107.Why propeller shaft is earthed

A difference in electrical potential between the hull and the propeller shaft will be generated due to the difference in materials and to the propeller being immersed in sea water.

In some cases, the difference in the electrical potential has caused spark erosion on the thrust and main bearings and journals of the crankshaft of the engine. In order to reduce the electrical potential between the crankshaft and the hull and thus prevent spark erosion, there must be installed a highly efficient shaftline earthing device.

The shaftline earthing device should be able to keep the electrical potential difference below 50 mV DC, and there must be installed a shaft to hull monitoring equipment with a mV-meter and with an output signal to the alarm system so that the potential and thus the correct function of the shaftline earthing device can be monitored.

Design description of the shaftline earthing device:

The shaftline earthing device consists of two silver slip rings, two arrangements for holding brushes including connecting cables and monitoring equipment with a mVmeter and an output signal for alarm.