Every boat owner knows the danger that corrosion poses to the hull of their vessel. A corroded hull not only poses a danger to the boat but to everyone onboard. The most effective way of combating corrosion in vital areas of your boat is to fit sacrificial or martyr anodes which will corrode instead of the hull or drive of the boat. The principle of sacrificial anodes is well understood, but there is often confusion about which type of anode you should consider fitting. Aluminium, zinc and magnesium anodes are all promoted, but the differences between them are not always clearly understood. The most appropriate type of anode for your craft will be determined by where you want to use your boat. If you want to protect a freshwater runabout or any other craft that will operate in rivers or other fresh water, then you should always choose magnesium anodes. Magnesium is far more active than aluminium or zinc and will be more effective at producing the required voltage than zinc or aluminium anodes under freshwater conditions.

Why are magnesium anodes best for freshwater crafts?

While the choice of magnesium over aluminium anodes might seem arbitrary, there are good technical reasons for making that decision. Saltwater is much more conductive of electrical current than fresh water. The greater resistivity of fresh water means that the greater driving voltage provided by magnesium can deliver a larger current than would be possible with a zinc or aluminium anode and therefore the anode will operate more effectively.

Why you should never use magnesium anodes in saltwater

As saltwater is more conductive, the amount of corrosion is significantly increased and a magnesium anode would not last very long at all. If you must use your boat in brackish water, or perhaps switch use between salt and fresh water, then an aluminium anode would be a good compromise option for protecting your hull in both types of water.

When should you replace your magnesium anodes?

While most boat owners understand the importance of sacrificial anodes, what is not always appreciated is that the anodes will require replacement from time to time. No anode will go on protecting your hull indefinitely but should instead be replaced when half of the anode has corroded. Should you discover that it takes less than a year for half of the anode to corrode, you should consider replacing your anodes with some that are significantly heavier to provide longer-lasting protection.