making turns for 15 knots

mah wrote:
In reading some of the novels and other books, you hear the sonar
operators saying "Making turns". How did you get a blade count
converted to knots?

Uh, that's classified (no, seriously).

But in layman's terms, think of it like this- say you have a
motorboat. If the engine is turning the propellor a certain rpm,
let's say 1000 prop rpm for the sake of argument. For any given prop
rpm, the boat will end up at a certain speed, in this case let's say,
30mph. The prop will make more or less the same sound in the water,
today, tomorrow, and a year from now. Let's say 1200 prop rpm will
make the boat go 35mph. If you have some way of listening to and
accurately measuring that sound, then when you hear the first sound
know it is probably your boat or one very similar to it going 30mph.
If you hear the second sound then it is probably close to 35mph.

And so on...

The secret squirrel part is how do you find out the blade rade to
knots correlation of everyone else's submarines.