Feathering propellers can significantly improve efficiency both under sail and under power, but demand particular attention to ensure the blades move freely as designed.
At first sight these may appear to fit into the “fit and forget” category of boating equipment. After all, they are made of robust marine materials with properties carefully selected for the purpose. However, the reality is that even a well looked after example may last only 10 years before serious problems becomes evident, while a neglected prop could fizz away in only a few months. An added complication is the increasing popularity of feathering propellers, which increase efficiency, but mean there are also moving parts that are permanently immersed in salt water to consider.
So what can go wrong? The biggest concern is one of inadequate anodic protection allowing electrolysis to eat away at the zinc content of the alloy from which propellers are made. A de-zincified propeller may at first sight look similar to one in good condition, but when tapped with a light hammer you will only hear a dull thud, instead of a clear ring like a bell. If the top layer of surface oxidisation is gently scraped away from the metal, the prop (or at least some areas of it) will be seen to have a pinkish hue. What you’re seeing here is the copper that’s left after the zinc has gone. What’s more, the prop won’t even be as strong as regular copper – there will be holes in the structure where the zinc is missing.
This highly polished prop shows the colour to expect once the surface oxidation has been removed. Any sign of a pink hue indicates dezincification and a significant weakening of the metal’s structure.
The solution is invariably a new propeller, but before fitting the replacement it’s important to ensure anodic protection is in place and is doing its job properly. A common problem is for the anode to drop off due to excess vibration. If you’re not lucky enough to regularly sail in waters where you can swim from the boat, the advent of inexpensive submersible digital cameras makes it an easy task to check that this anode is in place every couple of months.
A propeller that strikes an underwater obstruction such as a rope is likely to sustain damage to the edges of the blades. This will significantly impact on the prop’s efficiency and is also likely to through it off-balance, which will increase vibration in the drive system (including the propshaft, cutless bearing and gearbox, thereby increasing wear in these elements. However, the good news is this kind of damage is nearly always repairable, providing the propeller is otherwise in good condition.
Feathering props are available from a number of different manufacturers, so inspection and service procedures vary. However, as a minimum all should be inspected on an annual basis in order to the overall condition, and whether there’s wear or other damage to the bearings.