When hostilities commenced on September 3, there were seventeen ducks and one Type VII oceangoing boat, U-36, deployed in the North Sea for offensive and defensive operations. After the surrender of Poland, three ducks basing at Memel, and others in the eastern Baltic Sea, redeployed to the North Sea, so that in all, twenty-three of the thirty commissioned ducks patrolled the North Sea in the first month of the war.
Five ducks were assigned to lay minefields in four British ports and one French port. When Hitler barred attacks on French shipping, the mining mission to France was canceled. Four ducks laid a total of thirty-six TMB magnetic mines in four ports on the British east coast and in the English Channel. Two of the fields (Dover and Hartlepool) failed to produce. The field at Ordfordness, planted by Karl Daublebsky von Eichhain in U-13, produced handsomely: three ships for 27,203 tons sunk. The field at Flamborough Head, planted by Heinz Buchholz in U-15, sank two medium freighters for 4,274 tons.
The other ducks and the Type VII U-36 patrolled offensively and defensively in every nook and cranny of the confined waters of the North Sea. Two ducks, U-9 and U-19, hunted British warships off northeast Scotland. Several ducks reconnoitered the east end of the English Channel. One duck, U-14, patrolled the British naval base at Scapa Flow. Another, U-20, patrolled all the way north to Narvik, Norway, above the Arctic Circle. Others cruised the Kattegat and Skagerrak.
Owing to fuel limitations, the ducks could not stay out for more than about eighteen days. Many patrols, however, were shorter. Some ducks sailed from Wilhelmshaven and returned directly there. Others went on to Kiel via the Kiel Canal. Still others sailed from Kiel to the North Sea via the Kattegat and Skagerrak and returned to Kiel via that route. Most ducks required a week or more of repairs in the shipyards between patrols. In the interval, the crews were granted liberty and leave. Only a few ducks were able to mount two patrols in September.
The North Sea was aswarm with commercial traffic: British ships, neutral ships of Norway, Denmark, Sweden, Finland, Estonia, Latvia, Lithuania, and other nations. Many neutrals were transporting “contraband” (ore, chemicals, food, weapons, timber, etc.) to the British Isles. But the ducks were too small, too slow (13 knots maximum speed), and underarmed to stop and “visit and search” shipping in conformance with the Submarine Protocol. Moreover, for political reasons, Hitler had decreed that clearly marked neutrals in the North Sea were not to be stopped. As a result, ducks were compelled to limit attacks to targets they were permitted to sink “on sight” by surprise attack: British warships and submarines and British shipping unequivocally under escort.
On the rare occasions when permissible targets appeared, the duck skippers attacked boldly. Both Fritz Frauenheim in U-21 and Udo Behrens in U-24 shot at British destroyers off northeast Scotland. Frauenheim claimed—and was credited with—a kill, but neither attack was successful. Horst Wellner in U-14 and Werner Winter in U-22 both were wrongly credited with sinking Polish submarines. They subsequently attacked British submarines and both were again wrongly credited with kills. No duck sank—or even damaged—a British warship in September.
A few skippers found escorted or unescorted British merchant ships. Otto Kretschmer in U-23 fired a full-bow salvo of three torpedoes and one reload at an old 2,000-ton coaster under escort, but all four torpedoes malfunctioned or missed. Wilhelm Frolich in the Type VII U-36 stopped, searched, and then sank the unescorted 1,000-ton British coaster Truro, after the crew abandoned ship. Werner Heidel in U-7 also stopped and sank an unescorted British freighter, the 2,700-ton Akenside. These were the only two ships sunk by all the U-boats in the North Sea to September 22.
The vast flow of contraband in neutral ships in the North Sea infuriated Raeder and Dönitz. On September 23, Raeder met with Hitler in Danzig to urge relaxations in the U-boat rules, which not only would free the ducks to stop (i.e., attack) neutrals in the North Sea but would also reduce the risk and increase opportunities for the next wave of Atlantic boats. Having conquered Poland and despairing of persuading France and Great Britain to back out of the war, Hitler was then drawing plans to turn about and attack westward into France. He foresaw an early collapse of France. After that, he believed, Great Britain would seek peace.
A relaxation in the U-boat rules therefore fit Hitler’s plan to maintain pressure on the West. He approved tightening the screws on neutral traffic in the North Sea and other measures favoring the U-boats, which the OKM passed to Dönitz on the following day, September 24. These were:
• Restrictions on French shipping, including the mining of French ports, were lifted. “French ships are to be treated in the same way as British ships.”
• U-boats were authorized to use “armed force” against any Allied merchant ship which broadcast the submarine alarm, SSS. Such ships, without exception, were subject to “seizure and sinking.” However, “rescue of crews is to be attempted.”
• Allied “passenger” ships carrying 120 people or less (and hence presumed to be mainly cargo ships) could be sunk in accordance with the Submarine Protocol or prize rules. However, under no circumstances were “passenger” ships carrying more than 120 people to be sunk.*
The U-boat force cheered the relaxation of these rules. The authorization to sink French ships greatly reduced risks, opened the way for night surface attacks on Allied convoys, and increased the number of legal merchant-ship targets by about 500 ships of about 2.7 million gross tons. Inasmuch as all British ships were under orders to broadcast SSS upon sighting a submarine, the authorization to sink Allied ships making such a broadcast also greatly reduced risks by eliminating the requirement to search such ships before sinking them.
The relaxations were not all that was desired. Even though Allied ships broadcasting SSS could now be sunk without search, U-boat skippers still had to at least attempt to rescue the crews, a risky process at best. The prohibition against sinking a passenger ship carrying more than 120 people imposed on the skippers the difficult task of judging (by counting lifeboats, etc.) how many people might be on board. A request to sink on sight all armed ships and all blacked-out ships sailing close to the British Isles had not been approved.
Upon receiving the relaxed rules for the North Sea, the ducks fell upon neutral shipping in those waters with boldness and vigor. Heidel in U-7 sank two Norwegians. Harro von Klot-Heydenfeldt in U-4 sank two Finns and a Swede. Hannes Weingartner in U-16 sank a Swede. Joachim Schepke in U-3 sank a Dane and a Swede. Frolich, in the Type VII U-36, sank one Swede and captured another, which he escorted to Germany, concluding a month-long patrol. In all, the North Sea U-boats sank or captured ten neutrals for 13,000 tons in one week.†
The hue and cry from Oslo, Stockholm, Copenhagen, and Helsinki was so deafening that Hitler was compelled to back down. On September 30 he reimposed restrictions on the ducks which brought the attack on neutrals in the North Sea to a halt. Thereafter the ducks were restricted to offensive patrolling or minelaying in British waters.
British submarines patrolling in the North Sea hunted U-boats. Seahorse fired three torpedoes at U-36 and claimed a kill, but it was not so. Another British sub-marine (unidentified) shot at Otto Kretschmer in U-23 in the Kattegat but also missed. Several other ducks had close encounters with British submarines, but all evaded the attacks.
One duck was lost. She was U-12, commanded by Dietrich von der Ropp, age thirty. On September 22 Dönitz sent her into the English Channel to attack British troopships. On the basis of the earlier reconnaissance by ducks, Dönitz did not believe the British had mined the channel. But he was wrong. Between September 11 and 17, the British and French planted 3,000 floating (or anchored) mines on a line between Dover and Cape Gris-Nez. On September 25 they commenced augmenting the barrier with another 3,636 mines, which were controlled electrically from shore stations and monitored with banks of underwater hydrophones. On October 8 U-12 hit a mine and blew up with the loss of all hands. She was the first U-boat lost from which there were no survivors. Still disbelieving the Allies had planted a minefield in the channel, Dönitz incorrectly attributed her loss to ASW surface forces.
Had it not been for the two productive minefields planted by U-13 and U-15, the returns from the North Sea ducks and the Type VII U-36 would have been deeply disappointing. The twenty-three ducks (some making two patrols) and U-36 sank by torpedo or demolitions, or captured, only thirteen small ships for 16,751 tons: two British and eleven neutrals. However, the two minefields added five sinkings for 15,575 tons, raising the total kills or captures by all ducks and U-36 in the North Sea to a not inconsequential eighteen ships for 32,326 tons. But the greatest value of the North Sea patrolling was not in numbers. It provoked terror and convoying, and it was a perfect training ground for U-boat officers and men. Almost all the duck skippers—Kretschmer, Schepke, others—would soon “graduate” to Atlantic boats.