Just for the Health of It

Can I Use a Galvanic Isolator to Prevent Corrosion on My Boat?

Here is a short (but true) story from the past that accurately defines the effects of galvanic corrosion.

Hundreds of years ago, when wooden British fighting ships went to sea they were quickly affected by the Teredo Worm, a bivalve mollusc infamous for destroying wooden structures in sea water. This animal has killed many ships in the past and was a problem that had to be cured. The British navy believed that they had cured this problem by sheathing wooden fighting ships with copper, a ‘copper bottomed solution’ (it’s where the phrase came from). Sadly, they attached the copper using iron nails and without knowing it created, in effect, a battery. The copper (being more ‘noble’) became the cathode, and the iron became the anode. The iron nails became sacrificial and dissolved. If you are getting the picture, the anodes on your boat are sacrificial, and we know enough now that we can actually fit extraneous pieces of metal to our boat to prevent our very noble propellers and skin fittings from literally being dissolved.

corrosion prevention

An even earlier example of galvanic corrosion occurred when Royal navy ships hulls were sheathed with lead, again, in an effort to protect them from the Teredo seaworm. Lead is a more ‘noble’ metal than iron so when the ships were launched into seawater an electrical potential was created. The iron bands that secured the rudder of the ships became the anode, and the lead lining became the cathode. The flow of ions from the rudder to the hull caused the brackets to weaken and break. You can imagine the consequences of this to a ship at sea in a storm.

The problems associated with galvanic corrosion have been made worse by modern life. A huge percentage of boats that maintain a marina berth also have a mains power hook up for charging batteries and running our little luxuries, the TV and computers etc. Inevitably the marina’s power supply will be earthed and the earthed wire will generally terminate at a copper rod buried in the ground. This gives us an electrical potential in the region of 0.5 volts. This comes about because of dissimilar metals being connected in an electrolyte – the copper of the earthed rod (or cable) and the various steel or iron engine/boat parts. The electrolyte in this case being dampness in the ground connecting to water in the marina.

A galvanic isolator typically uses semiconductor diodes to block the 0.5 volt potential thus greatly reducing the damage to the less ‘noble’ metals of your boat. It should however, be noted that even with a galvanic isolator installed, you should still have sacrificial anodes on your boat’s hull. This is also the case on vessels that have no mains power on board. The metal fittings used on board that are submerged will still produce an electrical potential that will cause damage. You may not know this but other nearby boats in the marina can directly affect your own. If another boat nearby has no anodic protection it is quite possible that it will draw on your own anodic.parts. You will only become aware of this when your own boat is removed from the water for maintenance.

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