Cavitation: Causes and effects on ships’ propellers

In this post we are addressing the phenomenon of cavitation and its effects on the naval field.

First of all, let’s describe it.
Cavitation consists on the vaporisation, bubble generation and implosion which occur in a liquid because of the sudden variation in local pressure. The most exposed parts of a ship are the propellers due to the high rotation speed of their blades where the pressure might be lower than the saturated vapour pressure of the water.
There are different types of cavitation. They may be grouped as:
–          Tip vortex cavitation.
–          Sheet cavitation
–          Cloud cavitation
–          Bubble cavitation
–          Root cavitation
–          Face cavitation
–          Boss vortex cavitation
1. Test in a cavitation channel (Wikipedia)
Effects of cavitation
Damages caused by cavitation are produced when the bubbles collapse. They usually cause very strong local shock waves in the fluid, which may be audible and may even damage the blades. The cavitation stain is the phenomenon where the oxidized layer that protects the metal is erased by the bubbles of cavitation and, as a consequence, increases the oxidation of the blade.
Cavitation damages are easily detectable visually and can deteriorate severely or even destroy the propeller (image 2). Furthermore, it can affect to the efficiency and the thrust of the propeller.
2. Damages caused by cavitation on a propeller (Wikimedia) by Alex Axdahl
It can even produce vibration that can make the sailing experience uncomfortable or accelerates the wear of the components involved in the propulsion system such as bearings, shaft or engine.
In some cases, if there is much cavitation, it can affect to the rudder, damaging it as well as the blades of the propeller, causing corrosion problems on it and the course changes.
It is very difficult to avoid completely this effect in many cases but it is mandatory to reduce it as much as possible. The only phase of the life time of a vessel in which it can be done, is the propeller design. The load of the blades must be adequate according to the Keller criteria.
It is related to the number of blades of the propeller, diameter, thrust, depth of the shaft and a coefficient. During the design process, propellers are tested in the cavitation channel (image 1) to verify that the propeller behaviour is good enough and will not have cavitation problems.
Even though the propeller was well designed, a bad operation of the ship or changes on the revolutions (RPM) may cause the appearance of cavitation.
The anecdote
During the battle of the Atlantic, the German submarines were a nightmare for the American convoys because of their surprising attacks, impossible to predict, mainly in the beginning of the battle.
Although, the low number of blades of the German submarines forced the propeller to work at higher revolutions and, consequently, overloaded the blades and produced cavitation. When these bubbles collapsed underwater, produced a very slight sound, that could be detected by the ships sonar and locate the submarine. Thanks to this fact, the allies could strike back and destroy some of these submarines.
When the German Navy noticed that the reason why the allies could find their submarines was the cavitation, developed new propellers with more blades (image 3) in order to decrease the load on each blade.
3. 7 blades propeller of a submarine. (Wikimedia) by Absinthologue.
The importance of the prevention of cavitation is clear. The costs of the systems involved in the ship propulsion that might be damaged for this phenomenon are huge, and the inconvenience of the repairing due to the really long time that takes to do it can be a danger to the benefits of the ship operation.
A correct operation of the ship and periodic surveys are highly recommended to prevent possible damages.
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