Submarines depended on ambush because they were vulnerable in a direct surface fight with an escort. Some commanders accepted the high risk of entering the convoy itself, where the outer screen could not easily see them, and attacked from among the merchant ships. A simulation assembled by submarine officers, Wrens, and intelligence staff exposed the pattern that explained otherwise puzzling losses. Placing a Flower-class corvette or armed trawler inside the convoy then turned the U-boats' apparent sanctuary into a gun trap.
Notable Point · 32:19-36:14
How U-boats penetrated the center of a convoy
U-boats exploited an escort screen concentrated outside the convoy; stationing an armed vessel among the merchant columns threatened that route of attack.