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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 supply 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 proved to give the optimum electrical continuity.
Cathelco supply compact millivolt meters to monitor the potential between the shaft and the hull and verify the effectiveness of the system.
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.
The diagram above shows the problems which are encountered when a shaft earthing system has not been fitted. The impressed current leaves the anode and passes into the surrounding seawater where it ‘looks’ for the least resistive path, taking it to the propeller and then down the shaft line. When the shaft rotates, the resistance in the bearings is variable and for split second it can change from very low to very high. When the resistance is low, current ‘jumps’ onto the bearings and it is this factor which causes damaging ‘spark erosion’.
In the absence of a shaft earthing system, after the current has passed through the bearings there is still no easy way back to earth and therefore it travels back down the shaft line to the propeller blades. At the times of high resistance the seawater provides a less resistive path than through the bearings, so the current discharges into it from the tips of the propeller blades. This creates pitting in the blades as positively charged ions leave the surface of the metal. These pits enlarge as the propeller rotates, causing bubbles of oxygen to implode and create deeper cavities in the process known as the corrosion erosion cycle.
All of these problems can be avoided by having a shaft earthing arrangement which is correctly positioned on the shaft line.
Shaft earthing arrangements supplied by propulsion manufacturers are only designed to deal with the current that the electrical propulsion system impresses on the shaft line. The arrangement is also positioned close to the gear box when it should be as far aft on the shaft line as possible for the impressed current system earthing.
As the surface area of the slipring/brushes is not large enough to conduct the combined current from the propulsion systems and ICCP system, a situation arises where the excess current flows down the propeller shaft causing spark erosion and pitting of the propeller blades as described on the preceding page.
The diagram above shows the shaft earthing arrangement operating correctly. 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.
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 to only 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 wear resistance.
The high proportion of silver provides low resistance, whilst the graphite element gives the strength to withstand friction, ensuring a longer life and greater economy. Equally important is the design of Cathelco’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.