Submarine designers soon realised that well, there are other submarines around and you also need to hunt them down to protect your ships. Given the submarines now carry missiles, some of which carry nuclear warheads so hunting them down becomes very important. This role generally falls upon hunter killers, also called attack submarines and designated SSN for nuclear ones and SSK for conventional / AIP ones. There are some differences between hunting surface warship and a submarine and let us try to list them and discuss them.

Submarines are always doing sprint and stop. What it means is that submarines sail at transit speeds of 10-15kn for a fixed period of time and then slow down to 5kn or below to hear about everything in the new neighborhood. Also, they are looking for convergence zones which are areas of acoustic speciality where sounds from far of places can be heard. But remember, other submarines are doing the same thing so you need to catch them in either of those phases and it is incredibly difficult.

Range of detection is directly proportional the decibels of sound emitted by your target minus the sound generated by you because you have to hear them over your own powerplant and boat noises. Imagine two equally quiet submarines like two ships of the same class. One is emitting x db at 5 kn, the other is also emitting x db at 5kn so irrespective of range they wont be able to hear each other. Look at the case below where HMS Vanguard (top) and French SSBN Le Triomphant (bottom) collided in 2009. Now both these are SSBNs and are top of the line so incredibly quiet. They were so quiet they couldnt hear each other and collided.

So, the key to hunting other submarines is basically trying to catch them out your phase as in they are sprinting and you are listening. Hull mounted sonar sensors are suceptible to onboard sounds. A towed sonar array is a long cable with sonar sensors which are reeled out only at very low speeds to hear everything around you minus a bit of your own sounds. Modern sonar suites have algorithms that filter out your own noise from the captured passive sounds. Remember my earlier example? If you are emitting x db at 5kn and your target is emitting x db at 5kn with Towed Sonar Arrays you can hear their x db + <5kn of your own creating a differential and making hearing the other submarine easy. And, if they are out of phase so they are sprinting it becomes far easier.

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Victor-class Submarine: The Soviet Union’s Primary Attack Submarine

by Epsilon September 21, 2014

Did I say easier, scratch that it is not at all easy. Thermoclines are layers in water due to different salinity, temperature and flow directions. A steady body of water might have 100s of such layers and each is a different acoustic environment. You might be hearing a boat sprint and then lose it the next second because she dived under or went above such a layer. In that case you will have to follow it into that layer to keep track. But how do you track submarines?

This picture is from the movie The Hunt for Red October and is not classified. Sonar systems have such waterfall displays that show sound source of interest versus its bearing. A bearing is…. Imagine a compass around a submarine with its bow facing due north so 0 degrees. The bearing is the degree marking on the compass where the target is at. So if you hear someone saying bearing 120, means the target of interest is at 120 degree mark on the compass. To make it clear, here are a few examples.

• Case 1: If you are heading south so facing 180 degrees and the target is at bearing 120 it is ahead of you off 60 degrees to your port facing towards bow.
• Case 2: If you are heading west 270 degrees and the target is at bearing 60, it is behind you and 30 degrees off to starboard facing astern.

The picture above shows what direction the target is at and any movement like the zig zags on the graph show the target has a bearing rate so its turning with respect to you. It also shows vertical angular separation and if it gets faint, it is going far away and if it is getting louder it is coming towards you. If you both are going slow and you can hear it, they can most probably hear you to. So how do you evade?

• If you are a Russian submarine which can usually dive deeper than a western one, just go deep if the sea floor allows, find a thermocline layer and speed off to break contact.
• If you are an Alfa class submarine that goes 10 kn faster than anything else, just speed off no one can catch you.
• Generally the way to evade is to find a thermocline where you cant hear the other boat up or down that your hull allows and slowly move out of detection range.

Wait! We havent discussed Crazy Ivan! Without towed sonar arrays a submarine is especially blind to sounds behind it so while in the slow and drift phase you do maneuvers called clearing your baffles (a slang for stern). Soviet submarines would suddenly turn to port or starboard (goes starboard in the bottom of the hour Captain, ask me people where this comes from by resharing this article everywhere). This maneuver is called Crazy Ivan. There was also a more elaborate maneuver done by Yankee class submarines called Yankee Doodle. You know what is weird, the plot of a movie falls apart because in real life, the boat has something that the movie one didnt. If you don’t know, just share this article everywhere and people will tell you.