How Q-ships changed submarine risk calculations

First World War Q-ships were effective enough to alter enemy behavior even when they did not sink a submarine. Selecting capable officers and disguising weapons made a lone merchantman a potentially dangerous target. The possibility also complicated attacks on convoys because a submarine could not be certain that every armed escort had been identified. Britain considered reviving the concept in the Second World War, then spread ordinary defensive weapons across many merchant ships. The strategic effect was deterrence through uncertainty rather than a simple tally of ships destroyed.