For the month of April Allied losses fell to 56 ships of 327,943 tons. In May 1943 the battle reached a climax with the battles of Convoy ONS 5, Convoy SC 129, Convoy SC 130. Throughout the battles only two ships were sunk in convoy in the Atlantic while an air anti-submarine escort was present. Dönitz depended on the surface maneuverability of his U-boats to locate targets, assemble wolfpacks and the complicated business of positioning his forces ahead of a convoy for an attack. Allied air power determined where and when U-boats could move freely surfaced. It was the combination of convoy escorts and air power that made the Atlantic unsuitable for pack operations. The US Navy introduced the K-class blimp. They forced a commander to dive to prevent the vehicle marking his position or attacking directly. From 10 to 24 May 1943, ten convoys passed through the mid-Atlantic. Six of the 370 ships were sunk; three were stragglers. 13 U-boats were sunk; four by warships, seven by aircraft, and two shared.