@Baron:
I still have one question:
does a Sub on hunting warships missions, such as the IJN Subs you talked about, can be able to attack and sunk a destroyer?
Or a they too big fishs to fry for Subs?
And does such a feat was only possible in very special circumstances?
Example: such as dark night, sub having the surprise effect and DD traveling at half speed.
Or is it that, when a Sub have a Destroyer up on the cross-wire, their was always bigger and easier target, since DDs are escorting other warships?
This would explain why there is so few record of a Submarine sinking a Destroyer.
The different variables you list in your question pretty much summarize the answer, which is “It depends.” A destroyer is, in some respects, like any other surface ship: it floats because its hull displaces enough water to compensate for its weight, and it can be made to sink if you increase its weight (by filling it up with water) to the point where the water it displaces isn’t enough to hold it up. So in that sense, there’s nothing magical about a destroyer. As with any other ship, the best way to fill it up with water is to blow a hole in its hull below the waterline, using such weapons as torpedoes, mines or explosive shells. Torpedoes are arguably the most practical and versatile way of blowing a hole in a ship’s hull below the waterline, so let’s focus just on torpedoes for the sake of brevity. Let’s also just focus on submarine-fired torpedoes, since this is what the present rule discussion is about, even though torpedoes can also be delivered by surface ships and by aircraft.
So the two questions to consider become the following: to what extent is a destroyer more likely (or less likely) than any other ship to have a submarine put a torpedo into its side, and to what extent is a destroyer more likely (or less likely) than any other ship to sink if it’s been torpedoed?
Let’s take the second question first because the answer is fairly straightforward. Basically, I’d say that a WWII destroyer is more likely to sink from a torpedo hit than a WWII cruiser or a WWII battleship because destroyers were pretty fragile, as their nickname “tin can” conveys. They were basically the floating equivalent of a Zero fighter: fast and agile, but completely lacking in armour. Cruisers and battleships were armoured (to various extents) along their waterlines, and battleships typically had sophisticated anti-torpedo features such as liquid-loaded bulges designed to absorb and dissipate torpedo blast effects. On the other hand, a WWII destroyer was probably less likely to sink from a torpedo hit than a merchant vessel. Destroyers are combat vessels, so they’re designed with more watertight compartmentalization than civilian merchantmen, which helps to keep the ship afloat if it’s been torpedoed. Also, destroyers are manned by naval personnel, not merchant mariners, so this theoretically (but not invariably) means a higher standard of discipline and training, which can make the difference between life or death in a damage-control situation.
Now for the first question, which breaks down into two components: how likely is a sub to be taking a shot at a destroyer, and how likely is the sub to actually hit its target? To get back to my basic answer: it depends. Is the sub operating in the deep ocean? If so, it’s more likely to run into a convoy than a naval force because the convoys were more numerous. Is it operating near a commercial port? Again, the chances are that it’ll run into a convoy rather than a naval force. Is it operating near an enemy naval base? Here, the chances rise that it will spot a warship.
If the sub runs into a convoy, what will it choose to attack: the merchantmen or the escorts? The merchantmen were supposed to be the priority targets, so in principle a sub captain who has a target choice will, all other things being equal, attack the merchant ship rather than the escort vessel (unless he’s following Japanese naval doctrine). If things aren’t equal, the answer might vary. For instance, a U-boat captain who’s given a choice between a fast cargo vessel and a destroyer lying motionless in the water (let’s say, due to engine trouble or because it’s picking up survivors) might take a shot at the destroyer because it’s an easier target under those circumstances. Another variable is a U-boat is at the end of its mission and low on torpedoes, in which case the skipper might be more choosy about what to attack.
Target speed, target size, target distance, target orientation and target course are hugely important variables. WWII destroyers were at the high end of the surface-ship speed scale, so this made them harder to hit than slower ships…assuming they were operating at their maximum (flank speed) at the time a sub ran into them, which they probably rarely were because most warships (for fuel economy reasons) tend to operate at a slower cruising speed much of the time. WWII destroyers were near the low end of the size scale (though frigates and corvettes were smaller), so this made them harder targets than cruisers and destroyers. Distance and orientation are uncontrlable variables, since they come down to luck: a nearby destroyer lying perpendicular to a sub will obviously be an easier target than a distant aircraft carrier lying bow-on. Course is another big variable: a zigzaging ship is harder to hit than one traveling on a straight course, and a torpedo shot will be affected depending on whether the surface ship is headed toward the sub, away from it, or sideways across its track. Another variable: does the ship have lookouts who are on their toes (ready to spot incoming torpedoes promptly) and a sharp captain who will promptly turn his bow towards the torpedo to comb its track? If so, the odds of a hit are greatly reduced. Agile destroyers can obviously perform such maneuvers better than a tubby cargo vessel, so that’s a point in their favour.
So the upshot of all this is that there’s no single answer to the question of why destroyers were (or weren’t) sunk to a greater and lesser degree than other ship types. In a very general sense, the main factors at play were probably: a) that subs didn’t often take shots at destroyers since they were less important strategically than merchant ships; b) that destroyers tended to be hard targets to hit due to their speed and agility; and c) that successful torpedo hits on destroyers could very well sink or severely damage them, due to their relative fragility.