Frigates needed enough surface firepower to meet a U-boat using its deck gun and either drive it off or destroy it. A corvette's crew might fight hard without being able to win the same engagement. Sloops had greater combat power, but too few existed to cover every convoy, while destroyers were needed across many roles and theatres. The frigate filled the resulting tactical and numerical gap: it could be built in numbers and still contest a submarine above water.
Notable Point · 12:21-12:52
Why a frigate had to fight a surfaced U-boat
River frigates were intended to defeat a U-boat using its deck gun on the surface, giving convoys a capability that corvettes might struggle to provide.