Boating Advice: All about Anodes

Over our series of articles leading up to this point covering corrosion, we have gained a healthy insight into the problems and causes out there. We have explored the bonding system and the importance of this in the battle against corrosion. But without an anode you effectively don’t have a protection system.

Let’s backtrack a little... anytime you have metal objects in the form of skin fittings, legs/sterndrives/saildrives, propellers, shafts and rudders that are made up of dissimilar metal combinations and immersed in seawater you have a battery of sorts, supplying current flow between the two metals. If we leave this over time we have a perfect case of galvanic corrosion. 

To combat this and stay protected we need to add a third metal in the chain, in the form of an anode typically made up of zinc, magnesium and recently aluminium. The anode is basically lower in the food chain and gets eaten before the other combinations of metals.

ANODE TYPES
Anodes come in many sizes and vary for many applications. There are shaft, block, tear drop, pencil and so on. But one common factor in failings is poor connectivity; if the anode doesn’t contact the source of protection then it is basically useless.

I have seen many attempts to improve and protect from corrosion... even an anode dropped over the side of the boat on a piece of string!

If you’re not connecting metal to metal, you are not protected (see the slimy anode in the photo above, which clearly isn't working)

It is also very important that anodes are never painted over; they need to be exposed to provide protection.

LEGS AND DRIVES
Sterndrives, legs and saildrives, with the mix of dissimilar metals, are prone to corrosion. It is very important you keep an eye on these and have a professional inspection if you are going through anodes at a rate of knots.

It is recommended that all anodes are replaced when they have depleted by 50 per cent of their original size.

Engines also have anodes fitted generally to heat exchanges. These a generally of a copper alloy composition and a zinc anode is used. During your routine engine service these will be inspected and replaced.

THE NEW MADDOX ANODE
In recent years an Australian Company, Maddox has turned the anode world on its head. Zinc is no longer the preferred choice in the battle against corrosion. The Maddox anode is an aluminium-based anode and, from personal experience, is providing maximum protection.

If you are considering moving to a Maddox system, it is very important you have the installation done correctly, as it is not effective if used with a mixture of zinc anodes. The system works best with block anode and all fittings bonded correctly. (The Maddox anodes can be seen on the timber boat in the pics above).

IS YOUR ANODE WORKING?
Next time your vessel is up, have your repairer take note and, with the camera function on their phone, send you a picture. As they say a picture tells a story.

The amount of anodes we find during inspections that are full in size and have a white scale over them in considerable, and the comments coming from experts that it is great that the anode hasn’t depleted over a period is alarming. The anode in this case is in-active, more often related to poor installation and connectivity, (The sterndrive in the compilation photo has inactive white-crusty anodes).

On the flip side if your anode has rather large craters blown out of it, it means more than the anode is doing its job, it means you have an electrical leak in your system and this requires further testing by an expert. (The blown-out anode in the compilation photo shows the result of an electrical leak).

Timber boats are also at risk. Stern tubes and rudder tubes are the main issues found out in the field. More often the propeller, shaft and rudders are protected. On a timber boat it is a good idea to splash some vinegar on the timber around these fittings, if it starts to bubble you need to look at getting protected.

Have your repairer send you a picture: if your anode has a white,
scale-like coverage it is inactive; if it has craters blown out of it,
you
have a problem that needs urgent attention.
>> Have your system
inspected by a professional and have a corrosion analysis done. At the
time it may sound expensive, but it is a lot cheaper than a new
propeller, sterndrive, saildrive and transom shield.

NOTE: I have added this above-mentioned corrosion testing to all of my pre-purchase inspections (Ed: Gavin Clark is 'The Surveyor 'after all) and the findings have been beneficial to both buyers and vendors. Many issues have been simple fixes but have been on going for the current owner with no resolve. 

Prevention with education is the only tools we have to battle what is fast becoming a massive problem of corrosion in the industry. I always tell owners or potential owners you can only do what you can to protect your boat. By adhering to my corrosion advice and this ode to anodes we can, boat by boat, eliminate any outside influences and break the chain of corrosion.

Over this series, I would like to thank Brian Gatt from Logix Group. He has not only been a constant source of reference, but has gone over and beyond the call of duty to educate me in the testing procedures required to identify issues.

BoatPoint and boatsales would also like to thank Gavin Clark and Brian Gatt for these articles to date on corrosion and anodes. Search "The Surveyor" in our News section.

Photo credit: Gavin Clark from Slipstream Marine; Brian Gatt from Logix GRP; Maddox anodes and pics by Marine Protection Systems.

*Gavin Clark is a contributor at boatsales.com.au and BoatPoint.com.au who writes under the pseudonym 'The Surveyor'. He is a shipwright and marine surveyor with a business called Slipstream Marine based in Sydney. He has 27 years experience in the boating industry, is an avid boater and owner of a 33ft Sharpie (trawler). Gavin can be contacted at http://boatsurveying.com.au.