The submarine was powered by two MWM RS 127 S four-stroke, six-cylinder diesel engines of 700 metric horsepower (510 kW; 690 shp) for cruising, two Siemens-Schuckert PG VV 322/36 double-acting electric motors producing a total of 410 metric horsepower (300 kW; 400 shp) for use while submerged.

At 10 a.m. on 30 October 1939, U-56 avoids detection by ten British destroyers and battle cruiser Hood, protecting the Home Fleet west of the Orkney Islands and came within striking distance of HMS Nelson and Rodney.

In Captain Zahn's own account of the events, three cruisers were heading straight toward his U-boat's position, making any attack by him almost impossible, when suddenly they veered by twenty to thirty degrees from their previous course opening the field of attack and bringing him into a direct line of fire with HMS Nelson and HMS Rodney.

Rodney was the lead ship of the convoy and Zahn decided to wait until it passed and concentrated his sights on Nelson.

After the attack Zahn ordered the U-boat to descend to a deeper level to avoid depth charges, since the destroyers had by now detected its presence.

In the evening Zahn ordered U-56 to surface and subsequently sent a radio report to Berlin listing the targets in the group including HMS Rodney.

Had this delay in Zahn's report not happened, the German command could have sent U-58, which was in the area at the time, to renew the attack on the British targets.

Because of his failure to destroy the Nelson, Zahn became depressed and Karl Dönitz felt obliged to relieve him of his U-56 command and sent him back to Germany to become an instructor.

Later, in his memoirs, Dönitz called the failed attack by U-56 "an exceptionally serious failure" but did not blame Zahn whose daring, in the presence of the destroyers, he praised, saying "The commander who had delivered the attack with great daring when surrounded by twelve escorting destroyers, was so depressed by this failure, in which he was in no way to blame, that I felt compelled to withdraw him for the time being from active operations and employ him as an instructor at home".

[14] In a similar location, she sank the armed merchant cruiser HMS Transylvania 40 nautical miles (74 km; 46 mi) northwest of Malin Head on 10 August.

U-56 was attacked by the British submarine HMS Tribune about 15 nautical miles (28 km; 17 mi) northeast of St Kilda on 6 September.