WW2 Q-SHIPS

Q-ships were arms-laden decoy merchant freighters and tankers modified to lure U-boats during World War Two. Dangerous duty.

They were operated by the Navy, but in the later stages of war the Q-Ship USS Big Horn was transferred to the Coast Guard and renamed USGCC Big Horn (WAO-124). By that time it was primarily a Coast Guard weather ship but maintained its anti-submarine weaponry just in case it encountered U-boats.
 
All in all, the Q-ship program had no meaningful effect, with one decoy - USS Atik - losing a duel with a U-boat and sinking with its entire Navy crew of 26 on its first patrol in March 1942, about 300 miles east of Norfolk, Virginia.

Cutter Big Horn started plying the oceans as the merchant tanker SS Gulf Dawn, built in 1936 at Chester, Pennsylvania by the Sun Shipbuilding & Drydock Corp. and operated by the Gulf Oil Corporation.