Skipper’s Tips #6 – Anodes

As we head into Winter anyone with a boat should be thinking about servicing, anti-fouling and whether or not the sacrificial anodes need replacing?

“What on earth are sacrificial anodes?”

Let me explain in terms that even I can understand.  Different metals immersed in water will suffer corrosion as electrons erode.  The zinc sacrificial anode used in seawater, being less electro-negative than the metals used for props, shafts, rudders etc, will corrode and protect the running gear of your vessel. Vessels which are regularly taken from fresh water into salt water or even brackish water, will need to have a zinc/magnesium combination and those boats moored and used exclusively in fresh water will need to use magnesium sacrificial anodes alone.

If you were to let your anodes become completely eroded, the rest of the underwater metals on your boat would start to erode and as you might imagine this can get costly.  A few years back I bought a yacht which had not been properly protected and my first job looked like it was going to be replacing the prop. Fortunately, once I’d had it professionally checked and polished it turned out to still be usable but any more erosion and it would have been beyond repair and I would have had the additional expense of replacing it.

Therefore, my advice to you is for the first few months you own your new boat regularly check your anodes and indeed, the wiring on the inside of your boat that connects the anodes to the various bits of machinery which come into contact with the water.  Having regularly checked the erosion rate of your anodes you will start to understand how fast they are disappearing and be able to schedule less frequent checks going forward.

You may also find that there are plugs screwed into your engine block that will need replacing as they erode; check with whoever services your engine(s) that checking/replacing these particular anodes is part of the service and if it isn’t then you will either have to do this maintenance yourself or pay the extra to get it done on the engine service.

There is also likely to be an anode associated with your bow -thruster and if you have one, your stern-thruster too.  Clearly, you will need to check these anodes when the boat is out of the water.  If you decide to leave your boat in the water over Winter, then you will most likely be checking the underwater anodes twice a year – in November, as you prepare for Winter cruising and then again in April/May as you make your boat ready for the Summer; it is entirely your decision if you want to pull your boat out more frequently than this to check the anodes which are not visible from the pontoon.

Please bear in mind that, in some situations, anodes may degrade very rapidly. For instance, where you keep your boat can have a dramatic effect on the speed with which your anodes ‘disappear’. Anodes will often but not always, last for anything up to a year. However, in extreme cases I have heard of anodes having to be replaced every six weeks. This can be due to electricity leaking into the water near your boat and speeding up the erosion process.  Most likely, this is from a pontoon power supply or other electrical source connected with the pontoon or mooring.  Equally, it could be due to the type of metals used on the underwater gear on your boat in conjunction with insufficient anodes to provide effective protection.  Either way, you must surely see why I suggest regularly checking your anodes when you first get your boat.

As a rule of thumb, if the anodes you can see by looking at your boat from the pontoon look as though they are 50% or more degraded then it is time to organise the replacement of the anodes – unless there is a localised electrical fault causing faster than normal erosion, all the anodes fixed to your boat will all degrade at approximately the same rate, so the visible ones are your benchmark.


This has been another Captain Corbett’s Adventure.  If I’m not on Jersey teaching a private tuition Day Skipper theory or Yachtmaster theory course, then I’m either spending time with someone on their boat, giving them the confidence to take their boat out with their family and friends on board or I’m off somewhere exotic delivering a boat. Either way, I’ll write it up and put it on the Blog for you all to see, so keep popping back to see my most recent adventures.