Hiding in the sound
Underwater, sight is useless beyond a few metres. Hunters and hunted work by sound. Here an enemy escort circles on the surface, sending pulses down to find a British submarine.
The ping
The escort's sonar sends a pulse of sound from a dome under its hull. It travels through seawater at about 1,500 metres a second. The glowing shell is that pulse, and the bright line is where it is touching something: the hull, a mine cable, the seabed. Whatever it hits sends a faint echo back, and the time the echo takes gives the range. The British called this ASDIC, and fitted it to their own boats too.
The layer
Sun-warmed water sits on top of colder water. Sound bends where the temperature changes, so at that boundary, the thermocline, much of the ping is bent away, down and past what lies under it. Below it lies a shadow zone the sound barely reaches. A boat down there sends back only a whisper, and submarine captains dived under the layer on purpose. The layer's depth changes with the season and the sea; 180 feet here is illustrative.
The pressure
Every 33 feet of seawater adds about one atmosphere, another 14.7 lb on every square inch. The early T-class boats were riveted and rated to 300 feet. At that depth the sea presses on the hull with about 133 lb on every square inch more than at the surface.
Opened up
The outer hull is a thin, free-flooding shape for moving through water. A crew of around 60 lived inside the pressure hull, a steel cylinder split into compartments from bow to stern. Broadly: the tube space, torpedo stowage and the seamen's mess, the control room under the bridge, the wardroom and galley over the battery, the diesels, and the motor room.
The boat is modelled loosely on an early T-class, the Royal Navy's main ocean-going patrol submarine of the war: 84 m (276 ft) long, 53 built. It is shown with its original bulbous bow. That bulge held two of its ten torpedo tubes that faced forward, but it slowed the boats in rough seas; two boats had it cut away, and later groups were built with a finer bow. The Oerlikon aft of the bridge and the radar mast are mid-war additions. Depths are in feet, as the boats' own logs recorded them. The escort's range is in metres. Sound is shown about 36× slower than life so you can watch it travel. Sources are listed on the entry page.
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