Measurement of the extremely low frequency (ELF) magnetic field emission from a ship

The electromagnetic (EM) fields radiating from a ship are widely recognized as important components of underwater detection. Galvanic currents flowing in the water around the hull and in the hull generate an underwater electric field. This field is responsible for the extremely low frequency (ELF) EM emission. The rotation of the shaft(s) modulates the galvanic current passing through the shaft-bearing-hull and thus an ELF EM signal is generated into the water. The ELF EM signal from ships was analyzed using the cross wavelet transform and wavelet coherence. The phase angle statistics demonstrated the presence of an additional ELF magnetic field component that was identified as being generated by a rotating vertical permanent magnet. The only vertical part of the ship rotating with the same frequency as the shaft is the nickel aluminum bronze propeller, which contains 3–5% iron. The paper presents the measurements and the investigation methodology that led to the separation of an additional ELF component, and to the calculation of the magnetic moment that caused it.