A relatively cheap S-boat could impose costs without sinking a ship. Crews passing through threatened waters had to spend longer at alert stations. That vigilance displaced gunnery practice, other drills, and maintenance. Repeated alerts therefore wore down crews and reduced the time available to prepare their ships for combat.
Notable Point · 1:54:44-1:55:24
How the S-boat threat wore down convoy escorts
The possibility of S-boat attack imposed costs through fatigue and lost training or maintenance time even when no engagement occurred.