[Popular Science] Damage of Cavitation to Ship Propellers

[Popular Science] Damage of Cavitation to Ship Propellers

  Few people associate small bubbles with lethality. But in fact, those bubbles are extremely lethal for poorly designed propellers.

Propellers are generally one of the hardest parts on a ship, but they are often killed by bubbles. And here the bubble, its proper name is "vacuole".How on earth do vacuoles come into being? Why is it so destructive? Today, let's have a chat.

Causes of Cavitation

We know that water boils at 100 degrees Celsius at a standard atmospheric pressure, and boiling is the process of producing a large number of bubbles in water. At about two standard atmospheric pressures, boiling does not occur until 120 degrees Celsius, which is the temperature inside a high-pressure pot for families. On Mount Everest, water boils at a minimum of 69 degrees Celsius due to low pressure. The boiling point varies with the pressure.

Conversely, it also holds that the pressure of water boiling at different temperatures is different, and this pressure is the saturated vapor pressure.

  Comparing with the above table data, if the local low pressure below 2338.8 PA occurs in the 20 C sea water, the sea water will boil and produce bubbles, which are called vacuoles.

So the question arises. Why do low pressures occur around propellers in dense water?

It is not difficult to understand that when the propeller rotates at high speed, the pressure on the back of the blade will inevitably decrease due to the high speed movement of the blade, thus forming a suction surface. Low pressure will occur. When the pressure is lower than the saturated vapor pressure of sea water, a large number of bubbles will be generated, just like boiling.

  Damage of Cavitation to Propeller

Unlike boiling, the bubbles generated in boiling water will continue to grow because bubbles can continuously obtain heat from their surroundings. There is no heat input around the void, only the strong extrusion of the surrounding high-pressure seawater, which is the cause of the great power of the void.

In a few millionths of a second, the cavitation generated by the propeller shows explosive growth. Then, due to the high pressure of the surrounding sea water, the cavitation is compressed sharply until it collapses. When it collapses, an explosive jet will be produced. The velocity of the jet is over several thousand kilometers per hour, and even the hard propeller can not stand its long time. Years of shocks.

Explosive shock wave

And the jet is just one of the injuries. When the bubble bursts, explosive shock waves will also be produced. Looking carefully at the above picture, a visible shock wave is produced around the bubble when it bursts.

Therefore, jet and shock wave are the fundamental causes of cavitation damage to propeller.

A beer bottle filled with water hit the bottle hard at the mouth. At this time, the bottom of the beer bottle dropped. Why?

Let's look at the details.

The picture above tells us that when the top of the beer bottle is hit hard, the beer bottle will move downward rapidly, but the water inside the bottle can not keep up with this instantaneous high-speed movement, so the vacuole will occur. It's like when the propeller rotates at high speed, the water on the back of the blade can't keep up with the movement of the blade.

The void at the bottom of beer bottle lasts for a very short time, and when it breaks, it will have an impact on the bottom of the bottle, so the bottom of the bottle will fall off.