A convoy is a group of vehicles or ships traveling together for mutual support. Often a convoy is organized with armed support
for defensive support. During WWI, steaming at several times the speed of merchant ships and firing accurately at ranges of
several miles, a single battleship encountering a convoy could destroy dozens of ships before the remaining ships of the convoy
could scatter over the horizon. To protect a convoy against the enemy's capital ships would mean providing it with its own
escort of battleships: an unsupportable cost.
The fear of battleships was the main reason for the British Admiralty not adopting convoy at the start of the first Battle
of the Atlantic in World War I. But by the end of 1914, German capital ships had largely been cleared from the oceans and
the main threat to shipping came from U-boats. From a tactical point of view, World War I-era submarines were similar to privateers
in the age of sail: only a little faster than the merchant ships they were attacking, and capable of sinking only a small
number of vessels in a convoy because they could only carry a few torpedoes. The Admiralty took a long time to respond to
this change in the tactical position, and only in 1917, at the urging of the British Prime Minister, David Lloyd George, did
they institute a convoy system. Losses to U-boats dropped to a small fraction of their former level.
Actual analysis of shipping losses in World War I conclusively disproved all these arguments, at least so far as they
applied to transatlantic and other long-distance traffic. Ships sailing in convoys were far less likely to be sunk, even when
not provided with any escort at all, the loss of productivity due to convoy delays was small compared with the loss of productivity
due to ships being sunk, and ports could deal more easily with convoys because they tended to arrive on schedule and so loading
and unloading could be planned.
After July, the newly introduced convoy system was extremely effective in neutralising the U-boat threat. Britain was
safe from the threat of starvation. Even more importantly, April 1917 finally saw the formal entry of the United States into
the war, in response to the U-boat attacks.