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Maintaining your propulsion gear

In terms of how it would affect your cruising, having nothing to push you along is just about as bad as it gets.

Narrowboat propeller and rudder

In this latest maintenance article from the experts at River Canal Rescue, you can learn about your boat's propulsion, how to tell if it's not working properly and what to do about it.

The propulsion components comprise: propeller, prop shaft, stern gland, a thrust-bearing coupling, gear box and engine mounts.


If you hear odd noises, experience vibration or lack direction, the chances are the propeller is bent, chipped or damaged. If you hit something and hear a humming/whistling/grinding noise this suggests the prop is misshapen in some way. The vibration can cause the stern gland to leak or be damaged if left undiagnosed.

View from prop shaft end

Prop shaft

If you hit something and the shaft is damaged (bent) it will allow the propeller to oscillate. This puts pressure on the stern gland and can cause excessive wear or damage beyond repair.

The propeller and prop shaft right should not protrude out the back of the boat more than the width of the shaft. If it sticks out 5mm and the shaft is 2mm this leaves 3mm, which is susceptible to damage. If it's more than the width of the shaft it will need adjusting. It's best however, to ask an engineer to do this as it usually needs a coupling adjustment to resolve.

Stern gland

Stern gland

The stern gland is also known as the stern gear. There's a general misconception that stern glands stop water coming in, so naturally people want them as tight as possible. But there needs to be a small amount of water in order to cool the prop shaft. If they are over-tightened it causes friction on the turning shaft which causes overheating. If left undiagnosed, this will cause the ‘packing' to wear away sections of the shaft. If this wear occurs, it will cause the stern gland to leak more and increase the chance of prop-shaft damage. This only applies to traditional stern glands not stern seals.

To test if your stern gland is overhearing, after an hour's cruising, check if it's hot, if it is, the gland is too tight.


If the bolts connecting the propeller shaft to the engine are loose, any movement will either sheer them off, resulting in loss of propulsion, or make the coupling bolt holes oblong, resulting in delayed drive. Eventually the coupling will need to be replaced, and you may even have to change your prop shaft or gearbox if the coupling has damaged them. A simple check before each journey will stop this happening.

Gearbox/drive plates

If you hit an underwater object, the drive plate is usually the first victim of this underwater collision. However, if you've damaged the drive plate, it's unlikely you've damaged the gear box. General wear and tear appears to be another cause – and because canal boats don't have a clutch arrangement, gear boxes tend to receive a fair bit of abuse, so go easy and regularly service them.

Narrowboat Prop-shaft coupling
Engine mount

Engine mounts

Engine mounts affect all the above-mentioned components and if left without maintenance can cause extensive damage. These can be checked by observing from above whether the top nuts are touching the engine legs. The top nuts are set to the correct alignment and should not be touched or tampered with.

If there's a disconnect between the top nut and engine leg then this needs maintenance. There will be another nut below the engine leg which is used to make adjustments. Tighten the bottom nut until it compresses the engine leg onto the top nut. This has now re-aligned the engine, ensuring no further damage is being caused. Check regularly and if in doubt ask an engineer to do this for you.

Common problems

People often catch their mooring ropes around the propeller, so if they drop into the water, take the boat out of gear to prevent them being swallowed. Also avoid shallow water and be mindful what can be lurking beneath you – River Canal Rescue are often called to remove industrial fencing, barbed and razor wire, mattresses and tyres from propellers.

Be aware of the dangers of rudders – if you clip the rudder while turning, it will swing the rudder around which in turn impacts the swan neck causing it to change direction. If you're standing to the side of the tiller, rather than in front where you should be, you could be knocked off your feet by the sudden movement leaving you wet and embarrassed or, it could crush you between the tiller and the obstacle, which can cause serious injury. Worse still if the vessel is in gear, the boat could drive over you. Always stand forward of the tiller.

Last Edited: 05 July 2024

photo of a location on the canals
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