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Defeating the U-Boats
By J. Matchen
September 2012

In March 1943, the Allies suffered one of their worst months for shipping losses in the war. By August the U-boats had been beaten and would never again be a serious threat. How did it happen? Was it the result of a grinding war of attrition in which German science and industry simply couldn’t keep up? Or a climactic, Midway-like battle in which the submarines were soundly defeated?

The short answer is “yes.”

The War of Attrition

March 1943 was a spectacular month for the U-boat force. In the first three weeks alone over a half-million tons of shipping went to the bottom. The British Admiralty would later say that “the Germans never came so near to disrupting communications between the New World and the Old as in the first twenty days of March 1943.”

More important than the numbers alone, and more ominous for the allies, much of this destruction came from attacks on escorted convoys rather than the stragglers and independently sailing ships that had previously been the U-boats’ bread and butter. In one single battle — the five-day struggle against the convoys SC122 and HX229 — the wolfpacks sank 21 ships of 141,000 tons for the loss of only one U-boat (and that was sunk by a Liberator bomber at the end of the battle).

A convoy crosses the North Atlantic.

The growth in merchant sinkings masked other, less obvious factors. First, construction of new merchant shipping continued to grow, with the U.S. producing 21 million tons of shipping in the three years following its entry into the war. U-boat construction showed steady if undramatic growth but never achieved a battle winning critical mass.

The increasing numbers of U-boats were working harder for their successes. In October 1940 each U-boat in the Atlantic was sinking the equivalent of 920 tons of shipping per day. By August 1942 that number was down to only 149 tons. Casualties among U-boats were also rising and took a decisive turn in 1943.

The allies were also winning the intelligence war. Although both sides intercepted and decoded the other’s radio signals, the allies were able to gain a decisive advantage in rerouting convoys around the wolfpacks. Indeed, of 174 convoys crossing the Atlantic between May 1942 and May 1943, 105 were never even sighted by the U-boats.

New Tactics and Weapons

Along with the steady grinding of the attrition war (and of more importance to the SWWAS gamer) were new weapons and tactics that were coming to the fore by early 1943. First was the weight of numbers. With the end of end of escort and support missions for Operation Torch in November 1942, a large number of additional vessels became available to join the convoy war. These extra resources allowed the allies to establish escort support groups to supplement the convoy escorts. These groups consisted of a number of fast destroyer-type ships which sped across the ocean to join convoys which were facing wolfpacks.

These extra resources meant that the ships could now continue to press their attacks on the U-boats for the hours or even days needed to destroy the boats without facing the choice of leaving the convoy unescorted. The escorts were now also better trained in anti-submarine (AS) tactics such as the two-ship “creeping attack,” in which one ship would be directed over a submarine by a following ship which was able to maintain sonar (or ASDIC for the British) contact from a distance. The escorts also became more adept at air-sea combined tactics in which an aircraft would drive a U-boat under water and then direct the escorting ships in for the kill.

Air support improved in both quality and quantity with the invention of new weapons such as 10-cm-band radar which could spot surfaced U-boats and was, for a long time, undetectable by the subs’ radar warning devices. The invention of the Leigh light, a high-powered spotlight mounted on aircraft, allowed bombers to attack the U-boats at night, when they had previously been able to hunt relatively safely. Later the sonobuoy and acoustic homing torpedo would add to the U-boats’ woes.

Coastal command was also finally given (barely) adequate resources, and their long-range Liberator, Sunderland and Wellington aircraft could range far into the Atlantic and accompany the convoys for much of their passage. They also attacked the u-boats during their transit to or from their Atlantic hunting grounds, increasing transit times and reducing the number of boats on station.

Finally, the mass production of escort carriers (CVEs) allowed air cover throughout a convoy’s passage, as well as the formation of CVE hunter-killer groups which pursued and attacked the wolfpacks identified by allied intelligence.

An interesting stop-gap used by the British while they were awaiting their CVEs was the merchant aircraft carrier (MAC). These vessels were merchant ships (either a tanker or bulk carrier) which had a flight deck added and, while still carrying their cargo, could fly off and land a small air group of three or four Swordfish aircraft.

Battles Won

These factors culminated in several major convoy battles in mid-1943. From late March and through much of April, the Atlantic war slowed as many of the U-boats returned to base after their successful attacks in March. By late April, though, the wolfpacks had returned to the Atlantic to take up where they had left off.

One of their first victims was outbound convoy ONS5, which German intelligence had detected and which was stalked by several packs. At first storms protected the convoy, but on the night of May 4th the weather cleared and the U-boats struck. Five merchant ships were sunk that night and four the next day, despite the best efforts of the convoy’s escort and a powerful escort support group. On the night of the 5th, the submarine pack struck again but failed to sink a single merchant in some two dozen attacks. This time the escorts were more successful, sinking four U-boats. Overall losses for this convoy were twelve ships, but at a cost to the Germans of seven U-boats.

The wolfpacks’ next target was the fast convoy HX237, escorted by the British CVE Biter. This convoy suffered only three losses and destroyed three subs in return — one by Biter’s aircraft, one by shore-based aircraft, and one by a combination of shore-based aircraft and surface escorts.

The Biter escort group then moved on to support the slow convoy SC129. This battle was again a “push” with two merchants and two subs destroyed. The U-boats’ next target, slow convoy SC130, turned out to be the last Atlantic convoy seriously threatened by the wolfpacks. This convoy was attacked repeatedly over the period of May 15th to 20th without success. The attackers suffered the loss of five U-boats, and with this defeat Doenitz was forced to pull the U-boats from the North Atlantic.

The Germans tried attacking in other theaters, enjoying some success in the Indian Ocean, but were generally stymied by the Allies at every turn. In late August, the U-boats attempted a return to the Atlantic convoy routes on the strength of advanced technologies including acoustic homing torpedoes, various defensive countermeasures and the promising snorkel device. By this time, though, the Germans were well behind the development curve. They were never again able to pose a serious threat.

Bismarck Rules Variants

16.31 (addition): On a result of 10 the submarine makes contact and may attack any destroyer, destroyer escort, torpedo boat or merchant vessel in a convoy task force.

21.3 ASW (direct support variant): Land-based aircraft may be utilized for ASW in limited circumstances. Long-range level bombers and flying boats may be assigned to ASW missions in support of a particular convoy (a side note should be kept of this assignment). The convoy must be within half of the aircraft’s range from its base at all times during this direct support mission (i.e. a plane with a range of 16/2 could support a convoy 16 or fewer spaces from its base). The number of aircraft steps available for direct support may be limited by scenario rules.

A Wellington sports a U-boat-spotting Leigh light on its belly.

Leigh lights (16.42 variant): Beginning in June 1942 scenarios, a convoy may be directly supported by one step of long-range bombers during night turns. Convoys supported by Leigh light-equipped bombers do not suffer the –1 night ASW modifier. All other rules for direct support apply.

Air-sea cooperation — case 1: By mid-1943 aircraft and surface escorts had learned to work together well. To reflect this, for all scenarios taking place in April 1943 or later, aircraft steps (including escorting CV ASW and Direct Support aircraft) and surface escorts may be combined to reach the five steps/ships needed to gain the +1 modifier for ASW rolls. Note that rule 16.42, including the doubling of CV ASW steps, remains in effect. (Example: Three DE as escorts plus one step of Swordfish from an escorting MAC would receive the +1 modifier.)

Air-sea cooperation — case 2: For scenarios taking place before April 1943, rule 16.4 is modified to provide that the attacking sub is driven off on a die roll of 5 or 6 if the convoy has direct air support or CV ASW and the combined total of escorting air steps and ships is 5 or more. Note that this variation decreases the likelihood of a successful sub attack but does not increase the chance of destroying a u-boat as in Case 1 above.

Acoustic homing torpedoes: Beginning with the second half of August 1943, each German submarine (Type VII or higher) may have one torpedo factor designated as a homing torpedo. These torpedoes receive a +1 to-hit modifier.

Merchant Aircraft Carriers (MAC)

The British converted a total of 19 of these vessels as a useful stop-gap until they had a sufficient number of escort carriers (CVEs) available. They had a flight deck (those converted from grain carriers also had a hangar) and modest air capability while continuing to carry their cargo. They may operate one step of Swordfish aircraft which may be assigned to the ASW mission only. These aircraft are not eligible for night missions. Note that rules 16.42 (double steps for CV ASW) and Air-Sea Cooperation are in effect. Spare “Large Transport” counters may be used for these vessels with all ship values the same as the large transport (1 AA, Move 1+, 2 hull, 9 fuel). The actual ships’ names are below:

Oiler conversions:

  • Rapana
  • Miralda
  • Acavus
  • Adula
  • Alexia
  • Amastra
  • Ancylus
  • Gadila (Dutch crew)
  • Macoma (Dutch crew)
  • Empire Mackay
  • Empire MacColl
  • Empire MacMahon
  • Empire MacCabe

Bulk grain carrier conversions:

  • Empire MacAlpine
  • Empire MacKendrick
  • Empire MacAndrew
  • Empire MacDermott
  • Empire MacRae
  • Empire MacCallum

Click here to download the merchant aircraft carriers.

Escort Carriers (CVE)

The escort carriers were converted merchantmen produced in large numbers to provide wide ranging air support in situations where large carriers were unavailable or inappropriate. They were originally conceived for a convoy escort and ASW role, but served with distinction in many other capacities. Although the British and U.S. navies had disagreements over how to use them, the CVEs proved invaluable to both.

U.S. CVE Bogue class: Use counters and hit record sheets from Leyte Gulf. The carriers have a capacity of five steps, 2 AA, 1+ movement, 2 hull and 24 fuel. A usual air wing would be two steps of Wildcat (Martlet in British service) fighters and two or three steps of Avenger torpedo bombers. Early war air wings (particularly for Operation Torch) would include one or two steps of SBD Dauntless dive bombers in place of the Avengers. With their non-folding wings, the SBDs proved difficult in CVE operations and were later removed.

British CVE Attacker class: Counter values the same as Bogue class except a maximum of four steps of aircraft (the British CVE air wings were generally 18 to 24 aircraft).

Click here to beat the U-boats in Second World War at Sea: Bismarck.