Designing and Building the Mulberry Harbour

There are many worthy claims for whose idea the Mulberry Harbour was, Churchill proposed an seaborne invasion of the German islands of Borkum and Sylt using flat bottomed barges that would form wave barriers, and Operation Hush proposed flat bottomed invasion barges for an operation against Belgium.

In 1940 Lieutenant Colonel Wilson in the War Office remembered the Churchill idea and asked the engineer Guy Maunsell (of towers fame) whether it was feasible. In response he produced some outline sketches but nothing further was forthcoming.

The Welsh civil engineer Hugh Iorys Hughes proposed a solution for piers in 1941 that would allow ships to discharge vehicles onto a shoreline. In 1941 the British Army set up a small team called ‘Transportation 5’ or Tn5 for short to look at the problem of port repair, Brigadier Bruce White MBE, Royal Engineers, in charge.

Immediately upon appointment, he set about ensuring that this domestic remit was expanded to include possible future invasions of mainland Europe because he correctly surmised that utilising existing ports would be vital.

The Royal Navy officer John Hughes -Hallet was also credited with suggesting the idea after the Dieppe raid but by then, the other various ideas were already in their early stages.

Brigadier White also realised that in order to repair damaged ports an accurate survey would be required and due to time constraints, as much information prior to the on-site survey would be needed. Making use of a loaned office in the Institute of Civil Engineers he assembled a team of translators, engineers and industrial experts that would create a volume on every single port likely to be used in the forthcoming invasion.

What is certain is that over a period of time, these different strands and collection of engineers would eventually coalesce in one form or another to produce Mulberry although it is arguable that Guy Maunsell’s contribution was not fully recognised.

The US declared war on Germany on December 11 1941 and Russia was impatient for a second front against Germany but with painful memories of wasteful attrition warfare and failed amphibious operations from WWI, the UK was reluctant to launch ill conceived raids without thorough preparation, adequate logistics and overwhelming firepower.

Combined Operations HQ had since the mid thirties been researching amphibious raiding and under Admiral Sir Roger Keyes launched a number of small scale raids to ‘take the fight to the enemy’.

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The experiential base was being built in anticipation of future larger scale operations.

At the end of 1941 Commodore Lord Louis Mountbatten was placed in charge of Combined Operations.

Early Work

Initial experiments focussed on means of connecting ships to the shore such that they could unload vehicles, wave attenuation would come later.

Mountbatten was an advocate of the artificial harbour concept that had been bouncing around Combined Operations HQ that would allow the allies to attack where the Germans were weak but it was not a universally accepted position.

Work had continued in the UK through 1942 with various concepts and a progress update for Churchill prompted his famously impatient ‘Piers for use on Beaches’ memo.

Piers for use on Beaches

It is often thought that this memo from Churchill was the start of the process but it was not, conceptual work was already well underway but somewhat rudderless.

Churchill provided the vital impetus.

It was after this that the War Office Tn5 group was given a broader scope that included the development of piers for use on beaches, rather than simply means of repairing and expanding additional ports although as described above.

Although, it must be noted, Brigadier White was already doing that in any case.

Tn5 cast the net wide and set out to investigate three concepts, testing each before making decisions on moving forward.

Each of the concepts for connecting ships to shore for the purpose of rapid unloading of vehicles and stores; one each from the Admiralty, Tn5 and Hugh Iorys Hughes.

Before meaningful testing could be carried out, a suitable location was needed.

Garlieston in Scotland was chosen after an exhaustive search and survey process because it matched the likely Normandy beach gradients (1 in 200), tidal range (30ft) and soil conditions, was sparsely populated and located away from major cities.

Two beaches were used for the tests, Cairnhead and Rigg Bay.

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There are a couple of interesting stories about Garlieston and the Mulberry harbour.

First was the requirement for military personnel to act as ‘test subjects, essentially, spare bodies to play the role of invading soldiers and help with installation. Units were requested to send their best men but many apparently took this to mean ‘troublemakers you want to get rid of’. Stationed at Glasserton House the locals nicknamed them ‘goons’ because of their generally rowdy behaviour, behaviour that included petty theft and nearly burning the house down. After the great storm though, the ‘goons’ would be instrumental in returning Mulberry B to operation.

The other story involves the efforts of an MI5 agent posing as a tramp who tried repeatedly to gain information about what was going on from the local residents and military personnel, none was forthcoming, typical of the time.

The disastrous August 1942 raid on Dieppe was a test of tactics and equipment at scale.

The failure of this raid should be viewed in the context of it providing answers to a number of key questions those planning the main invasion had. It also emboldened Hitler’s position on the effectiveness of static defences as defined by the ‘Atlantic Wall’ against the opinions of his Generals who thought investing resources in a mobile reserve and counter attacking force would be the better option.

After building prototypes, testing started in Garlieston.

First, was the Admiralty design called Swiss Roll that used a flexible roadway constructed of timber and canvas, secured using steel cables.

Swiss Roll

The tests showed that although the concept was sound, it could not support anything heavier than a lightly loaded small truck

It was discounted from further development although actually used very briefly for personnel offload during D Day at Mulberry B (file that one under ‘not a lot of people know that’)

The ‘Hughes Pier’ used a series of concrete caissons called Hippos that would sit on the sea bed with rigid steel roadways attached, called Crocodiles.

Hippo and Crocodile - Mulberry Harbour

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Hipo and Crocodile Drawing 08[/tab] [tab title=”Hughes 2″]

Hipo and Crocodile Drawing 07[/tab] [tab title=”Hughes 3″]

Hipo and Crocodile Drawing 06[/tab]

Hipo and Crocodile Drawing 05[tab title=”Hughes 4″]

Hipo and Crocodile Drawing 04[/tab] [tab title=”Hughes 5″]

Hipo and Crocodile Drawing 03[/tab] [tab title=”Hughes 6″]

Hipo and Crocodile Drawing 02[/tab] [tab title=”Hughes 7″]

Hipo and Crocodile Drawing 01[/tab] [/tabs]

Tests revealed that the caissons would not form a sufficiently stable platform for the roadways, which suffered severe bending and distortion.

It too was eventually discounted.

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The Design Matures

At a dinner at Chequers, Bruce White explained to Churchill how he could help with his ‘piers for use on beaches’ problem, recalling the particulars of a storm at Valparaiso harbour in 1924 during which a ship belonging to the same firm he worked for had survived when no other ship had simply because it was equipped with spud legs.

The ships master had seen the damage that the storm waters were doing to other ships so simply raised his above the waves on its spud legs.

The ship was the rock cutting dredger, the Derocheuse, built in 1888 by Lobnitz and Co in Renfrewshire.

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Rock cutting dredge, DEROCHEUSE built by Lobnitz of Renfrew as used in the Suez Canal
Rock cutting dredge, DEROCHEUSE built by Lobnitz of Renfrew as used in the Suez Canal
[/tab] [tab title=”Derocheuse drawing”]

Derocheuse[/tab] [/tabs]

The idea of using spud legs and a floating roadway to shore evolved over several months with the Lobnitz company of Renfrew playing a key role in the design of the pier head, with Major Allan Beckitt RE and WT Everall RE working on the roadway.

The pierhead was based on a Lobnitz dipper dredger called the AB95 Lucayan. The Lucayan used the same type of electrically driven spud legs as the Derocheuse, built in 1923.

It was not scrapped until 1980.

AB Lucayan

The pierhead was to be a relatively simple design, steel construction, 200 foot long by 60 foot wide with each corner having a 90 foot spud leg that could be raised and lowered by electric motors, thus raising or lowering the platform.

A pair of diesel generators provided power for the motors.

MOD 951

Because it rested on its spud legs, it would provide a stable platform for ships to unload onto to, in effect, a quayside.

Although it could be raised clear of the waterline and waves the normal mode of operation would see it partially submerged, water providing support, reducing the load to be transmitted to the seabed by the legs.

Once a ship had unloaded its vehicles or stores at the pier head they would need to be transferred to shore by some form of flexible roadway or bridge span.

Mulberry Harbour Whale 02

The Tn5 concept, neatly summed up in the diagram above, used floating roadways attached to pier head platform that would rise with the tide and be supported either by buoyant pontoons when floating or the beach when not. Because the pier head and beach would be fixed in position the total length of the causeway would change between the high and low water mark.

The solution proposed was a telescoping span at the pier head end.

Bruce White continued to build his organisation, recruiting many specialist consulting engineers and technicians; the technical and industrial foundations of Mulberry were being built, and built well.

One of the innovations was the use of two committees, a technical committee and a manufacturing committee, the former discussed and agreed the designs and proposed a range of solutions to the manufacturing committee, who examined them in terms of materials and labour requirements.

The best technical solution would be no good if it could not be built in war ravaged Great Britain.

If the pier head was relatively simple, the roadway was far from it.

They would be required to accommodate lateral as well as vertical movement in each span and as a whole. Torsional flexibility would also need to be considerable and this would require the use of complex spherical bearings developed by William Everall and Allan Beckett in conjunction with the Military Experimental Engineering Establishment (MEXE).

Even the tread pattern on the bridge decks was subject to much discussion and testing.

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Tn5 Design Bridge Span on Test 15[/tab] [tab title=”Roadway Testing 2″]

Tn5 Bridge Section[/tab] [tab title=”Roadway Drawing 1″]

Mulberry Harbour Whale 01[/tab] [/tabs]

Supporting bridge spans would be a series of floating pontoons, each required to support a total weight of 56 tons each, including a 25 ton single load, usually a tank.

The original concept used a design derived from a Thames barge and testing these initial concepts at Garlieston provided invaluable insight not only into their suitability and use but also their towing characteristics and general seaworthiness.

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Tn5 Design 06[/tab] [tab title=”Initial Testing 2″]

Tn5 Design 02[/tab] [tab title=”Initial Testing 3″]

Tn5 Design 04[/tab] [tab title=”Initial Testing 4″]

Tn5 Design Testing[/tab] [tab title=”Initial Testing 5″]

Tn5-Design-09[/tab] [tab title=”Initial Testing 6″]

Tn5-Design-10-1[/tab] [/tabs]

After continuous testing the designs evolved.

To allow multiple pier heads to be connected together so that more than one ship could be accommodated and a ‘one way’ configuration for vehicles an intermediate pontoon was designed that connected using one of the telescoping bridge spans.

A further addition allowed the unloading and loading of ramp equipped LST and LCT’s (x2) using a sloping deck pontoon called a buffer pontoon that was connected using a flexible hinge. Shock absorbing fenders and a ramp assembly would allow simultaneous unloading from the LST’s front and side ramps.

A collapsible mooring dolphin was designed and built but not used.

[tabs] [tab title=”Pierhead 1″]

Spud Pierhead 03[/tab] [tab title=”Pierhead Ramp”]

Spud Pierhead 02[/tab] [tab title=”Intermediate Pontoon”]

Intermediate Pontoon[/tab] [tab title=”LST Buffer Pontoon”]

Spud Pierhead 07[/tab] [tab title=”Connected Pierheads”]

Tn5 Design 14[/tab] [/tabs]

Because of a shortage of steel, many roadway pontoons were made using Ferro concrete, a technique pioneered by Messrs Wates at the Vickers Barrow in Furness shipyard.

Steel pontoons were still used and modified with simple spud legs for areas where rocks may be prominent. The telescopic span was also modified to include an ‘erection tank’ that allowed faster coupling to the pier head.

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Tn5 Design 07

[/tab] [tab title = “Beetle Design 1”]

Mulberry Harbour Beatle 02[/tab] [tab title = “Beetle Design 2”]

Mulberry Harbour Beatle 01[/tab] [tab title = “Erection Tank”]

Mulberry Harbour Whale 03[/tab] [/tabs]

During trials it was found that correct mooring of the, by now called, Whale and Beetle components was vital and a specially developed anchor designed by Allan Beckett was used, the now familiar kite anchor.

These anchors were placed and maintained by small low profile boats called shuttles or Surf Landing Under Girder (SLUG) boats.

The importance of SLUG boats was stressed many times, although there are scant references to them in much of the published history.

Wave attenuation, or the production of sheltered water, was a particularly difficult problem for the design team.

The committee worked on a number of options for the production of sheltered water, block ships, concrete caissons, air bubbles (called the Brasher System after the American engineer, Philip Brasher) and a partially submerged balloon called a ‘Lilo’, the latter concept being developed by the Admiralty.

The goal of ‘effective joint working’ was not yet resolved.

A minute of a committee meeting on the 18th August 1943 records;

It would appear that Lt. Commdr Steele’s main object in attended the Committee had been to acquire information for the Admiralty regarding the activities of this committee rather than for the purpose of sharing information with them

Work continued, the optimal design for the air bubble system used short lengths of pipe suspended from floats rather than anchored or fixed to the seabed. It was also found that having the perforations facing down produced the best effect. A great deal of experience and input came from the compressor company, Ingersoll Rand.

A letter to a member of the committee on 9th September 1943 records their confidence in one of their engineers that would be travelling to Garlieston;

As well as assisting with the layout of the individual compressors and main supply lines he will take charge of the complete compressor plant, supervise operation and maintenance during the period of your experiments. We feel sure Mr Ashby can relieve you of all all anxiety regarding the supply of air.

Blockships offered the advantage of being self propelled but they were no more effective than concrete caissons and in any event, would need tugs for placement. The concrete caissons would provide excellent wave attenuation but likely be difficult to tow. The air bubble system showed a great deal of promise but the amount of power needed to generate sufficient air volume was thought too considerable so it was dropped at the end of 1943.

The floating breakwater was originally of an inflatable design from Robert Lochner, hundreds of experiments confirmed it was capable of reducing waves if used with a suitable concrete anchor.

Despite its promise, vulnerability to enemy gunfire or accidental damage resulting in further experiments with rigid construction.

Mulberry Harbour - Flexible Breakwater

Lilo became Bombardon, the by now familiar cruciform steel shape.

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Bombardon 01[/tab] [tab title=”Bombardon 1″]

Bombardon 02[/tab] [tab title=”Bombardon Test”]

Mulberry Harbour - Bombardon under test[/tab] [/tabs]

The Admiralty thought the combination of block ships and Bombardon would be sufficient but the civil engineers on Tn5 thought otherwise, in fact, they were less convinced about Bombardon because of the significant quantity of steel required and block ships because of their unpredictability when sinking, and when sunk.

The Admiralty on the other hand, thought the civil engineers on Tn5 simply could not appreciate the difficulty in towing large concrete caissons and that none would survive the journey.

The concrete caissons went through a number of design revisions but fundamentally, they were large concrete hollow forms that would be towed into position and flooded. Resting on the seabed, their sheer mass would provide sheltered water for the pier heads. The final design was a joint UK/USA one that would displace 6,000 tons and 200f feet long by 50 feet wide. A shortage of materials meant compromises were made, reinforcement and wall thickness were often the subject of much discussion but the driving factor was their short intended lifespan and the simple fact that they had to be available to the required quantity in time for D Day, perfect was most definitely the enemy of good enough.

[tabs] [tab title=”Concrete Caisson 1″]

MOD 86[/tab] [tab title=”Concrete Caisson 2″]

MOD 581[/tab] [tab title=”Concrete Caisson 3″]

Phoenix Caisson 01[/tab] [tab title=”Concrete Caisson 4″]

Phoenix Caisson 04[/tab] [/tabs]

There were many arguments and disagreements between Tn5 and The Admiralty on the subject of providing sheltered water that would result in counterproductive delays and poor use of finite resources.

Ironically, it was Air Chief Marshal Sir Charles Porter that provided the view of risk v resource that helped settle the matter.

In the end, all three systems were used; concrete caissons (Phoenix), floating cruciform (Bombardon) and sunken block ships (Corncob).

Each system was used to their advantage, block ships for example, worked better in shallow water and were used for providing sheltered water for landing craft, the ‘Gooseberries’

93 Bombardons were made, consuming a total of 20,000 tons of steel. 147 Phoenix were required in 6 depth variations for D Day but as the Mulberry Harbours operation time was extended more were needed to double bank them, these latter types also had top covers to alleviate the problem of overtopping causing internal pressure to burst them.

Some of the PHOENIX caissons had anti-aircraft guns and barrage balloons, with the necessary crew quarters built into the structure.

At the end of August 1943 the Quebec conference was convened between the allies. During the voyage, Lord Mountbatten staged a makeshift demonstration of the effectiveness of breakwaters. Professor J.D. Bernal was one of Mountbatten’s scientific advisers and responsible for the lecture theatrics in one of the bathrooms. A fleet of paper ships were floated in of the large baths, providing the wave motion, using a broom, was Lt Cdr Grant.

Upon the command of ‘more waves’ the broom was vigorously pumped up and down in order to simulate the English Channel swell.

The paper ships were all lost.

The exercise was then repeated but this time, the replacement ships were protected with an inflated Mae West life preserver, simulating the breakwaters. Of course, despite efforts of Lt Cdr Grant, no ships were lost.

Everyone was thus convinced.

The complete concept was duly presented at the Quebec Conference in 1943.

Winston Churchill requested that Brigadier White and a small team join the conference and after flying over in a bomber, the discussions had a greater level of detail that originally planned.

Half a dozen meetings later, the decision was made to produce two complete Mulberry harbours, A (American) and B (British).

Mountbatten soon realised the enormity of the task would be beyond Combined Operations and so he reorganised the project with more involvement from the War Office.

After returning from Quebec, Brigadier White found a memo on his desk titled ‘Artificial Harbours’ describing the decision from the conference, he was horrified at the complete lack of operational security and immediately went to the War Office, requesting a meeting with the head of security to report the breach.

The ‘big book of codenames’ that was used for assigning operation names was consulted.

Next available from the list was Mulberry, and so, the Mulberry system of artificial harbours and landing piers was officially born.

Between the end of 1942 and end of 1943 most of the effort was design and testing, perfecting the final system to be produced.

Upon the conclusion of this phase, the Allies had agreed on the design of the four key elements of Mulberry;

  • The means of producing sheltered water (block ships,floating breakwaters and concrete caissons)
  • A pier head that allowed a number of different ship types to quickly offload
  • A pier that could accommodate the movement of wind, wave and tide
  • A connector from pier to beach (this was vital, but like the SLUG boats, not often heard about)

The objective offload for Mulberry A at Omaha Beach was 5,000 tons per day and Mulberry B at Arromanches, 7,000 tons per day.


Production started in December 1943, 6 months to D Day.

23 pier heads were ordered including 14 buffer and 24 intermediate pontoons. 10 miles of floating roadway (Whales) would need 120 eighty foot spans and 670 floating pontoons (Beetles).

Hundreds of manufacturers and tens of thousands of personnel were involved in the immense production engineering challenge and it was far from plain sailing.

Despite continued protests from Bruce White, the Admiralty continued to insist on Bombardon. Building Bombardon caused a number of unwanted secondary effects.

First, the use of dry docks for Bombardon meant they were unable to be utilised for Phoenix construction, the King George V dry dock in Southampton could be used to build 8 Phoenix in one go for example. This meant Phoenix were often constructed in unsuitable locations which caused a number of losses.

Second, the amount of steel needed meant an increasing number of Beetle pontoons had to be constructed on concrete, an unsatisfactory compromise.

[tabs] [tab title=”Beetle Build 1″]

Beetle assembly at Marchwood 01 [/tab] [tab title=”Beetle Build 2″]

Beetle construction at Wates Engineering[/tab] [tab title=”Bombardon Build”]

Bombardons under construction King George V Southampton[/tab] [tab title=”Whale Build 1″]

Whale spans at Marchwood under construction 02[/tab] [tab title=”Whale Build 2″]

Mulberry Whale spans at Marchwood[/tab] [tab title=”Whale Build 3″]

Whale spans at Marchwood under construction 01[/tab] [tab title=”Phoenix Build 1″]

phoenix[/tab] [tab title=”Phoenix Build 2″]

Phoenix caisson under construction 07[/tab] [tab title=”Phoenix Build 3″]

Phoenix[/tab] [tab title=”Phoenix Build 4″]

phoenix[/tab] [tab title=”Phoenix Build 5″]

phoenix[/tab] [/tabs]

Construction of the Mulberry Harbour components is worthy of a book on its own, it was an incredible achievement in any case, but even more so in light of it being a wartime project with limited manpower and materials undertaken in a country that had been at war for 3 years and to an almost impossible timescale.

The unsung hero of this phase was Sir Harold Augustus Wernher deploying his bombastic and single minded personality with great effect.

The post war record of Mulberry often fails to mention the critical contribution of Harold Wernher


Table of Contents


Designing and Building the Mulberry Harbour

D Day Plus


The Rhino Pontoon and DUKW


Post War

Summary and Final Thoughts

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Derek Henry Hooper
Derek Henry Hooper
October 21, 2020 5:44 pm

My father trained hundreds of U.S. and Canadian servicemen welding in order to speed the Mulberry construction programme.

Paul Bicknell
Paul Bicknell
December 22, 2020 8:28 pm

What were the cylindrical tanks used for on the side of some of the Pheonix caissons ie the ones with the guns fitted. Was it fresh water and fuel for generators for the gun crews.

My father aged 17 in 1944 worked on the river Thames in tug boats and saw some of the caissons being constructed and towed about but had no idea what they were for at the time.

Peter Hill
Peter Hill
March 20, 2021 7:59 pm

I recall seeing that the cylindrical “tanks” are actually concrete anchors. I can’t remember the need for deployment

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