Dilandu wrote:Even primitive subs would make close blockade much more dangerous; the US civil war clearly demonstrated, that even the hand-powered submarine could destroy relatively modern warship.
While that is true, they were also suicidal vessels, as the spar torpedoes used nearly always sunk the attacking submarine as well as the target ship(if they sunk anything). And overall success rate was very poor.
Of course, that can be greatly improved upon relatively easy, but it´s still unlikely to be possible to achieve anything close to a WWI sub. Too many things that needs electrics.
Dilandu wrote:The historical main function of submarines were to attack enemy ships. Basically it's the only thing they could do effectively.
That´s overstating it. Japan used subs to keep their outer island garrisons supplied quite well.
German "milk cows" supply submarines were also reasonably effective.
And coastal shelling by subs were done a number of times, only reason they mostly didn´t do much was due to operational mistakes, or simply trying to do something the sub wasn´t really meant to do(a single 7.5cm or 10cm cannon isn´t going to do all that much damage to a harbour or other important target, if instead you have 2-4 subs each with 2 15cm guns however, that can make a noticeable dent in most things).
Using submarines to land commando strikes could be a VERY effective way of using them.
Submarines for scouting is also very realistic, except of course that wont work here since there´s no good communications.
Also however, it should be remembered that the success of submarines historically have been in sinking MERCHANT vessels, most of the time, submarines attacking warships were asking for trouble. While not a general truth, it is something to keep in mind.
Dilandu wrote:And historically, the underwater warfare was the main weapon of weaker navy against more powerful.
USA used submarines to extreme effect against Japan, so that´s not exactly true.
It´s the main weapon of choice against an enemy relying on shipping.
Note that while Japan started WWII with the best submarine tech in the world, their subs generally had very little effect on the war.