How submarines prevent cavitation at the propellers - the injection of gas in front of the propellers seems counterintuitive to me, since that would lower the density of the water and reduce power. But the rubber membrane on the front of the propellers makes sense:
More methods than I had expected:
Reduced Propeller Speed: Operating the propellers at lower rotational speeds helps to reduce the pressure differences in the water, thus decreasing the likelihood of cavitation. However, this can impact the submarine's speed.
* Increased Operating Depth: Cavitation is less likely to occur at greater depths due to the higher ambient pressure of the water. Submarines often operate at depths where cavitation is minimized.
* Air Injection/Masking: Some submarines have systems that release small amounts of compressed air near the propeller. This air fills the vacuum bubbles as they form, preventing them from collapsing violently and creating noise. This is mentioned as a standard technique in the US Navy and possibly other navies.
* Degassing Water: Research has shown that releasing degassed water near the propeller's low-pressure side can prevent cavitation without altering blade speed or pitch. This method is highlighted as simple and cheap to implement.
* Nozzle Systems: Some designs use nozzles that spray compressed air in front of the propeller to create a large bubble, known as a "macro bubble," which helps reduce hull vibrations caused by cavitation.
* Air-filled Rubber Membranes: An alternative to nozzle systems involves placing an air-filled rubber membrane directly behind the propeller. This also creates a pocket of air to prevent cavitation without the need for compressors.
Toroidal propellers provide more surface area that reduces the amount of cavitation for a given amount of thrust.
