"I also thought that they were running silent. So was there a sounding really or no? If so, did the sounding really show anything?"
The boat was reported doing 30 knots...at that speed you will be heard in Moscow; It's called cavitation.. you are not running silent...silent is slow - 8 knots or less...If he was running at speed, I don't think he would have heard the echo of his own sonar....he truely was flying blind and
totally relying on the charts....Two mistakes: relied on the charts (hard not to) and keeping the old ones to be found later........
clarification. I had read that there to be no active sonar so as to avoid detection.
But then why would he be under these rules, if he were running above 8 knts?
Not so. With modern prop designs and running deep you do not cavitate.
Cavitation is also dependent on the depth. As the surface of the propellor goes through the water, there is a local pressure drop. So, the local pressure at that point would be static pressure due to depth minus the pressure drop due to the Bernoulli effect. Bubbles will form if the local pressure is less than the vapor pressure of water (Pv is also temperature dependant). What causes the cavitation is as the bubble travels to a point where the local pressure goes above the vapor pressure (back to the static head pressure) the bubble implodes. Makes a lot of noise. Where it implodes also matters. If it implodes in close proximity to a solid object, the local pressure due to the implosion can be on the order of 100,000 psi. That's high enough to cause some severe local erosion. Nothing secret, I run into a similar phenomenon with control valves (power plant, refinery).
So, if the boat was running at xx knots (yes, I'm censoring here!) at below 500 foot depth, cavitation would be less likely than at a 150 foot depth.