Ow effective was the German U boat campaign of the First World War

Question: How effective was the German U boat campaign of the First World War ?

This should be a short essay on the effectiveness of the campaign as a means of prosecuting the war. The focus should specifically be on Germany and should make a clear argument using relevant sources.

Referencing Requirements:
Please include the following references:

Business in great waters: the U-boat wars, 1916-1945 by John Terraine

World War I: the global revolution by Laurence Sondhaus

Total Rhetoric, Limited War: Germanys U-boat campaign 1917-1919 by Holger H Herwig in Great War, total war: combat and mobilization on the Western Front, 1914-1918 by Stig Forster