Even with operational problems great success was achieved in American waters. From January to July 1942, Dönitz's submarines were able to attack unescorted ships off the United States' east coast and in the Caribbean Sea; U-boats sank more ships and tonnage than at any other time in the war. After a convoy system was introduced to protect the shipping, Dönitz shifted his U-boats back to the North Atlantic. The period, known in the U-boat Arm as the "Second Happy Time", represented one of the greatest naval disasters of all time, and largest defeat suffered by American sea power. The success was achieved with only five U-boats initially which sank 397 ships in waters protected by the United States Navy with an additional 23 sunk at the Panama Sea Frontier. Dönitz put the successes down to the American failure to initialize a blackout along the East Coast of the United States and ship captains' insistence on following peace-time safety procedures. The failure to implement a blackout stemmed from the American government's concern it would affect tourism trade. Dönitz wrote in his memoirs that the lighthouses and buoys "shone forth, though perhaps a little less brightly than usually".