ATLANTIC CONVOY DUTY

Photo: U.S. Coast Guard
U.S. Coast Guard Treasury-class cutter Spencer on convoy duty. 

Photo: U.S. Coast Guard
View of convoy from cutter Spencer.

Photo: U.S. Navy
North Atlantic convoy steams through heavy sea in December 1941. Possibly photographed from cargo ship USS Aquila (AK-47). Ship in foreground is a tanker.

U.S. Coast Guard cutters and Navy ships escorted North Atlantic convoys of merchant vessels during World War Two to fend off German U-boats and keep supply lines open.

"HX" merchant convoys originated in Halifax or New York and were bound for ports in the United Kingdom, according to Wikipedia. "ON" convoys originated in the UK bound for North America. 

Often times German U-boats would employ the "wolf pack" tactic, unleashing coordinated attacks on convoys.

For Coast Guard and Navy escorts, the weapon of choice was the underwater depth charge, which unleashed a lethal "hydraulic shock" against the raiders.

Treasury-class cutter Spencer (WPG-36) sank the German submarines U-633 on March 8, 1943 and U-175 on April 17, 1943, according to Wikipedia.

During a 1941 convoy rescue off France, the U.S. Coast Guard cutter Modoc (WPG-46) sighted the German battleship Bismarck, which had evaded British forces. A patrol aircraft located the Bismarck, based on Modoc's report, enabling the British fleet to pursue and sink the battlewagon.
"The Modoc’s position reports had been intercepted by the shore-based German naval signals interception service," according to a U.S. Naval Institute account of the incident, and while the Bismarck's commander "knew of the Modoc’s presence, it seems that little or no mention was made" to his officers.

Photo: Wikipedia
British convoy, 1943. 

Photo: U.S. Navy

Cargo ship[ USS Aquila (AK-47) rolling heavily in the North Atlantic, while steaming in convoy, December 1941. A Russian transport is right astern. Photographed from the sky lookout positioning, while leaving Halifax, Nova Scotia.