Submarine Guide


Submarines are a powerful unit in HOI2, which the AI finds very difficult to use. Some players consider optimal use of submarines to be a gamey exploit.

Surface Battles

Many players consider that there is little historical justification for such results. In World War Two, most submarines acted independently. German 'wolfpack' attacks were generally made by less than a dozen units, and only exceptionally by 20-30 submarines. Coordinating attacks by a large number of submarines was, and remains, a difficult logistical challenge.

A HOI submarine unit represents several (probably about 5) boats. For this reason, many players consider that a stack of a dozen SS units (60 subs) is not only ahistorical but also unrealistic: a fleet of 60 submarines has not only never occurred but would be very difficult to keep together and find a coordinated attack impossible.

Some players have suggested adjusting the stack limits for submarines to adjust for this. As an interim solution some adopt a 'house rule' of a maximum stack size for subs of two or three units.

Convoy Raiding

Submarines can conduct convoy raiding attacks with considerable effect. However, many AI countries, particularly large ones such as the UK, build up very large stockpiles of all goods early in the game. This considerably reduces the value of convoy raids, because industry can continue even if no more resources are being imported. Historically, the UK was on the verge of running out of some important goods at many times thanks to sub attacks.

But even if submarines do not hurt the IC of a country directly, a successful submarine campaign forces the victim to build convoys and escorts so there is much less IC left for more dangerous stuff like ships, planes and troops. Additionally, troops at oversea territories don´t get supplied and so starve to death or will be evacuated.

AI use

As of version 1.3b, AI development and construction of submarines is disabled. This is because developers felt that the AI was unable to use its submarines effectively, and that switching off their development would result in a more challenging game.

AI nations retain their existing sub fleets, but will build no more.

Players who want a game with a more historical naval construction plan are advised to use patch 1.3a.

Quote from the HoI2 Lead Script AI Developer Lothos

"..Subs are not supposed to be able to attack surface fleets head on...I have my own house rule of no more than 6 subs in a fleet..."

Submarines in Hearts of Iron 2

Submarines have a high Surface Attack value. A large stack of submarines is often able to defeat a significant enemy surface fleet, sinking core units like battleships and carriers. This is particularly true when attacking at night, when submarine-based doctrines have been researched, and when other attacks have reduced the number of destroyer and light cruiser screens. Note that this is not true in Doomsday, where sub surface attack values have been nerfed and a convoy attack value has been added.

Submarines in Doomsday

Doomsday has seriously crippled the surface attack rating of submarines, and has introduced the convoy attack rating to offset this. The convoy attack rating is the same as the surface attack rating for all sea units except for submarines, who have higher convoy attack ratings. Doomsday also introduced Light Carriers with high sub attack ratings, and the ASW upgrade for other ships.

This means that subs are no longer a viable option to destroy enemy fleets. They are still useful for convoy raiding though, just remember to pay attention to the battle menu, and retreat immediately when your subs are engaged by a surface fleet. All in all, I have found Naval and Tac bombers (flying from Spain) to be a better option for convoy raiding in DD.

Submarines in Armageddon

Version 1.1

Functionality essentially unchanged from Doomsday.

Version 1.2

The Amageddon patch v1.2 changed submarine behavior from v1.1

  • Submarines do not remove FOW in adjacent land areas
  • Small units (1 to 3) of submarines seem more vulnerable.
  • Large units (24+) of submarines seem more powerful.

In reviewing Combat range it becomes apparent that submarines should be grouped into singular models. Combining different model submarines will result in the whole fleet attempting to fight at the range of the shortest range submarine.