@Baron:
I have the impression that submarines’ targets were rarely Destroyers. That most of the time, it wasn’t warships but merchants ships. And when attacking warships, it was mainly Carriers, Battleships and Cruisers. Destroyers were too fast, maneuverable, and too well equiped to try to sink them. Am I wrong?
In the Battle of the Atlantic, the primary job of German submarines was to sink merchant ships. This made good strategic sense. Britain imported all of her oil, many of her raw materials and much of her food, so if those supplies could be cut off (or greatly reduced), Britain would be starved into surrender. This made the merchant ships much more valuable U-boat targets than warships. Think of it this way:
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If U-boats had (for the sake of argument) sunk all of Britain’s warships but none of Britain’s merchant ships, Britain would have continued to survive (and continued to fight its air war against Germany).
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If U-boats had (for the sake of argument) sunk all of Britain’s merchant ships but none of Britain’s warships, Britain would have starved and would have run out of fuel, at which point all of its warships would have been put out of action because of empty oil tanks (or would have been compelled to go elsewhere for fuel, like Canada or the US, which would basically have made it very awkward for them to operate in British home waters).
So that’s why the U-boats prioritized targeting merchantmen. On the other side of the battle, the job of the Allied convoy escort ships (and convoy-protection aircraft, when flying range allowed it) was to protect the merchantmen. In that sense, both the U-boats and the Allied combat vessels and aircraft regarded the merchant vessels as “the prize” (to put it crudely) for which both sides were fighting. For U-boats, sinking a warship was a nice bonus, but it wasn’t their primary objective.
In the early days of the war, the Royal Navy and the Royal Canadian Navy misunderstood to some extent what their primary mission was. At first, they placed too much emphasis on sinking U-boats. Only later did they realize that their key objective was “the safe and timely arrival” of the convoys. In other words, they realized that a (hypothetical) situation in which every convoyed ship arrived safely but no U-boats were sunk would be a huge success for the Allies, while a (hypothetical) situation in which great numbers of U-boats were sunk but every convoyed ship was lost would be a massive disaster for the Allies.
This explains why long-range aircraft like the Liberator had such a huge impact on the Battle of the Atlantic. The Allies realized that these planes could greatly increase the safety of convoys even without sinking any U-boats. Why? Because U-boats feared aircraft more than anything else (they had weak anti-aircraft batteries, and their deck guns and torpedoes were useless against planes), so when a plane appeared overhead they usually pulled the plug and crash-dived. And that was the Achilles’ heel of WWII diesel-electric submarines: they were fast and long-ranged on the surface, but spending time under water slowed them down (for hydrodynamic reasons) and also ate into their operating range because diesel fuel would have to be wasted on recharging the batteries after they surfaced. Also, submerged U-boats had a harder time torpedoing convoys than surfaced U-boats, in part because their slow submerged speed gave them just a small window of time to line themselves up for a torpedo shot before a convoy slipped away. So to a large degree, the Allied air strategy was to try to drive the U-boats underwater as often as possible and keep them there for as long as possible.
Don’t get me wrong: the Allies were always happy to sink U-boats, and some aircraft (like the Liberator, when it was equiped with the Leigh Light and when suitable tactics for its use were perfected) proved quite adept at killing subs. But the Allies did their best (not always successfully) to keep firmly in mind that sub-killing was only a means to an end, not an end in itself, and that the job of “getting the convoys through” did not inherently require sub-killing.
In the Pacific, we have an interesting variation of what happened in the Atlantic. The American subs, by and large, concentrated on sinking Japanese merchant ships for precisely the same reason as the Germans tried to sink British ones: to strangle Japan. Japan in WWII was strikingly similar to Britian, in the sense that both countries were resource-poor, highly-populated, industrialized islands located very close to a large continent – so it’s hardly suprising that both countries were major naval powers, since their survival depended on their merchant fleets.
Japan, unlike the Americans, focused its Navy’s attention far too much on sinking enemy ships. Its subs spent too much time hunting American warships and not enough time attacking the American supply ships that kept the US Navy operating thousands of miles from Hawaii. Japanese destroyers spent too much time in warship combat and not enough time on the boring but essential job of protecting Japanese merchant ships from US submarines. It’s a very curious blind spot that the Japanese had about the value of merchant vessels (their own and those of the enemy), possibly because their warrior culture at the time made them feel that the proper job of a warship was to fight another warship, not to look after the interests of Japan’s gross domestic product.
The American subs in the Pacific, incidentally, were never shy about taking a shot at a Japanese warship when a target of opportunity came along – just look at what happened to the Shinano. They just kept in mind that sinking merchant ships was a potentially war-winning task, and they carried it out so successfully that, by 1945, Japan was nearing the economic bottom of the barrel and US subs were starting to run out of targets.
