What impact did german u-boats have on allied shipping during ww1 ?

Generally speaking German U-Boats had a devastating effect on allied shipping during WW1, since the Germans practiced "unrestricted submarine warfare"--meaning that no target was off limits. 

The question of what caused the full effects of the radical stage, also called the second french revolution by some historians, which shook the political and social reforms in france have caught the attention of many scholars, particularly because of principles implemented during this stage which is contradictory to the fundamental principle of the french revolution itself, by using dictatorship to create equality.