Cavitation - The constraining factor

Transportation across the oceans has attracted the attention of mankind since the dawn of history. What started with a simple bundle of logs has evolved into a huge sophisticated vessel of today. Earlier, wind (sails) and human power (by oars) were the only means of propulsion but since the advent of 19^th century Mechanical Propulsion began to be used in ships. Most of the modern ships have quite similar propulsion arrangements consisting of a wheel rotating about a transverse axis with radial blades or paddles to impart astern momentum to the water around, giving it forward thrust . 

As the size of vessels grew, the thrust required from these screw propellers also increased. Consequently, the rpm of these propellers escalated. When the  propellers’ diameter is restricted and they are required to produce very large thrusts at high rpm’s  the propellers are likely to experience a phenomenon called “Cavitation”.

Cause 

Water begins to vaporise when its vapour pressure equalises the saturation vapour pressure. The vapour pressure of water is 1.704 kN/m^2 at 15 °celsius and 101.325 kN/m^2 (atmospheric pressure) at 100 °celsius. If the pressure inside water falls to the vapour pressure the water at that point begins to vaporise forming voids filled with water vapour. The formation of such vapour filled cavities is termed as cavitation.Cavitation also depends upon the purity of water. The impurities in sea water such as suspended solid particles and dissolved gases act as nuclei for the formation of cavities.Vapour cavities are formed locally but when pressure increases clear of this region the vapour cavities collapse or “implode”. This implosion occurs with a high impact force with stresses nearing 2800 N/mm^2. 

A propeller generates thrust by creating a pressure difference between the face and back of its blades. The pressure on the back of the blade falls below the ambient pressure and the pressure on the face rises above it. Hence, the back face of the blade becomes prone to cavitation.

Types of Propeller Cavitation

The type of cavitation occurring has two bases for classification :

   1.Classification on the basis of the region where cavitation occurs:

• Tip Cavitation
• Root Cavitation
• Boss or Hub Cavitation
• Leading Edge Cavitation
• Trailing Edge Cavitation
• Face Cavitation
• Back Cavitation

    2.Classification on the basis of the nature of the cavities or appearance:

• Sheet cavitation
• Spot cavitation
• Streak cavitation
• Cloud cavitation
• Bubble cavitation
• Vortex cavitation

Cavitation exhibited in a cavitation tunnel in a stroboscopic arrangement

Hub Vortex Cavitation

Leading  Edge Cavitation

Tip Vortex Cavitation

Effects 

1. Cavitation in marine propellers results in the increase of blade rpm without any appreciable increase in speed of the ship hence reducing the efficiency and thrust of the propeller. 
2. When the bubbles implode they generate very strong local shock waves in water which generate a lot of noise and do mechanical erosion of the blades in form of pitting. The frequency of collapsing is generally around 15,000 Hz to 20,000 Hz. The collision intensity depends upon a number of factors-: 

• Life span of cavitating bubble which is about three milliseconds which makes the event rapid. The faster the surrounding water collides, the greater the energy it possesses.
• The size of the cavitating vapour bubble. Generally, the vapour bubble formed by cavitation at 68° Fahrenheit is about 35 times larger than the one produced at 212° Fahrenheit. A larger bubble corresponds to a greater amount of water colliding.
• The quickness and amount of water together represent the total kinetic energy with which the imploding bubble attacks the metal.
• It also accelerates corrosion in blades due to ‘Cavitation stain’. Cavitation Stain is a phenomenon where the oxidised layer that protects the metal is removed by cavitating bubbles hence making the metal vulnerable to corrosion. Corrosion damage is visually detectable and can eventually destroy the propeller.  

• In the given figure a series of 18 shots show progressive events of flattening of a spherical bubble and finally imploding with the release of enormous energy.

Series of 18 shots show progressive events of cavitation

Pitting Due to Cavitation

Propeller Pitting due to cavitation

Prevention of Cavitation

Due to the detrimental effects of cavitation propellers are designed so as to restrict its level to a value so that its effects are negligible. It is achieved by following three methods:

• Increasing the cavitation number.
• Decreasing the loading on the propeller
• Designing the propeller for uniform loading according to Keller criteria, which is related to the number of blades of the propeller, diameter, thrust, and the depth of shaft.

Supercavitation and Supercavitating Propellers

Generally it has been observed that it is difficult to avoid cavitation completely in high speed crafts where the propeller rpm crosses 1000 rpm. So instead of avoiding it some propellers are being designed such that they use this property to create a sheet of vapour around the propeller blades which increases the effective diameter of the propeller and thereby increases the thrust as well as the efficiency of the propeller. Cavitation is not a problem for these propellers as they are designed to work in cavitating zone. Although before achieving the design speed (when cavitation occurs) these types of propellers demonstrate very low efficiency due to their non-streamlined (wedge shaped) sections.

Super cavitating propeller with wedge shaped training edges

Supercavitation is even used for torpedoes and other high speed objects in marine environment. The vapour sheet formed due to cavitation corresponding to the high speed of torpedo encompasses the object thereby greatly reducing the drag and hence enabling higher speeds. They are being developed by defence forces in fast supercavitating torpedoes and war projectiles. For instance, the soviet torpedo named VA-111 Shkval, German torpedo Barracuda, weapons like SPP-1 underwater pistol, APS amphibious rifle.

VA-111 Shkval using the phenomenon of cavitation

Thus cavitation though has some serious detrimental effects, it can be used as a beneficial tool in particular cases. It is one of the most important factors determining the diameter and rpm of the propeller blade by placing major constraints. An optimum design and balance needs to be maintained between the blade diameter and rpm to obviate the risk of cavitation.


Article By - Soumya Sameer