From World War II at Sea, by Craig L. Symonds (Oxford U. Press, 2018), Kindle pp. 251-255:
On December 9, 1941, the day Hitler unleashed the U-boats for use against American shipping, Dönitz asked OKW to release twelve of them for a campaign in American waters. The German high command allotted him only six, keeping the rest for service off Gibraltar, further annoying an already disgusted Dönitz. Moreover, one of the six boats developed an oil leak, so that in the end, only five of them departed in December to take up positions off the eastern coast of the United States. Dönitz also sent ten of the smaller Type VII boats, packed with extra fuel and supplies, to the waters off Nova Scotia, which was just within their operational range. Those fifteen boats represented a substantial portion of his entire U-boat flotilla.
Crossing the Atlantic in a surfaced U-boat was harrowing. Peter-Erich Cremer, skipper of the U-333, recalled that “the waves were as high as houses.” The boats pitched wildly, banging down on each successive wave with a jarring thump, often knocking crewmen off their feet. They also rolled side to side by as much as 120 degrees. When the seas became so violent as to threaten the safety of the boat, the captain could submerge into the relatively calm waters below the raging surface, but that reduced the boat’s speed to about five knots, which dramatically lengthened the transit time and used up precious fuel, food, and water supplies. Dönitz wanted all of the boats to begin simultaneous attacks on January 13, and running submerged for any length of time jeopardized meeting that deadline.
While the British and Americans squabbled, Operation Paukenschlag [Drumbeat] got under way, though not quite with the kind of devastating impact Dönitz had envisioned. Mainly this was because the five Type IX U-boats did not all manage to get into position by the target date of January 13. Hardegen’s U-123 sank the Panamanian tanker Norness off Long Island on the fourteenth, but the last of the five boats did not arrive at its assigned position off Cape Hatteras, North Carolina, until the eighteenth.
The Carolina capes constituted a critical choke point for American coastwise trade. In January 1942, 95 percent of the oil pumped from the Louisiana and Texas oil fields made its way to the Eastern Seaboard in tanker ships that necessarily had to pass around Cape Hatteras, where the shoals narrowed the shipping channel to a mere thirty miles. Eventually the United States would shift much of its domestic oil transport to rail cars and pipelines, but when Dönitz’s U-boats arrived off Hatteras on January 18, the shipping there was so abundant that upon surfacing, Hardegen was astonished to see “no fewer than twenty steamers, some with their lights on.” That night he sank four of them.
In accordance with Dönitz’s suggested protocols, the U-boats lay quietly on the bottom of the continental shelf during the daylight hours, surfacing at night to look for passing freighters, and especially tankers. Not only did the targeted ships proceed independently, but many, as Hardegen noted, still had their running lights on, making them irresistible targets. Even those ships proceeding blacked out were often starkly silhouetted against the lights that were still burning on shore, since most cities from Miami to New York did not enforce nighttime blackouts. German U-boat skippers, who had been at war for more than two years, were dumbfounded by such carelessness, and bemused by the sight of car headlights passing along the coastal roads. Peter Cremer, commanding the U-333, recalled that “through the night glasses we could distinguish equally the big hotels and the cheap dives, and read the flickering neon signs.” Peering into New York harbor through his binoculars, Hardegen jokingly told his crew that he could see dancers atop the Empire State Building. In such an environment, the U-boats, few as they were, had a field day. In the last two weeks of January, they sank twenty-three ships, thirteen of them tankers. Counting the ships sunk in Canadian waters by the smaller Type VIIs, the U-boats of Operation Paukenschlag dispatched forty-one Allied ships displacing 236,000 tons in just two weeks. The losses were shocking, all the more so in that many of them occurred within sight of the American coastline.