75 Years Ago, a Brilliant Invention Decided When D-Day Would Occur
06 June 2019
Posted by: Thomas Wakelin
The calculation of Tides for D-Day was very important for Navigation both to and from the beaches and for getting Ships, troops and equipment onto the beaches and over the obstacles placed in their way. The calculations were carried out at the Liverpool Tidal Institute, by the brilliant Arthur Doodson who was using one of Lord Kelvin's Tidal prediction machines.
In October 1943, the oceanographer Arthur Doodson, head of the Liverpool Tidal Institute, got a letter from his friend Commander Ian Farquharson. For months, Doodson and the machine he’d perfected had provided the British Navy with key tidal information to launch their attacks on Nazi-occupied France, but this time Farquharson’s letter was far more cryptic: It listed no specific latitude or longitude, and it referred to a new location he called “Position Z.”
“The place is nameless and the constants inferred,” he wrote to Doodson. “There is in fact, very little data for it, I am gambling on the inferred shallow water constants giving something like the right answer.”
Position Z was the northern coast of Normandy, where Farquharson had surreptitiously been sending two-manned submarines to take water measurements in the hopes they’d help predict the tides at potential landing sites for D-Day. Since it is the 75th anniversary of D-Day and France is still France, we know that Doodson and his machine were able to calculate “something like the right answer” using Farquharson’s measurements. Doing so, however, was an extremely difficult feat.
What Doodson had perfected was a tide-predicting machine, an extremely complicated invention that, as it was developed over the years, took into account an immense amount of data on tides. Fortunately, Doodson was an internationally respected genius when it came to doing this tidal math. Accounts from the Biographical Memoirs of Fellows of the Royal Society describe his ability to do all this calculating in his head. For the tidal calculations at “Position Z,” however, even the smallest error would have resulted in the allied forces plunging headlong into a series of underwater booby traps laid out by German forces. And so, Doodson created two machines to mimic what he did in his mind.
The two machines he kept in separate rooms in his Liverpool observatory included the simpler Kelvin 10-component tide-predicting machine and the 1.2-ton Roberts Lége Universal model, which summed up all of the components using over 30 different pulleys and wheels.
A tide-predicting machine on display in Japan. This machine was modeled after the one used in Liverpool to predict the tides at Normandy.