‘Mulberry’ Floating Harbours

Co-ordinating the effort required awesome organisation and logistics. The Allies had to marshal and maintain over 2 million men, 11,000 aircraft, and 7,000 ships in England. The prodigious industrial output to meet their requirements had to be matched by efficient distribution. The engineering work behind the landings was staggering, and thousands of construction workers were recruited to work night and day. The Petroleum Warfare Department pioneered PLUTO (Pipeline Under The Ocean), ready to pump millions of gallons of petrol across to the invaders. To get the astonishing volume of men, equipment and supplies ashore in north-western France, Churchill’s pet project, the technologically ingenious ‘Mulberry’ floating harbours, were essential. Two were to be constructed off Normandy. Over a hundred enormous 6,000-ton reinforced concrete caissons called ‘Phoenixes’ (each 60 feet high, 60 feet wide and 200 feet long) would be towed across the Channel from Selsey Bill and Dungeness by some of the fleet of 132 tugs and then filled with sand from ‘Leviathans’ so they sank to form a breakwater in the Bay of the Seine. Outside this artificial reef was a floating line of ‘Bombardons’ towed from Poole and Southampton to calm the waves, and inside, in shallower water, a line of ‘Gooseberries’, formed from two dozen redundant merchant navy vessels, Liberty ships and one old dreadnought that were scuttled and sunk where needed. In the calmer waters within the two-square-mile Mulberry harbour, strong Lobnitz or ‘Spud’ pier heads were sunk deep into the sand which allowed long bridges or floating roadways to the shore, known as ‘Whales’, to float up and down with the tides. The menagerie of code-names was augmented by power-driven pontoons called ‘Rhinos’ and amphibious vehicles known as ‘Ducks’.

Artificial Ports

When the Allies prepared for the Normandy landing, it became evident to them that the Germans would do everything possible to prevent their French ports from falling into enemy hands. They consequently decided on a surprise approach, which involved bypassing existing ports and landing on a bare expanse of beach. Two structures were designed to accomplish this purpose: landing craft of various types that were to be deliberately run aground on the beach and then opened to discharge their cargo, and artificial ports.

Churchill had conceived the idea of artificial ports as far back as 1915. In 1940, as prime minister, he thought of it again. On May 30, 1942 he elaborated on his concept in a note dispatched to Mountbatten, who for some time had pondered over the problems that would be posed by a military landing. During a meeting of the chiefs of staff, Mountbatten declared; “If there are no employable ports, we can build them piece by piece and tow them over.” After reconnaissance information about Dieppe confirmed the need for “mulberries” (the code name given to these artificial ports), two of them were constructed in June 1944 for use at Arromarches and Vierville. Their use came as a complete surprise to the Germans, who had never even suspected their existence. Carried over the English Channel piece by piece, these two ports, complete with breakwaters, loading platforms and mobile jetties almost Vs of a mile in length, had a storage capacity exceeding the port of Dover. They could handle daily cargo unloadings of 6,000 tons of equipment and 1,250 tons of vehicles. The construction of these brilliant examples of British naval engineering in gcnuity, weighing one million tons each, required the labor of 20,000 men over a period of eight months, as well as 100,000 tons of steel and 8.75 million cubic feet of concrete. The Vierville port, constructed under extremely bad weather conditions, turned out to be useless. So did the Cherbourg “mulberry,” which was finished on June 27, 1944. But the Arromanches mulberry was able to discharge cargoes of 680,000 tons of equipment, 40,000 vehicles and 220,000 men between mid-July and October 31, 1944.

The artificial port used at Arromanches consisted of four basic elements. The first was breakwaters of three different sorts. One line of breakwaters was formed by filling 60 old ships with 500,000 tons of cement, which, of course, caused them to sink. Next came 146 open caissons of reinforced concrete. Six different types of caissons were made, ranging in weight from 1,672 tons to 6,044 tons; they were deployed at different levels as the depth of the ocean floor increased. The largest could be used where the ocean floor reached a depth of 30 feet. These caissons, armed with Bofors guns to protect personnel, were towed across the Channel by 1,500-horsepower tugs. At the proper moment the caissons’ valves were opened; they then filled with water and sank. The outermost breakwaters, cylindrical metal floats 225 feet long and 16 feet in diameter, were assembled side by side in groups of three and supported by a concrete “keel” weighing 750 tons, the upper part of which emerged six feet from the surface of the water. Anchored at a depth of 65 feet, they were placed end to end to form a floating breakwater a mile long, which took the first shock of the waves, before they struck the caissons.

The second element of the mulberries was fixed oi sheltering jetties. Floating caissons were placed end to end and sunk in lines at right angles to the shore. They protected the port from waves, and from attacking midget submarines or frogmen, and served as loading platforms for small ships.

Next were wharves made of pontoons. To keep the floating platforms at a steady horizontal level, which was necessary to avoid complications in unloading ships, they were anchored to steel braces on the ocean floor by a system of pulleys and cables. To compensate for the tides, pontoons were raised or lowered by winches. The winch operation was controlled by extensometers that were connected to the cables securing the pontoons to the steel braces.

The most difficult problem was how to maintain a continuous connection between the pontoons and the shore, which, at high tide, was about 3,000 feet away.’ In spite of the breakwaters the sea was in such constant turmoil that there was doubt as to how long floating jetties made up of several sections would behave. Exhaustive studies beginning in 1941 led to the construction of a jetty supported by floating caissons. Each 100-foot section was composed of two girders in an extremely rigid “bowstring” form, transversely connected at the center by an equally rigid strut and at different points on the strut by a number of articulated braces. The striated sheet metal deck was fixed to the various braces in such a way as to permit expansion. Thus, the ends of the two master girders could take various positions relative to one another. The whole structure was made of high-elasticity steel; its components were riveted together.

The variation of the tides was such that the floating jetties needed the capacity to lengthen or shorten. For this purpose they were fitted with telescoping sections, each consisting of twin girders whose ends were enclosed in a central unit into which they could slide. In this way each section could be lengthened by nine feet.

The installation of an artificial port required a graded shoreline, depending on its composition-i. e., whether it was primarily wet sand, pebbles, marsh etc. Roads in some cases needed to be constructed from prefabricated material shipped across the English Channel and swiftly laid by engineering crews. The road-building material was sometimes prefabricated metal mats, or perhaps of mineral or vegetable origin. In this last instance a British Valentine tank was sent forward with an enormous drum dispensing several hundred feet of coconut matting. A roadway laid down in this fashion over wet or dry sand proved excellent for wheeled and tractored vehicles.

Further amazing engines onshore also sprang from Churchill’s ‘inflammable fancy’: armoured tank bulldozers and ploughs, special fat-cannoned Churchill tanks for blasting blockhouses, other ‘Crocodile’ Churchill tanks that could squirt petrol and latex flames over a hundred yards, great machines for laying fascines across mud or barbed wire, or for thrashing their way with flailing chains clear through exploding mine fields. These devices came from Churchill’s direct encouragement and protection of a brilliant maverick, Major General Sir Percy Hobart of 79th Armoured Brigade, and were collectively known as ‘Hobart’s Funnies’.

The deception plan for NEPTUNE, the cross-Channel attack, was called Plan FORTITUDE, and its object was ‘to induce the enemy to make faulty dispositions in North-West Europe’. FORTITUDE NORTH aimed to keep Hitler worrying about Scandinavia, and the danger to Germany posed by an Allied attack on Norway and Denmark. Dummy wireless traffic and bogus information from double agents indicated that the (notional) British Fourth Army in Scotland, supported by American Rangers from Iceland, was going to attack Stavanger and Narvik and advance on Oslo. British deceivers also worked hard on the neutral Swedes. The commander-in-chief of the Swedish Air Force was asked for ‘humanitarian’ assistance in the event of an Allied invasion of Norway. As his office was being bugged by the pro-Nazi chief of Swedish police, this information went straight to Berlin. When Hitler read the transcript he ordered two more divisions to reinforce the ten already in Norway. Thus 30,000 more soldiers were diverted away from France.