I believe there was a thread on this topic not too long ago. One big obstacle is that the UK would certainly notice Germany building hundreds of U boats and modified their naval development to counter. A bigger obstacle is the fact that Germany just did not have the resources to build that many submarines in the first place.Doenitz maintained after the war that if the resources used to build the Bismark and other capital ships had been invested instead in U boats, that he would have been able to bring Britain to its knees in 1940. He reckoned that he needed 200. If the Germans had focussed exclusively on U boats then maybe they could have achieved this target without affecting their production quotas for the Kriegsmarine and the Heer.