Small convoy escorts could try to resist a surface raider with their four-inch guns and anti-submarine weapons, but they were likely to be swept aside. Destroyers formed the next defensive layer, though they were not intended to defeat a heavy raider unaided. That task fell to cruisers and battleships, with air support and escort carriers where available. Their usefulness depended on their position, whether they could reach the convoy in time, and whether aircraft could cover the interception area. PQ 17 occurred before all of these mutually dependent forces were available in enough numbers to make the system reliable.
Chapter · 20:40-21:57
Why small escorts could not solve the surface-raider threat
Corvettes and trawlers could defend a convoy against submarines, but a surface raider required destroyers, cruisers, battleships, air cover, and better positioning.