Submarines usually come when you and your enemies already have battleships. Submarines have a strenght of 24, while battleships have 40, making submarines rather chanceless against battleships. If we consider that the price to make them isnt really different (150 vs 225) I think submarines arent worth the money invested compared to battleships, or even destroyers. I have only found limited use of themtalking about playing against people in Multiplayer)
-harassing enemy sea resources( the enemy can send a destroyer and a battleship to his most important strategic resource, like oil. So submarines become quite useless again)
-Infiltrating enemy territories with spies( you dont need more then one submarine for this, nor you have to effectively use the submarine)
-I am not sure about this, but can you block naval trade routes, and prevent the city working on it's sea tiles, when a submarine occupies it? (just like the way you can do the same with ordinary navy units.) This would uncover you sub's location :/
-Weakening enemy battleships. If you can catch the two flanking promotions, you can get a 80% (50%+10%+20%) retreat chance. You decrease the strenght of the enemy navy, with small submarine losses, so your main batleships can destroy them a LOT easier. I still think that they are not worth 150 hammers for this task, you are better off having 2 battleships with +20% strenght, then having 3 submarines, each with 80% retreat chance. The cost should be decreased to let's say hmmm 120hammer.
-harassing enemy sea resources( the enemy can send a destroyer and a battleship to his most important strategic resource, like oil. So submarines become quite useless again)
-Infiltrating enemy territories with spies( you dont need more then one submarine for this, nor you have to effectively use the submarine)
-I am not sure about this, but can you block naval trade routes, and prevent the city working on it's sea tiles, when a submarine occupies it? (just like the way you can do the same with ordinary navy units.) This would uncover you sub's location :/
-Weakening enemy battleships. If you can catch the two flanking promotions, you can get a 80% (50%+10%+20%) retreat chance. You decrease the strenght of the enemy navy, with small submarine losses, so your main batleships can destroy them a LOT easier. I still think that they are not worth 150 hammers for this task, you are better off having 2 battleships with +20% strenght, then having 3 submarines, each with 80% retreat chance. The cost should be decreased to let's say hmmm 120hammer.