From the most recent game update:


The naval combat balance was adjusted, especially in terms of how Destroyers are fighting Submarines. Prior to this update it was too easy for Destroyers to destroy Submarines due to their attack range combined with their high speed and ability to reveal. Destroyers now get their attack range removed and are compensated with higher damage values and lower costs, among other changes:

• Destroyers:
  
  • The attack range was removed
    
  • The research, building and upkeep costs were reduced
    
  • The damage against submarines was increased.
    
  • The view range was increased from 50 to 70.
    
• Submarines:
  
  • The research, building and upkeep costs were increased
    
  • The hit points of Submarines were decreased.
    

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As things stand, submarines are too easy to kill.

This is not because destroyers have an attack range. That range makes them useful in ship-to-land bombardment. This is a feature, not a bug.

The issue is that destroyers serve as scouts for other, bigger ships. A stack of 9 battleships with 1 destroyer can roam around demolishing subs all day long.

This update makes subs weaker and less useful while not addressing the core problem in any way. Goodbye, subs.
(Click to set source)
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Level up your cruisers and battleships to 2 or 3, stack them with 1 destroyer, and they dispatch single subs very quickly indeed. Fast enough to clear a path for an advancing troop convoy. Of course large stacks of subs take longer, but you can just go around those. Or if the opponent is offline, take a few hours to bombard them, at no risk.

High combat speed + detection at a distance = dead submarines in 1.5

Unless the opponent is online 24/7 to baby sit his subs.