As with Midway, operating submarines requires a lot of skill and micromanagement. Although the AI can manage basic things, like moving and firing, at least somewhat effectively, the AI does not do an effective job with diving to the proper depth, as well as evading attacks when attacked underwater. They are also rather slow, requiring patience to get in firing position, and even more to set up the perfect shot for your torpedoes. Submarines have only a few torpedoes and replenish themselves rather slowly, so submarine commanders must make their torpedoes count.
The major threats to submarines are destroyers, depth charge-equipped dive bombers, and most devastatingly, floatplanes and flying boats. The latter two are the most devastating since these planes sport sonar - it is impossible to hide from a floatplane or a flying boat aside from going to crush depth. Not only that, but they also sport their own depth charges, allowing them to attack unhindered. Because of these aerial threats, any submariner will always appreciate a squadron of fighters overhead.
One of the major problems in Midway was the effectiveness of air-dropped depth charges. These charges easily killed submarines in one or two hits. In Pacific, air-dropped charges cause considerable damage, but a single squad will not kill a submarine. Air-dropped charges must be dropped accurately in order to cause maximum damage, but will still cause some damage at near misses.
Submarines do not have an engine element - it is impossible to stop them from moving. Their engine is replaced instead by a periscope. The periscope can be damaged if you bump into a ship with your periscope raised. Ordering crew to repair the periscope will allow you to get your periscope back and running quicker (although how crew can repair the periscope while underwater, with the sub in motion, is unknown). Periscopes now have a limited ability to look up. Submarines can now spot aircraft with their periscope as a result.
Unfortunately, because all units short of carriers can be respawned instantly in Pacific, submarines are not as valuable as they were in Midway. Their main goal now is to simply damage enemy ships as they appear; sinking them is generally pointless.
Submarines are therefore best used singularly or in tandem to intercept ships that your forces cannot defeat easily. For example, in Swirl: an enemy player selects the Montana-class or Super Yamato-class unlock. Your team only has access to the Renown-class or Kongō-class battleships, which are no match for these high-class battleships. A single 100 point submarine can critically damage these super battleships without fear of retaliation from them, allowing ordinary battleships or even heavy cruisers to finish them off.
Submarines are also perfect for hunting down enemy carriers; if victory hinges on the destruction of a carrier, a submarine is perfect for this.
Submarines operate at one of four depth levels. The surface and dive controls will allow you to control which level your submarine operates at. Submarines also have a hull component that can be destroyed. If the conning tower of your submarine is destroyed, your submarine will take minuscule but constant damage at manoeuvring depth as well as crush depth.
- Surface: While your submarine has the most weaponry and functionality on the surface, try to keep your submarine's surface visits brief. Submarines have little to no armour on the surface, and a few cannon shots or well-placed bomb can easily fry a surfaced submarine. Players attempting to get places in a hurry should stay at the surface, as subs are always the fastest on the surface.
- Attack Depth: Also known as periscope depth. Attack depth is where your submarine should be to launch torpedoes underwater. Going to attack depth renders you invulnerable to cannon fire, but at the same time remain extremely weak to depth charge attacks of any kind. Torpedoes launched from this depth must rise to the surface, so point-blank torpedo shots will miss at this depth.
- Manoeuvring Depth: Your submarine should be spending most of its time at this depth level. A submarine is rather resistant to depth charge attacks here, but it can still be targeted. If the conning tower of your submarine is destroyed, however, your sub will take minuscule but constant damage at this depth.
- Crush Depth: This depth level is beyond what your submarine can take in terms of pressure. While your submarine will take damage until it is destroyed at this depth, you are completely invulnerable to depth charges and invisible to sonar. Be careful, however, as your own sonar goes dead and you will be unable to see anything aside from the seabed, even visually.
Although today's submarines have advanced electrolysis techniques to provide a near infinite supply of oxygen, WW2 submarines didn't have this luxury - they had to periodically surface for air.
All submarines have an air supply, used by crews and the ship's engines. The air supply slowly depletes while the submarine is underwater, and will automatically replenish at the surface. Running out of oxygen while underwater won't kill a submarine, but instead force it to the surface.
Fortunately, submarines can stay underwater for a long time - usually several minutes. The Type A midget submarine, being small, naturally has a considerably shorter oxygen supply than the larger fleet submarines. It takes about 30 seconds to a minute to refill completely, so a skillful submarine commander can time his or her visits well to avoid unnecessary damage and conflict.
A submarine's primary weapon is the torpedo. These self-propelled underwater missiles are guaranteed to breach a target's hull and cause considerable damage. Torpedoes have been sped up considerably from Midway, making them easier to use. They also cause a lot more damage. The submarine AI effectively employs torpedo waves against enemies now, making the submarine AI more effective. In general, however, it is still best to attempt to micromanage submarines where possible.
When launched at depth 2, the torpedoes will take about 2 seconds to rise to the surface. Until then they simply swim under anything, so firing a torpedo at point-blank range will, most often than not, actually result in your torpedoes being too deep to strike the enemy hull.
Like ship-launched torpedoes, submarine torpedoes can steer themselves to a small degree. You can fire torpedoes approximately 30 degrees to the left or right of the bow or stern. When launched, the torpedoes will travel straight until they clear the sub, at which they will turn toward the crosshair, rising to the surface at the same time.
The advantage to torpedoes is their sheer power. Two torpedoes can sink a destroyer (assuming its crew isn't at water repair) or a cargo ship. Four can sink a heavy cruiser. You have over 30 torpedoes. Be happy, submariners.
While in Midway it was difficult to, as a submarine, sink another submarine, in Pacific it is entirely possible. Torpedoes can be launched horizontally at other submarines while both are underwater. These torpedoes cannot steer themselves left and right but will automatically descend and rise to meet their target submarine.
Once a submarine has acquired another submarine as a target, torpedoes sent in the target submarine's direction will automatically be launched horizontally. They will also, as mentioned previously, automatically rise and descend to meet their target submarine if necessary.
Because submarines are more versatile than depth charge equipped recon planes, a submarine can prove to be an effective counter to another submarine. Once the enemy submarine threat has been neutralized, the sub can proceed to attack the enemy fleet.
The Head-On Attack
Most destroyers, once they spot you, will make a beeline straight for you, obviously intending to get on top of you and sink you with depth charges. You can turn this against them by performing a head-on attack. A head-on attack is difficult to do, but a master of the manoeuvre can sink enemy destroyers with a submarine, inverting the typical role between subs and destroyers.
The head-on attack is just what it sounds like — going head-to-head with a destroyer. If the enemy is in a straight line for you, you can't miss since the enemy warship isn't moving laterally. Once you're almost right on top of each other – at 0.3 nautical miles or less (but not so close your torpedoes don't rise to the surface in time — that's what makes the move tricky) – unleash two or more torpedoes (more if captained by another player).
If all goes well, your torpedoes should hit the bow of the enemy warship, which registers as a hit. Dive to crush depth to avoid a counter-attack (and to prevent your periscope from being smashed off — fortunately this goes away after a while). If two or more torpedoes connected, the enemy destroyer should be heading for the bottom.
In Pacific, torpedoes can no longer be fired in rapid succession - they now have a cooldown of about a quarter to half of a second. Be sure to account for this when attempting to perform the head-on attack.