To this day, international shipping is how much of the world gets its goods. During WWII, the chain of supply stretched across the Atlantic as the US sent supplies to the Allies even before their formal entry into the war. These ships were under constant threat from enemy attack. In this article, Warlord Games looks at merchant convoys in Victory at Sea.
From the article:
One of the main reasons to maintain a navy is to deny use of the oceans to the enemy, preventing the movement of industrial goods and materials, troops and supplies. Commerce raiding formed a critical part of the strategy for some nations, and indeed represented the Axis’ best hope for victory over Britain. That victory was almost achieved, while in the Pacific US submarines gradually starved Japan of raw materials by much the same methods.
Preventing attacks on defenceless merchant ships is the other main role of the navy, and it was here the war was fought, day in and day out, by the humble corvette, frigate and destroyer escort, and later by escort carriers. These vessels battled the submarine threat for the duration of the war, and at times were forced to do what they could against a major surface raider, usually resulting in being sunk.
There were, however, other ways to defend merchant ships, or to give them a measure of self-protection capability. Grouping ships into convoys meant there was more expanse of empty ocean out there – hopefully raiders would not even find the convoy. It also made escorts more effective, but in the event a convoy was hit by a surface raider such as a heavy cruiser or battlecruiser, the concentrated target would be devastated in short order. Nevertheless, the convoy system helped a great deal.
Other measures included mounting a few light guns on merchant ships, often with army or navy reserve crews. While a couple of 4-inch guns in open mounts would be no use against a serious warship, they might be able to deal with a submarine. Many submarine attacks were carried out on the surface with guns, in order to save torpedoes. This practice became dangerous and Q-ships (armed merchants with concealed weapons) were deployed. Revealing her armament at the last second, a Q-ship could quickly sink a submarine if it could be lured in close enough to the ‘defenceless’ merchant. This was one reason why unrestricted submarine warfare was the only effective strategy.
Armed merchant ships could also function as raiders. Germany made extensive use of such vessels, with mixed success. Even less successful were Armed Merchant Cruisers created by adding thin armour plate and a few guns to a liner or fast merchant vessel. Big, slow and horribly vulnerable, such vessels were no match for a real warship but were deployed for lack of anything better.