When the Commander of the German U-boat arm Karl Dönitz got the approval of Hitler for his ambitious plan to build a number of bomb-proof U-boat pens in the port of Lorient, the first phase of it was off to an ambitious start in February 1941. On the location of the Keroman Fisherman’s Wharf, two “Dom” bunkers were built to provide a bomb-proof shelter for U-boats on its repair lines. While at the same time only a stone throw further to the Southeast the construction of another bunker of massive proportions designated “Keroman 1” was started.
Construction
The construction of Keroman 1 or K1 was supervised by Organisation Todt (OT), which contracted a mix of German and French engineering and construction companies, together with thousands of Forced Labourers that were housed in camps in the area. K1 was completed in September 1941, just 7 months after the start of the construction. The structure roughly measures around 120 meters long by 101 meters wide and is 18.5 meters high. The walls vary between 2 meters to 3,5 meters thickness and the roof is approximately 3.5 meters thick.
In 1942 the bunker was enlarged with a new section on the South side. This section housed extra facilities for emergency-power, heating and oil supplies.
Starting from May 1941, the construction of Keroman 2 or K2 was started parallel to this adding 7 extra submarine bays to the whole.
Operation
With a distance of around 200 meters from the water, Keroman 1 was designed as a dry-dock facility for maintenance and repairs of German U-boats. In order to move the submarines to the docks and back to the water again, the engineers used a similar slipway design as the one at Keroman’s fisherman’s wharf. Instead of a rotating tilting bridge, a rail system was used to transport the submarines along the dry-docks.
The schematic drawing of the floor plan underneath illustrates the full U-boat transportation procedure.
Slipway
The Keroman 1 bunker has a large U-boat entrance at the mouth of the Ter, giving access to a wet dock with a slipway. The dock could be closed and the water pumped out, lowering the submarine on a chariot with the help of a mobile crane operating from underneath the bunker roof. The chariot was then hauled on the slipway ramp by a winch.
The slipway measures about 200 meters from its entrance at the Ter estuary to the ramp.
Underneath a picture of the slot where the door would seal off the slipway from the estuary to pump the water out and lower the u-boat on the chariot.
Rail system
Once hauled up the slipway ramp, the U-boats could be transported onto a mobile platform by a tow tractor. Then the mobile platform could be moved along a rail system located on the Northwest side of the bunker to transport it to one of its five bomb-proof submarine dry-docks designated K1 to K5.
The picture underneath shows the mobile platform being tested during construction of K2.
Most of the rail system running across from the dry-docks have been concreted over. But a small part near the slipway is still in its original state. The picture above shows the French post WW2 submarine “La Flore” on the mobile platform.
Underneath the stern of submarine La Flore is the last remaining 38 ton tractor of the two originally built in 1941. It was used for the last time in 2001 to pull La Flore from the K2 dry dock.
The following picture gives a better view of the mobile construction used to transport the mobile platform or cart inside dock K6a (of Keroman 2) for storage. One of the steel doors of bay K6a of K2 at the far end is openend.
Dry docks
Keroman 1 counts five bomb-proof submarine dry-docks designated K1 to K5. Before the U-boats could enter the dry dock, it had to have its periscope removed. In the picture underneath you can see a mobile crane pulling out the scope of U 67.
Bunker defense
For the defense of the Lorient U-boat base, Keroman 1 had several bunkers added to it’s construction.
FlaK bunkers
Above the slipway entrance of Keroman 1 a large Anti-aircraft bunker was placed on the roof. The bunker has two FlaK positions on top for anti-aircraft defense and an MG embrasure at the front to defend the slipway entrance from the roof. Another FlaK tower is a little further back on the roof above the slipway and not visible from the ground.
PAK bunker
On the Southwest side of Keroman 1 near the entrance on the water side there is a large bunker that has a large embrasure for a PAK gun facing the estuary.
Visit
It is possible to walk around the massive structure. The interior however is used by various companies and is therefore restricted.
Excellent information, as always. Very interesting.