Here is why military submarine propellers are kept secret. Among the primary design objectives of the modern submarines, is the objective of noise elimination. 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.