Rudder cavitation damage
The rudder of a ship is particularly prone to damage caused by erosion and corrosion. Although it may affect any ship, it is more prevalent in high speed container carriers and other fast vessels.
The turbulence of the ship passing through water together with pressure changes due to the propeller, creates hydrodynamic cavitation which produces forces large enough to erode steel. The unprotected, eroded surface is further degraded by corrosion causing rapid damage to the rudder: a dry dock repair or replacement is the consequence.
The Belgium based company Ecospeed recently treated the rudders of four container vessels owned by three different owners claiming to give lasting protection against cavitation damage. According to the manufacturer, the rudders will not require repainting as the surface coating is guaranteed for ten years and will remain intact for the lifetime of the vessels, thereby preventing similar damage from recurring.
The effectiveness of Ecospeed as a barrier to cavitation damage was verified in tests sponsored by the French Ministry of Defense carried out in Grenoble, France. Tests were carried out in a flow channel and were divided into 6 stages during which the coating was exposed to an increasing pressure drop, producing a growing cavitation force. After the last stage no erosion was present on the test patch coated with Ecospeed.
Many attempts to prevent cavitation damage to rudders over the years include repositioning the rudder, mounting a stainless steel or other material mantle on the rudder, redesigning the rudder, using a twisted rudder, all with little or no prevailing success.
The Ecospeed solution is to apply a cavitation damage proof coating to the rudder surface using a Surface Treated Composite (STC) of specially formulated glass-flake vinyl ester resin. Preparation and application requirements are simple, the surface must be grit blasted to remove any previous coating and create an adequate bonding surface. It is applied in two coats of about 500 microns thickness adding up to a total of at least 1000 microns dry film thickness (DFT). Additional coats can be used to fill pitting: severe damage should be repaired by welding. Three hours should be allowed between coats and the vessel can leave drydock 24 hours after the final coat. The coating has very low VOC content and an antifouling coat is unnecessary.
Ecospeed’s first candidate for treatment was the RoRo MV Elisabeth Russ in 2004. Since then their treatment has been applied to 110 rudders. Not one has suffered from further cavitation damage and none has needed to be recoated.
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