Mulberry Harbor

Mulberry harbours
In 1942, when the Allies began to plan their return to mainland Europe, they accepted that they could not rely on capturing a harbour in working order. During the planning for Operation Overlord, it was decided that two artificial harbours would be constructed off the Normandy beach-head, one in the British sector at Arromanches and the other in the American sector at St Laurent.

Codenamed 'Mulberries', the harbours would consist of two breakwaters offshore and flanking made from hollow, ferro-concrete caissons called 'Phoenixes'. These massive prefabricated components would be towed by tugs across the Channel from England and, once laid end to end to form the breakwaters, would be sunk. Within each of the protected areas of the two harbours would be three floating piers, connected to the shore by floating steel roadways. Each Mulberry would be supplemented by 70 blockships.

From the summer of 1943, shipyards around Britain employed 20,000 workers around the clock to build more than 150 caissons, each 200ft (61m) long and 60ft (18m) high. The project eventually absorbed in excess of two million tons of steel and concrete. It also involved huge numbers of British tugs, plus others requisitioned in the United States, to tow the Mulberries into position.

From 4 June, in Operation Corncob, the tugs began towing the completed Phoenixes towards Normandy from their assembly point near Lee-on-Solent, at a speed of 3-4 knots. When the invasion was postponed for a day, the tugs remained stationary. On 6 June, when the invasion finally got the go-ahead, they were anchored about five miles off the French coast.

On 7 June D+1 the Phoenixes were brought to about a mile of the shore and turned over to a fleet of harbour tugs. These held the caissons in position while their sea valves were opened to flood the interiors. In addition, each Phoenix was topped with an anti-aircraft gun emplacement

When completed, each Mulberry was about one mile long and stood about 30ft (9m) above sea level at low tide and 10ft (3m) above at high tide. Up to seven Liberty Ships could tie up simultaneously to each one to unload their cargoes.

Narrow steel floats (bombardons) 200ft long had been moored off the artificial harbours to break up incoming waves. However, despite this, between 18 and 22 June both Mulberries were badly damaged by the worst Channel gales for 40 years. The harbour at St Laurent was never used again, which delayed the Allied build-up within the bridge-head at a critical time when German reinforcements were arriving at the front. Eventually the Americans were able to land more over the open beaches than could be handled by the repaired British Mulberry at Arromanches.

Nevertheless, the Mulberry concept was sound, providing sheltered waters behind the breakwaters and enabling the British to land difficult loads.