How successful was the German U-Boat campaign against Britain?

After unsuccessful attempts in 1915-16 with the U-boat blockade, in January 1917 Germany took the gamble of announcing unrestricted submarine warfare. Labeled the U-boat campaign, the aim was to sink all enemy and neutral merchant ships in the Atlantic and starve the Allies into surrender. This was almost successful, losses between February and July 1917 averaged 600,000 tons per month and in April Britain was restricted to 6 weeks wheat supply. Unfortunately in the same month the USA joined the war, and David Lloyd George came up with the convoy system. This involved larger numbers of merchant ships sailing together as warships provided protection. This was extremely successful, losses were substantially decreased and the Germans gamble failed. The British navy had played a significant role in the eventual defeat of Germany. Ahh.... & then there's WW2. Again, for a limited period, the predation of the U boat was very serious indeed. Many, many merchant ships were lost. As in WW1 the most used method of destruction was on the surface at night. Until the Allies were able to close the 'Air Gap', the parts of the Atlantic Ocean not overflown by long range aircraft detecting submarine movements, the wolf packs had a free run in destroying Allied shipping. The most successful U boat captain was Otto Kretchmer. Eventually the life expectancy of the U boat crew became very, very limited.