Ship impressed current cathodic protection - modulations of system current outputs by propeller/shaft rotation on physical scale model hull

Physical scale modelling was employed to examine the effect of propeller/shaft rotation on the current outputs of ship impressed current cathodic protection (ICCP). When a one-hundredth scale metal hull was protected in diluted sea water, propeller rotation produced low frequency modulations of output. Two propellers rotating resulted in two sinusoidal frequencies, the higher frequency 'carrier' being amplitude modulated by a lower frequency envelope. There was only the 'carrier' waveform present when only one propeller was rotated. The 'carrier' frequency (4.67-0.83 Hz) was related to the speed of shaft rotation and was apparently determined by the faster propeller. Envelope frequencies (0.48 and 0.42 Hz) corresponded to differences in speed of the two propellers. Possible causes of this corrosion control phenomenon are proposed, together with the requirement for further experimental investigation and waveform analysis.