<b>Galvanic Cathodic Protection</b> The principle is based on the fact that the underwater hull to be protected is short-circuited with a less noble metal, with the steel becoming the cathode and the less noble metal becoming the sacrificial anode. In the case of corrosion protection with sacrificial anodes, no adaptation to the required protective current is possible. Zinc sacrificial anodes only provide partial protection, i.e. the protection is not sufficient for larger, scoured outer skin surfaces. A comparable full protection with sacrificial anodes, as is achieved with potentially controlled impressed current anodes, is not justifiable for reasons of weight and cost. Openings in the outer skin, such as sea chests, bow thrusters and the ballast water tanks, must also be protected by galvanic anodes. When the sacrificial anodes have been worn away, they must be replaced, which is usually the case after two years.