Exploring the stealth techniques submarines used to evade detection through passive sonar
Silent running (or ultra-quiet) is a stealth mode of operation for submarines that was developed to evade discovery through passive sonar detection.
During silent running, propellers operate within a characteristic RPM band in which no cavitation noise arises. Since this rotation speed is relatively low, the first electric submarines were equipped with special “silent running” engines designed for optimum performance at reduced speed. Nuclear submarines can operate even more quietly at very low speeds by turning off active reactor cooling during silent running, allowing the reactor to be cooled by natural convection of water alone.
The silent running protocol, which had been in use since the latter part of World War I when hydrophones were invented to detect U-boats, aimed to eliminate superfluous noise. Nonessential systems were shut down, speed was greatly reduced to minimize propeller noise, and crews were urged to rest and refrain from making unnecessary sound.
Slamming Hatches and Banging Wrenches on Metal
Michael Denney, a former US Navy submariner, described the practical reality of silent running on Quora. A typical day involved 6 hours standing watch, 6 hours off-duty, and theoretically 6 hours of rest. Any maintenance or ship-wide drills could occur during rest periods. During ultra-quiet operations, however, off-watch crew members enjoyed uninterrupted rest time.
At the end of war games with other submarines and ships, engineering watchstanders would deliberately roam around slamming hatches and banging wrenches on metal to reveal their position. They would typically be found within half an hour of this noise-making, though it didn’t inspire regular tiptoe discipline among the crew.
Photos by Photographer’s Mate 2nd Class Lynn Friant and Photographer’s Mate 1st Class Kevin H. Tierney / U.S. Navy
