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Corrosion protection

Shaft Earthing systems

Even on ships fitted with ICCP or sacrificial anode systems, propeller shaft bearings are vulnerable to corrosion. This is because turning shafts are electrically insulated from the hull by the lubricating oil film in the bearings and by the use of non-metallic bearings in the tail shaft. 

The problem can be eliminated if the shaft is earthed to the hull using a propeller shaft slipring. Cathelco supplies complete shaft earthing assemblies consisting of a pair of high silver content/graphite brushes mounted in a balanced brush holder, running on a copper slipring with a solid silver inlay track. This combination has been proven to give optimum electrical continuity. 

The effectiveness of the shaft earth arrangement relies on the quality of the shaft earthing brushes and slipring. That is why it is important for customers only to use Cathelco OEM replacements which are designed to give the optimum level of performance. Cathelco brushes are made from 80% silver and 20% graphite, a specification that provides the best combination of conductivity and wears resistance. 

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. 

  • Complete shaft earthing assemblies
  • An optimum level of performance
  • Low maintenance need

  • Prevents spark erosion in the bearings 
  • Prevents cavitation of propellor blades 
  • Low maintenance 

  • Analogue or Digital meters available 
  • Single or dual shaft meters available 
  • Alarmed 
  • 4-20mA output 

Impressed current leaves the anode and passes into the surrounding seawater where it ‘looks’ for the least resistive path to ‘ground’ which is the hull of the ship. In these circumstances, the propeller is the largest area of ‘uncoated’ metal and therefore the current flows to the propeller as it offers the least resistance. 

On reaching the propeller the current flows down the shaft line until it reaches the earthing arrangement where it is directed back to ‘ground’ (the ship’s hull). Because the shaft earthing arrangement has been fitted correctly, the current has been able to find the path of least resistance throughout its journey without discharging into areas where it could cause damage. 

Shaft Earthing kit comprises of  

  • Slip ring – Made from copper with a solid silver inlay track to provide excellent conductivity when in contact with the brushes. 
  • Brushes – Produced from 80% silver and 20% graphite, a specification that provides the best combination of conductivity and wear resistance. 
  • Double brush holder – The sprung brush holder ensures that the correct amount of pressure is applied to the slipring at all times. The double brush holder ensures that damaging currents are directed back to ‘ground’ – the ship’s hull. 
  • Millivolt Meter – These provide a means of monitoring the performance of the shaft earthing equipment. 
  • Single brush holder – Single brush holders are supplied for use with millivolt meters. 
Evac people on a service project onboard a vessel

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