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

At 3.16 a.m. on the morning of 21 September, U-48 had the convoy
HX72 in sight. Just over two hours later, Rolf picked up an SOS from a ship
called Elmbank, a large freighter, carrying timber and sheet metal, which
had been hit by U-99. Peering through the periscope, Bleichrodt had seen
flashes and the sound of dull explosions. Half an hour later, they were also
in position to fire. Their first torpedo missed, fired at too far a range, but the
second, fired twenty minutes later, hit, with huge detonation columns
erupting into the air. Almost immediately, Rolf picked up another SOS
signal – they had hit the Blairangus. Fires now broke out on the ship as her
cargo began to explode. Lifeboats were hurriedly launched but the skies
were grey with squally showers and after a while, the stricken ship
disappeared from view. U-48 now turned her attention to a tanker, although,
again, the torpedo missed. Another immediate shot was not possible
because they discovered one of the torpedo fins had been dented. Turning
around they now took over from U-47 as the convoy shadow, radioing
position reports to the other U-boats.
All day, U-48 kept up with the convoy, waiting for darkness, as were the
others in the wolfpack. Several were low on torpedoes, U-47 and U-48
included, but at twenty minutes before midnight Bleichrodt attacked with
his last tube, hitting Broompark, which soon began to lilt.
That night the carnage really began as HX72 struggled on, the wolfpack
snapping at its heels. U-99 sank two, including a 9,200-ton tanker. U-47
then surfaced and with U-99 finished off the Elmbank, which had limped on
all day, using their guns. In an extraordinary feat of daring, U-100 then
manoeuvred into the heart of the convoy and sank three more. All through
the next day, the wolfpack continued to keep up with the convoy, picking
off one ship after another, U-100 sinking a further four. By the time both U-
47 and U-48 turned for home, eleven ships had been sunk and a further two
badly damaged. That was more than a quarter of the entire convoy.
A further 295,335 tons of Allied shipping were sunk by U-boats that
month, and 56,328 tons by long-range Focke-Wulf Condors. The wolfpack
mauling of HX72 proved indisputably the value of concentration of force;
this principle that held true for the army and Luftwaffe applied equally to
the U-boat arm, just as Dönitz had long been arguing.
‘We lost another large number of ships off the Bloody Foreland last night as
well as the night before,’ noted Jock Colville on Sunday the 2nd. ‘These