Why Germany could not secretly build an Atlantic-winning navy

How much would Germany have needed to spend to have a real chance of winning the Battle of the Atlantic?

The central obstacle was not a price tag but the impossibility of secretly creating hundreds of submarines, supporting surface forces, and the shipyard capacity to build them without provoking a British response.

Spending the entire German defence budget on the navy would cripple Germany's land-war plans without guaranteeing Atlantic success. Delaying the war to finish more ships would also give Britain more time to expand its own forces. A fleet of roughly 300 submarines would require visible yards, workers, and infrastructure that British intelligence could detect. Even a surprise submarine advantage would be one-dimensional and invite convoys and an anti-submarine response, so Germany would also need numerous surface raiders.