The Threat of the U Boat A blockade simply prevents something from passing. In the case of World War I, blockades were established to limit the amount of supplies arriving at enemy ports. In previous history, surface ships did this by simply blocking ships entering or leaving a port. In February 1915, unrestricted submarine warfare was established. This meant that any ship bound for an English port would come under fire. This is because some ships carrying food were also thought to be carrying supplies for the war effort.2 b. Since the Germans could not break the British naval blockade in the North Sea against German ports, they turned to the use of submarines to starve the British into submission, destroying Allied ships carrying food and raw materials. Source 4 shows that German Admiral Tirpitz thought that by also blocking our supplies, Britain would face a similar situation to their country. The graph from Source 2 shows three main points. First, the graph starts at just 0.3 million tons of losses in Allied shipping. This gradually increases due to the increase of German U-boats. Secondly, it can be seen that in 1917 the reintroduction of unrestricted submarine warfare had a large effect on the amount of Allied ships sunk. The graph reaches 6.25 million tons. Finally, it is evident that anti-submarine tactics dramatically reduced the amount of ships lost. Depth charges were very effective, second only to mines. The decrease is also believed to be due to destroyers traveling in large convoys to protect them.4. There were some serious food shortages and riots across Britain. This information... middle of paper... of bodies. Adults weren't the only ones on the receiving end of this propaganda. Patriotic books and toys were made for children, to encourage further support for the war effort. Another important factor in defeating the U-boat threat was the threat that the Germans did not have enough of them. Historian Holger Herwig stated that with an average of twenty or thirty U-boats in the vast expanses of sea around Britain they were simply "insufficient to do the job". However the German Admiralty may have realized that at a 50% loss rate they would need several hundred U Boats and knew that the raw materials for building the U Boats could be used elsewhere, such as in the production of munitions and weapons . The Germans may also have been discouraged from building more U-boats as Britain quickly overcame the immediate threat with the new technology.