Rudder anodes

The rudder is provided with anodes on both sides these should be fixed either at the level of the propeller hub or as far as possible above and below on the mdder blade. There are specially shaped rudder anodes which are welded to the front edge of the rudder. Sea chests and scoop openings should be specially considered and provided with anodes because of their increased current requirement. 

Fig. 17-4 Distribution of galvanic anodes for different shapes of stem, (a) Steamer stern one propeller, one balance rudder, (b) Steamer stern one propeller, a suspension rudder, (c) Tugboat stem two propellers, a Kort nozzle mdder. (d) Transom stem two propellers, a suspension mdder. (e) Transom stem two propellers, two suspension mdders.

Anodes are not attached to the rudder but are situated between the rudder shaft and the ship s wall and connected via a copper strip. The propeller is protected via a slip ring on the shaft. To achieve a low-resistance contact, the divided copper or bronze ring has a rolled silver-bearing surface on which metal graphite brushes slide. The transmission voltage should be below 40 mV. 

The ship has two propellers and two rudders. The propellers were made of nickel-aluminium-bronze alloy (NAB) and modelled as solid disks with a surface area equivalent to the real propellers. The shaft is made of carbon steel and the propellers and shafts were assumed to be uncoated because of turbulence engendered by propeller movement. The ship s hull and rudders are also made of carbon steel, which were coated to prevent corrosion. The ICCP system evaluated included four anodes and a centre controlled power supply. The half of ship BEM model was shown in figure 2. 

Special propulsion also requires relevant calculations and distribution of the anodes. For Kort nozzles, the total surface area of the rudder is determined and a basic protection current density of 25 mAm imposed. The anodes are attached on the external surface at a spacing of 0.1 r to 0.25 r at the region of greatest diameter. Internally the anodes are fixed to the strengthening stmts. With Voith-Schneider propellers, the anodes are arranged around the edge of the base of the propeller. 

It has been observed that many fishing vessels call into a floating dock once a year to carry out a complete inspection/repair/overhaul of equipment on board. These repairs and overhauls are normally carried out by yard workers or specially contracted personnel. These vessels also come in for dry-docking every 3 to 5 years (depending on the condition of the vessel) to carry out repairs on seawater valves, replacement of hull anodes, inspection of propeller, tail shaft and rudder and any other fitting which lies beneath the water line. 

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