Among the primary design objectives of the modern submarines, is the objective of elimination of noise. The noise that the screw (what propeller is called on a submarine) gives off can give away the position of the submarine. The noise is generally created because of cavitation.
That is why when it comes to the designing of submarines, the blades are designed so that they are capable of creating thrust while keeping cavitation to a minimum. Older submarines could manage speeds of only 2 or 3 knots and still stay quiet. Whereas, the modern submarines are able to achieve speeds of eight to ten knots while maintaining the same noise level.
The propeller or screw (as it is called on a submarine) is a basic device that works by pumping fluid and gas. You can either have the screw rotate faster or increase its size. Making it rotate more quickly makes it noisy, and the size can only be increased to a certain extent. A smaller propeller or screw rotating at higher speeds is prone to cavitation.
The shape of the submarine propellers is kept a secret because it can indicate the speed at which it will be rotating for the sake of remaining stealthy. If the enemy is aware of the shape and speed of propellers, it can calculate the output frequency along with the engine type and RPM. This leads to the enemy fine-tuning its sensors and listening to disturbances in a particular frequency range. This can lead to the enemy finding out about the location of your submarine even when it is not moving at its noisiest speed. For instance, if a submarine can manage 8-10 knots and remain quiet but the enemy has made adjustments to its sensors for detecting the submarine’s specific hardware; the enemy might up the submarine when it is operating at the speed of 7 knots.
The shape, section, and pitch of the propeller change the envelope in which the screw can work without creating cavitation. Thus getting these details will help the enemy to ascertain the performance of the submarine and also give them the tactical advantage of picking it up on their sensors. As for the curious ones, who want to know the shape of a submarine’s propeller; modern ones feature a radial setup that runs throughout the length of the submarine – think of a churro!
5 Stars of 5. Very Informative.