SM UC-42

About 5.5 kilometres south-east of Roches Point, in only 27 metres of water, lies the wreck of the German minelaying submarine SM UC-42. She is one of roughly fifteen First World War submarines known to have been lost around the Irish coast, but her wreck is unusually well documented because it was found once in 1917, effectively forgotten, and then rediscovered almost a century later. ¹

A submarine built to lay mines

UC-42 belonged to the Type UC II class, a series of submarines designed specifically to place mines in hostile coastal waters. She was launched by AG Vulcan at Hamburg in September 1916 and commissioned two months later. At just under 50 metres long and around 400 tonnes displacement on the surface, she was considerably smaller than the large ocean-going U-boats which have come to dominate the popular image of Germany's submarine campaign. ²

Her principal weapon was not the torpedo but the mine. Six vertical mine chutes were fitted into the forward part of the hull, each containing three UC 200 mines. The tubes were open to the sea and the mines were released directly downwards as the submarine passed across the intended minefield. UC-42 nevertheless retained three 50 cm torpedo tubes and an 8.8 cm deck gun, allowing her to attack shipping conventionally when the opportunity arose. ^{2 3}

The mine chutes made the UC boats highly specialised vessels. Their ability to lay a concealed minefield close to a harbour entrance or shipping route gave a comparatively small submarine the potential to disrupt traffic long after she had left the area. It also introduced complications of its own. Every mine released from the forward tubes altered the submarine's displacement and trim, something which had to be compensated for as the operation continued. ³

UC-42 entered operational service with the Flanders Flotilla and completed six patrols. During those patrols she was credited with sinking thirteen merchant vessels and one auxiliary warship and damaging another warship. By September 1917, under Oberleutnant zur See Hans-Albrecht Müller, she had been ordered west again, this time to mine the approaches to Cork Harbour. ²

The last patrol

UC-42 left Flanders on 1 September 1917. German records subsequently gave 10 September as the date of her loss, which is also the date adopted by INFOMAR. Her objective was the entrance to Cork Harbour, where the Royal Navy base at Queenstown — now Cobh — made the surrounding approaches an obvious target for a minelaying submarine. ¹

What happened once she reached the Cork coast remains uncertain. The traditional explanation is that UC-42 was destroyed by one of her own mines while laying a field off the harbour entrance. That account appears in the INFOMAR wreck sheet and has been repeated in several later histories of the submarine. ¹

It is a plausible explanation, particularly given the conditions in which the submarine was working. The Cork approaches are shallow, and UC-42 would have been operating slowly while releasing mines beneath the forward part of the boat. The repeated change in buoyancy as the mines left their chutes required careful control of trim. In less than 30 metres of water, there was relatively little room in which to recover from a serious mistake. ³

The wreck, however, complicates the story. The damage visible today is not obviously consistent with the detonation of a full-sized anti-shipping mine immediately beneath the hull. That does not rule out an accidental mine explosion, but it means the familiar version of UC-42's loss should not be treated as proven. ³

The submarine nobody knew was already there

On 31 October 1917, more than seven weeks after UC-42 had left Flanders, Royal Navy torpedo boat TB055 was operating with minesweepers near the entrance to Cork Harbour when an oil slick was noticed on the surface. The slick was followed to its apparent source and hydrophones were used in an attempt to determine whether a submarine was lying beneath it. Mechanical sounds were reportedly heard from below, variously described as hammering or turbine-like noises. ⁴

The obvious conclusion was that a German submarine had been caught beneath the surface. TB055 dropped a depth charge onto the contact. More oil and bubbles appeared, after which the armed minesweeper Sarba was brought in and dropped another charge. The area continued to produce oil and was watched overnight. ⁴

On 2 November dockyard divers finally descended to the wreck. They found a German submarine lying on the seabed with damage around the stern. A brass plate on the conning tower carrying the marking “C42, 1916” identified the wreck. ⁴

UC-42 had therefore been found only weeks after she disappeared. The problem was that the Royal Navy's own intelligence made it effectively impossible for the crew still to have been alive when TB055 attacked. The submarine had left Belgium on 1 September, while the longest recorded operational endurance of a UC boat in home waters was considerably shorter than the two months which had passed. Whatever noises the hydrophones detected, they were not the crew of UC-42 working inside the wreck. ⁴

The depth charges dropped at the end of October therefore did not sink UC-42. They struck a submarine which had already been lying on the seabed for weeks.

A wreck found twice

Despite the Royal Navy investigation, the precise position of UC-42 did not remain widely known. The wreck was effectively lost again until 6 November 2010, when Irish divers Ian Kelleher, Niall O'Regan, Philip Johnston, Eoin McGarry and Timmy Carey located a submarine south-east of Roches Point. ²

Its rediscovery renewed the question of what had actually caused the loss. The wreck did not display the catastrophic stern destruction which might reasonably be expected from an anti-shipping mine detonating directly beneath it. Later examination instead found much of the pressure hull comparatively intact, with external structure stripped away and damage concentrated towards the stern. ^{1 3}

Some of that damage could have been caused by the depth charges dropped by the Royal Navy after the submarine had already sunk. Divernet's later wreck interpretation also raises another possibility: UC-42 may have lost control of her trim while laying mines and struck the seabed stern-first. Releasing the mines from the forward tubes would progressively make the bow more buoyant, requiring the crew to compensate throughout the operation. ³

There is not enough evidence to replace one definite explanation with another. The safest conclusion is that UC-42 was lost while operating in the Cork approaches, almost certainly while engaged in minelaying, but that the precise mechanism of the sinking remains uncertain.

The wreck

INFOMAR surveyed UC-42 during survey KRY12_04 in the approaches to Cork Harbour. The wreck lies at approximately 51°45′02.59″N, 08°12′57.14″W, around 5.5 kilometres south-east of Roches Point, in a general water depth of approximately 27 metres. The surviving wreck measures around 45 metres long, five metres wide and 3.7 metres high. ¹

The submarine lies on her port side, orientated north-west to south-east with the bow towards the south-east. Much of the outer casing has disappeared, exposing the pressure hull beneath. INFOMAR describes the inner pressure hull as essentially intact, although the outer structure has largely gone apart from the surviving port saddle tank. ¹

The wreck remains recognisably a submarine rather than simply a cylindrical pressure hull. One of the external bow torpedo tubes lies partly buried beside the wreck and elements of the hydroplanes remain visible. The conning tower has become detached and lies alongside the main hull. Damage around the stern also preserves evidence of the subsequent depth-charge attack. ^{1 3}

The mine-laying equipment is particularly significant. Diving photographs show the surviving mine chutes and the remains of at least one mine still present within the structure. The wreck is consequently not merely an example of a minelaying submarine; part of the equipment associated directly with her final operation remains on the seabed. ⁴

Identifying UC-42

The identification made by the Royal Navy in 1917 was already unusually strong because the divers recovered the conning-tower plate carrying the submarine's number. The modern rediscovery allowed that identification to be checked independently rather than simply accepted from the wartime record.

Divers cleaned marine growth from the bronze propellers and located manufacturers' serial numbers which could be compared with surviving German records. The numbers corresponded with UC-42, providing a second and quite separate means of identifying the wreck. ⁴

The documentary chain is therefore unusually complete. UC-42 can be followed from her departure from Flanders, through her disappearance, the Royal Navy's accidental rediscovery of the wreck in October 1917, its identification by dockyard divers and finally its relocation and confirmation by Irish divers in 2010.

Ironically, the part of the story which remains least certain is the event that put her on the seabed in the first place.

Twenty-seven men

INFOMAR gives the complement at the time of loss as 26 or 27 men, while the surviving crew list reproduced by Nautical Log contains 27 names under Hans-Albrecht Müller. None survived. ^{1 5}

Following the rediscovery, a commemorative plaque was placed at the stern by the divers who located the wreck. UC-42 is treated as a war grave and, as a wreck more than a century old, is protected under Irish underwater-heritage legislation. INFOMAR notes that diving the site requires an archaeological licence. ^{1 5}

That context matters when looking at how complete the submarine remains. The pressure hull visible in the modern survey is not simply an interesting surviving example of Type UC II construction. It is also the place in which most, if not all, of UC-42's crew died in September 1917.

A strange second sinking

UC-42 has an unusual history because the submarine effectively appears twice as a wartime casualty. The first loss occurred in September 1917, unseen by anyone who survived to explain it. UC-42 disappeared while operating off Cork and settled onto the seabed with her entire crew.

The second episode came seven weeks later. Oil rising from the wreck convinced British vessels that they had located a live German submarine. Hydrophones appeared to confirm the suspicion, depth charges were dropped and only afterwards did divers discover that the target had already been dead for weeks. ^{3 4}

The modern wreck preserves traces of both events. Whatever damage originally caused the submarine to sink is now mixed with the effects of the later depth-charge attack, while more than a century of corrosion has removed much of the outer casing and exposed the stronger pressure hull beneath. ¹

That leaves an unusually clear identification attached to an unusually uncertain loss. We know with considerable confidence that the wreck is UC-42, where she lies and what she had been sent to do. Whether she detonated one of her own mines, lost control while laying them or was lost through some other failure remains unresolved.

What can be excluded is the explanation briefly believed on 31 October 1917. TB055 did not find UC-42 alive. It found a submarine which had already been lying on the bottom of the Cork approaches for weeks.