Stern glands and shaft care
Ben Sutcliffe-Davies celebrated 40 years in the marine industry in September 2018. He started boat building with his father and uncle and initially built wooden craft; over time he has also built craft in GRP, steel and aluminium. He has been a marine surveyor for more than 20 years and is now a full member of the Yacht Designers and Surveyors Association (YDSA) as well as of British Marine and the Inland Boat Building Association.
FOR many boat users the connection of the gearbox to the propeller never gets much thought, I’m sure. The only times it very likely does is when either a leak develops through the gland or something gets fouled around the propeller.
There are a number of options available on how to connect the drive shaft to the gearbox; they include what is known as hard coupling where no protection to the gearbox is provided. Personally I do like some form of coupling where there is a combination of protection to the gearbox and a little bit of alignment assistance.
Should the propeller strike something hard then the shock load would be taken by the coupling arrangement rather than the gearbox and therefore save you or your insurance company an expensive bill for a replacement gearbox and propeller etc.
There are several good brands out there with various levels of protection. While on the fitting of a coupling it’s important to ensure that suitable bonding has been provided to allow the cathodic connection to continue; some have this built in or supply bridging cables.
Greasing point
Back from the coupling arrangement some craft, depending on size of shaft and length of the run, will have a form of midway bearing to help support and ensure no unnecessary whip starts to develop. Some boat builders fit one regardless. Some manufacturers provide a greasing point for the midway bearing and it is essential to regularly force grease into it, others are sealed and impossible to lubricate. Some of these don’t have a waterproof seal and so if you get standing water in the bilge can start to ‘squeal’.
The next bit of the drive train to me is one of the most important, the stern gland arrangement. This is where the shaft is passed through the hull to the outside underwater area with an appropriate watertight connection to the stern tube. It’s worth pointing out that at survey this is one area that is not possible to inspect.
With age they can suffer from corrosion. Some stern glands are connected with a hard bolt-on method and some have a special rubber hose fitted; the latter needs to have suitable hose clamps provided to secure the hose to both stern tube and gland packing. When tightening the gland packing it’s essential not to over-tighten and it will load the stern tube hose; in some cases I’ve attended craft where this has torn.
The hose condition should be regularly checked as failure will result in rapid flooding. When I apprenticed some 40-plus years ago it was pretty simple: almost all craft had some sort of ‘stuffing gland’ arrangement. A packing gland rope would be fitted with a good quality gland grease and the packing would be forced into the gland by either a machined capping nut or a flanged compression sleeve with two nuts, lock nuts and bolts to each side.
Almost all of these gland arrangements have a means of greasing while under way but not always. If you do tighten the gland it’s important not to overtighten to avoid the bearing becoming too hot. Like everything there is a happy medium between a tiny infrequent drip and a shaft bearing running too hot. If you need to add more gland packing it’s important to do this correctly.
When under way the greaser should be occasionally turned down. If you are having to constantly force grease in then it’s time to tighten the gland or, as mentioned, add more stuffing! There are several good websites that show how to add gland packing. Basically, the gland packing rope needs to be the right size and has to be taken around the shaft to make a single ring that is then cut at a chamfer to ensure that it is almost like a thick washer. Normally when repacking, depending on the size of the gland, you can often add two or three rings of packing with the chamfers on opposing sides. Lastly be aware that if you see deep scores or wear in the area of the gland packing it is likely the engine alignment is not correct (that’s another subject!).
Lip seal
The other types of stern glands that have been used for the last 25 years or so have some lip seal arrangement within and are quite popular. They are all virtually maintenance free throughout their lifespan and are generally reliable but generally need replacement at around 5-7 years depending on manufacturers’ recommendations.
Some when the craft is launched require ‘burping’; this is quite simply pinching the rubber at the end and allowing the air to free from the stern tube. Forgetting to do this will cause issues with potential damaging of the seals from lack of water cooling in the shaft arrangement to the seal. Some now have a breather to help with water flow up the stern tube and prevent crevice corrosion too.
The drive shaft passes through a stern tube and externally the shaft will be supported in a bearing. These will wear with use, typically in some of the canal systems where fine silt can wear the rubber. Generally, on a 40mm shaft there shouldn’t be more than about 2mm of play; if when the craft is out of the water it’s getting near that, then it’s often better to change while she is out than pay the costs of dry docking or lift out later.
Most bearings have a simple grub screw arrangement to hold them in place; it’s often hard with blacking or antifoul applied to find them. To change the bearing the propeller will need to be removed as well. Before fitting a new bearing, check that the shaft is not worn in the area of the bearing as at this point any excessive wear will just accelerate wear in the shaft bearing going forward. When refitting the propeller don’t forget a little grease will help with removal next time and will ensure the keyways are in good condition and the propeller nut is properly seized.
The repacking of a stern gland isn’t too difficult to do but obviously should not be done afloat unless you know exactly what you are doing. As always, if in doubt get a professional.