The first Tide Predicting Machine was produced in 1873. By the 1914 outbreak of WWI they were being used widely for official tidal predictions and navigation. They proved so valuable during the war that subsequently the US classified their very existence.

Use of Tide Predicting Machines throughout World War II

During the summer of 1940 a German plan – Operation Sea Lion (Unternehmen Seelöwe) – for invasion of England from the sea was on the cards, and tidal predictions proved very important. The British believed the Germans would land at high tide to minimize the length of beach they would have to cross under fire. The admiralty consulted charts of the tides and moons to establish on which days conditions will be most favourable for a seaborne landing. They looked for occasions when high water would occur near dawn, with no moon, and selected dates in July and August near eight English ports – the information was used to help protect these locations. By September, having failed to destroy the Royal Air Force, Hitler cancelled the operation.

In November 1942, the Allies undertook their first amphibious landing, at Casablanca in French Morocco. They used tidal predictions produced by the US Coast and Geodetic Survey using the Harris Tide Predicting Machine. Over the next two years, that machine also provided tidal predictions for American amphibious landings on various Japanese-held islands in the Pacific.

The Admiralty banned the publication of all tables of tidal predictions that might help the Germans.

In Physics Today, September 2011, Dr Bruce Parker writes:

“All the Admiralty tide and tidal-current predictions for the war effort were produced by Arthur Thomas Doodson at the Liverpool Tidal Institute. The 53-year-old Doodson was at that time the world’s leading authority on tide prediction. He used two tide-predicting machines: the Kelvin machine, built in 1872 but overhauled in 1942, and the Roberts-designed machine, built in 1906.

The two machines were put in separate rooms at the observatory to minimize the chance of a bomb destroying both. That was a very real possibility. Worry heightened during one of the Nazi propaganda broadcasts by the infamous ‘Lord Haw Haw’ (the British traitor William Joyce). He promised that ‘by morning, Bidston Observatory will be no more.’ Many bombs did fall near the observatory. Hundreds of windows were shattered, and many doors were damaged, including the entrance to Doodson’s bunker. But both tide-predicting machines survived and were kept running from early morning to late at night, seven days a week.

As a precaution, predictions for all the ports in the Admiralty Tide Tables were completed two years ahead of time. But the machines were also required for many additional predictions for wherever around the world the Allies needed them. By 1943, because of the war effort’s demands for educated personnel, the technical staff at the Liverpool Tidal Institute had been reduced to just Doodson and six young women who carried out the thousands of tabulations and arithmetic computations required for tidal analysis. Their additional duties included night-time fire watch on the roof in tin helmets and trench coats and carrying buckets of water in case an incendiary bomb hit the observatory.”

Tide Predicting Machines were then critical to the success of the 1944 D-Day landings, the largest naval, air and land operation in history. They provided accurate predictions for five Normandy beaches used during the landings – some of which had a tidal range of six metres (which would expose significantly greater areas of sand to traverse under German fire at low tide than high).

An interesting wartime development in the history of Tide Predicting Machines was the development of portable versions. One such had been developed in Germany around 1935 – then during World War II, twenty of these were manufactured. The machines worked in a similar way to their large counterparts, but were only capable of representing ten harmonic constituents. The German navy used them to calculate water levels at sea in areas where no calculations were available.

After the war there were plans to manufacture machines for use by hydrographic offices and on survey vessels when approximate tidal predictions were required. These were to be copies of the German portable models. However, nothing came of this and later on, during the 1950s, Doodson designed hit own portable machine that was manufactured by the Légé company. It is not clear how useful any of these portable machines really were.