Five of the submarine freighters were converted into long-range cruiser U-boats (U-kreuzers) equipped with two 15 cm (5.9 in) SK L/45 deck guns, including U-151 which was originally to have been named Oldenburg.

U-151 left Kiel on 14 April 1918 commanded by Korvettenkapitän Heinrich von Nostitz und Jänckendorff, her mission to attack American shipping.

She arrived off the United States East Coast on 21 May, laid mines off the Delaware Capes and cut the submerged telegraph cables which connected New York City with Nova Scotia.

On 2 June 1918, known to some historians as "Black Sunday", U-151 sank six American ships and damaged one off the coast of New Jersey in the space of a few hours.

[7] On 14 June U-151 followed this with the sinking of the Norwegian barque Samoa, en route from Walvis Bay, South-West Africa, to Perth Amboy, New Jersey, with a cargo of copper ore, by gunfire 90 miles (140 km)s off the Virginia coast.