Silent running (or ultra-quiet) is a stealth mode of operation for submarines.
During silent running, the propellers have a characteristic RPM band in which no cavitation noise arises. Since this rotation speed is usually relatively low, the first electric submarines had special “silent running” engines designed for optimum performance at reduced speed.
Nuclear submarines can run even more quietly, at very low speeds only, by turning off active reactor cooling during silent running. The reactor is then only cooled by natural convection of the water.
The aim of silent running (a protocol that has been in use since the latter part of World War I, when hydrophones were invented to detect U-boats) is to evade discovery by passive sonar by eliminating superfluous noise: nonessential systems are shut down, as explained above speed is greatly reduced to minimize propeller noise and the crew is urged to rest and refrain from making any unnecessary sound.
As already explained (CLICK HERE to read the article) ‘Being “quiet” is more than not talking.
Slamming hatches and banging wrenches on metal
‘A normal nuke day is 6 hours standing watch, 6 hours maybe off, theoretically 6 hours in the rack, and repeat. If any maintenance was due, it happened during the 6 hours ‘off’. Any ship wide evolutions (drills or all hands clean up) could come during your rack time. During ultra-quiet, if you weren’t on watch it was sweet, sweet bunky time.
‘I’ll leave it to the sonar techs to say how much they could hear, but at the end of every war game with other subs and ships, if it was time for the other guys to find us, the engineering watchstanders would roam around slamming hatches and banging wrenches on metal.’
‘They’d usually find us after a half hour of this, but certainly didn’t inspire us to tiptoe on a regular basis.’
Photo credit: U.S. Navy