I hear what your saying but tend to disagree but i'm not a naval person, your coming at it from an audible point of view - see below, where's all the noise/clutter astern? Raise the tilt angle & you'll see it no doubt but then if you wanted to be looking there its tune-able out also. I don't swallow you going to hide a 100m sub astern of me & i won't know its there.
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Edit - Anyway i guess its a moot point discussion as theres no real need for a sub to go knocking on a ships hull other than a PR stunt anyhow.
My submariner friend says he has no clue what that screen is trying to tell anyone. It looks Chinese or Japanese too and could even be a radar plot from an aircraft. The position is also just east of Kobe in Japan.
Active sonar will transmit forward, sideways and down and can be deflected off layers which submarines use to hide under. There is no warship in the world that tries to transmit sonar through their propellor wash.
Passive sonar does not work astern of any vessel because of propellor wash unless they are using a towed passive sonar array that will be about 500 metres or more behind them but will severely restrict their ability to perform fast turns.
Subs do hide under ships to mask their presence.
My former submariner friend says an SA Navy sub went directly under the Soviet Minsk carrier and her task force in the 80's and recorded all their noise signatures. The sub surfaced after all had passed and all hell broke loose with helicopters being launched, lights flasshing morse code and a destroyer that turned around to sail past the sub and contacted her requesting an exercise. It was politely refused.
Minsk: