The Great North Sea Mine Barrage


When my father, Rear Adm. D. Pratt Mannix 3rd, died in 1957, he had served as a midshipman on a square-rigger and lived to see the atomic bomb dropped on Japan. Born in 1878, he had fought in eight wars, been awarded six medals, and had seen action against Moro pirates and the Imperial German Navy. He had watched the United States grow to be the most powerful country in the world. As the U.S. Navy was responsible for much of this growth, he had had an opportunity to see, firsthand, history being made. His most hazardous and most important duty came during the First World War when he was given command of a minelayer—one of the tiny fleet of ships with orders to lay a mine barrage across the North Sea from Scotland to Norway, thus “bottling in” Germany’s dreaded U-boats. This ambitious scheme had been first envisaged by the assistant Navy secretary, Franklin Delano Roosevelt, within weeks of America’s entry into the war in April 1917. When, at Roosevelt’s prodding, the technology had been devised to make possible mining on such a vast scale, the operation got under way in June 1918. Many experienced naval officers considered the whole idea madness. To the best of my knowledge, Father’s account of what happened is the only eyewitness record of this little-known episode in our history.

—Daniel P. Mannix 4th


In April 1917, when America entered the war, by far the greatest danger to Allied ships came from the German U-boats, the “stiletto of the seas” as they were called. Conventional naval tactics against them were useless, and they were sinking 800,000 tons of shipping a month. Since plans were made to send thousands of American troops to reinforce the reeling Allies in France, the submarines presented a special danger. Slow-moving transports, loaded with men, would have been the ideal target for the torpedoes of the deadly underwater ships.

At this time I was stationed at the Philadelphia Naval Yard, and I learned of highly secret plans to lay a minefield across the North Sea from Scotland to Norway, a distance approximately as far as from Washington to New York. It would be the greatest undertaking of its kind in history. The English Channel had already been successfully mined by the British, thus blocking it to U-boats, but the boats could still leave the German coast and swing north of the British Isles, attacking the shipping lanes between England and America.

To block this wide passage would require a minefield 250 miles long and 900 feet deep. This meant a total of 400,000 mines. There were not enough mines in the world for such a field and no chance of manufacturing such a vast number in the time available, yet already an announcement had been made in the House of Commons that Great Britain had only enough food for another month. A new type of mine was needed that could be used in deep water and did not actually have to be struck by a ship in order to explode. Such a mine had indeed been devised. It was attached to an anchor by a long copper cable, and if a submarine touched the cable, the mine exploded. A hundred thousand such mines would be sufficient to form an effective barrage.

The mines were to be loaded with a new and terrible explosive, far more deadly than dynamite, called TNT. Little was known about TNT and its effects. Both the British and the French had found the substance too dangerous to use, so there were few volunteers for a minelaying squadron. In any case, professional sailors disliked minelaying on principle; it was often referred to as “rat catching” and lacked the glamour of shooting it out with broadsides from the big guns. There was also the consideration that if you were beaten in ship-to-ship conflict, you could always surrender. But if during minelaying one of your mines exploded, that was it. “You made a hole in the water that it took three months to fill up” was the popular phrase.

I saw a great opportunity here for advancement, so I volunteered for the Minelaying Squadron. I was accepted and given orders to proceed to New York, where I was to take command of the Jefferson . I had no idea what the Jefferson was, but when I arrived in New York, I quickly found out.

She was an old ex-merchantman with canvas-covered decks like a ferryboat, and all her internal fittings were of wood. For twenty years she had carried passengers and freight between New York and Norfolk, Virginia. As her engines were completely shot, she had been retired from service. I could not believe that this frail ship was supposed to cross the Atlantic, weather North Sea gales, and actually survive the ordeal.