The first meeting of the Sealion Planning Group on 12th September 1939 might well have been the last. Vice-Admiral Marschall insisted that invading Britain after a successful French campaign was simply impossible because the British Navy would sink anything that tried cross the Channel. Geisler, for the Luftwaffe, admitted that he could not promise to sink more than a small fraction of the British Fleet. Marschall pointed out that they woudn't sink anything in the dark. Jodl attempted to intervene by suggesting that coastal artillery could protect a crossing of the Straits of Dover. Marschall turned to his second, Captain Hellmuth Heye, and asked him to explain “the problems”. Heye explained that it was unlikely that coastal artillery would hit enemy ships sailing through the Straits in daylight. He noted that the dispersion of a salvo from naval guns was generally at least 1.5 % of the range and thus that at the 40 km range required, shells would land somewhere in a circle with a diameter of nearly a kilometer. Worse they would take around 2 minutes to get there and it would be very hard to predict where the enemy ship would be as a cruiser or destroyer would sail around 2 kilometers while the shells were in flight. The only hope, in his view, was to lay minefields to protect the invasion fleet and even that would be very difficult.
“Difficult indeed”, noted Marschall. “Let's work out how many mines you will need,” he continued.
“We have to lay two barrages, one to block the Straits of Dover, which is around 40 km, and one from Cherbourg to Weymouth, which is around 150 km. If we had one mine every 50 metres, we would need 4,000 mines but of course we need many more than that or the first few enemy ships will clear a gap for the rest. In fact, even if we could lay the mines, which would be difficult, the enemy will be able to break through by streaming paravanes or simply sending minesweepers ahead of their force.”
“If we use the new magnetic mines, the British will not be able to gain anything by using paravanes and may not be able to sweep the mines. There is also the advantage that magnetic mines can stop enemy submarines,” argued Heye.
“Perhaps, they cannot sweep them now but they will surely have learnt how to sweep them by next summer,” replied Marschall.
“Only if we allow them to practise,” returned Heye. “We need to build up a very large stack of magnetic and, if they are ready, the acoustic mines, and start laying these mines in the week before the invasion. We must ensure that the British do not believe that we have magnetic mines until it is too late for them to develop ways of sweeping them.”
“Even ignoring the planned invasion, I agree that there are good reasons for waiting until we have and can lay very large numbers of magnetic mines before we use them. I am happy to recommend that delaying their use and also that their production is added to our list of priorities.”
“OK! I will report that to the Führer,” agreed Jodl.
“However, there are still several issues. Firstly, if we are to make the 40,000 mines that we seem to need, we need as much explosive as the Luftwaffe can drop in more than a month at a time when explosives are in very short supply. Secondly, how are we going to lay them? S-boats can each lay 6 mines per trip, so if we can build 100 S-boats, they will still only be able to lay 600 mines near Weymouth in a single night. A destroyers can lay 60 mines, so our destroyer force may be able over a thousand mines per night but like the S-boats, they are needed for other jobs. The destroyers cannot carry both mines and depth charges for example. Thus we will depend on converted merchant ships to lay most of our mines and we will need over a hundred to be converted into minelayers. Even then we will not be able to lay 40,000 mines and the minelayers will be very vulnerable.”
“We need to use the Luftwaffe to drive the British warships out of the area, we then use the KM to take control of the area for 24 hours, this allows the minelayers to establish barrages and finally the mines, the LW and the KM have to defeat the British fleet. If we cannot lay enough mines to make both barrages completely effective, we also have to devise a strategy to bring the main British force down their East Coast onto the shorter Dover barrage and make that barrage strong enough to stop them,” concluded Heye.
OTL the RN had received reports of German magnetic torpedo fuses and the Oslo Letter mentioned magnetic mines but few counter measures were started before an air dropped mine was examined in November.
