The Type VII U-boat: General Features
The Type VII U-boat was a single hull design, meaning that its pressure hull was also its outer watertight hull. Around this, forward and aft, was a thin, non-watertight structure which provided streamlining and room for ballast tanks. On each side were long, bulbous saddle tanks that provided room for additional ballast tanks and, in all but the A model, fuel bunkerage. Along the top of the boat ran a flat-topped deckcasing which provided a flat stable surface for any work that had to be done on deck. The space between the deckcasing and the pressure hull was used for torpedo storage, air trunks, ready ammunition stowage and, later in the war, for the stowage of inflatable life rafts. The tower was also a multi-layer structure. Inside was the watertight conning tower from which the torpedoes were aimed and fired when the boat was submerged. Surrounding it was the external tower structure, which provided a protected site for the lookouts. The air trunks that fed air to the diesels ran up the after part of the external tower structure.
Of the eight sections of rolled galvanised sheet steel that made up the pressure hull, only the two central sections were cylindrical. The remaining sections were truncated cones. The sections of the pressure hull were constructed by welding the rolled sheet steel that made up the skin of the pressure hull to a set of circular ribs spaced at regular 60cm intervals. The only exceptions were the very ends of the hull which were prefabricated of three rolled and stamped pieces, two half conic sections and a spherical section end cap.
Forward torpedo room
The bow of the boat was given over to the forward torpedo room. The four torpedo tubes projected almost four metres into this room. They were arranged in two vertical banks of two, the upper tubes being slightly outboard of the lower two. Outboard of each tube was a two-metre long compressed-air cylinder, which stored the air needed to launch a torpedo from that tube. Aft of the torpedo tubes, as the diameter of the hull increased, room became available for the storage of spare torpedoes and the equipment to handle them. Beneath the removable deck plates, storage room existed for four torpedoes. At full load, two torpedoes usually hung from the overhead, one on each side.
The sleeping accommodation for the crews was almost an afterthought. Two rows of three bunks were laid out on either side of the compartment. These folded out of the way to provide room to handle or stow torpedoes.
Below the torpedo storage, the hull was divided horizontally by a curved bulkhead, below which were located the forward trim tank and the two forward torpedo tanks used to counterbalance the weight of fired torpedoes.
The next section aft was the forward accommodation and this was more spacious, the crewmen who lived there held higher rank. Aft of the bulkhead dividing the torpedo room and the forward accommodation was the forward head on the port side and a small food locker on the starboard side. Next aft came the small space to accommodate the four CPOs. Another thin bulkhead and hatch separated the CPOs from the officers, whose accommodation came next aft. A drinking water tank was on the starboard side of this area. Aft of the wardroom, on the port side, was the CO's compartment, the only private space on the boat. Even here the privacy was minimal, as only a heavy felt curtain separated the CO's room from the wardroom. This was intentional, as the most important activities of the boat were located within earshot of the CO's quarters. The radio and sound rooms were just across the passageway, on the starboard side of the boat. The control room was just aft of the CO's quarters, and was separated by a watertight bulkhead.
The area below the deck plating in these sections was large. Below the CPOs' and Officers' rooms was the forward battery compartment. Aft of this was the magazine, where ammunition for the deck and Flak guns were stored. Outboard of these compartments on both sides was a forward extension of the large forward fuel oil tank, which connected through the watertight bulkhead to the main body of that tank.
The command of a Type VII U-boat, at least when submerged, was conducted from the control room. Here were located the controls that guided the boat on the surface and submerged, the valves that controlled the flooding or venting of tanks and most of the mechanical devices that drove these controls. The centre of the room was dominated by the shafts for the two periscopes. Along the starboard side of the control room were located the two planesmen's positions that controlled the vertical movement of the boat. The navigator's chart table sat just behind them. The after end of the starboard side of the control room was taken up by the auxiliary bilge pump. The port side was completely given over to machinery: the periscope motor, the banks of valve handles controlling the flooding or venting of the boat's ballast tanks and the main bilge pump. The valve handles were painted red or green to indicate the location of the tank they controlled. A small drinking water tank was located on the port side and a similarly sized hydraulic oil tank on the starboard.
A downward-curved horizontal bulkhead separated the control room proper from the circular hull section. This space, crescent moon in shape, was given over to the large internal tanks that were so characteristic of this design. This tank space was further divided into three sections by two vertical partitions. Forward and aft were the main fuel oil bunkers. The main ballast tank was located in the middle.
In the control room overhead, between the two periscopes, was a watertight hatch and a ladder giving access to the conning tower. This tiny compartment, projecting above the circular pressure hull for a height of only slightly more than two metres, contained the equipment needed to aim the boat's torpedoes. This included the attack computer, a compass repeater and the attack periscope. Above was a watertight hatchway leading to the bridge.
There were two periscopes - the small one to the rear was the attack periscope and the large one to the front was the sky periscope. The attack periscope had a smaller head than the sky one so as to reduce the chance of it being spotted by the enemy under attack. The sky periscope had optics that allowed the user to scan the sky from horizon to vertical for enemy planes prior to surfacing. The larger head of the periscope allowed more light into the optics, though the realistic chances of seeing an aircraft through this periscope were pretty slim. The small attack periscope, which had an electrical drive, was of the "all height" type and was viewed from the conning tower. "All height" means that the eyepiece stayed at the same level even as the head moved up and down. The U-boat commander sat on a seat and turned the periscope by hitting foot pedals to drive the periscope right or left. Incorporated into the optics was a stadimetric ranging system that used split optics to find the range to the target.
After accommodation and galley
The after end of the control room, like the forward end, was sealed by a watertight bulkhead pierced by a circular hatch. Immediately aft of the bulkhead, on the starboard side, was a tiny refrigerated compartment, aft of which were stacks of small storage lockers for the petty officers. Next aft came the petty officers' bunks. Outward of the bunks, the after internal fuel oil bunker extended upward, taking up the space between the bunks and the pressure hull. The after end of this section was given over to the galley. Outboard on the starboard side was the after pantry. The after head, most often used as additional food storage, and the after circuit-breaker cabinet took up the port side of the galley space.
The after battery compartment was located below the decking of this section. Outboard of the battery compartment, the diesel oil bunker continued aft to the thin bulkhead that separated this section from the engine room.
Engine room and motor room
The two large diesels which drove a Type VII U-boat were 6-cylinder 4-cycle manufactured to near identical designs by MAN or GW. They were extremely lightweight and powerful for their size, and proved to be extraordinarily durable in operational use. They were located side by side in the engine room. The narrow central passageway gave access to the engines' valves and cylinders. Small compressed air and carbon dioxide tanks were located outboard of the engines, the former to start the engines and the latter to fight fires.
The motor room contained the two small electric motors, which were aligned on the drive shafts from the diesel engines. These electric motors were designed to work on the direct current supplied by the storage batteries. The control panels that monitored the boat's electrical power were located above these motors. The two large air compressors were located aft of the motors, the electrical compressor on the port side, and the smaller, diesel-driven Junkers compressor on the starboard.
Aft of this was the single after torpedo tube and its associated compressed air tanks. The space below the decking between the motors provided stowage for the single reload for the after torpedo tube. The hoist for this reload was centred on the overhead. The after torpedo tank and trim tank were located below the decking aft of the torpedo stowage.
Streamlining the boat and providing room for the myriad of tanks and trunks, the deck-casing and forward and aft hull made up a second, outer skin that covered much of the outside of a Type VII U-boat. Most of the area between the pressure hull and the external hull was free-flooding, but there were compartments and tanks outside of the pressure hull that were watertight. The holes for the Gruppenhorchgerat (GHG) - group listening apparatus - were located on each side in a semi-circle around the foreplanes. Above these was one pair of the circular diaphragms of the Unterwasser Telegraphie (UT) - underwater telegraph - with the other pair 50cm aft.
The deckcasing was made of thin steel, and other than at the extreme bow and stern, was covered with hardwood planks, primarily because a metal surface would ice up much more quickly in freezing weather. The lower edge of the deckcasing was marked with a long line of free-flooding holes to further facilitate the entry and exit of water. If there had been no free-flooding holes to allow water in between the thin deckcasing and the pressure hull, the pressure exerted upon the boat when at depth would have crushed the deckcasing.
The space forward between the deckcasing and the pressure hull was used primarily for storage. The motors and gearing for hauling in the anchor were located at the bow. Aft of this was the forward torpedo storage tube.
Features on the forward deckcasing included retractable bollards that were extended only when mooring the boat. Aft of the forward bollards was a retractable capstan, which was only needed if the electrical anchor hoist should fail. Further came the hinged access door to the forward torpedo hatch. Aft of the torpedo hatch was the forward battery hatch, which was bolted shut and only opened in the dockyard for the purpose of replacing battery cells. Next aft came the watertight container for ready ammunition for the 88mm deck gun, and the supports for the gun itself.
A major feature of the after deckcasing on the VIIA was the external torpedo tube, which was deleted in all subsequent variants. Despite the removal of the Flak gun from the after deckcasing, all Type VIIs until the middle of the war retained the watertight container (right aft of the tower) that was originally used to store the barrel of the 20mm Flak gun. Aft of that came the hinged deckplating giving access to the cook's hatch, the hinged hatchway over the torpedo hatch and the removable deck plating over the after torpedo storage tube.
The area between the after deckcasing and the pressure hull was largely taken up by the ventilation trunking running from the tower to the engine room, and the exhaust trunking and mufflers that ran to the exhaust ports on each side.