STRUCTURE

All German U-boats used a double-hull design. The inner hull was the pressure hull, a steel cylinder with a steel hemisphere at each end. Atop the pressure hull, roughly amidships, was the conning tower, cylindrical with a hemispherical top. The crew lived within the pressure hull and conning tower, and worked in them when the U-boat was submerged. 

The pressure hull maintained 1 atmosphere pressure, despite the depth of the U-boat, until the U-boat passed the crush depth, the depth at which water pressure exceeded the ability of the pressure vessel’s steel hull to resist it. Then the hull collapsed, crushing its contents. The crush depth was an unknown quantity for each individual U-boat, but it was known to be below the vessel’s test depth – the maximum depth an intact U-boat could safely dive. For Type VII and Type IX U-boats the test depth was 230m (750ft), deeper than any other nation’s submarines could safely dive.

The pressure hull was pierced by numerous openings. There were four hatches: the forward and aft torpedo hatch, through which torpedoes were loaded; the galley hatch, allowing stores to be taken directly to the galley; and the conning tower hatch, used almost exclusively when the U-boat was at sea. (The other three hatches were on the deck, easily swamped by waves.) There were also torpedo tubes – four in the bow and one (for Type VII U-boats) or two (for Type IX U-boats) in the stern. Finally a multitude of small openings existed, through which cables, pipes, and control shafts penetrated. All had to be sealed before the U-boat dived and strong enough to resist water pressure at depth to prevent leaking. Damage limited the depth that could safely be reached, as higher water pressure at lower depths triggered leaks.

The pressure hull was divided in half horizontally along the forward two-thirds of the hull. Below were the bilges and batteries, above was the crew working space. In the aft third of the U-boat the diesel engines and electric motors occupied the entire pressure hull.

Surrounding the pressure hull was the outer casing, forming a hydrodynamic shell around the pressure hull. While the casing streamlined the hull, it was not designed for optimal underwater movement. It was topped by a flat deck on which deck guns were located, along with containers for external stowage of spare torpedoes and other items (including escape life rafts). The bridge superstructure rested on the deck, as did platforms for anti-aircraft guns. Fuel and ballast tanks were located on the sides between the two hulls. Ballast tanks could be filled with water or air to allow the U-boat to dive or surface.

U-549

 PROPULSION

Every U-boat was equipped with two diesel engines. The Type VII had two supercharged six-cylinder, four-stroke engines, but the engine used depended on the U-boat variant. Type VIIA, most VIIB, and a few VIIC U-boats had two MAN six cylinder, four-stroke M6V 40/46 diesels, each producing 1,050–1,155 brake horsepower. Starting with the Type VIIB U-boat, Germaniawerft six-cylinder, four-stroke F46 diesels were installed, each producing 1,400–1,600hp. The Type IX U-boats were equipped with two MAN 40/46 supercharged nine-cylinder, four-stroke diesels, each producing 2,200hp.

U-boats were also fitted with two electric motors. Early Type VII, all Type VIIA, and around half of the Type VIIB U-boats were outfitted with Brown, Boveri & Cie (BBC) GG UB 720/8 double-acting electric motors, together totalling 750hp. The rest of the Type VIIB, early Type VIIC, and Type VIID U-boats were fitted with AEG GU 460/8-276 electric motors. Later Type VIIs, as well as the Type VIIC/41s, had BBC GG UB 720/8, Garbe, Lahmeyer & Co. RP 137/c, or Siemens-Schuckert-Werke (SSW) GU 343/38-8 electric motors which produced 375hp each. Type IX U-boats had two SSW 500hp GU 345/34 electric motors. All of these motors ran off power stored in banks of batteries in the lower half of the pressure hull forward of the engine room.

Two sets of engines were required to enable prolonged submerged activity. Diesels were more efficient and more powerful than electric motors; but they were internal combustion engines requiring air to burn fuel, and producing toxic combustion gases that needed to be exhausted. If run while the U-boat was submerged, without access to the surface to draw air and exhaust burnt fuel, they soon asphyxiated the crew. The solution was to run the diesels while surfaced, using them to drive the U-boat’s twin propellers through geared connections, and drive a generator to charge the storage batteries.

The diesels were shut down when the U-boat submerged. It was then propelled by the electric motors, drawing electricity from the previously charged batteries. The electric motors could also drive the U-boat when it was surfaced, using the diesels only for the generators; but this was less fuel-efficient – and no competent U-boat skipper wasted fuel.

WEAPONRY

The U-boat’s primary weapon against  a destroyer or destroyer escort was its 533mm (21in) torpedoes. Although U-boats carried deck guns, a gunnery duel with a destroyer was a mismatch. Not only did the warship’s main battery outnumber the U-boat’s deck guns, the destroyers had a large suite of anti-aircraft guns, effective in supressing any U-boat gun crews. By 1942, U-boats were having their deck guns removed to allow more anti-aircraft guns – useful against attacking aircraft – to be carried. 

When the US Navy began fighting U-boats in fall 1941, the two main torpedoes in service were the G7a T1 compressed-air torpedo and the G7e T2 electrically powered torpedo. Both carried a 280kg (617lb) Hexanite-filled warhead and contact triggers, which detonated upon striking a ship. More powerful than TNT, a single hit by a Hexanite-filled warhead could cripple or sink a US Navy destroyer. 

Both the G7a and G7e torpedoes were also fitted with Federapparattorpedo (FAT; spring-operated torpedo) gearing during 1941 or Lagenunabhängiger Torpedo (LUT; a more sophisticated version of FAT) gearing after mid-1944. Both permitted a torpedo to zigzag back and forth after traveling straight for a preset distance. They were effective against convoys, but generally of little use in stopping a destroyer intent on attacking a U-boat. The un-aimed looping torpedo was easily evaded once a destroyer was aware of its presence.

The G7a torpedo had a range of 8,000m (8,749yd) when set for 40kn or 14,000m (15,311yd) at 30kn. It left a visible surface trail of bubbles as it traveled, potentially alerting targets of their approach. The G7e torpedo could reach 5,000m (5,468yd) at its 30kn setting, if preheated. The G7e left no trail of bubbles and was simpler to manufacture.

You can read more in DUE 127 US Destroyers vs German U-Boats: The Atlantic 1941–45