The tide predictions for D-Day were (a) vital to the operation’s success, and (b) performed on Victorian-era brass analog computers:
As an Allied cross-channel invasion loomed in 1944, Rommel, convinced that it would come at high tide, installed millions of steel, cement, and wooden obstacles on the possible invasion beaches, positioned so they would be under water by midtide. But the Allies first observed Rommel’s obstacles from the air in mid-February 1944. “Thereafter they seemed to grow like mushrooms . . . until by May there was an obstacle on every two or three yards of front.”
The obstacles came in a variety of shapes and sizes. In figure 1 we see rows of half-buried logs pointed upward at a low angle, some with explosive mines on them. There were also so-called hedgehogs, each consisting of three 2-meter iron bars crossed at right angles, and “Belgian gates,” 2- by 3-meter steel frames planted upright.
The Allies would certainly have liked to land at high tide, as Rommel expected, so their troops would have less beach to cross under fire. But the underwater obstacles changed that. The Allied planners now decided that initial landings must be soon after low tide so that demolition teams could blow up enough obstacles to open corridors through which the following landing craft could navigate to the beach. The tide also had to be rising, because the landing craft had to unload troops and then depart without danger of being stranded by a receding tide.
There were also nontidal constraints. For secrecy, Allied forces had to cross the English Channel in darkness. But naval artillery needed about an hour of daylight to bombard the coast before the landings. Therefore, low tide had to coincide with first light, with the landings to begin one hour after. Airborne drops had to take place the night before, because the paratroopers had to land in darkness. But they also needed to see their targets, so there had to be a late-rising Moon. Only three days in June 1944 met all those requirements for “D-Day,” the invasion date: 5, 6, and 7 June.
A 6-meter tidal range meant that water would rise at a rate of at least a meter per hour—perhaps even faster due to shallow-water effects. The times of low water and the speed of the tidal rise had to be known rather precisely, or there might not be enough time for the demolition teams to blow up a sufficient number of beach obstacles. Also, the low-water times were different at each of the five landing beaches (from west to east, they were code-named Utah, Omaha, Gold, Juno, and Sword). Between Utah and Sword, separated by about 100 km, the difference was more than an hour. So H-Hour, the landing time on each beach, would have to be staggered according to the tide predictions. Tidal currents, the along-shore flow due to the changing tide, were another important consideration. Strong tidal currents could easily push amphibious craft down the beach, away from their intended landing spots. But tidal currents were much harder to predict than the tides themselves, because they were much harder to measure.
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 (shown in figure 2), and the Roberts-designed machine, built in 1906.
These were interesting machines; I understand that some of them were in use until the 1970s!
Actually there were two separate machines used for tidal prediction: the first one resolved historical tide information into its components of different frequencies; the second synthesized the predictions going forward.
Absolutely fascinating. Thanks for the great post.