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

Hitler scrapped the Z Plan and ordered the beginning of a massive U-boat
Command, with the emphasis on the Mk VIIs that Dönitz had been urging
for several years. Indeed, such was the urgent importance of now building
U-boats, they were to take priority over even key projects such as the Ju 88,
while skilled shipwrights drafted into the Wehrmacht were to be sent back
to the shipyards right away. Hitler wanted Britain out of the war and he now
believed that the U-boats were the key to achieving this, more so than
Göring’s much-vaunted Luftwaffe. The new plan called for the production
of U-boats at a rate of thirty to forty a month. Dönitz, it seemed, had been
right all along, and, as if to prove the point, six weeks after the outbreak of
war one of his U-boats made an emphatic statement of intent, with the
sinking of just one boat. Not any old boat, however, but one of Britain’s
mighty 50,000-ton battleships: HMS Royal Oak.
The best submarine commanders had many crucial attributes but a cool
head, decisiveness and intuition – a sixth sense – were essential. Günther
Prien had them all, as well as a ton of experience: half his life had been
spent on the sea, travelling the globe, where he had learned telegraphy,
navigation, about weather, and, crucially, the art of leadership.
Making a periscope attack was not easy, as a number of considerations
came into play. For a torpedo to successfully hit a target, a calculation had
to be made taking into account the speed, range and course of the enemy
ship, the submarine and the torpedo. The Mk VII had two periscopes, a sky
periscope as well as an attack and night periscope. The Captain would look
through the latter, and, with the help of a graduated ring around the lens,
make a number of visual calculations. Range was calculated by reading the
angle between the waterline of the target and its bridge or masthead. This
was tricky because an estimate had to be made having already assumed the
size and class of the ship, not an easy task when simply peering through a
periscope. Although an estimate of speed was made visually, sonar helped
with this calculation.
The Captain would call out his estimates, which would then be passed
to the navigating officer, so that he could start to plot a course for a suitable
interception. The crucial factor was working out the director angle (DA), or,
in plain terms, the ‘aim-off’ needed in order to hit a moving target. The
torpedo, effectively a mini-submarine itself, was most effective when
hitting a target at somewhere close to ninety degrees. As the Captain