The orders involved submarine screening actions for the German invasion fleet and reconnaissance - particularly off Narvik and Trondheim.
The operation's reconnaissance and screening objectives succeeded for the most part, however Hartmut is notable for the large number of faulty torpedoes fired and four U-boats sunk.
The deployment of U-boats during Operation Weserübung is occasionally criticized as a waste of manpower and materiel for this reason.
Speculation persists that massive torpedo failure during this campaign provoked German naval high command into improving the weapon's performance.
BdU Karl Dönitz intermittently sent ships to reconnoitre areas of the Norwegian coast and to try to locate portions of the British fleet.
12 smaller Type II U-boats were called up, despite that the Norwegian coast limited their operating time.
German submarines operating near the Norwegian coast made numerous attacks on British ships, although many proved unsuccessful due to faulty torpedoes.
[5] After docking in Bergen for a short time, U-13 then sank the Danish vessel Lily that had been taken as a prize by the British after the German occupation of Denmark.
[10] A British fleet of cruisers and transport ships were anchored near Narvik offloading troops and war materials.
[11] U-48 under Herbert Schultze made multiple attacks on a fleet of three cruisers on 10 April, but the torpedoes failed to arm or detonated prematurely.
On 19 April, commander Dietrich Knorr attacked the French cruiser Émile Bertin, but both torpedoes missed.
[13] Hans-Gerrit von Stockhausen commanding U-65 attacked a group of British destroyers, but the torpedoes missed or failed to arm.
Cold water in the North Sea and Norwegian fjords hampered the use of electric torpedoes used at the time, which required heating to reach suitable operating temperature.
Most commanders were used to operating at sea where the effect of tides and currents over short distances is negligible between a boat and its target.
These U-boats began patrols during the timeframe of Operation Weserübung, but were not fit for service (either due to damage, command changes or other reasons) during the beginning of Hartmut.