How to Put a Permanent End to Costly, Repeated Rudder Repair and Replacement

In the new Hydrex White Paper No. 6:  Rudder Cavitation Damage Solved

Finally the well known and heretofore very resistive problem of rudder cavitation damage, a real thorn in the side of ship superintendents, operators and owners has been solved. This White Paper discusses the problem and the solution in depth. 

Part I. Overview of rudder cavitation damage

As any shipowner knows, a ship’s rudder is particularly prone to damage caused by erosion and corrosion. The problem features more prominently in high speed container carriers and other fast ships, which are more seriously affected than slower vessels. However, it is a potential problem and hazard for all ships and boats. This problem results in frequent, costly repairs to or replacement of this vital part of the ship’s underwater equipment. So far, the bulk of efforts to relieve this problem have not been fully effective.

Why the rudder?

A ship’s rudder, placed directly behind the propeller to give the ship maximum maneuverability, is particularly prone to erosion followed by corrosion. The erosion in this case is caused by hydrodynamic cavitation.

Hydrodynamic cavitation is a phenomenon that accompanies turbulent fluids. The  turbulence in the fluid, in this case caused by the ship’s motion through the water but more particularly by the action of the ship’s propeller, results in areas of greatly reduced fluid pressure. (The physical laws involved are clear and well documented, but not relevant to this White Paper which is intended for shipowners/operators, not scientists.) Due to the low pressure, the water vaporizes. This causes small vapor-filled cavities or bubbles in the fluid up to about 3 mm in diameter. The cavities travel through the water and the pressure around them increases, causing them to collapse suddenly. The implosion of the cavities is accompanied by a complex set of physical processes. It is the collapse of the cavities which is accompanied by very high pressure pulses, speeds and temperatures in the water, that cause the damage. 

The forces involved are very large. It is as if the surface affected has been subjected to repeated, heavy blows from a hammer, as well as high temperatures. This causes what is known as cavitation erosion as the surface material, first paint and then steel, begins to flake away. This process can be greatly magnified by the presence of gravel or other hard particles in the water. 

One need only examine a ship’s rudder that has been subjected to cavitation damage to see that, whether one understands or subscribes to the theory, in practice very real damage is caused by this phenomenon. Rudders become deeply pitted; paint coatings and hard steel simply disappear; whole plates can fall off and the rudder practically disintegrate altogether, all as a result of this cavitation damage.

Cavitation is caused by the flows from the motion of the propeller, the cavities imploding on the propeller blades or being transported rapidly back to implode on the rudder surface. But the cavitation can also be caused by the turbulence around the rudder itself, and the collapse of the cavities can occur almost immediately after the cavity is created. So the rudder is subjected to cavitation damage from two main sources: the turbulence caused by the propeller and that caused by the water flowing over the rudder itself. 

Cavitation damage is not limited to the ship’s rudder. The propeller is also subject to the phenomenon, as are stabilizers, the vessel’s hull and other parts of the underwater vessel where the water flows are particularly swift or turbulent. But the rudder is particularly prone to this phenomenon due to its position and form. 

The process is gradual, but not necessarily slow. This process can occur in a remarkably short period of time. Sometimes six months is all it takes for serious rudder damage to be present. The first step is that the cavitation causes the paint coating on the steel to erode, eventually exposing bare steel. The erosion of the steel is then accompanied by the electro-chemical process of corrosion because the steel is no longer protected. The effect is multiplied as the cavitation continues and the erosion it causes is added to by the natural corrosion of bare steel exposed to water – the electro-chemical process and the oxidation which this brings about.

Attempted solutions

Rudder cavitation damage is a well known and extensively documented phenomenon. There is a vast amount of literature on the subject. High speed video has been used to capture the process of cavitation in action so that it can be studied. Computer programs have been developed to model the effects of cavitation and predict where the most damage will occur, depending on the construction and shape of the rudder. Many scientists have investigated the phenomenon and scientific papers on the subject abound.

There have been many attempts to prevent the damage caused by cavitation. In the main these attempts fall into the following categories:

1. Change the position of the rudder so that it is not behind the propeller. This reduces cavitation on the rudder, but is impractical since the ship loses its maneuverability. The ideal placement of the rudder so that it provides maximum control of the ship is directly behind and in the wake of the propeller. The more rapidly moving water makes the rudder more effective. In other words, positioning the rudder so that it carries out its function in the best possible way renders it most liable to cavitation damage.

2. Redesign the rudder so that it is less affected by the flows and turbulence. Some inventors have developed a twisted rudder which is marketed and in use. The twist is an attempt to reduce the turbulence caused by the flow of the water from the rotation of the propeller by changing its angle of attack on the rudder. This has met with some success but has not eliminated the problem.

3. Strengthen the surface of the rudder to increase its resistance to cavitation erosion, often with some other metal. This has only partially relieved the problem, and can in fact be counterproductive if the combination of metals increases the electro-chemical/corrosion factor. The difference in potential between metals can cause very rapid corrosion to occur. Historically, the most dramatic example of this was perhaps the attempt to put copper sheathing on steel hulls to protect them from fouling. The proximity of the two metals resulted in very rapid corrosion of the steel. There have been attempts to reinforce the rudder with a stainless steel plate over the steel, only to have the welds or fasteners holding the plates in place corrode completely so that the plates simply dropped off.

4. Use cathodic protection systems to reduce the electro-chemical/corrosive effects. Since the corrosion only sets in after the protective coating has been eroded by cavitation, this is like putting a lock on the barn door after the horse has been stolen. It may reduce the corrosion, but it does not address the primary cause, which is the erosion damage caused by the cavitation.

5. Develop better coatings and rudder protection. It is in this area that the solution presented in this White Paper lies. There have been many attempts to devise a better protection system for the hull. Most of these have been ineffective. But not all of them.

Click here to read and download Hydrex's White Paper No. 6 in full