I think it helped lengthen the war. America was supporting the Entente powers so much before the Lusitania that I think we were just looking for an excuse to get involved. I think the primary objective of the submarine campaign was not to shatter the blockade but rather to interdict British supply lines between America and Britain. They did a pretty good job too and I think drawing America into the war was inevitable. Allowing the US to continue to "secretly" supply Britain was unacceptable for Germany and without a strong surface fleet they had no option but to revert to submarine warfare. The only way the British were able to protect themselves, even after the US entered the war, was to break up their Navy and do convoy escorting. The Brits followed the Mahan doctrine and sought decisive battle with Germany to "end" the German Navy. They thought they finally had it at the Battle of Jutland, but Jutland was a very indecisive battle with no distinct victor. Jutland was the biggest naval battle of WWI but it had very little effect on the war aside from confirming the German policy of avoiding fleet to fleet battles. This did a lot to disprove some of Mahan's theories.
