Convoys counter surfaced U-boat attacks

Convoy defense exploited the attacker's need to approach the protected ships and reveal itself when firing. Early in the war, U-boats sometimes made daring night surface runs through the middle of a convoy, a tactic Britain had not expected. Merchant ships received more guns, and a Flower-class corvette could wait inside the formation for a boat that penetrated the screen. A surfaced submarine was fast and dangerous, but it had no armor. Damage to the pressure hull could make diving impossible or fatal.