The effectiveness of the defensively armed merchant ships shouldn't be measured by the numbers of U-boats sunk or damaged by them. While it was certainly important for the morale of the merchant sailors to be able to fight back, nobody expected them to hunt down and destroy the attacker - the armament acted as deterrent and in this role it proved to be quite effective. Only the armed guards aboard UNICOI were credited with the destruction of a U-boat, when U-576 was left unable to dive after being hit by their gunfire and was subsequently sunk by aircraft. Another example of a merchant ship successfully defending itself is the action between FORT BINGER and U-588: [uboat.net]

U-boat commanders generally attempted to ambush their targets, firing torpedoes unseen while submerged or at night. Once detected most commanders disengaged as the element of surprise was lost. They seldom risked a gun duel with an armed merchant ship as the U-boat wasn't a very stable gun platform and damage to their U-boat had to be avoided at all cost - one lucky hit could leave it unable to dive. I'm certain that the losses of unescorted merchant ships would have been considerably higher if they would have been unarmed.

Let's not forget that the U-boats were not the only enemies of the merchant ships, to have anti-aircraft guns on each of them gave a convoy much more firepower to defend itself against air attacks. Furthermore, the mutual destruction of the Liberty ship STEPHEN HOPKINS and the German auxiliary cruiser STIER showed that even a merchant ship could fight back if forced to do so.

Best regards
Rainer

Crew member of uboat.net