Prop-shaft coupling maintenance

Prop-shaft couplings need regular maintenance to make sure they stay secure. When the bolts on the coupling become loose or disconnect all together, this can cause extensive damage. The worst-case scenario is the engine being thrown off its mountings potentially damaging the stern gland, prop-shaft, coupling, gearbox and engine mounts, plus any external components that may become trapped when the engine shifts. The cost to rectify can go into the thousands.

Unfortunately, this kind of occurrence is still relatively common due to boaters overlooking maintenance in this area. Everyone knows filters and fluids need maintaining, however ensuring bolts are tight tends to be less recognised. The worst part is, this is one of the simpler maintenance tasks and easier to identify than a loose engine mount or slack fan belt.

So what should be done?

Simply undertake a weekly visual check. When you get into the engine room to check oil levels etc., just take a peek at the prop-shaft components and ask yourself the following questions:

1. Has the engine been noisier or' clunkier' recently?
2. Has the sound of the engaging gear become more pronounced or ‘snappy/sharp'?
3. Does the engine sound smoother than it used to?
4. Is the stern gland leaking excessively?
5. Are there problems getting full power from the engine?

If you answer yes to any of these, it's a sign your prop-shaft coupling may be coming loose or is already loose and causing damage. By spotting a potential problem now, you'll avoid further expensive damage later.

How to fix it

You will need access to a set off spanners (or a spanner wielding friend).

Make sure your engine is turned off and that the engine room is safe to enter before you start any work:

• Locate the stern gland and prop-shaft. Follow the prop-shaft towards the engine until you hit the first component. This should be the prop-shaft coupling (it may be thrust bearing which will be bolted to the boat itself, with the prop-shaft travelling through the component). To identify the component is definitely the prop-shaft coupling, it will only be bolted to the prop-shaft and gearbox (not the boat ‘engine bearers') and the prop-shaft will go into the component but not come through the other side
• All visible bolts will need checking. Do not tighten them if they are not loose. If they are loose, tighten them up tentatively or, ask someone more experienced to do it for you. Over tightening can be as damaging as under tightening. Do not engage gear unless you've completed this task

If this becomes a regular occurrence it's time to question ‘why'? Ask an engineer in to check the engine over as the engine maybe out of alignment, have damaged engine mounts or a faulty coupling. Awareness is the key to maintenance in this case.

With thanks to River Canal Rescue for their advice.