[Aktungbby]

Originally Posted by iambecomelife

Yep - that's correct. I read a book called "Night of the U-Boats" that talked about the problem created by mixed nationality convoys - the author says UK sailors hated neutral ships; the giant flags & bright colors could give away a convoy's location. In HX-79 British tanker "Sitala" was sailing near the neutral Swedish tanker "Janus" on a moonlit night - both got sunk because the bright paint on "Janus" helped submarines find their convoy:

https://www.uboat.net/allies/merchants/ship/641.html

https://www.uboat.net/allies/merchants/ship/638.html

THe bright paint is moot on a moonlit night in a two convoy attack:

HX 79 was an east-bound convoy of 50 ships which sailed from Halifax on 8 October 1940 making for Liverpool with war materials. On 19 October, 4 days from landfall, HX 79 was entering the Western Approaches, and had caught up with the position of SC 7, which was under attack.
The escort for the crossing had been meagre, being provided by two armed merchant cruisers against the possibility of attack by a surface raider, but even these had departed when HX 79 was sighted by U-47, commanded by submarine ace Kapitänleutnant Günther Prien.
At this point HX 79 was unescorted; Prien sent a sighting report and set to shadowing the convoy, while Konteradmiral Karl Dönitz ordered the pack to assemble. Those U-boats which had attacked SC 7 and were still able to fight (three had departed to re-arm having expended all their torpedoes) were directed to the scene. Four did so, U-100 (Joachim Schepke), U-46 (Engelbert Endrass), U-48 (Heinrich Bleichrodt) and U-38 (Heinrich Liebe) joining U-47 during the day.
However the Admiralty, concerned by the fate of SC 7 and anticipating an attack, rushed reinforcements to the scene; throughout the day a large escort force of 11 warships also gathered to provide cover.
Action
Undeterred by their presence however, the pack attacked as night fell; using the darkness to cover an approach on the surface, Prien penetrated the escort screen from the south to attack from within the convoy, while Endrass (who had learned his trade as Prien's 1st officer), did the same from the north.
Over the next six hours, 13 ships were torpedoed; 6 by U-47 alone (4 of which were sunk). 10 ships were sunk from the convoy, and 2 stragglers were lost later in the day. These were Shirak, which had been torpedoed in the night, and Loch Lomond, sailing with the convoy as a rescue ship. Another, Athelmonarch, was damaged but was able to make port.
HX 79 had lost 12 ships out of 49, a total tonnage of 75,069 gross register tons (GRT).
None of the attacking U-boats were damaged.

In short the Convoy had happened upon a convoy attack in progress (SC 7) ; was spotted by Prien and shadowed and the other Uboats, all commanded by top aces, closed in to commenced night attacks in moonlight. HX 79 may be considered to have gotten off lightly when compared to SC 7:

SC 7 had lost 20 ships out of 35, of which seven fell to Kretschmer's U-99. The total tonnage lost was 79,592 GRT. The arrival of convoy HX 79 in the vicinity had diverted the U-boats and they went on to sink 12 ships from HX 79 that night. No U-boats were lost in either engagement. The loss of 28 ships in 48 hours made 18 and 19 October the worst two days for shipping losses in the entire Atlantic campaign. The attack on SC 7 was a vindication of the U-boat Arm's wolfpack tactic, and was the most successful U-boat attack of the Atlantic campaign. The convoy escort was ineffective in guarding against the attack. Convoy tactics were rudimentary at this early stage of the war. The escorts' responses were uncoordinated, as the ships were unused to working together with a common battle-plan. Command fell to the senior officer present, and could change as each new ship arrived. The escorts were torn between staying with the convoy, abandoning survivors in the water, as DEMS regulations demanded, and picking them up, leaving the convoy unprotected and risking being torpedoed themselves.

When U say Happy Time BBY, this was the Apogee of the entire Uboat campaign!