The Images below show the results of Over Protection within the confines of a small water ballast tank.


Anode Calculations for Iron and Steel Hulls.


The surface area of the hull and anode determines the amount of protection given. The voltages of all metals will vary depending on the salinity of the electrolyte and the total weight of anodic material determines the useful life of the protection system. On a metal hull, it is common to use the standard formula for sea water immersion:


For the hull’s wetted surface: in square metres it is usually accurate enough to use Kirk’s formula. The current density: mA per square metre is approximately 35 for steel, the immersion hours factor is (8760 hours/year times the number of years intended service (usually 2) times the fraction of the year the vessel is in the water


Anode Weight = (Hull’s Wetted Surface) x (Current Density) x (Immersion)/(Energy Content) x (1000mA/Amp)


and the energy content in amp- hours is 368 for zinc anodes at 99% efficiency. The factor for other metals is usually obtainable from the Galvanic Table.


For FRP hulls it is necessary to determine the amount of exposed underwater metals of different type, but easier to install standard anodes and then attach fish anodes until the desired potential is reached.


40 | The Report • March 2017 • Issue 79


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