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What is the purpose and importance of a shaft earthing device?
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A turning propeller and propeller/intermediate shafts are separated - and insulated - from the ship’s hull by a thin layer of lubricating oil film in the bearings (M/E bearings, stern tube bearings).
In this case, the ship’s cathodic protection system is not able to protect effectively the the propeller and propeller shaft, because there is no continuity in the circuit.
As a result, electrical potential is created between the propeller shaft and the ship’s hull, due to the fact that they are made of dissimilar metals.
The electrical potential between the propeller shaft and the hull can cause electrical current to flow through the propeller shaft, intermediate shaft and M/E crankshaft and cause spark erosion and deep pitting on the surface of the M/E bearings.
In order to reduce the negative effects and the risk of spark erosion, the propeller shaft can be grounded using a shaft earthing device.
The shaft earthing device consists of a pair of brushes made from a high silver content/graphite compound mounted on a brush holder. These brushes run on a copper slip ring with a solid silver track installed on the propeller or intermediate shaft.
M/E makers always recommend the installation and correct maintenance and operation of shaft earthing devices in order to protect the bearings.
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