Propellers are complicated and repairs should be done by specialists but owners can carry out checks and some routine maintenance themselves when the boat is in the boatyard. A propeller is critical to a boat’s performance, fuel consumption and ride, so it makes sense to keep a propeller in good working order.
Here are some checks and maintenance to carry out:
- Clean the propeller thoroughly, removing all marine growth. Using a plastic scraper to remove barnacles is preferable to a metal one to avoid scratching the blade surface. I find mixing up a paste using white vinegar, flour and salt works well. Apply the paste to the surface and leave it for about 15 minutes. Then rinse with warm water and polish, being prepared to use plenty of elbow grease. Use superfine wire wool to help things along if absolutely necessary but whatever you do, don’t use a grinder.
- Grease the propeller as recommended by the manufacturer’s instructions. Note that folding and feathering propellers need to be greased and the manufacturers provide advice and a kit for doing this.
- Check the surface of the blades for any dents, scratches or nicks, paying particular attention to the tips of the blades.
- Check for any hairline cracks in stainless steel blades, especially around the edges. These can be repaired for much less than the cost of a new propeller, but if ignored then the propeller will soon deteriorate to the point that it will have to be replaced.
- Minor nicks and scratches can be filed smooth but take care not to grind too much as you could easily alter the blade geometry. Filing the nicks smooth will be worthwhile as the smallest of imperfections will affect performance.
- Check the propeller and prop shaft stern bearing for signs of fishing line caught around it, which can easily damage shaft seals. This may require removing the propeller to carry out the check.
- Check the condition of the propeller sacrificial anode if fitted and replace according to manufacturer’s recommendations – it is normal practice to replace an anode when half of it has been wasted away. Anodes do an essential job protecting bronze or aluminium propellers from galvanic interaction with stainless steel propeller shafts. A missing or badly wasted anode will result in very rapid corrosion of the propeller as soon as it is back in the water. Make sure you clean the contact point between the anode and the propeller shaft before replacing it.
- Check for areas of pitting in the blades of faster boats that can be caused by cavitation. Cavitation is a highly complex phenomenon that causes bubbles to form on the back of a propeller blade due to the negative pressure that is created when a propeller spins at high speeds. Over time these bubbles hammer away at the blade and can cause pitting damage, which leads to vibration. Ask an expert’s opinion on the best course of action to repair the blade and whether they think cavitation is the cause.
- If a blade has been bent out of shape or chunks of metal are missing then this will not only affect performance but also cause vibration, risking damage to struts, bearings, seals and drive shafts. The solution is to remove the propeller and ask your yard to recommend a specialist who can repair it for you. They will have the right equipment to get the propeller back into the correct shape – if it is repairable. Trying to hammer it back into shape yourself is clearly not an option.
There are quite a few possible causes of vibration in addition to propeller blades being out of shape and cavitation, as outlined in the checks above. Vibration from the engine and drive system is not good news and several other checks can be done at the same time as checking the propeller if vibration has been an issue:
- Check the integrity of the bracket or strut supporting the propeller shaft and propeller. If this is at all loose then this should be looked into.
- The propeller shaft may be worn or slightly bent, so check this over carefully for any signs of wobble when turning it.
- The engine and shaft may be misaligned – it will probably be best to have this checked over by an engineer unless you have the skills to do this yourself.