The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

Following Munich and the instigation of the massive rearmament drive,
the OKM produced the Z Plan, which although it called for 233 U-boats
also proposed building six battleships, eight cruisers, four aircraft carriers
and a number of surface vessels. The U-boats and battleships were to be
completed by 1943, which was still five years away. The Z Plan was flawed
for many reasons, but it missed an important truth. Germany’s geography,
with only a stretch of coastline in the narrow Baltic, made it very difficult
for surface vessels, particularly large capital ships, to break out into the
Atlantic, where any war on Britain’s lifeline would have to be conducted.
The English Channel was impassable in a time of war owing to mines,
aircraft and British shipping, but the route into the North Sea and around
the north of Scotland was also easy for the British to block. The only
vessels that could adequately reach the hunting grounds of the Atlantic were
U-boats. The Royal Navy had large fleets of surface vessels but their role
was primarily to protect those lifelines, for which fast, powerfully armed
surface vessels were well-suited. Germany’s task would be to destroy as
much merchant shipping as possible – and for that, Dönitz believed, U-
boats were the best tool available. Furthermore, there was another massive
advantage. Unlike battleships or aircraft carriers, U-boats were
comparatively easy, quick and cheap to build. Lots of them could be
produced in a comparatively short time. And they used less fuel.
By the summer of 1939, Dönitz was ever more convinced that the
Germans would soon be at war with Britain. Germany had already publicly
renounced the Anglo-German Naval Treaty and the hurried rearmament
programme had been instigated because of the threat in the west, rather than
the east. Britain had made its pledge to Poland. With war thus looking
increasingly likely, Dönitz asked Admiral Raeder to convey to Hitler his
continuing concerns about the size of his U-boat fleet, which had only
twenty-seven ocean-going boats, of which just nineteen were ready for war.
In contrast, Britain had fifty submarines and France, seventy. In July,
Raeder conveyed Hitler’s reply. ‘He would ensure that in no circumstances
would war with Britain come about,’ noted Dönitz. ‘For that would mean
finis Germaniae. The officers of the U-boat arm had no cause to worry.’
Why Hitler was so anxious to keep his navy in the dark is not clear, but
with the subsequent outbreak of war seven weeks later he soon began to
recognize the important role U-boats could play, a role already outlined in
some detail in a paper earlier submitted by Dönitz. Thus, in September,