Sacrificial Anodes and Galvanic Isolators

 

Propeller corrosion

Help! My Propeller's Been Eaten!


Sounds a bit extreme? It may sound extreme, but it's true, and I mean true in real life, it is not a magical internet myth. Your metal propeller can become a victim of galvanic corrosion, and it can be fatal if not treated. The picture above is real, and it could be yours. If your boat is out of the water now, strike a propeller blade with your knuckle. It should ring like a bell with a clear note. If all you hear is a dull thud, you may be too late already.

Galvanic corrosion is the enemy of all boaters. It can affect metal parts both inside the boat and outside. The major problem areas can be propellers, rudders and propeller shafts. It doesn't finish there though. In steel hulled boats, the hull can be affected too, along with aluminium outboard engine castings and even drive legs.

The problem has increased in direct proportion to the number of boats on marinas that offer a shore power connection. Many boats are connected to mains power while ever they are in the marina, and inverter mains while they are away.

Anodes are a way of drastically reducing corrosion to your hardware providing that they are fitted to your boat in the first place! Boats in your own marina that are connected to the mains, but do not have anodes fitted will actually corrode yours. When your anodes are gone, galvanic corrosion will attack other parts of your boat, the propeller is always a target in this case. This is one of the reasons that your boat should be lifted out of the water regularly, to check the anodes. Of course you could also snorkel or dive down to check them.

Anodes come in 3 main types, Aluminium, Zinc and Magnesium. They are all suited to different applications, but Aluminium seems to support the widest number of applications, particularly the Aluminium compound known as 'Navalloy', which carries a military specification. Zinc is good too, but as it erodes its own corrosion forms a waterproof barrier, preventing it from working effectively. If your zinc anodes seem to be lasting a long time get busy with a file or grinder, they've stopped working because the corrosion has 'sealed' them. Magnesium is also a good material to use as an anode, except that the magnesium disappears (or is sacrificed) way too quickly.

Normally I would include an Amazon link to increase your choice, but there really isn't much of a choice there. Instead, here are two ebay links with masses of choice. The first link, the text link, is International in that it connects automatically to your own ebay site as described in the link itself. The second carousel link is currently UK only.

 


Galvanic Corrosion - What it is and How to Stop it


Modern boats have to be built to comply with CE standard EN ISO 13297. This means that The shore earth cable must be connected to your boats bonding system which includes all of your boats major metal items. This is to ensure that if a fault occurs on your mains electrical system your RCD will operate and prevent electrocution. Clearly, that is a very good thing. However, this life saving rule presents us with its own problems. The earthing arrangement now shares our boats metal parts with everything connected to our marinas earthing system. This includes metal pilings and importantly, other boats.

If you find that your own anodes are disappearing at a high rate it is likely that other boats near you are insufficiently protected and are 'leaching' from your anodes. The only realistic cure is to install a galvanic isolator. The Ebay link below will take you to galvanic isolators on their site, and you will find that the first half dozen or so are from Sterling. This is intentional as they have an excellent reputation in marine electronics. However, they are only the first few. Scroll down the Ebay page for lots more choice.