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First, let's say that in 1910 or so, the German Navy decides that Germany could not keep up with British battleship building and so puts all the resources into building subs and expanding the sub personel. Now, unlike during the inter-war years, this change of focus would not mean the RN would focus on anti-sub warfare and resources in response, for the simple reason that very few people understood what role in naval warfare subs should play and how effective they would be. After all, it would take the experience of WWI and millions of tons of ships lost at sea before the RN would institute convoys and other anti-sub tactics and techs. There's no reason to assume that the RN would feel particularly threatened by subs, no more than it felt particularly when the French built torpedo boats like crazy in the belief that battleships were obsolete.
Therefore, in 1914, Germany has hundreds of subs, but less battleships and cruisers than in RL. When the British institutes the blockade, Germany counters with unrestricted sub warfare on all merchant marines. How would the war then progress?