines. Various materials are used for the packing, traditionally flax, hemp and cotton, but more modern materials include graphite, acrylonitrile and Kevlar. Packing materials may be impregnated with grease or PTFE lubricants. Packing is normally square in section and comes in a variety of dimensions.
See this link and this one for more detailed coverage of packed stern gland repacking.
|Packed stern gland types|
This drawing shows what is possibly the most common packing arrangement. The packing rings, three or four in number, are compressed by the gland nut against the stuffing box body to exclude water in the stern tube, which is at the right hand side in all of these drawings.
In this design the stuffing box is bolted to the stern tube at the flange. This means that the shaft is not free to move within the stern tube, which must therefore have a bearing at its inboard end. In many designs the stern tube is connected to the outboard bearing carrier by a threaded joint. In many designs the outer bearing housing is unscrewed from the stern tube after removing the mounting bolts. The outer bearing carrier can then be removed, as shown in a photograph below, centre.
Greasing helps to lubricate both the packing and the stern tube bearing but care should be taken not to over-lubricate. Excessive greasing can cause packings to run hot. The diagram shows a grease nipple but various different arrangements will be seen below. Some installations do not use grease at all, a positive water supply to the stern tube being substituted.
Here the gland nut has been tightened to compress the packing rings. The tightening method is described below. Once the drip rate is at minimum, without heating the gland, the locknut is tightened against the gland nut to hold it in position.
Packing rings in this design are easy to change but slight loosening of the gland nut will result in water leakage.
Here the design of the stuffing box is identical to the top one except that the bolted flange attaching it to the stern tube has been replaced by a rubber hose. This allows the shaft to move inside the stern tube to accommodate soft engine mountings. There is normally no bearing on the upper end of the stern tube in this design.
The rubber hose should not be the sole means of resisting torque between the stern gland and stern tube. Castellations, known as 'dogs', are formed at the mating ends of the stern gland and stern tube to prevent this. There is a photograph below.
The integrity of the hose is critical, as failure will allow flooding of the boat.
This design differs from those above in that the packing rings are pressed inside the body by a top-hat shaped sleeve.
Once fully tightened the packing rings are compressed within the body. This makes them more watertight but more difficult to remove. Special tools are available for the job but it is usually possible to drive a screw into each ring in turn, allowing them to be pulled out.
| || |
A typical packed stern gland, flanged type bolted to the stern tube. A compression fitting on the top connects a remote greaser tube for the inboard bearing.
The white cloth is a Pampers babies' nappy, a highly absorbent device that can be very useful in absorbing water.
Photo: Graham Jones
This gland is attached to the stern tube by a short length of hose, which allows a certain amount of movement to accommodate flexible engine mountings. In this case the greaser is remote, supplied via the nylon hose.
Unusually, the gland and locking nuts are tightened with a C-spanner,
Note also the wire to the stern tube connected to an external anode and the grease tube.
Photo: Mike Newby
| || |
Perhaps the most common arrangement in general use. The greaser is mounted on the stern gland, a simple cap full of grease screwed down to force grease to the packing. Both gland and locking nuts are tightened with spanners.
Photo: Steve Pavey
A standard design stern gland but with the inboard end of the stern tube supported by a small bulkhead on a Contessa 26. In this case there is no rubber hose. Engine mountings must be stiff unless a special coupling is used. A connection to an anode protecting the stern tube can be seen in the foreground. A remote greaser hose goes to the stern tube.
Photo: Tom Grant
| || |
Mike Newby's sketches and words about his stern gear arrangement. (Sketches are high res and can be read at 200% zoom)
The stern tube housing is all phosphor bronze and contains a bearing aft.
Many people make the mistake of just replacing the stern tube rubber with ordinary rubber piping, this is not correct or safe, as it is special reinforced stern tube rubber, in this case it is 3/8” wall x 3” long (available from T Norris & Co).
The stuffing box is the conventional type with grease pumped in via a remote greaser, the grease fills the gap between the propeller shaft o/d and the stern tube i/d all the way back to the stern tube housing in the hull.
Once set up and usually ‘adjusted twice’ (so the stuffing box does not run hot) after re-stuffing the stuffing box, it does not drip, maximum after 3 hours engine running a saucer full. Grease is pumped at the rate of 1.5 turns after each day's trip.
| || |
Instead of a gland capping nut this design has a flanged compression sleeve that is tightened by two nuts on studs attached to the gland body. The packing is water-lubricated by a pressurised supply.
Photo: P Catterall
Another variation on the basic design. In this case the gland nut has an internal sleeve that applies force to the packing material.
Photo: Matt Hughes
| || |
An elderly and completely troublefree stuffing box – water lubricated. Flanged compression sleeve with a traditional packing. Has only been adjusted three times in 20 years.
Photo: Roger Gaspar
Cutless bearing carrier at the outboard end of the stern tube on the traditional boat shown on the left. The housing can be removed after unscrewing from the stern tube inside the boat. The cutless bearing can be removed from the housing on the bench.
Photo: Roger Gaspar
| || |
An almost completely metal-free stern gland. The main parts of the packing are made from a plastic composite, only the bolts being metal. The source of the rust is the carbon steel washers.
Photo: Ian Pirie
Component parts of the standard stuffing box. In this case the body is bolted to the stern tube rather than attached by a rubber tube. The cup is shown on the left. This compresses the packing rings against the flat face of the body, shown on the right. The locking ring is in the centre.
Photo: John Tetlow
| || |
A packed gland installed in a 1983 32' motorboat. The hose attachment at the aft end is a water feed that is tapped into the log. The water feed comes from a dedicated factory tapping, on the injection elbow just after the main water injection pipe. This is on a VW Marine engine and is located as advised by VW. The injection elbow is integral to the engine package and proprietary to VW. A trickle of water passes at idle and more at speed.
This photo shows the rubber hose pulled forward to reveal the dog clutch style interface.
Photos: Keith Ireland
| A packed gland on a Seamaster 925 sailing boat probably dating from the original 1978 build. The packing is inside the gland nut, compressed against the end of the threaded part of the gland. The length of the nut suggests that there may only be one turn of the packing material inside.|
Photos showing the internal appearance of the gland nut with its packing (top) and the end of the threaded part with the locking nut (bottom).
Problems with packed stern glands
The most common problem that will be encountered is leakage of water into the boat. A small leakage, perhaps one or two drops per minute when the engine is running, is sometimes advocated as desirable. My own experience is that it is perfectly possible to achieve no drops into the boat without overheating of the gland. The initial action is to tighten the packing to take up any leakage.
With the gland nut type, first loosen the locknut. These nuts are quite large, perhaps 50 mm across flats, and it is worth making or buying spanners to fit them before problems arise. Tighten the gland nut a little at a time, turning the shaft by hand as you do so. With the compression sleeve type adjust each nut equally, checking that the flanges on each part remain parallel. Keep tightening until you begin to feel some resistance to shaft turning. At this stage it is quite likely that there will be no water drips at all. Then run the engine in gear, checking that the water flow into the boat has stopped or is maximum 3 or 4 drops a minute, but also that the gland does not get warm. If flow is higher but there is no temperature rise it is safe to take up a little more.
When everything has settled down after a couple of hours of motoring the shaft should rotate freely by hand and the gland should be cool. Now the packing can be nipped up very slightly to eliminate drips altogether, but check for heating from time to time. Advice from some sources is that warmth, but not hotness, in the gland is acceptable. I prefer no warmth at all and have always operated in this way.
A serious problem can occur with the type of stern gland that is attached to the stern tube with rubber hose. Failure of the hose could result in flooding of the boat.
Friction between the packing and the shaft causes a torsional stress in the rubber. This should be resisted by the engagement of castellations, or 'dogs' between the stern tube and the gland body. In this case the dogs were not engaged and the rubber hose was subjected to torsional shear, which resulted in its failure
(Not my photograph).
Stern tube showing 'dogs' intended to engage with the stern gland, relieving the rubber hose from torsional shear. Note the remote greaser tube on the stern gland.
Use of general purpose hose rather than the special one made for the purpose
The boat was bought in poor condition with the intention of gradual improvement. The stern gland was not replaced before launch. An initial leak, source not known, was modest, letting in manageable amounts of water. Leakage increased rapidly to a much higher rate. Over the space of two weeks the boat filled up to almost two feet deep and was close to sinking. After salvaging it and drying it out the source of the leak was found. The rubber hose in the photo was split beneath, and found to be generally in very poor condition. It was probably as old as 20 years or more.
When the gland was finally removed to be replaced, it was found that there was no positive means to prevent the frictional torque from being taken by the rubber. No dogs were present and a bolted arrangement for taking the torque had apparently been removed by a previous owner.
Photos: Luke Smith
| || |
| A most unusual failure of the gland nut of a stuffing box. The photo above left shows the arrangement as installed. A conventional fixed stuffing box, grease lubricated. The locknut and gland nut remain attached in their normal position. Immediately to the right of the gland nut can be seen the sleeve that is used in this design to apply pressure to the packing rings. Further to the right the top of the gland nut can be seen around the shaft, with a security hose clip further right.|
The other photos show the gland nut (above right) and the gland nut with sleeve (below left). The top part of the gland nut has fractured from the threaded section. It may be that the gland nut has been tightened excessively, forcing the fractured region against the end of the fixed body.
The gland nut was repaired by brazing (below right). In cases where the repaired component is permanently immersed in seawater this would be bad practice, as galvanic corrosion is almost inevitable, but here it is perfectly safe.
Photos: Tim Granville
| || |
This photo shows a seriously neglected stern gland, so heavily fouled up with old grease that it is difficult to identify its type. It appears to be a packed gland with greaser aft of the packing. The fact that grease, mixed with water to judge by its colour, is entering the boat shows the packing to be in need of taking up. Grease will not normally enter the boat, but accumulates at the forward, upper end of the stern tube. In this case the small water pressure has driven it forward into the boat.
Some water is lying on the stern tube and grease has been splashed up the hull during motoring.
The stern gland appears to be directly connected to the stern tube, without a hose.
Photo by The_real_jaykay, whose blog can be seen at http://www.goodtarp.com/
Other stern gland types
Although packed glands can be wholly reliable and almost maintenance free, a range of more technical, maintenance-free seals has entered the marketplace in the past 20 years or so. These are based either on rubber lip seals or on ceramic-to-metal seals similar to industrial pump mechanical seal designs
Lip seal stern glands
| || |
The Volvo seal is entirely constructed from rubber, combining the hose that attaches to the stern tube with lip seals that prevent leakage of water into the boat. An internal bearing is water lubricated while the lip seals need to be greased occasionally from a small sachet available from the manufacturer. The lips run directly on the shaft, which needs to be smooth and damage free. Volvo seals are reliable and long-lasting, a life of 10 years being common.
The seal is not vented in any way. On immersing the seal, whether at first launch or after drying on a tide, air will accumulate in the bearing. Running the shaft in this condition will cause a squealing noise and rapid wear. It is necessary to 'burp' the seal, compressing it in the lips area by hand to allow air to be dispelled. A small amount of water should be allowed into the boat to ensure that no air remains.
A shaft seal would normally be replaced with the boat on the hard. It is possible to change a Volvo seal (or most other types) afloat, as this video shows. Not advisable without some experience of doing it in easier circumstances.
Photo: Andy Gray
| || |
A sectioned Volvo seal showing the various parts. On the left is the hose that attaches the seal to the stern tube with a wide clamp. In the centre is a water-lubricated bearing that maintains axial alignment of the seal assembly with the shaft. Between the two the recess in the rubber moulding allows small relative movement between the shaft and stern tube. To the right are two lips that bear on the shaft, retaining water when stationary or rotating. In the photograph these are heavily worn: 2-3 mm longer when new.
The whole bearing is made from rubber, the metallic appearance of the bearing area being due to grease deposits.
Photo: Andy Gray
| ||The Radice lip seal is similar to the Volvo but with the addition of a vent to overcome the necessity for burping.|
A lip-seal type, possibly one made by Bukh or of a very similar design. Inside the gland there are three rubber seals that need to be changed periodically. The seals on the example shown have been changed three times in around 15 years. Provided the seals and shaft remain in good condition this can be a very reliable seal type.
The hose at the top of the seal connects to a small reservoir containing about 100 ml of oil. This reservoir is mounted at above normal sea level, providing a small positive head to counteract the pressure of water acting on the other side of the gland.
Photo: Mike Bryan
| || A lip-seal type of gland on a boat bought in the Netherlands. The housing towards the camera, forward on the boat, is believed to contain one or more lip seals running on the shaft. The reinforced hose carries seawater from the raw water pump that cools the seals and obviates the need for burping. This particular gland was built by Allpa http://www.allpa.nl/and is described as 'water lubricated'.|
Similar types are made by Vetus, and another was fitted as original equipment on the Albin Ballad, shown here http://www.ballad.no/pakkboks.htm although these contain grease.
Another make, this one labelled BMW. It also contains two lip seals supplied with oil from a reservoir.
The unfortunate result of insufficient hoseclip tightness was that the seal rotated, tearing the vent tube and allowing water into the boat. Fortunately the situation was recognised immediately and rectified with the wooden plug shown in the photograph.
|Tides Marine Seriesone shaft seal|
Tides Marine seal installed on a Moody 31. The tube on this lip seal is connected to pressurised water downstream of the raw water pump. This provides a constant supply of cooling water to the seal so that burping is not necessary.
Beware of this case history:- Mine was dripping - not from the lip seal but from the point where the articulated hose meets the black "collar". When I removed it yesterday the black part had sheared right across about 10mm inside the hose. So 10mm and one jubilee clip were all that were keeping the water out, Tides Marine advised this was caused by overheating from running dry. Cause was a failure of the pressurised water feed.
Photo: Roger Smith
A very similar arrangement, this time on a Moody 346.
Photo: Jeff Williams
| || |
The Halyard seal is no longer manufactured but many are still in service. Oil delivered to the seal from an external reservoir is contained within lips in the body of the seal. Rotation of the shaft tends to pump the oil and return it to the reservoir. The lip seals bear on a highly-polished stainless steel sleeve that fits over the propshaft, retained by grub-screws. O-rings between the sleeve and propshaft prevent entry of water by this route. The oil used is multigrade 10W/30, monograde 30 or ATF.
Photo: 'Old Varnish'
A Halyard seal. The tightness of the small grub screw(s) on the right hand side of the unit is critical- These clamp the sleeve to the prop shaft. If they work loose the sleeve can move, until on reaching its maximum point of travel a loud metal-on-metal squealing noise begins. A sailing magazine a few years ago reported a Halyard seal which started squealing during a trip before destroying itself.
Photo and words: David Wray
Face seal stern glands
|PSS seal - older version|
The PSS seal works on the same principle as the mechanical seal in a centrifugal pump. The fixed face is a ring of carbon (graphite) material, spring loaded by the elasticity of the rubber bellows. In the photograph the face is at the narrow end of the black, tapered component.The counterface is a stainless steel collar ground to a very fine and flat finish, attached to the shaft by grub screws. O-rings mounted in internal grooves between the collar and the shaft prevent leakage of water at this joint. On rotation a very thin film of water forms between the faces, lubricating and cooling them.
This is an older version of the PSS seal, that does not have a vent tube. Any air trapped in the seal after launching or drying out must be 'burped' by pushing the two faces apart briefly.
Photo: Bjorn Forsman
| || |
An example of current production of the PSS seal, The photograph at left shows the seal as installed with the bellows fully expanded. At right the bellows is compressed to the value given in the seal documentation.
The main difference between older and current production is the vent tube. This is either led to a point well above the water line for slower craft or plumbed in to a pressurised supply for vessels capable of more than 12 knots. The vent tube obviates the need for 'burping' of the seal after drying the boat.
A useful video of the setting process for the PSS seal is shown at
Photos: David Coxon
|PSS seal corrosion problems|
| Photo shows the rotating stainless steel face, reversed after the problem was found in order to use the opposing face. Crevice corrosion has occurred between the stainless steel and the graphite fixed face. This may have taken place when the boat was ashore over winter, or perhaps in service. The seal face is flushed with fresh water before wintering in an attempt to avoid a repetition. Two examples of this problem are known.|
A narrow line of crevice corrosion that has taken place between the shaft and the rotating seal. Adding a smear of sealant before the rotating seal is slid into place may overcome the problem but care must be taken as the face is very sensitive to contamination.