When steel is immersed in sea water (e.g. a ships hull) small galvanic currents are initiated at anodic areas of the metal surface, causing corrosion. Such corrosion predominates at the stern of a ship, where the combined effects of increased turbulence and differential metals results in accelerated corrosion rates.
Therefore, to overcome this undesirable condition of arcing, whilst at the same time ensuring that cathodic protection is extended to the propeller, it is general practice to install Shaft Grounding System.
The importance of shaft earthing
It is essential that the cathodic protection current provided by the sacrificial anodes or the ICCP system is correctly earthed from the shaft line back to the hull. The earthing arrangement for the cathodic protection needs to be located as far along the shaft line as possible to earth the current at the location nearest to its source. The ideal location is between the stern seal and the furthest aft bearing.
Vyas Metal’s - high quality brushes and sliprings
are made from 80% silver and 20% graphite, a specification that provides the best combination of conductivity and wear resistance.
Equally important is the design of Vyas Metla’s sprung brush holder which ensures that the correct amount of pressure is applied to the slipring at all times.
The slipring, itself, is made from copper with a solid silver inlay track to provide excellent conductivity when in contact with the brushes. All of these features ensure that damaging stray currents are safely conducted to earth without causing damaging ‘spark’ erosion in bearings.
Damage which can be caused
• Spark erosion resulting in damage to the bearings.
• Cavitation damage to the tips of the propellers.
• Formation of deeper pits in propellers due to the corrosion/erosion cycle.
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