In 1942 Dönitz summed up his philosophy in one simple paragraph; "The enemy's shipping constitutes one single, great entity. It is therefore immaterial where a ship is sunk. Once it has been destroyed it has to be replaced by a new ship; and that's that." The remark was the green light to unrestricted submarine warfare and began the tonnage war proper. BdU intelligence concluded the Americans could produce 15,300,000 tons of shipping in 1942 and 1943—two million tons under actual production figures. Dönitz always calculated the worst-case scenario using the highest figures of enemy production potential. Some 700,000 tons per month needed to be sunk to win the war. The "second happy time" reached a peak in June 1942, with 325,000 tons sunk, up from 311,000 in May, 255,000 in April and the highest since the 327,000 tons sunk in March 1942. With support from the Royal Navy and Royal Canadian Navy, the new convoy systems compelled Dönitz to withdraw his captains to the mid-Atlantic once again. Nevertheless, there was still cause for optimism. B-Dienst had cracked the convoy ciphers and by July 1942 he could call upon 311 boats, 140 operational, to conduct a renewed assault. By October 1942 he had 196 operational from 365. Dönitz's force finally reached the desired number both he and Raeder had hoped for in 1939. Unaware of it, Dönitz and his men were aided by the ULTRA blackout. The addition of a fourth rotor to the Enigma left radio detection the only way to gather intelligence on dispositions and intentions of the German naval forces. German code breakers had their own success in the capture of the code book to Cipher Code Number 3 from a merchant ship. It was a treble success for the BdU.