Merchant raider
Merchant raiders are ships which disguise themselves as noncombatant merchant vessels, whilst actually being armed and intending to atttack enemy vessels.
See also:
Hilfskreuze Atlantis
Marauders of the Sea, German Armed Merchant Raiders During World War 2 (http://members.tripod.com/mackenziegregory/log/macslog/MaraudersoftheSea2GermanA.html)
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