The German Imperial Admiralty converted Yarrowdale into a commerce raider, arming it with guns taken from decommissioned ships and fitted two torpedo tubes.

The ship was put into service as SMS Leopard and the new captain, Korvettenkapitän Hans von Laffert, sailed in early March 1917 to relieve Möwe.

Möwe (Fregattenkapitän Nikolaus zu Dohna-Schlodien) had evaded the Northern Patrol, assisted by a rudimentary underwater wireless link to a submarine.

Yarrowdale carried 117 vehicles, 30,000 coils of barbed wire, 3,300 long tons (3,400 t) of steel bars and 6,300 boxes of small-arms ammunition.

Dohna-Schlodien liked the potential of Yarrowdale as a commerce raider in size, speed [23 kn (43 km/h; 26 mph)], room for a large crew and its "unremarkable-ness".

[1] Under the command of Acting Leutnant Reinhold Badewitz, the ship was sailed to Germany, unwittingly assisted along the way by a strike by Liverpool boilermakers, which contributed to the Northern Patrol being reduced from 23 to six vessels at sea.

[4] The Kaiserliche Admiralität provided a great deal of information to Laffert on the number, dispositions and movements of the Northern Patrol, which had been gleaned from deciphered British wireless messages.

The sky was covered by dull grey clouds, being pushed northwards by a south-easterly wind, with occasional darker patches bringing snow or freezing rain; the temperature was 3.3 °C (37.9 °F).

Day was ordered to send an examination party to inspect the ship, which aroused his suspicion because it flew a Norwegian flag and had the name Rena but had a large "N" painted on the hull upside-down.

Day could see that woodwork had been removed, no visible wireless equipment and had managed to steam at 13 kn (15 mph; 24 km/h) for some hours, unusual for a merchant ship.

Day voiced his suspicions to Lieutenant Frederick Lawson Royal Naval Reserve (RNR), an Australian, who volunteered to lead a boarding party because of the inexperience of the officer who would normally deputise for the second in command.

As Day waited for Larson to signal, he moved Dundee, to keep on the weather (upwind) quarter, at right angles to the unknown ship, ready to fire broadside.

Achilles was 4 nmi (4.6 mi; 7.4 km) distant to the east-north-east with Dundee in grave danger if it opened fire on the strange ship.

A search for survivors was not conducted for fear of submarine attack; the boat that had carried the boarding party was recovered by a merchant ship several months later.