Hum, here's another example of unrealistic results as a result of more or less unrealistic physics...let me explain that.
Active Sonar can't detect a submarine at periscope depth (Reason: The node where the sound impulse comes from is below the bow of the destroyer and points slightly downwards), and passive sonar (hydrophones) can't either unless it is very close (Reason: Surface noise).
Since SH2 I have seen people develop irrealistic tactics that consisted essentially in submerging very deep, then coming up for the shot. That is not historically correct as far as I know, no sub commander would do that for several reasons:
1.- All the time you are below periscope depth you are virtually blind and your only means of knowing what the target is doing is hydrophones. The convoy might well zig zag away and you would never notice it until it is too late
2.- You can't collect aditional critical data through visual observation to set up a fire solution: AOB, distance, speed
3.- The amount of time required for a sub to go down and then up is enough to ruin any decent attempt of positioning yourself well for the attack.
4.- There were in real life much more chances of being detected at medium or deep depth (Where active sonar reaches well and passive is not distorted by surface noise ) than at periscope depth, except in two cases: A) A heavy termal duct at shallow depth, which you can position your sub below, and B) Broad daylight in an aircraft covered zone, as you can be detected submerged at periscope depth by a plane visually.
The irrealistic tactics describe above have been more than often the result of the use of auto targeting in SH2/3/4, where you get "magically" the perfect fire solution with just a click of the mouse. Sometines in conjunction with astonishing manouvers like "blow ballast" to climb faster, then level at periscope depth
. A real submarine would go out of control when blowing ballast heavily, and surface uncontrolled
In real life the destroyers patrolled all around the convoy perimeter continuously with a stop and sprint tactic not for fun, but because the only means of detecting a submerged submarine at periscope depth was mainly listening or casually stumbling upon her. If you submerge deep and enter the area where their active sonar reaches more and their passive is more effective, you are losing an advantage. You have already a big speed disadvantage when submerged. If you worsen it by going into the area where escort sensors are more effective and you are blind for not being able to use the periscope, the result is obvious.
Unfortunately, SH2/3/4 have not always simulated well the escort sensors, and some mods have been allowing them also to be operational at periscope depth, which is not correct. While that probably might have been done to compensate other flaws, it has lead to the use of historically incorrect tactics in some cases.
In real life commanders would hunt on surface by night, and only attack at periscope depth if a target of oportunity is found at broad daylight and no alternative (Following it) is possible. The US submarines doctrine at the start of the war was exactly the opposite (Submerge all day and attack at sonar depth, with no visual aim) at the beginning of the war, and the results were a plain disaster. The germans employed the oposite tactics and had incredible results.
When things changed later with the adition of radar, air cover, and convoy escort screens grew thicker, the only means of attack for submarines was submerged, be it day or night, which didn't necessarily mean "go deep and come up when the convoy is above you". Instead, just the fact of being unable to do anything else than moving at 2 knots at periscope depth when the convoy was already 20 miles away was enough to ruin any decent firing solution. How would you be able to position yourself properly at 2 knots when you are engaging a convoy who is moving at 8 knots and zig-zagging?? That's what killed the success of the german u-boots, and not the improved ASDIC. The improved active sonar helped hunting and destrying them once located, but the real reason they were no longer able to closen the enemy convoys enough to fire a torpedo (With a run length of some 2000-3000 metres at most if you want to hit anything) is that they were forced down by aircraft and surface radar.
