The main water types: salt, fresh and brackish call for different protection needs for your boat. Zinc and aluminum anodes protect in salt and brackish waters and never mix zinc and aluminum on the same vessel.
Zinc= Salt Water only
Aluminum= Salt or Brackish Water
Zinc and aluminum sacrificial anodes are for use on hulls, propeller shafts, rudders, trim tabs, outboard engines, stern drives and in the cooling system of most inboard engines to protect metal parts from galvanic corrosion. Check your manual for recommended size needed and check them on a regular basis as environmental conditions can accelerate deterioration.