Cavitation is a fluid dynamics phenomenon where tiny bubbles are created in the water and then collapse forcefully on the metal surfaces of the ship. This is caused by the movement of the propeller blades. The effect is comparable to a steel tipped hammer that is repeatedly hit against a steel surface at exactly the same location; after a while damage will occur. Due to their position just behind the propellers, rudders are particularly prone to cavitation forces and damage. The cavities created by the cavitation will then grow deeper and deeper and in the end can eat right through the rudder (or other affected areas). Large eroded areas can occur this way. Besides the mechanical damage caused directly by the cavitation process, it immediately opens the door for corrosion damage.
What needs to be done is either prevent the cavitation or make sure that it has no damaging effect. Efforts to prevent cavitation through design changes to ships have had some effect but have not solved the problem. This leaves proper protection as the workable avenue. The Ecospeed glass-reinforced ship hull coating has been found to provide a very thorough and lasting protection against cavitation and corrosion. The coating provides the rudder with an impenetrable protective layer while its flexibility enables absorption of the forces that are produced by cavitation, thereby preventing the damage normally caused by this phenomenon. If the cavitation cannot pierce the coating then no erosion of the steel can occur and no corrosion can follow. The object that is coated with Ecospeed will not suffer from corrosion.
Tests conducted in a flow channel have confirmed that Ecospeed performs extremely well under severe cavitation. These tests were divided into six stages during which the coating was exposed to an increasing pressure drop, creating a growing cavitation force. Even after the last stage no erosion was present on the test patch coated with Ecospeed. The tests were organized by the French Ministry of Defense and were carried out in Grenoble.
Protection should start at the newbuild phase
When a vessel comes into drydock maintenance of its stern area, especially cavitation damage repair, can take a long time. There are strict procedures concerning blasting, painting, welding and propeller and stern tube seal work. This makes it impossible to carry out most of the repairs that need to be done in these areas simultaneously. Painting is then usually assigned to the end of the schedule. As a consequence it may not get done at all or else prolongs the stay in drydock. Taking into account the tight drydock schedule of most vessels this is often problematic. With an Ecospeed application one can avoid these problems from day one because no full repaint of the underwater hull will be needed during drydocking, and that includes the rudder. Ecospeed will remain intact for the lifetime of the vessel and is guaranteed for ten years. At the most, touch-ups amounting to less than 1% of the surface area will be required. Planning the maintenance of the vessel’s stern area therefore becomes much easier.
The newbuild phase is the perfect time to apply Ecospeed, but the coating can also be used to protect vessels that have been in service for some time and are already facing cavitation and corrosion damage.
Ecospeed’s flexibility makes it easy to adapt the application schedule to the rest of the activities scheduled at the shipyard or drydock in a way which does not interfere with them. Overcoating time can be as short as three hours, which means that for smaller surfaces such as rudders or bow thrusters the two coats required can usually be applied in one single day.
Ernst Russ successfully ends rudder cavitation damage problems
Founded in 1893 Ernst Russ is a Hamburg based, family owned shipping company. The company has a fleet of ten vessels including five roro cargo ships. Due to the higher propeller revs of these roro cargo vessels, their rudders suffered particularly from cavitation damage.
The rudders of the five roro ships were originally coated with a standard epoxy coating. During the first intermediate docking, between two and three years from launch, extensive cavitation damage had already appeared on the rudders.
During a 2004 docking, Ecospeed was applied experimentally on the Elisabeth Russ. The trial was successful beyond all expectations. When the ship next came out of the water in 2007 it could be seen that no further cavitation damage had occurred. As a result, the rudders of the remaining four roro ships were coated with Ecospeed, all with similar results.
The most recent drydocking of the Elisabeth Russ in 2011, confirms that the original Ecospeed protection applied in 2004 is still holding firm and the rudder is intact, free from any further cavitation damage.
“In general everybody is looking to be in drydock as short as possible and to get all the work done as quickly as possible, ” explains Ernst Russ superintendent Grzegorz Girjat. “Additional hot work on the rudder inevitably results in some collisions with other jobs. I would say for me it is quite clear. Had we not applied Ecospeed on the rudders, we would certainly have extensive work to do in drydock.”
One year after the Ecospeed underwater hull coating had been applied on her rudder, the 197- meter bulk carrier Gypsum Integrity came into drydock in Setúbal, Portugal. At the same time Ecospeed was applied on the rudder of her sister vessel, Gypsum Centennial at the same location. The excellent condition of Ecospeed on the Gypsum Integrity confirmed the owner’s decision to have the coating applied on the rudder of their second vessel.
A few months later the 195-meter container vessel Marie Delmas, owned by another company, came into drydock in Dubai five years after receiving an identical treatment. The coating on the rudder of the Marie Delmas was still in excellent condition. Since then the rudders of several other ships owned by this company, CMA Ships UK Ltd., have also been coated with Ecospeed.
These are just a few of many, many cases where rudder cavitation has been completely dealt with or prevented through the application of Ecospeed.
If one takes into account the costs of the temporary underwater repairs and the regular inspections required by a condition of class until the next drydocking, it becomes clear that the investment in a coating system that offers extra protection from day one is easily won back. For this reason more and more owners have Ecospeed applied on the rudders of a large part of their fleet or have it put in the rudder specs of their newbuild vessels. These owners invest in the right coating system to protect their rudders because they know that the savings will be enormous.
A large number of rudders have now been coated with Ecospeed with 100% success. In all cases where Ecospeed has been standardly applied, the rudders suffered no cavitation damage and did not need to be recoated.