US hunter–killer groups extended their patrols to the central Atlantic in the summer. They sank 15 U-boats from June through to August 1943. A number of supply submarines were destroyed crippling the Germans' ability to conduct long range operations. At the end of the summer, practically all supply U-boats had been destroyed. In September 1943, Dönitz ordered his submarines back to the North Atlantic. U-boats were equipped with the G7es torpedo, an acoustic torpedo, which the grand admiral hoped would wrest the technological initiative back. The torpedo was the centrepiece to Dönitz's plan. Great faith was also placed in the installation of "Wanze" radar to detect aircraft. It was intended as a successor to the Metox radar detector. A number of his boats were later retrofitted with the submarine snorkel, permitting the submarine to stay submerged. Dönitz placed much faith in the Type XXI submarine. He accepted that the older submarines were obsolete now that Allied defences in the air were complete. He required a "true submarine", equipped with a snorkel to allow his crews to stay submerged, at least to snorkel-depth, and evade radar-equipped aircraft. Dönitz was pleased with the promised top speed of 18 knots.