The term “stern gear” encompasses propellers, propeller shafts, shaft couplings, rudder tubes, rudder assemblies, propeller shaft brackets, propeller shaft seals, stern tube assemblies, bearings and more.
The stern gear of a boat needs to be checked carefully when the boat is ashore as this is something that can only be done when it is out of the water. The same applies for any maintenance and repairs that may need doing, so it is best to check it all over as soon after an end of season lift out as possible.
There are a number of ways that inboard boat engines can be connected to their propellers. The conventional system has a straight line of components leading back from the engine including a gearbox, engine coupling, propeller shaft, stern tube (or shaft log) then through the hull to the propeller. Other arrangements include saildrives, sterndrives, hydraulic transmission and water jet propulsion.
The stern tube can be made of metal and built into the deadwood at the stern of a hull, or embedded in a resin glass moulding through which the propeller shaft passes. The forward end of the stern tube has a watertight stern gland and a bearing for the shaft may also be incorporated.
The stern gland, or stuffing box, is the clever part of the stern gear that prevents water from entering the hull while at the same time allows the propeller shaft to rotate at high speed – it is an ingenious type of seal in other words. The gland is packed with three or four rings of compressible material around the shaft (traditionally greased flax) that enables the shaft to turn without abrading the metal and prevents water from getting into the hull. More modern materials used include graphite, acrylonite and Kevlar. The packing material is compressed by a large nut which encompasses the shaft and this can be tightened as the packing material slowly wears away. In time the packing gets worn away to the point that it needs to be replaced with new packing material.
It should be noted that while this is a clever and well proven system, the traditional stern gland arrangement is not 100% watertight and is designed to allow a tiny bit of water to drip into the bilges – in the region of 1 drop every minute or so when the engine is running. The gland needs to be adjusted just tight enough to prevent the shaft from overheating, enabling it to turn. If the packing is over-tightened then the friction on the shaft is increased and it will heat up as a result, which is not at all desirable. Worse still, the shaft will struggle to turn and may fail completely, which is not pleasant to contemplate.
There are many variations of stern glands and it is worth have a good study of your boat’s particular arrangement, taking expert advice if in any doubt about how it works. Most stern glands incorporate a greaser that allows grease to be injected into the gland through a tube. The grease is applied every few hours of engine running time by turning a small handle on the stern tube greaser reservoir. Here are some stern gland maintenance tasks and tips:
- If your boat has a traditional type of stern gland then it needs to be checked and tightened if necessary, when the boat is ashore.
- Remember not to over tighten the packing.
- Care should be taken not to over lubricate as this can cause packing to run hot.
- Check and re-fill the stern greaser with waterproof grease.
- It is recommended to change the packing in most stern glands every two or three years.
- Many arrangements include a section of heavy duty rubber hose in the stern gland which is held in place by stainless steel jubilee clips. If your boat’s system has this, check the condition of the hose as this can perish. If it is in bad condition, it will need to be replaced ,which is probably best done by a professional as it is a tricky job which involves disconnecting the prop shaft which puts everything out of alignment.
Aside from the traditional stuffing box type stern gland there are two other groups of seals which are more recent developments and are collectively referred to as dripless shaft seals (DSS).