"Capturing Enigma - How HMS PETARD Seized the German Naval Codes" 
by Stephen Harper, 2002

HMS PETARD's capture of ENIGMA material from U-559 on 30th October 1942.
  
In an action for which posthumous George Crosses were awarded
to Lieut. Fasson and AB Grazier, and a George Medal to 16-year-old NAAFI
Assistant Brown, in which they recovered, amongst many other things, a
current Kurzsignalheft (short signal book) and the 1942 Wetterkurzschlussel
(short weather cipher).  Most of the captured material arrived at Bletchley
Park on the 4th November, but for some unexplained reason the critical (to
cryptanalysts) short weather code did not arrive at Bletchley until the 24th
November, three weeks later.  Briefly the Kriegsmarine had used 3 rotor
Enigmas at the beginning of the war, and Bletchley was well versed in
decoding many of the signal types, but on 1st February 1942 the Kriegsmarine
introduced the 4-rotor M4 Enigma into its U-boat TRITON network, blinding
Bletchley temporarily.  Following a mistake by a U-boat radio operator
Bletchley managed to deduce the wiring of the fourth rotor.  This enabled
4-rotor bombes to be developed.

When the first four-rotor Bombe became operational in June 1943, these
earlier developments enabled Bletchley Park to solve the keys and read
TRITON traffic successfully on only three days before they broke the code
with the U-559 material. As code-breakers at Bletchley Park began studying
this material, captured by HMS PETARD far from the menace in the Atlantic,
the biggest U-boat offensive was reaching a climax and was close to gaining
the upper hand.

The second edition of the short weather cipher was precisely what the
cryptanalysts had been hoping for. From this key document, when it finally
reached them, they learned that the four-letter indicators for regular
U-boat messages were the same as those used as three-letter indicators for
weather messages except for one added letter. This meant that once a daily
key was found for a weather message the four-rotor signals needed to be
tested on the Bombes in only twenty-six positions to find the full key.

The Germans named their new M4 Enigma TRITON after the son of POSEIDON, the
Greek ocean demigod who blew through a twisted sea shell to stir storms.
Bletchley Park gave it the code name SHARK. The vital key for the first
decrypts of SHARK signals was found on Sunday 13 December, just six weeks
after the sinking of the U-559. It had taken constant work and thousands of
Bombe runs before this success was achieved. Later that day, solutions to
the hitherto impenetrable four-rotor Enigma messages between U-boats at sea
and the U-boat Command started to emerge. Only an hour after the first
decrypts were made intercepts of U-boat signals were sent to the Admiralty's
Submarine Tracking Room. They revealed the positions of fifteen U-boats.

Convoys from the United States carrying supplies for their forces in North
Africa began getting through without loss, evading the known positions of
U-boat wolf-packs revealed by ULTRA. It was estimated that reading TRITON
signals saved between 500,000 and 750,000 tons of shipping in December 1942
and January 1943 alone. In the first few days of 1943, ULTRA located a
wolf-pack of six U-boats lying in wait between the Azores and Madeira.
Having none of his usual B-Dienst intelligence on this convoy, mainly
organised by military signals, Doenitz knew nothing about it until a lone
U-boat chanced upon the convoy, and signals for it to trail the convoy and
orders gathering other U-boats into the area were decrypted at Bletchley
Park.

A convoy of nine fully laden tankers designated TM1, escorted by the
destroyer HMS HAVELOCK and three corvettes, was ordered to divert sharply to
the south, a route which would have avoided the U-boat ambushes by 100
miles. But the destroyer's captain, Cdr R.C. Boyle, unaware of the `golden'
source of the intelligence on which the detour was based, decided to
maintain his original course rather than head for rougher weather he knew to
be in the south, because of the imminent necessity of refuelling the
Corvettes. That doom-laden decision was tragically followed by one of the
occasional interruptions Bletchley Park suffered. The Germans changed the
Triton setting at noon and Bletchley Park was unable to read further U-boat
traffic for 48 hours, and there was a flurry of U-boat messages for the next
three days. Before ULTRA was restored, seven tankers totalling 55,000 tons
and carrying 100,000 tons of badly needed fuel, were sunk - a 77 per cent
loss and the highest proportion of any convoy lost throughout the war. But
from then until the war ended Bletchley Park was able to read U-boat traffic
on a nearly current basis.