The secret drawings by Welsh engineer Hugh Iorys Hughes will be sold at Bonhams in New York on the day before D-Day next month.
The Mulberry harbours, one of which still sits on the beach at Arromanches and can be seen a low tide, played a crucial part in the D-Day landings 70 years ago as they enabled the Allies to land heavy vehicles such as tanks.
Hughes first proposed the idea of a floating harbour to the War Office in 1941 and his brother who was a Royal Navy Commander told senior staff about the idea who took it to Winston Churchill.
In May 1942, some two years before the D-Day landings, Churchill ordered that the idea should be brought to fruition and Hughes was chosen to head the project. The plans going on sale were drawn up between June and August 1942, and are the basis of what eventually became the Mulberry harbours used for the invasion of France.
The documents are Hughes’ own 1:44 scale copies which he kept and in them he described them as ‘proposed landing jetties’. They were tested under great secrecy near Hughes’ home town of Conwy in North Wales and almost a thousand men working on the project were unaware of its true purpose.
The finished prototypes were subsequently towed to another secret test site in south west Scotland and Hughes’ ideas were integrated with prototypes developed by two other teams, including Scottish engineer Robert Lochner.
Bonhams said that the sale features 350 lots, including important manuscripts, famous photographs, crucial wartime technology, authentic weaponry, personal mementos and battlefield souvenirs. Taken together, the offerings paint a vivid picture of the 20th century war that forever changed the course of human history.
Other highlights include a German Army Enigma enciphering machine and several lots of memorabilia related to the British flying ace Douglas Bader. Other D-Day related items include a flag flown from an American LST-493, one of a handful of surviving flags flown from D-Day ships and a series of the hourly bulletins of D-Day action from The Dow Jones Hourly News Digest.
‘Every object in the June 5 WWII sale has an incredible story to tell,’ said Tom Lamb, Bonhams’ business development director for the books and manuscripts department in New York.
‘They bear witness to the war’s unparalleled turmoil, and testify to the bravery of many individuals as well as entire nations. Presenting the sale on the eve of the 70th anniversary of D-Day is a way of honouring this momentous day and recognizing the tragic losses on all sides,’ he added.