Here is something writen in his book "Memoirs Ten years and twenty days" page 56 and 57:
3. On sept 6th 1939 a Uboat (U-38) was fired upon for the first time by a merchant vessel on sight. On Sept 26th Winston Churchill, First sea lord og the Admiralty, announced the arming of all of the British merchant navy. In a broadcast on Oct 1st, 1939, the admiralty called upon all steamships to ram German submarines.
Soon afterwardswe learned the that neutral shiping also followed suit.
U-3 reported from the north sea: 'Swedish ship
Gun stopped at night in bright moonlight. Captain came aboard, papers not satifactory. 36 tons explosives stated to be for Belgian Ministry of War. Steamer, however, on course 280 degress and 15 miles south of the naze. While prize crew were taking over, steamer made sudden attempt to ram with rudder hard over at full speed. U-boat managed to avoid by maximum use of diesals. Steamer was then sunk by prize crew.
4. As a general rule all merchant vessals sailed without lights in the same way as warships. At night it was very difficult in a U-boat when sighting a shadowy form to decide whether it was an auxilary cruiser or an ordinary merchantman. In order to distinguish details the u-boat was forced to close to a comparatively short range. If it then made a signal by lamp ordering the ship to stop or made use of its searchlight , the glare interfered with its own readiness for action and at the same time betrayed its own position. If that ship turned out to by armed then the u-boat stood a very good chance of being destroyed by a sudden burst of enemy fire.
All these obstacles confronted us in our endeavours to wage the war in accordance to the provisions of the Prize Ordinance agreement.
Sept 4, 1939 U-39 sank the British liner
Athenia. The ship was steaming without lights and was on an unusual course and zig zagging. The u-boat commander concluded that she was an auxilary cruiser and sank her. As a result of this occourance that same evening the follwing order was issued: By order of the Fuerer and until further notice no hostile action will be taken against passenger liners even when sailing under escort.
------------------------------------------------------
Sinking neutrals was always a political hot potato even in the beginning of 1941 where Hitler was still trying to woo other nations to his cause.
Identifying neutrals is all part of the game
It would have meant a reprimand if a U-boat commander got it wrong.
The book is Admiral Karl Donitz Ten years and twenty days ISBN 0-304-35686-7 published by Cassell military paperbacks. Great book gives a good understanding from a command perspective.
Kpt, Otto
type IIb
U-23
GWX 1.03