Non-nuclear powered submarines are typically referred to as conventional submarines and make up the vast majority of submarines in service worldwide. Compared to nuclear submarines they have several differences:
- Cheaper and easier to build and maintain
- Typically slower speeds (submerged cruise speed tends to be < 10kt)
- Reduced operational range
- Typically smaller size
- Smaller crews
- Most require time at or near the surface to recharge their batteries (see Propulsion below)
- Very quiet when operating on battery power
Due to their slower speeds and limited operational range, conventional submarines are often employed in littoral waters (i.e. shallow waters close to land) and are particularly effective in natural chokepoints like straits and channels. While conventional submarines lack the speed and endurance of nuclear submarines, their quietness and stealth allow these relatively inexpensive platforms to pose a major threat, even to extremely heavily defended targets (as evidenced by the famous case of a Swedish Gotland-class submarine 'sinking' the USS Ronald Reagan in an early 2000's exercise ).
Conventional submarines have diesel combustion engines that they use to operate on the surface, as well as batteries which are used while the submarine is submerged. The batteries are charged by excess power from the diesel engines.
In C:MANO a submarine will automatically switch between diesel and battery propulsion depending on the depth of the submarine; at periscope depth a snorkel-equipped submarine will deploy the snorkel and switch to diesel propulsion, while a submarine without a snorkel is required to be on the surface in order to switch to diesel propulsion. If neither of those conditions are true (i.e. the submarine is submerged) the submarine will run on battery power. When battery reaches a certain threshold (set in the Unit Doctrine menu) the submarine will automatically come to a depth where it can operate the diesel engines to recharge the batteries (unless there is a nearby threat).
A snorkel is--in basic terms--a tube that allows a submerged submarine to vent exhaust gasses into the atmosphere. Snorkels typically have a range of design features that prevent the ingress of sea water and simultaneously allow the submarine to be ventilated with fresh air (exhaled carbon dioxide builds up in submerged conventional submarines--this however is not modelled in C:MANO... yet) while venting exhaust gasses.
Prior to the German Type XXI submarine coming into service in 1943 conventional submarines were not typically fitted with snorkels (although some types were retrofitted after this date) and required surfacing to run the diesel engines and recharge the batteries. After the Second World War the addition of snorkels to new submarine designs became almost universal, allowing conventional submarines to operate on diesel power while at periscope depth and therefore greatly reducing their vulnerability to observation.
In C:MANO a submarine fitted with a snorkel will automatically deploy the snorkel and switch to diesel propulsion when at periscope depth or shallower.
Some more modern (2000's and beyond) submarine types are fitted with air-independent propulsion (AIP); an umbrella term for a variety of technologies that allow conventional submarines to operate submerged for extended periods. As they typically rely on volatile fuel sources like hydrogen and liquid oxygen, AIP systems typically cannot be recharged at sea.
In C:MANO a submarine fitted with AIP will not use AIP unless it is in combat and the battery is depleted--this is to ensure that AIP isn't 'used up' performing mundane tasks like transiting or patroling. Once AIP is engaged the submarine will be limited to creep speed.
Diesel submarines are much quieter than nuclear submarines (when operating on battery power, that is--running the diesels is extremely loud) but lack the speed and endurance of their nuclear cousins. To make up for the lack of speed, it is recommended that off-board support from reconnaissance assets such as maritime patrol aircraft be used to locate targets and identify their probable path of intended motion. The conventional submarine can then be moved into position ahead of time to wait for the target to enter their weapons employment envelope.
Chasing targets with a diesel submarine is not a good practice; traveling at 'high' speeds (which are embarrassingly slow compared to nuclear submarines) will negate the quietness of conventional submarines and rapidly discharge the batteries. In situations where an intercept cannot be made at a reasonable speed ('cruise' speed in game terms) a better option may be to use submarine launched missiles, or forego the attack entirely and wait for a more suitable target.
Conventional submarines as an ASW platform
Because of limited speed and submerged endurance, conventional submarines are not ideal ASW platforms. A nuclear submarine has a real chance of outrunning a heavyweight torpedo, while the chance of a conventional submarine outrunning even lightweight torpedoes is marginal at best. If your opponent manages to detect you and fire on you, your chances of surviving the encounter plummet. With this in mind, it is important that when conducting ASW operations using conventional submarines a commander lays the best foundation for a 'one-shot kill' of the target submarine. Ideally you will fire from behind the enemy submarine (the baffles) at a range where even with rapid acceleration to flank speed they will not be able to escape the torpedo. Since you are behind the enemy submarine and in their sonar baffles an ideal outcome is that they will not detect you on firing, will not detect the torpedo until it is too late, and will not be able to return even a bearing-only shot since they will be accelerating away from the incoming torpedo (i.e. their weapons will be pointing away from you).
While conventional submarines are vulnerable to other submarines and--to a lesser extent--surface ships, aircraft are the biggest threat to conventional submarines. Their high speed allows them to rapidly respond to submarine detections by other platforms, and often the first indication you will have that they have found you is active sonobouys appearing nearby, or a torpedo in the water. Once your position is known, it is only a matter of time before weapons are in the water in an effort to destroy you.
The weakness of an aircraft of an ASW platform is its limited endurance; finite fuel means that an aircraft will have to return to base at some point to refuel. If you are being harassed by an aircraft the best option is to attempt to obfuscate your position (either by remaining quiet, frequently altering course, making high speed dashes or a mix of all three) while moving away from the aircrafts base to reduce its time on station; once the aircraft leaves the area you should change course and move away from your current position as fast as possible. There will no doubt be another aircraft coming to continue the hunt, and the more distance you can put between your last known position and your current position the less likely you are to be found.
Some submarines are equipped with surface-to-air missiles; if you are being harassed by ASW aircraft you can opt to surface and attempt to destroy them. This is a bold but not always successful gambit; the SAMs carried on submarines are usually short ranged and often have a low probability of kill (PoK), and being on the surface not only exposes you to return fire in the form of rockets, guns, bombs and missiles, but also definitively gives away your identity and position--it is very satisfying to shoot down that annoying MPA though!