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  • of d-day series.

  • 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 codenamedcorncob”.

  • 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 theRochers 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

  • up for the CuriosityStream and Nebula bundle deal. Here you will get a year's access to

  • both platforms for just 14.79 a year. An insane deal that will get you access to this series,

  • ad free videos from your favourite YouTube creators, Nebula exclusives like Tom Scott's

  • Money, and access to CuriosityStream's award winning documentaries, like this one called

  • 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

  • want to see that, please sign up to the CuriositySteam and Nebula bundle deal.

  • As always, thanks for watching and thank you to all my patreon supporters. If you would

  • like to see more from me, the links to my twitter, instagram, discord server and subreddit

  • are below.

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B2 中高級 美國腔

D日的浮动港口(The Floating Harbors of D-Day)

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    joey joey 發佈於 2021 年 06 月 02 日
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