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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
with berths for up to 217 troops.
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
directly onto the beaches of Normandy.
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.
Let’s see how they did it.
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
to the 712th Survey Flotilla.
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,
dubbed Mulberry Harbour B.
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
the map.
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
wares in the relative calm.
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
detonated the charges with the men still aboard. There the ships sank and the crew members
remained at action stations. Providing valuable first aid, refueling and repair services for
the smaller boats under their protection.
The next component of the breakwaters, the Bombardons, were designed for deeper water.
The designers of the Mulberry harbours wanted breakwaters placed further from the coast,
which would provide some protection to deeper draught ships like Liberty ships, which would
unload their wares onto smaller ships to transport to the beaches.
These breakwaters underwent several design iterations, namely in the quest to reduce
the level of concrete and steel required to build them. Building a structure that lay
on the seafloor in water this deep was not feasible, so some innovative designs were
trialed.
One idea was a pneumatic breakwater. [1] This idea proposed the use of compressed air fed
through perforated pipes along the seafloor to create a wall of air bubbles that would
block the motion of waves. This sounds like an outlandish idea. How could air bubbles
possibly block the immense power of the sea?
Two surface currents are created with this method. One opposes the incident waves, the
other inner harbour facing current has no effect on the wave action.
This was a tried and tested method of creating a breakwater. [2] This is a video of the effect
in action. However, it takes an enormous amount of power to pump the volume of air required
against the pressure of water bearing down on the pipes. Estimates of the power required
for the two mulberry harbour pneumatic breakwaters were placed at an astonishing 3 million horsepower.
[3]
This being rather obviously unfeasible, left the Allies with one choice. A series of floating
breakwaters. Waves are, after all, surface phenomenons, and an adequately designed floating
breakwater can provide shelter once anchored sufficiently.
Initial designs for these floating breakwaters consisted of 60 metre long flexible rubber
hulled breakwaters. These flexible breakwaters however could transmit waves through to the
other side if not adequately stiff and were vulnerable to tearing, from gunfire, ship
collisions and waves.
Rigid floating breakwaters were needed. These cross shaped steel breakwaters were chosen,
as they minimized the quantity of steel required, even still each one contained 250 tons of
steel. The bottom 3 arms were porous to water, while the top was enclosed to allow it to
float. Each was 60 metres long and 7.6 metres in both width and depth.
Leaving large gaps between these floating breakwaters was not an option, as the waves
would simply pass through the gaps by diffraction and rebuild themselves inside the harbour
with most of their energy retained. During the design phase the engineers tested a range
of gaps, optimizing the area covered and the wave transmission and found that a 15 metre
Two rows of these breakwaters were moored 250 metres apart which provided a reduction
in wave height by 70% and reduction in wave energy by 90%. [Reference: Page 77 book] These
breakwaters were assembled about 2 kilometres offshore for both Mulberry Harbour A and B
in the 6 days after D-Day. Providing Liberty ships some protection as they offloaded cargo
to smaller ships capable of offloading directly onto the beaches.
The final line of defence was the phoenix Caissons. These provided the brunt of the
protection for the harbour. These were essentially boats made of concrete.
There were 6 different sizes, each tailored for the depth of water they were designed
to be sunk in. The largest being the A1 type, which was 18 metres high, 24 metres wide,
63 metres long [4] and weighed an astonishing 6 thousand tonnes.
These larger caissons even had an anti-aircraft gun attached to provide anti-aircraft cover
for the harbour, which was not needed in the end.
Each end of the phoenix caissons was upswept to allow for easier towing across the channel.
The lower level of the concrete boats consisted of a series of watertight compartments, which
were flooded to sink them.
In fact, once each caisson was complete the Allies sunk them offshore to hide them from
German planes, until the time came to refloat them and transport them across the channel.
147 of these phoenix caisson were manufactured on beaches, excavated baisons along the Thames,
and in actual dry docks around Britain. With a workforce of over 20,000 people taking 150
days to complete the massive breakwaters. The skill of these workers varied greatly
and as a result, the concrete was of varying quality.
Upon sinking many caissons broke their backs , but at this stage it mattered little. Their
purpose was to sit on the seafloor, but removal efforts would prove difficult and to this
day many of these massive concrete structures remain in Normandy.
The next portion of the Mulberry Harbour design, the piers, required vastly more engineer forethought.
They needed to function at high and low tide and every level of tide in between. That’s
a great deal of mechanical movement along the length of the pier and that’s not even
accounting for wave action, as the breakwaters couldn’t feasible stop all waves from getting
through. These structures needed to deal with these forces and manage to provide a steady
road way for trucks filled with supplies and tanks travelling across. It was a huge engineering
challenge.
The first structure needed was an offloading pier. This couldn’t be a floating platform.
It needed to be rigid and stable to accommodate the shift weight of trucks, loading cranes
and supplies.
So, each platform came with 4 legs which could be controlled independently of each other.
Each leg could be lowered to the seafloor, allowing the pier to lie evenly on the surface
even if the seafloor was uneven. The platform was capable of moving up and down these legs
at a rate of about 0.75 metres per minute, more than enough to quickly adjust to tide
levels. The platform could float, but it was typically kept a short distance above the
water to keep wave rocking to a minimum.
Around the 4 edges of these Spud Piers were attachment points for floating pontoons that
could expand the berthing space for ships like this modular slipway designed to allow
LSTs to offload their cargo directly from their front ramp. Each spud pier also had
eight bearing seatings to attach bridge spans to the pier. These bridge spans were the next
challenge. The motion of the sea would place rolling,
pitching and twisting action along the entire length of these bridges. If the bridge was
not capable of moving with the ocean, it would inevitably break apart.
The bridge sections were relatively stiff along their length thanks to their H beam
design. Ensuring the bridge spans could support the 25 tonne tanks that would be crossing
them.
The tapered H beam design, where the side girders narrow towards the ends, also allowed
the sections to accommodate a great deal of twisting motion, up 40 degrees along its length.
Essential for dealing with the rocking ocean.
The bridge sections were modular. Allowing additional length to be added by simply adding
additional sections. This was achieved through spherical bearings at each end that slotted
into seating in pontoons called Beetles.
The bearings were designed to allow 24 degrees of deflection between each section, allowing
bridge sections to pitch with the waves and accommodate up to 48 degrees of slope when
laying directly on the seabed. To ensure the bridge sections were not simply ripped apart,
wire cables were attached to the beetle pontoons and wrapped around a hole in the girders end
section.
These beetles, like the Phoenix Caissons, were primarily constructed with concrete,
to save precious steel. Only pontoons which would be laid over the tidal zone were constructed
from steel, and ones designated for rocky sections of tidal zones also came with adjustable
spud legs.
These flexible floating bridge sections connected the spud piers to the shore line and allowed
them to operate through the full tidal range without issue.
With all of these components designed, tested, manufactured and hidden from sight, the Allies
were ready. As D-Day approached, the various components were transported to these locations,
before sailing towards the area zebra, the entrance point to the shipping lanes the Allies
had cleared of mines just a day before D-Day.
Construction of the breakwaters began on D-Day itself. Mulberry Harbour A, the American Harbour
off the coast of Omaha beach, was the first to begin and finish construction. The Bombardons
anchored down on June 6th and the first Pheonix Caissons were in place by June 9th. All corncob
ships were scuttled by June 10th and the entire breakwater was finished on June 11th, just
5 days after D-Day. By June 18th 4 spud pier heads were completed and offloading cargo,
with two whale and beetle piers connecting them to the shore.
Mulberry Harbour B, the British harbour was slightly slower in construction. By June 18th
the eastern pier was completed with four Spud pier heads, while the centre pier and the
LST pier were partly completed. These structures were protected by a partially completed breakwater.
Only the 25 of 82 phoenix caissons were in place by June 18th, with no protection on
the eastern or western flanks of the harbour.
June 18th is a pivotal date, because on June 19th an unprecedented storm arrived in Normandy
and raged for 2 days.
Winds tore down from the North East through the Strait of Dover, with waves as high as
4.5 metres being reported. Far exceeding the maximum design expectation of 2.4 metre waves.
[7]
It is said that this was the worst storm the region had seen in 40 years.
The American Harbour, Mulberry A, was completely broken beyond repair. 21 of the 28 phoenix
caissons were destroyed. With 4 more being badly damaged. The whale and beetle piers
were smashed and twisted by the lashing waves. Seven of the thirteen blockships broke their
back while others sunk below waves as a result of the ground below their hulls being swept
away. Bombardons and ships alike had broken their moorings and drifted ashore, sometimes
crashing into the harbour itself and causing further damage.
Mulberry Harbour B, by some miracle, avoided the worst of the waves thanks to the natural
breakwater of the “Rochers du Calvados”, located directly in the path of the waves.
However, this did not save the Bombardon breakwaters located further to sea. These were broken
apart completely and many came ashore to the west of the harbour, while 6 of the 25 caissons
already in place were destroyed. Three of the four pier heads were also damaged, but
not beyond repair. Parts were scavenged from Mulberry A to not only repair Mulberry B,
but to strengthen it.
Mulberry B, soon to be dubbed Port Winston, took whale bridges from Mulberry A to construct
a second stores pier, an LST pier, and a shorter pier to allow Rhino barges to offload. By
the end of October fourty thousand vehicles and nearly a quarter of a million personnel
used this harbour to come ashore, while an average of seven thousand tons of supplies
were discharged everyday. An immense effort, far exceeding the original plans.
In fact, the harbour operated far beyond its original operational target date.
Breakwaters were reinforced with an additional 40 Phoenix caissons. This extra protection
allowed Port Winston to stay in continual operation through the autumn and winter until
it’s closure in December 1944, by which time the capture of Cherbourg, Dieppe, Ostend
and Antwerp made the use of the temporary harbour unnecessary.
Detractors of the harbours point out that the US forces managed just fine without their
Mulberry Harbour. Thanks to a greater than expected performance from the LST’s and
other landing craft capable of supplying directly onto the beach. Omaha beach was managing 10,000
tonnes of supply per day at its peak, while Port Winston achieved 6,750 tonnes per day
at its peak. This criticism ignores the need for landing craft in the Pacific war and upcoming
southern invasion of France, not to mention that redundancy and planning for the worst
is a vital part of war logistics.
With a foothold in Europe established, the Allies could begin streaming into the continent.
2 months of intense fighting in Normandy was fueled by these supply lines, inch by inch
the Allies scrapped forward, but by the end of July the tides turned.
The breakout arrived and the front line began to race ahead of established supply lines.
Operation Cobra drove south towards Brittany reaching it’s border by July 31st. By August
7th Brittany was overrun, by August 25th Paris was liberated and by September 15th nearly
the entirety of France was free, while the Allies pushed towards the German border. But
this rapid advance brought with it new problems. Supply lines from harbours to the front line
were being stretched, and they would be on the verge of breaking point until Antwerp
could be captured. The Allies had been successful in their campaign to cripple German supply
lines prior to their invasion, but this was the very infrastructure they now needed. Vital
train supply lines would take time to reconstruct and the Germans were readying themselves for
one final massive counter offensive. The battle of the bulge. This battle would test the Allied
supply lines to the limit and without one simple but pivotal innovation, the Allies
could have been stopped in their tracks. The Red Ball Express, which is the subject of
the final episode of the Logistics of D-Day series, available exclusively on Nebula.
This series has been a passion project of mine for the past 9 months and the episode
you watched today has featured tidbits from each of the 9 episodes. We have entire episodes
on why Normandy was chosen, which is also available here on YouTube. The seven others
are exclusive to Nebula and include episodes on the deception tactics to keep things like
the Mulberry Harbours secret, an episode detailing the methods the Allies used to clear a way
through the minefields of the English Channel and subsequently punching a hole in fortress
Europe, an episode each for the amphibious and airborne landing operations, one detailing
the close air support logistics where engineering battalions rushed ashore on D-Day to construct
new airfields, and another detailing it insane oil supply pipelines that were laid across
the english channel on D-Day.
This series, which is over 2 hours long in total, would not have been possible without
the funding and support of Curiosity Stream and Nebula,the steaming platform I created
with over one hundred of my fellow YouTube creators.
The best way to watch, and ensure that future projects like this can be funded, is by signing
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“Hidden Traces”, which explores the scars of D-Day still present in Normandy. Like those
massive concrete breakwaters, many of which remain in the shallow waters of the Seine
Bay to this day.
For now, we are taking a break from this series and will begin working on upgrading our animation
abilities and researching for Season 2. We want to hear your suggestions for the next
battle we should focus on, but for now I am veering towards covering the Logistics of
the Battle of Britain. An aerial war that tested two nations ability to not only build
planes and train pilots, but tested their wartime planning and strategy. A decisive
battle that turned the tide of World War 2 and prevented the German’s from gaining
the air superiority needed to begin their own amphibious invasion of Britain. If you
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