How the Mulberry Harbours kept D-Day afloat

In the summer of 1942, the Allies planned a daring raid. 6,000 men, mostly of the 2nd Canadian Infantry Division, would land at the port of Dieppe, capture it, gather intelligence and then destroy the dock, before heading back to British shores. But when the raid actually took place on the 19th of August 1942, almost nothing went to plan. The preliminary bombardment had not softened up the German defences as hoped and the brand new Churchill tanks got stranded in the shingle. Of the men who went ashore 3,500 thousand were killed, wounded or captured. It was a catastrophe.

However, the debacle at Dieppe inadvertently led to the creation of one of the Second World War's greatest unsung inventions, one that kept the Allies in the fight when they returned to French shores in 1944. This is the story of the Mulberry harbours, the invention that kept D-day afloat.

Before we get to that though, a reminder to subscribe to the Imperial War Museums YouTube channel, for more videos just like this every two weeks.

The lesson derived from the raid on Dieppe was that an attempt by the Allies to seize a defended port on the French coast would result in disaster. They knew they had to answer the question of how to supply an army which had landed on an open beach.

And it wasn't just the landings, when the Allies invaded France they would have to quickly break out from their beachhead to avoid being pushed back into the sea. The volume of supplies needed to do that was massive.

They were going to need thousands of tons per day of ammunition, food, medical requisites and of course replacements in men for those who'd been wounded or captured.

The Allies realized that if they couldn't capture a French port they would have to build their own, float it across the English channel and construct it on the French coast. A huge undertaking, but one that would help decide the Second World War. Those harbours were codenamed Mulberrys. But stepping back fora moment, what is it that's so important about ports in the first place?

The principal facility that a port provides or harbour provides is shelter from the sea for the ships within and in particular, with regards to the channel which can have rather unpredictable and quite extreme weather conditions, that's very important.

Crucially a port would also allow larger ships to unload their cargo in the deep water of the harbour rather than the slow process of transferring those items to smaller craft to land on the beach. So how did the allies fulfil those two requirements and build the Mulberry harbours?

Well, this is a composite aerial photograph of the second Mulberry harbour at Arromanches in France, let's see how it works.

The outer harbour had three constituent parts. First were the 'Bombardons', large floating steel sections which when chained together produced an outer breakwater about 1.5km long. Next, were these immense concrete casings
codenamed 'Phoenixes', which were then flooded to form part of the inner breakwater.

These had been sunk off the south coast of England in order to conceal them from Luftwaffe reconnaissance. Like the bird of mythology risen from the ashes, three days before D-day they emerged from the depths.

The final part were codenamed 'Corncobs'. These were old obsolete ships which were sunk alongside the 'Phoenixes' to create what was called a 'Gooseberry', basically the calm seas of the inner breakwater.

This method proved very effective and there are photographs that show the water both on the outside of the breakwater and inside.

And this ingenious design continued inside the harbour where supplies were actually unloaded. Again this came in three parts. First were the pier heads where large vessels would dock.

These pier heads had been cleverly designed because at each corner there was a leg these could be adjusted up and down independently of each other by means of a clever winching mechanism which made sure that the pier head would remain at a standard height above the sea within the harbour.

Next the supplies would move along 'Whale' roadways which connected the pier heads to the shore. Over a mile long, these roadways could flex and move with the waves making them ultra durable.

An example of a 'Whale' can be seen at Imperial War Museums Duxford and in fact that particular 'Whale' was so durable that after the war it served as a bridge in Pont-Farcy in northern France.

Finally, the 'Whales' rested upon floating pontoons which were codenamed 'Beetles'. These helped bear the weight of those enormous Sherman tanks, so vital to allied success. But did the Mulberrys work? Well, on the 6th of June 1944, after two years of planning, the Mulberrys were finally put to the test on D-day. Over 130 000 troops landed on the beaches of Normandy, establishing a beachhead and fighting towards their objectives further inland.

Archive Clip: "The army has seized the beaches and poured in but the tremendous problem of communication and supply still remained."

Following them were the constituent parts of the Mulberry harbours weighing over 600,000 tonnes. With the initial beachhead secured, it was a race against time to get these prefabricated ports up and running.

Archive Clip:  "At the earliest possible moment, the port construction units get busy. Here you see one of their surveyors at work making a final recce of the beach in readiness for Mulberry."

Mulberry A for the Americans was established at Omaha beach, while Mulberry B for the British and Canadians was built at Sword beach near Arromanches. Within days they were bringingin the much-needed supplies to keep the Allied armies moving, but then disaster struck.

The Mulberry harbours had only been operational for a matter of days when on the night of the 19th to 20th of June a severe storm struck
the coast.

Archive Clip: "A June gale, this was an enemy more deadly than the Germans. It blew all day, all night, all next day and the next night."

It was deemed that Mulberry A was too damaged to be repaired. Some of the components from Mulberry A were used to repair the damage done to Mulberry B

Archive Clip: "The harbour still lived and the men who had built it now set about the job of mending it."

Which continued to function until the November of 1944.

After the storm the Americans continued to land stores on the beach itself using DUKW and landing craft at the site of Mulberry A. The calm seas created by the outer breakwater made this far more effective than expected. Together the Mulberrys landed over 2m men, 0.5m vehicles and 4m tonness of supplies.

Although the allies captured Cherbourg and they captured smaller ports along the French coast. Until Antwerp was captured by the allies, they were still dependent upon the efficiency of the Mulberry in bringing supplies ashore.

Operation Overlord was a huge risk, particularly landing on an open beach. The Mulberrys were the insurance against that risk and luckily for the allies they paid off. On the 1st of September 1944, the 2nd Canadian Infantry Division once again entered Dieppe, this time as liberators.

Archive Clip: "The Canadians were again in Dieppe looking out across the beaches where so many of their gallant comrades fell in the rehearsal just two years before."

A service was held to honor those who had lost their lives during the raid in 1942. The lessons learned from that disaster created the Mulberrys and paved the way for Allied success in 1944.

The importance of the mulberries is perhaps best summed up by the nazi's former armaments minister Albert Speer. After the war, he said: "To construct our defences in two years we used 13 million cubic tonnes of concrete and one and a half million tons of steel. A fortnight after the landings by the enemy this costly effort was brought to nought by an idea of simple genius."