The underwater extremely low frequency electromagnetic (ELFE) signal, which results from the modulated current flow between the cathodic protection system and the ship propellers, can be detected by influence mines and trigger their detonation. The propagation distance of an ELFE signal from a typical untreated ship is sufficient to make ELFE-capable influence mines a serious threat.

Influence mines which have the facility to install ELFE signal sensors and signal processing exist today.

The primary source of the vessel underwater electromagnetic field is the electric current that is produced by hull corrosion protection systems. These systems use either passive (zinc) anodes or active (powered) anodes to produce a current that flows into the hull where protective coatings have worn away. This current also flows onto and through the propeller shaft.

• 
• ELFE field from Active Cathodic Protection

The electric potential between the anode and the cathode (the propeller or other bare metal) creates a Static Electric (SE) field (also known as Underwater Electric Potential or UEP). The magnitude of the field is proportional to the level of current flowing between the electrodes and the distance between them.

There is a current path from each anode to the propeller shaft, which then flows along the shaft before grounding to the hull through the shaft bearings. Current variability results from variations in bearing resistance as the shaft rotates and the electrical current becomes modulated at the frequency of the rotating shaft. An alternating current creates an electromagnetic field at the same frequency, which is at the shaft frequency and its harmonics. In the case of a rotating propeller shaft, this is termed the Extra Low Frequency Electromagnetic (ELFE) field. Additional components to the ELFE field can be contributed by the propeller blades. As the propeller turns, the distance from the propeller blade nearest the anode varies and the signal becomes modulated with the propeller blade passage frequency. Poorly filtered power supplies on ACP systems can also allow electrical "noise" to contribute to the signal, introducing onboard power frequencies into the ELFE field.

Davis supplies an ELFE influence mine countermeasure called Active Shaft Grounding (ASG). As shown in the interactive graphic below, the ASG system, when activated, provides for a low resistance path for the current to bypass the shaft bearings.

Main benefits of ASG system:

• • Virtually eliminates ship underwater ELFE signature
• • Reduces corrosion to shaft bearings
• • Resilient to deterioration in slip ring brush contacts
• • Compatible with passive and active cathodic protection systems
• • Comprehensively tested to military standards

•  

• 
• ASG installed to eliminate ELFE

The ASG system uses electronics to actively ground the shaft to the hull through a slip ring brush assembly. The system measures the shaft to hull potential through one slip ring assembly, and then uses this signal, after amplification, to control a high current power supply which draws a current out of the shaft through a second slip ring assembly. The circuitry reduces the variation in shaft-to-hull resistance, which in turn results in small variation in the current between the cathodic protection anodes and the hull. The ELFE signature is then virtually eliminated.

• 
• ELFE Signature Measurements with and without ASG engaged