The auxiliary cruisers of the Kriegsmarine were converted merchant, equipped with guns and torpedoes to attack Allied merchant shipping. The military equipment on board was camouflaged in a way that those ships still appeared to be "normal" merchant ships.
They carried enough material and paint to change their outer appearance often, like adding a second funnel, modifying the superstructure and repaint the whole ship. They often gave themselves the look of an existing allied merchant to deceive other allied ships.
Those ships archived some remarkable results. During their operations, the nine auxiliary cruisers sunk over 140 ships with over 700000 t. They operated in all major Oceans of the world, some of them had were temporarily stationed in Japan for Pacific operation. The longest voyage was the one of the Atlantis
, during its 622 day mission the ships sailed 102000 sea miles.
With their six 15 cm guns, they were even capable to fight against "real" warships if they could surprise them. When the Australian CL Sydney
wanted to inspect the auxiliary cruiser Kormoran
and closed up to 1000m, the Kormoran
opened fire and was able to sink the Australian ship (but was lost because of too much damage during this battle).