From: Winnipeg, MB
"Convoy" (Middleton?) is a very good book about March 43. There were actually 3 eastbound convoys at sea covered by the book: HX229, HX229A,and SC107. March 43 was the time when convoys began to get very long range aircraft coverage of the Greenland Air Gap. The results were immediate.
In addition both the USN and the USAAF changed their tactics: rather than hunting submarines just any old place they started to commit modern DDs/DEs to hunting submarines near the convoys and forming more (convoy) Support Groups and CVE Hunter-Killer Groups. The USAAF spent 1942 bouncing their bombs off of the sub pens in France. Committing the initially limited number of long range bombers to support of the convoys worked much better.
Also previously the USN tended to view ASW as defensive (similar to the IJN) and committed only the oldest DDs and a few USCG cutters to convoy escort duties (where by 1943 they were getting overwhelmed by the relative hordes of Uboats).
Yes, a number of things came together at that point in the war to finally defeat the U-boats. VLR Liberators, CVEs, DEs, Black Swan type Sloops and Frigates, HF/DF, radar on smaller escort vessels, DCs capable of 600 foot depth, better intel, better tactics, better training, etc. Did you know that scientists were hired to figure out the best D/C pattern for an attack. The optimum trade-off between effectiveness and D/C expenditure turned out to be 10: two off the racks, two from the throwers, two more off the rack, two from the throwers and a final two more from the rack.
As for the use of old vessels, the British and Commonwealth countries were in the same fix when their war started. At least the US had some time to observe what was happening and start the planning for all the ships it needed to build when the time came. The result was a flood of escorts (and merchantmen) that the Germans never imagined could happen.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth