It was a prearranged plan on the part of the three U-boats operating in the English and Bristol Channels to communicate with each other by wireless at noon and at midnight. U75's wireless had a range of about 180 miles, and although it could be "jammed", the call could not be tapped by vessels other than the one for which it was intended. To make doubly sure, the messages were sent in code.
For nearly ten minutes U75 "made her number" without eliciting any reply. Perhaps it was well that Kapitan Schwalbe did not know what had happened to her consorts. U74 was at that moment lying on her side at the bottom of a Welsh harbour, her crew poisoned by the chlorine fumes from her batteries—the result of a rash curiosity on the part of her Lieutenant-Commander to investigate the approaches to the anchorage. As for U77, she was flying blindly for safety, with a couple of destroyers hard on her track, and a naval sea-plane overhead to direct them in their search.
Foiled in her efforts to get in touch with her consorts, U75 remained awash. The heave of the sea made it most difficult for her to use her periscope with certainty, for she had chosen a bad pitch on her ascent—the furious "overfalls" or "tide-rips" to the west of Lundy Island.
"We'll pay another visit to St. Mena's Island, Herr Rix," decided Kapitan Schwalbe, after the two officers had discussed the sinister matter of their futile attempt to make use of the wireless. "To-night at nine o'clock ought to suit. If we cannot get von Ruhle to see our signals—for my own part, I doubt whether he is in these parts—we'll have to do our best to get ashore. Meanwhile, keep a bright look-out. If we see any likely vessel coming this way, we'll try our luck once more."
"Message just received, mein herr," announced the wireless operator.
"From whom?" enquired Kapitan Schwalbe eagerly. He was devoutly hoping that either U74 or U77 had been able to "call up".
"I cannot say, sir," replied the man as he handed a code message to his superior.
Decoded, the "wireless" was as follows:
"Station 41 to unterseebooten. Two hundred gallons of fuel available here. Will be on the look-out for signals at 1 a.m."
The message was a "general call" for a secret petrol depot to any German submarine operating in the vicinity. Reference to the list of stations showed that "41" was at Port Treherne, a remote cove on the North Cornish coast about fifty miles from St. Mena's Island.