[citation needed] During her trials it was found that the Type IA submarine was difficult to handle due to her poor stability and slow dive rate.

Oberkommando der Marine (OKM) had ordered her loaded with mines and to be ready to put in place a minefield in Portland Harbour.

She was deemed unfit for combat duties but was none-the-less to stand ready to engage shipping with torpedoes upon completion of mining operations.

[3] With the war underway, on 4 September, U-26 began to penetrate the harbour but was slowed by antisubmarine patrols which were intense, as Dönitz had feared, and forced the first two attempts to be aborted.

The Admiralty had claimed the sinking of a mine-laying U-boat on 8 September and attempts to contact U-26 went unanswered, leading Dönitz to fear that U-26 and her valuable Enigma machine may have been sunk in shallow waters from which they may be recovered by the British.

Three freighters totaling 17,414 tons were sunk (one each of Greek, Belgian, and Dutch nationality) and the corvette HMS Kittiwake was damaged severely, though this last result was successfully kept from the Germans.

However, 6 of them were killed on 22 July 1940, along with 2 British servicemen, when a Heinkel He 111 of Kampfgeschwader 26 jettisoned its remaining bombs when returning from an inshore anti-shipping sortie and accidentally hit POW Camp 5 at Duff House, Banff, Scotland.