Final answer:
The exact number of ships that left British ports and were sunk by German submarines is not specified, but it is known that by 1918, German U-boats had sunk nearly 5,000 vessels, including 11 British destroyers during a particular ten-day period in 1940.
Step-by-step explanation:
During World War I, German submarines engaged in a form of naval warfare that had a significant impact on shipping. By 1918, U-boats had sunk nearly five thousand vessels. However, the specific number of ships sunk that left British ports is not provided in the historical record presented. In a particular act of aggression, the sinking of the RMS Lusitania in 1915 by a German submarine caused the death of nearly 1,200 civilians and became a crucial event in influencing public opinion and political reactions of the time. Furthermore, during a ten-day period in July 1940, German submarines sank eleven British destroyers, showcasing the continued threat that submarines posed throughout both World Wars.
The Atlantic warfare during World War II saw the use of U-boat wolf packs, which were incredibly effective. For example, in the first seven months of 1942, they sank close to 700 Allied ships in the Atlantic. This aggressive strategy highlighted the Germans' reliance on submarines due to their inferior surface fleet when compared to the British navy.