The "U-flak" boats were four VIIC boats (U-441, U-256, U-621, and U-951) modified to be surface escorts for the attack U-boats operating from the French Atlantic bases. They had greatly increased anti-aircraft fire-power.

Conversion began on three others (U-211, U-263, and U-271) but none was completed, and they were eventually returned to duty as traditional VIIC attack boats.

The modified boats became operational in June of 1943 and at first appeared to be successful against the surprised Royal Air Force. Seeing their potential, Dönitz ordered the boats to cross the Bay of Biscay in groups at maximum speed. The effort earned the Germans about two more months of still-limited freedom, until the RAF developed counter-measures. When the RAF began calling in surface hunters to assist the aircraft, the U-flak boats were withdrawn and converted back into fighting vessels.

The concept of the U-flak began the year before, on August 31, 1942, when U-256 was seriously damaged by aircraft. Rather than scrap the boat, it was decided to refit her as a heavily-armed anti-aircraft boat intended to stop the losses in the Bay of Biscay inflicted by Allied aircraft.

Two 20 mm quadruple Flakvierling mounts and the experimental 37 mm automatic gun were installed on the U-flaks' decks. A battery of 86 mm line-carrying antiaircraft rockets was tested, but this idea proved unworkable. At times, two additional single 20 mm guns were also mounted. The submarines' fuel capacities were limited to Bay of Biscay operations only. Only five torpedoes were carried, preloaded in the tubes, to free the space needed for the additional gunners.

In November 1943 -- less than six months after the experiment began -- all U-flaks were converted back to normal attack boats, fitted with Turm 4. The standard anti-aircraft armament for U-boats was no longer much inferior to U-flaks, and the U-flaks had not been particularly successful. Even with massive anti-aircraft firepower, a U-boat was still vulnerable to having her pressure hull punctured; her best bet when encountering aircraft was simply to dive. According to German sources only six aircraft had been shot down by U-flaks in six missions (three by U-441, one each by U-256, U-621, and U-953).

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Type VIIC/41

Type VIIC/41 was a slightly modified version of the successful VIIC and had the same armament and engines. The difference was a stronger pressure hull giving them a deeper test depth and lighter machinery to compensate for the added steel in the hull, making them actually slightly lighter than the VIIC. A total of 91 were built; all of them from U-1271 onwards lacked the fittings to handle mines.

Today one Type VIIC/41 still exists: U-995 is on display at Laboe (north of Kiel), the only surviving Type VII in the world.

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Type VIIC/42

The Type VIIC/42 was designed in 1942 and 1943 to replace the aging Type VIIC. It would have had a much stronger pressure hull, with skin thickness up to 28 mm, and would have dived twice as deep as the previous VIICs. These boats would have been very similar in external appearance to the VIIC/41 but with two periscopes in the tower and would have carried two more torpedoes.

Contracts were signed for 164 boats and a few boats were laid down, but all were cancelled on September 30, 1943 in favor of the new Type XXI, and none was advanced enough in construction to be launched.

It was powered by the same engines as the VIIC.

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Type VIID

The type VIID boats, designed in 1939 and 1940, were a longer version of the VIIC with three banks of five vertical tubes just aft of the conning tower, rather like a modern ballistic missile submarine, except that these tubes ejected mines rather than missiles.

On the surface the boat used two supercharged Germaniawerft, 6 cylinder, 4-stroke F46 diesels totalling 3,200 bhp (2,400 kW) at between 470 to 490 rpm. When submerged the boat used two AEG GU 460/8-276 electric motors giving a total of 750 shp (560 kW) at 285 rpm.

These boats did not fare well: only one survived the war; the other five all went down with all hands.

U-213 -- U-214 -- U-215 -- U-216 -- U-217 -- U-218

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Type VIIF

The Type VIIF boats, designed in 1941, were primarily built as torpedo transports. They were the largest and heaviest type VII boats built. They were armed identically with the other Type VIIs except that they could have up to 39 torpedoes onboard and had no deck guns.

Only four Type VIIFs were built. Two of them, U-1062 and U-1059, were sent to support the Monsun U-boats in the Far East; U-1060 and U-1061 remained in the Atlantic.

It used the same engines as the VIID.

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Specifications

Class	VIIA	VIIB	VIIC	VIIC/41	VIIC/42	VIID	VIIF
Displacement surface	626 tons	753 tons	769 tons	769 tons	999 tons	965 tons	1084 tons
Displacement submerged	745 tons	857 tons	871 tons	871 tons	1099 tons	1080 tons	1181 tons
Length overall	64.5 m	66.6 m	67.1 m	67.1 m	68.7 m	76.9 m	77.6 m
Length pressure hull	44.5 m	48.8 m	50.5 m	50.5 m	50.9 m	59.8 m	60.4 m
Beam overall	5.85 m	6.2 m	6.2 m	6.2 m	6.85 m	6.4 m	7.3 m
Beam pressure hull	4.7 m	4.7 m	4.7 m	4.7 m	5 m	4.7 m	4.7 m
Draft	4.4 m	4.74 m	4.74 m	4.74 m	5 m	5 m	4.9 m
Power surface	1,700 kW[1]	2,400 kW[2]	2,400 kW[3]	2,400 kW[4]	2,400 kW[5]	2,400 kW[6]	2,400 kW[7]
Power submerged	560 kW[8]	560 kW[9]	560 kW[10]	560 kW[11]	560 kW[12]	560 kW[13]	560 kW[14]
Surface speed	17 knot (31 km/h)	17.9 knot (33 km/h)	17.7 knot (33 km/h)	17.7 knot (33 km/h)	18.6 knot (34 km/h)	16.7 knot (31 km/h)	17.6 knot (33 km/h)
Submerged speed	8 knots (15 km/h)	8 knots (15 km/h)	7.6 knots (14 km/h)	7.6 knots (14 km/h)	7.6 knots (14 km/h)	7.3 knots (14 km/h)	7.9 knots (15 km/h)
Surface range	11,470 km	16,095 km	15,170 km	15,725 km	23,310 km	20,720 km	27,195 km
Submerged range	175 km	175 km	150 km	150 km	150 km	130 km	140 km
Maximum operating depth	220 m	220 m	230 m	250 m	270 m	200 m	200 m
Crush depth	230 to 250 m	230 to 250 m	250 to 295 m	275 to 325 m	350 to 400 m	220 to 240 m	220 to 240 m
Complement	42 to 46	44 to 48	44 to 52	44 to 52	44 to 52	46 to 52	46 to 52
Deck gun	C35 88 mm/L45 with 220 rounds						none
Anti-aircraft guns	C30 20 mm		Various			2×C30 20 mm with 4,380 rounds	3.7 cm Flak with 1,195 rounds 2×C30 20 mm with 4,380 rounds
Bow tubes	4 [15]
Stern tubes	1 [16]
Torpedoes (maximum)	11	14	14	14	16	14	14 / 39 [17]
Mines	22 TMA mines or 33 TMB mines	26 TMA mines				15 SMA mines in vertical chutes and either 26 TMA mines or 39 TMB mines	none
Number commissioned	10	24	568	91	0 [18]	6	4

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Notes

1. ^ 2 MAN, 6 cylinder, 4-stroke M6V 40/46 diesels totalling 2,100 - 2,310bhp. Max rpm: 470-485.
2. ^ 2 supercharged MAN, 6 cylinder, 4-stroke M6V 40/46 diesels totalling 2,800 - 3,200bhp. Max rpm: 470-490.
3. ^ 2 supercharged Germaniawerft, 6 cylinder, 4-stroke M6V 40/46 diesels totalling 2,800 - 3,200bhp. Max rpm: 470-490.
4. ^ Same as VIIC
5. ^ Same as VIIC
6. ^ 2 supercharged Germaniawerft, 6 cylinder, 4-stroke F46 diesels totalling 2,800 - 3,200bhp. Max rpm: 470-490.
7. ^ Same as VIID.
8. ^ 2 Brown, Boveri & Cie GG UB 720/8 electric motors, totalling 750shp. Max rpm: 322.
9. ^ 2 AEG GU 460/8-276 electric motors, totalling 750shp. Max rpm: 295.
10. ^ Same as VIIA or VIIB, 2 Siemens-Schuckert Werke GU 343/38-8 electric motors, totalling 750shp and max rpm: 296 or 2 Garbe Lahmeyer RP 137/c electric motors, totalling 750shp and max rpm: 296.
11. ^ Same as VIIC
12. ^ Same as VIIC
13. ^ 2 AEG GU 460/8-276 electric motors, totalling 750shp. Max rpm: 285
14. ^ Same as VIID.
15. ^ A small number of VIIC boats were fitted with only two forward tubes
16. ^ A small number of VIIC boats were fitted with no stern tube
17. ^ 39 Torpedoes were carried in the transport role
18. ^ None of the boats were ready by the end of the war

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References

• Stern, Robert C. (1991). Type VII U-boats. Annapolis, Maryland (USA): Naval Institute Press. ISBN 1-55750-828-3.
• Uboatwar.net. The VII Class. Retrieved on 6 April 2007.
• Uboat.net. U-Boat Classes. Retrieved on 6 April 2007.

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