A few years ago we made the change from magnesium anodes to aluminium anodes.
Magnesium anodes are for fresh water use only and with our ever growing confidence at sea we have found we are spending more and more time on salt water, the poor old magnesium anodes were struggling to last a year with three or four weeks a year of salt water immersion. Up until fairly recently our only other option would have been zinc anodes but these would have been next to useless in fresh water, so we would have been changing our anodes mid season each year.
With advances in technologies a third option came to light of aluminium anodes. These were originally designed for brackish waters such as those found on the Norfolk Broads but have been found to offer the same protection in both fresh and salt water situations. They certainly have worked for our situation cruising in a mixture of fresh, brackish and salt waters throughout the year. The current set which have been on for three years could have been cleaned up and reused for another year, however the ring anode was showing more signs of wear then the other three anodes so we have decided to change them all for new. For the sake of £40 for all four anodes it isn't worth scrimping and saving, after all it would cost much more to remedy the damage caused by not having adequate anode protection of the stern gear.