Thread: The Mulberry Harbours

To enable the forces already on the Normandy coast on D-Day to be reinforced and kept supplied with utmost speed it was necessary that two invasion ports, each roughly the size of Dover Harbour, should be constructed in England, then be transported across 100 miles of sea and placed in position off the enemy occupied coast. The story of this staggering achievement was told in the “War Illustrated” in November 1944.

Long before the invasion was launched it was obvious that vast quantities of stores and equipment would have to be landed on open beaches. It was estimated that about 12,000 tons, plus 2,500 vehicles of all kinds, would have to be handled in this manner each day for 90 days at least. There was only one possible plan; this was to have ready prefabricated sections which could be assembled at the spot where two vast invasion ports would be most useful. A harbour for British use was to be located at Arromanches, another for American use, at Port-en-Bessin. And each approximately the size of Dover Harbour, which took seven years to complete! These two were to be set up in days, to give full shelter and all usual port facilities to vessels whilst unloading. This entailed the construction of 150 caissons, enormous hollow blocks for the harbour walls. The largest of these caissons had a displacement of over 6,000 tons. Each contained crews quarters for use during the passage, the crew being partly naval, and partly from the Royal Engineers (or American Seabees) for carrying out the operation of sinking. At a later stage, Bofors guns, 20 tons of ammunition, and rough shelters for a guns crew, were placed on the top of most caissons as additional A.A. protection for the harbour. Caissons were towed across the Channel, each by a tug of about 1,500 hp


Mr. Jack W. Gibson, 57 year-old Yorkshire man and Director of Civil Engineering at the Ministry of Supply, one of the chief figures on the construction side of the prefabricated ports.

On arrival they were manoeuvred into position with the help of small tugs, then special valves were opened in each, allowing water to fill it and sink it where it was to remain. It took roughly 22 minutes to sink the largest. These had all been built during the winter months, a few in graving docks, the rest in emergency basins constructed behind river-banks, the banks were then dredged a way so that the partly completed caissons could float out and be towed to wet docks for completion of the concrete work. Towing of these to the Normandy coast commenced on D-Day plus one. On D-Day blockships sailed to where this work was to be completed, to provide breakwaters for the immediate shelter of hosts of small craft. Sixty ships of various types and sizes were earmarked for this purpose, including old warships. These made the crossing close behind the assault forces and all arrived safely. They were sunk by explosive charges, and their crews were then brought back to England. To complete each port, internal equipment such as piers was essential. It was no easy matter to construct a pier hundreds of feet long on a flat beach, with a rise and fall of tide of over 20 feet, and which sometimes may be floating and at other times be resting on sand or, worse still, rock. This problem had been under examination for many months, and in 1942 the Prime Minister took a personal interest in the matter. In typical Churchillian style he wrote, “Piers for use on beaches. They must float up and down with the tide. The anchor problem must be mastered. Let me have the best solution worked out. Don’t argue the matter. The difficulties will argue for themselves.”

Pier-Heads Built as ships

The result was eminently satisfactory. After severe sea tests the equipment went into production amounting to 7 miles of pier and all necessary appurtenances. Pier heads were built, as ships, in various ports on the coast, from Leith round to Glasgow, four by the R.E’s at military ports in Scotland. Most of the remaining equipment was prefabricated all over the country, and then assembled at the Army depots at Southampton and Richborough. About 240 contractors were employed on this particular task, and 50,000 tons of steel were used. The work of assembly-on-the-spot went on whilst the sinking of caissons was being completed. By D-Day plus 12 more than half of these were in position, and the harbours were already an impressive sight. Up to this time the operation was going according to plan. Very few accidents had occurred, and the air superiority was such that enemy interference had caused little trouble. Floating breakwaters, consisting of steel floats, had previously been moored end-to-end in a long line, the object of these being to have a damping effect on the sea in strong winds; 15,000 tons of steel were used in their construction. The Army Fire Service helped in sinking the pier heads to the correct level, as well as manning and floating land based fire stations.

And then, on D-Day plus 13, there occurred the biggest June gale for 40 years. It blew from the worst possible direction, the harbours were exposed to its full force, and these were only at the half-way stage. The American harbour suffered very severely and the breakwaters were largely broken up, so much so that the work on this harbour was discontinued. All the pier equipment which was on the voyage across when the gale started was sunk, but only one caisson failed to weather the journey.

Gigantic prefabricated pier for one of the home made invasion harbours being towed across the Channel. Average speed for this operation was 4 knots, the towing distance for each unit averaging a hundred miles. The total tug fleet available for the assembly of the harbour parts off the Normandy coast was 85, varying from vessels of 1,500 hp to those of 600 hp, the latter not generally used in the open sea. Each round trip took about three days and the transport of well over 1,000,000 tons was involved.

After the gale subsided the work of construction continued on the British harbour at Arromanches, though a long spell of rough weather prevented pier equipment from being towed over, so that the remaining harbour was not unloading to maximum capacity until well into July. But even on the worst day 800 tons of petrol and ammunition, as well as many troops, were landed over the piers. Eventually it was completed, and a port bigger than many with famous names had been built in a few weeks against a lonely French beach. Day after day, in all weathers, scores of ships of all sizes had moored within its shelter or berthed in unbroken lines along its quays. Never, even at the height of a peacetime trade boom, had so much shipping used such limited accommodation at one time. As a result of the craftsmanship of vast numbers of British workmen hundreds of thousands of tons of vital supplies, scores of thousands of men and many thousands of vehicles had been put ashore in the most rapid military build-up ever undertaken. The prefabricated port made possible the liberation of Western Europe. Furthermore, when the concrete sections were assembled for trial, in shoal water off Dungeness shortly before D-Day, they were seen by German air reconnaissance and led the German High Command to the erroneous conclusion that the Pas de Calais, with its flying-bomb launching sites, was the objective. This inaccuracy caused the retention of large enemy forces in the wrong parts of Northern France, enabling the landings to go ahead before German troops could be switched to the real zone of operations.

Without this crowning example of the British genius for invention it is doubtful if Hitler's West Wall would so readily have been pierced: towed in sections from Britain to Arromanches in Normandy, a prefabricated harbour provided vital accommodation for the Allied shipping. One of the enormous concrete caissons seen above, about to be towed across the Channel to Normandy. The caissons in position (bottom), forming part of the main deep water breakwater, with the A.A. guns on platforms to guard the harbour.

Steel roadways carried on steel girders run out from the beach at Arromanches to distant pier-heads at which our invasion craft, including 7,000-ton vessels, discharged their urgent cargoes. A pier-head, with a displacement of 1,000 tons, complete with crew's quarters, generating sets, and storage accommodation, at the end of the cross-Channel tow (above). Ambulances formed part of the traffic using the piers, a convoy (bottom) proceeds from shore to hospital ship.

Looking towards the invasion coast, long lines of pier roadways straddle calm sea inside the two-miles-long harbour formed by sinking the concrete caissons, in who’s making vast quantities of rubble from blitzed sites of London and other Cities were used. This sheltered water played, a tremendous part in enabling the Allies to achieve so successfully and swiftly "what Philip of Spain failed to do, what Napoleon tried and failed to do, what Hitler never had the courage to do."

Final erection of the second harbour, for American use, was discontinued, but not before many vehicles and much equipment had been landed (top). At the British harbour (bottom), in the construction of which the soldiers were sailors and sailors were soldiers and Royal Marines were both, casualties were being evacuated whilst Army vehicles were being discharged at the end of one of the floating piers, so devised as to rise and fall with the 20-foot tide.

There are 2 caissons left in Portland Harbour near Weymouth, Dorset. They are part of thye D-Day Museum.MukberryHarbour-1_050507.jpgMulberryHarbour_060717.jpg

Seeing those boats along the side of the caissons gives a better picture of the actual size of these monsters ..

A picture showing the mulberry harbour being constructed in England before it was towed over the sea to Normandy.

This picture shows the wreckage of the mulberry harbour at Omaha beach after a large storm on 19th June 1944, leaving only the British harbour which came to be known as Port Winston at Arromanches.

A picture showing part of a section of the floating roadway being towed over the sea to Normandy.

Section of a "Mulberry" port in the making in England: 6,000-ton concrete caissons, two of 150 which went to the construction of the famous prefabricated D-Day harbours (code word Mulberry) towed over to Normandy, as seen while nearing completion in a British dockyard, by the official Admiralty artist, Sir Muirhead Bone. His drawing shows workmen engaged on the huge structures mounted on their floating bases "and lying in a shallow dock where bulldozers (centre foreground) and a "grab" (right) are dredging the river-bed so that the caissons may float out in preparation for the astonishing Channel crossing. Some of the units of the harbour had to be moved to the South Coast from as far away as Scotland. Workmen in their hundreds swarm over the steel-tubing scaffolding; while from, the quayside gigantic cranes unload material into the caissons decks and under-decks. These caissons, towed across the Channel and then sunk in position off the enemy held shore of Arromanches, helped to form a great harbour, later known as “Port Winston,” whereby a preliminary force of 250,000 men was landed on Hitler's French seaboard, affecting a complete strategic and tactical surprise on the Germans. The whole vast organization of preparing, towing and placing in position was handled, under the direction of Admiral Sir Bertram Ramsay (killed in an air accident in France on January 2nd 1945) by Rear-Admiral W. G. Tennant On December 6th 1944, Mr A. V. Alexander, First Lord of the Admiralty, disclosed that the first person to suggest the use of prefabricated harbours for a European landing was Hughes-Hallett in the summer of 1943.

It was great ideas like this that helped the Allies in winning the war much sooner than anticipated.