Name: The Floating Harbors of D-Day
Uploaded: 2021-06-02T02:00:25.000Z
Duration: 24 min 56 s
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When learning the history of D-Day, we are most frequently met with tales of valor of

soldiers rushing the beaches. Our collective vision of that day opens with the lowering

of Higgins boat ramps on Omaha beach. The fury and violence of battle taking centre

stage. In this chaotic vision, it's hard to believe that what was happening was a carefully

choreographed plan. One that required hundreds of thousands of people. Labourers in factories,

merchant sailors transporting goods across treacherous oceans, engineers dreaming up

novel solutions to new problems, and of course the military strategists that conceived the

invasion plans. Each played their part in the lead up to D-Day.

By the end of July, just a little under 2 months after the first men came ashore, six

hundred and seventy five thousand personnel were ashore with one hundred and fifty thousand

vehicles, six hundred and ninety thousand tons of supplies and sixty nine thousand tonnes

of fuel. The planning and background support required to make this happen was immense.

Some of these men and supplies were air dropped into the fields of Normandy, but the vast

majority of these supplies were brought ashore by landing craft ranging in size from the

personnel landing craft like the iconic Higgins boat, all the way up to massive ships like

the LST, or Landing Ship Tank. These behemoths could carry 18 Sherman tanks or 33 trucks

These ships would prove invaluable in maintaining the supply chain in Normandy, however the

logistics of direct beach supply came with a host of potential and inevitable delays.

All of the landing craft, like the Higgins boat, needed shallow draughts to allow them

to get close enough to the water's edge to offload their cargo. This made them incredibly

unstable and in rough seas many capsized.

Some, like the LSTs, had ballast tanks that could be filled or emptied to alter their

draught. Filling them to increase their draught for the open ocean, increasing the ships stability,

and emptying them to decrease it and come close to shore.

Yet, even LSTs faced logistic problems. Arrive at low tide and you are racing the tide while

you unload your cargo into deeper and deeper water. Arrive at high tide and you are racing

the tide as it leaves you stranded on the beach until the next high tide arrives.

On top of all this, the vast majority of war material was coming from the United States

aboard Liberty ships. These ships needed deep water harbours to dock. Without a harbour

of this kind in France, Liberty ships were forced to dock in Britain and transfer their

goods to smaller ships like the LST. If these ships could simply dock in France it would

save a great deal of time and effort in this logistical supply chain.

Cherbourg was prioritised for this very reason. It's deep water harbour was a key strategic

target and the Allies choice of Normandy was largely based on its proximity to the Cotentin

Peninsula, a peninsula which could be cut off from reinforcements from Utah beach westward.

However, this campaign would take some time. The Allies needed a deep water port as soon

as possible. Without a steady stream of supplies and reinforcements, the small Allied beachhead

risked being forced back into the ocean.The Allies were faced with a difficult dilemma

and their solution is one of the most remarkable feats of the second world war.

Two floating harbours were constructed and towed across the English Channel to be placed

One for the British and one for the Americans, each with an equivalent capacity to the port

of Dover. This is the story of the Mulberry Harbours.

Harbours serve some key functions to allow ships to unload their cargo in safety. The

first role a harbour must fulfill is to act as a safe refuge for the ships. Protecting

them from the chaotic weather of the sea. To do this, harbours need large breakwaters

and other protection to halt waves in their tracks, leaving the inner harbour relatively

calm. Next, they must provide water deep enough

to allow ships to moor directly next to the pier and unload their cargo. Regardless of

tide levels. Two simple functions that often require an

immense engineering effort, dredging of the seafloor to increase the depth of water, construction

of large concrete structures and general planning often taking years to complete.

Yet the challenge the Allies faced was much greater than a typical harbour. This harbour

needed to be built in complete secrecy, floated across the English Channel and assembled in

the middle of a war zone. It was an insanely ambitious plan.

Before making any detailed plans, the engineers would need detailed surveys of the ocean floor.

This information was vital for the design stage.

Surveying the ocean floor would inform the designers where to locate deep water offloading

points for ships, how high breakwaters needed to be to rest on the ocean floor and sit above

the waves at all tide levels, and finally it gave the vital seafloor topology information

required to design a floating pier flexible enough to deal with the rough seas and lay

on the seafloor without buckling and breaking.

Taking soundings while Germans watched and listened from the shore was a job entrusted

They made a total of 6 trips on moonless nights between November 1943 and January 1944, setting

sail from Cowes on the Isle of Wight. Data was gathered in two locations. Here to the

east of Omaha beach, the designated location for the American harbour, dubbed Mulberry

Harbour A, and here to the west of gold beach, the designated location of the British harbour,

Typically, three small LCP landing craft were towed by gun boats across the channel. Once

they reached a point, about 48 kilometres off the coast, they slipped their tows and

began their stealthy approach to the shore. These LCPs were modified with underwater exhausts

to muffle the sound of the engine, tarps to allow the crew to work with light without

being seen from shore, and each boat was equipped with echo sounders to survey the ocean floor,

along with the necessary navigation equipment to match the soundings with coordinates on

This work provided depth charts, like this one, which gave the engineers the information

they needed to accurately plan the layout of the harbours.

The first components they needed to address were the breakwaters. These would provide

shelter to both the ships and floating piers contained within. The breakwaters took several

forms. The simplest being codenamed “corncob”.

The Allies had a huge inventory of aging ships which were no longer fit for use. Planners

sought ships that were at least 30 years old, like the first world war super dreadnought

HMS Centurion, a twenty five and a half thousand tonne ship, which had been previously used

as a remote control target ship for Naval aiming practice in the nineteen thirties.

It was scuttled as part of the breakwater for Mulberry A.

This was just one of 14 ships that formed what was termed a Gooseberries at Mulberry

Harbour A. One of these break waters were formed at each and every landing beach. Shielding

the landing beach from the worst of the waves, and allowing landing craft to unload their

The scuttled ships were also ballasted and sunk in a manner that ensured their superstructure

remained above the water line. Infact, the crews of these ships had no idea of their

fate as they set sail for Normandy. Their useful equipment was removed, two explosive

charges were laid and they were then informed of their destination, with no further information.

Upon arriving and anchoring off the beaches, a wrecking officer boarded their ship and

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