Stern Glands

Stern Glands

The conventional stern gland is usually part of the inboard shaft bearing assembly and its sole job is to keep the water out whilst allowing the shaft to turn. They fall into two main types:-

- "PATENT" TYPES" - These do not need packing, are supposed to last for "life". They often have a telltale bottle. There are a number of designs like Deep Sea Seals & Carbon ring seal. Will they seal if the shaft is bent? Dunno....

Some modern, non-packed, stern glands have an oil bottle attached by a short hose. This is NOT FOR LUBRICATION - it is a telltale - in case the gland starts to leak and shows up by either loss of oil or water appearing in the oil.

- "PACKED TYPES" - In this type (the vast majority), the shaft disappears either through a large, longish nut, or through a sleeve which is held in place by two nuts/bolts. These glands have a "tacky" rope like material inside them which is squeezed up by the big nut or sleeve to stop leaks.


Packing Stern Glands - First of all you must recognise that there are many different types of non-packed stern glands being fitted today and if one of these is leaking you may well be faced with taking the boat out of the water and either pulling back or totally removing the propeller shaft. Some packed glands are associated with a "rubber hose" as part of the assembly, so the fact that there is "rubber" present cannot be taken as any of indication of gland type.

Locating leaks. If a boat is taking water from the general area of the stern gland many people's first reaction is to tighten the gland, however there are other things that should be checked. Clean the black "gunge" from the face of the gland and inspect the hole from which the shaft emerges. If the shaft is not leaving the gland concentrically with the hole and gland (as shown on the right) the gland is worn and may never stop leaking for any length of time. On boats with flexible engine mounts this is often because the rubber mounts have collapsed with age and the engine has not been re-aligned with the shaft every few years. If the mounts collapse on a GRP boat and the situation is ignored the shaft can wear right through the bronze shaft log. This creates a leak (with a sharp edged to the hole) at point Y. Likewise if the rubber hose on this gland is not checked for condition it can perish and leak. If it splits there is a good chance that the boat will sink. If you have an adjustable gland a slight bow in the shaft or even an engine "jumping about a bit" can cause the ball joint to work loose and then the O rings get damaged and the joint leaks.

Adjusting the gland. Convention has it that a gland should drip between once and twice a minute, and if you have a "yellow metal" shaft this is vital to prevent unacceptable shaft wear. However if you leave your boat for long periods you have to rely upon an automatic bilge pump, rely upon a film of grease to keep the gland watertight, or tighten the gland before leaving the boat and slacken it for running. To carry out the adjustment slacken any lock nuts and then tighten the adjusting nut until the gland drips as you require. On glands like Fig 1 please ensure that you tighten the nuts each side of the gland evenly. Remember to re-tighten the lock nut(s) when you have done. Never adjust so far there is no more adjustment left for a later occasion, if you do the gland will leak and you will be unable to stop it. When you have about 12mm (half an inch) of adjustment left it is probably time to think about re-packing the gland.

Preparation for packing. Having decided that it is now time to pack the gland you must first of all consider how much wear you found when you inspected the front of the gland. The greater the visible wear the more worn the bearing surfaces are likely to be and thus the greater volume of water is likely to leak in when the packing has been removed. If the gland has a remote greaser (as most canal boats) tightening this down may well stop almost all leaking. If you are concerned about the amount of water that may leak in on a narrowboat you can remove the weed hatch and wrap cling-film or mastic bandage into the space between propeller and back of boat - but please remember to remove it before putting the boat in gear!

Removing the old packing. Some people simply add more packing on top of what is already there. This may be fine in an emergency, but the old packing will have lost a lot of its "body" and is likely to be hard and worn. It is best to repack from scratch, so you need to get all the old packing out. If you bought a pre-pack you may find instructions inside telling you to buy a special removing tool. Remove the nuts and any "pusher" and screw the screw into the old packing (Fig 11). Then use pliers on the screw head to pull the screw and old packing out. Repeat until you can feel the screw scraping on bare metal. Now the water may be jetting in, but it will be at low volume. This is where you will be grateful that you prepared the packing before taking the old stuff out! You have lots of time - so don't panic.

Repacking the gland. Conventionally, take a piece of packing and push it down the hole using a blunt instrument (I had a length of mahogany I kept for the purpose, but nowadays I use a screwdriver). Note the position of the cut/join. Put the next length in with the cut about 120 degrees away from the first one. Repeat for the next length and so on. Do not totally fill or overfill the void. If you do you will have problems getting the parts back together and may well cross thread the big nut or jamb the pusher. Leave at least 3mm of space above the top packing ring. The leak should now have virtually stopped. Unconventionally, wrap the packing around the shaft in the direction noted before and just keep pushing it fully home, one bit at a time. Note, with this method the gland may leak a bit in reverse.

Adjusting the gland. If the gland is still leaking gradually tighten the nut(s), turning the shaft with your hand* until the drip either just stops or drips once or twice a minute. Now you can pump out the bilge and double check that all lock nuts are in place and tight. Run the engine in ahead and astern for half an hour or so. Keep checking for leaks and adjusting (with engine stopped) as required. Take the boat for a run. Keep feeling the gland for overheating and checking for leaks. Adjust if required.

Packless glands. Certain packless glands utilise a rubber boot on the shaft that holds a ring seal against the face of the stern tube. As with any "rubber" component one must take care to ensure it is changed as soon as there are signs of the "rubber" deteriorating. Certain types are designed so you can fit two onto the shaft. A working one and a back up one some way up the shaft. If the working one fails you can cut it off and then fit the spare in its place without taking the boat from the water. The gland must be adjusted whenever it starts to drip to excess. They are supposed to drip about once every minute or so, but privately owned boats which spend a lot of time out of use would either sink or flatten their batteries via the automatic bilge pump. Most private boats would adjust the gland by tightening the big nut/sleeve until the drip just about stops, the stern tube greaser is then used to finally seal the drip at night and when the boat is left. If the nut/sleeve is fully tight or if the gland will not stop leaking, the gland needs re-packing. If the engine has dropped or been misaligned the gland and bearing may well be worn egg shaped. This will prevent the packing sealing, or, more likely, cause the packing to be shredded inside the gland. Always carefully inspect the bore of a gland which will not stop leaking to make sure it is not worn

The Greaser. This is often referred to as the stern GLAND greaser but, in fact, it actually lubricates the inboard shaft bearing and in boats with plain outboard bearing, that also. Whatever type of greaser you have fitted your lecturer would NOT ADVISE the use of "water pump grease" especially if you have a plain outboard bearing. This grease is too thick and is difficult to get into the bearing. Plain outboard bearings use the grease or emulsion of grease and water passing down the stern tube for lubrication, so ensure they get an adequate supply. Rubber (cutless) outboard bearings should be grease resistant, but you I would not put them to the test. With this type of bearing be more sparing with the stern gland greaser.

Shaft alignment - This topic follows stern glands and shaft logs because, as already stated, an out of alignment shaft will cause bearings and glands to wear, and thus leak. They also cause overheating of the gland and bearing and excess vibration and noise. The easiest way to avoid these problems altogether is to use a suitable flexible coupling so the engine and shaft does not have to be aligned.

Flexible Couplings. There are three types of flexible couplings:

1. Anti-torsional vibration couplings - these are characterised by some form of rubber inserts and are quite stiff, in fact they are usually impossible to bend by hand. They MIGHT allow a small amount of angular misalignment, but all one can be sure of is that they smooth out the torque from the engine and cushion the engine from anything which tries to stop the prop. This type of coupling should be aligned.
2. One piece flexible couplings - this type allows a limited amount of angular and radial (vertical/horizontal) misalignment. They are more flexible than type one. "Centaflex" typifies this type. Never exceed the amount of misalignment quoted by the manufacturer. The gearbox and shaft should be initially aligned with this type AND checked every year. A solid dummy coupling is often used to replace the flexible coupling whilst aligning the shaft.

3. Flexible "jack shaft" types - typified by "Aquadrive" or intermediate shafts containing a pair of vehicle universal joints. These is the only "fit and forget" couplings as far as alignment is concerned. They might need greasing regularly (if they are U.J. types I would insist they have a grease nipple fitted). They will require a thrust block to transfer prop thrust to the hull, otherwise they will "dog leg". The thrust block might be part of the assembly (Aquadrive), otherwise the thrust block will also need regular lubrication. These types allow the use of very flexible engine mounts if required.

Aligning the shaft - This is a very bad name - you actually align the engine. It is far easier to demonstrate the procedure than to explain it in text. First, loosen the stern gland so the shaft can be slid back and forth, then remove the bolts holding the half couplings together (fit dummy coupling if required). Slide the shaft back so the male "land" come free from its recess. The shaft will now "flop about" to a greater or lesser extent - much greater if you have a flexible shaft log. Using scrap block of wood pack the shaft so it will still slide, but is held centralised within the free play. Slide the shaft forward until the couplings meet. The land should slide into its recess. If it does not the shaft is out of radial alignment. This engine needs lowering to align the couplings. The engine might also have to be moved sideways on its beds to cure radial misalignment.

Next use feeler gauges all round the gap in the couplings to check the angular alignment. To cure this misalignment the front of the engine has to be raised, but in doing this the gearbox coupling will move down, so the engine will also have to be lifted. This procedure of raising and lowering various parts of the engine will eventually bring the shaft into alignment. Try and get the gap to less than 0.1mm. Slide the shaft forwards, it should slide home easily, recheck with feeler gauges. Check tightness of engine mounts and recheck alignment. Bolt up couplings and tighten stern gland.

Raising & lowering the engine. Some engines are supported on long, threaded studs between two nuts. These are the easiest to raise and lower, they are also the most likely to drop when the nuts work loose. Another method is to use a pair of (metal I hope) wedges under each foot which are slid over each other to raise or lower the engine. These can slide out by vibration and cause the engine to drop. The traditional way is to cut shims from scrap metal shaped like a square U. The engine mounts are loosened and the engine levered up. The shim is slid in place with the open end pointing downhill (towards the stern). These work well but the engine needs tightening every time a shim is inserted to ensure it is absolutely flat.