Deciding which sacrificial anode is suitable for your boat depends on the composition of your boat, your drive assembly and the water you are in.

Ideally the best way is to test your boats potential is with a voltmeter but this is rarely done and in the main, not required.

 

Firstly the Hull material.

A fibreglass boat with an inboard engine and bronze and stainless metal parts needs less protection than a boat with an aluminium stern drive.

Steel, Ferro cement or Aluminium hulls may need more protection again.

Zinc or Aluminium anodes can't overprotect for this type of hull no matter the volume attached. Magnesium would also work with a fibreglass boat but only in freshwater.

Aluminium, wooden or steel boats can however be overprotected with magnesium anodes and can cause damage by spooling the paint off.

Sterndrives and outboards use either zinc or aluminium anodes, with aluminium being more versatile as it can be used in both fresh and sea water.

Water Type.

Zinc anodes are most suitable for applications in sea water environments, but will form a protective coat in freshwater that renders it useless.

Magnesium anodes are widely used in freshwater on steel and fibreglass boats.

If magnesium is used in seawater it will over react and quickly disappear.

Aluminium works in both environments and is widely used on stern drives and outboards in both sea and fresh water.