Effect of warship impressed current cathodic protection configuration on low frequency current output modulations associated with propeller/shaft rotation

Physical scale modelling was employed to examine the effect of reference electrode (RE) location upon the modulations of warship impressed current cathodic protection (ICCP) Current output resulting from propeller rotation. A one hundredth scale painted Metal model of a frigate hull, fitted with two rotating propellers, was protected by ICCP in sea water diluted to an electrical conductivity one hundredth that of standard sea water. All 'all aft' four anode ICCP configuration was employed, with the RE of one of three locations in the after cut up. Propeller/shaft rotation produced amplitude modulations of ICCP current outputs. The data shows that both the waveform and the magnitude of the modulations were determined by where the RE was sited relative to the propellers. A different output waveform was associated with each RE location. A superimposed 'beat' rhythm was produced only with one position. The RE location also determined not only total current outputs but also the absolute and percentage magnitude of the modulations. Possible mechanisms for the phenomenon and requirements for further studies are discussed.