Why armed merchantmen struggled with U-boats inside convoys

Why use a corvette inside the convoy instead of relying on Q-ships or armed merchantmen?

Merchant ships had guns, but nighttime silhouettes concealed the submarine; a corvette inside the formation could search for it with radar.

Armed merchant ships did try to engage submarines inside convoys. At night, a surfaced U-boat could be obscured by the silhouettes of surrounding merchant ships, making it difficult to find and engage. A corvette stationed inside the formation could turn on its radar and search for the surfaced contact. The follow-up explains the visibility problem that prompted this escort tactic; it does not give a separate assessment of Q-ship effectiveness.