Submarine Glossary — Terms & Definitions
From AIP to wolfpack, this comprehensive glossary covers the essential terminology used in submarine operations, design, and warfare. Whether you are researching submarines for the first time or brushing up on technical terms, this reference has you covered.
41
Design, Operations, Weapons, Navigation
Beginner to Expert
A
AIP (Air-Independent Propulsion)
A propulsion technology that allows conventional (non-nuclear) submarines to operate without access to atmospheric oxygen for extended periods. Common AIP systems include hydrogen fuel cells (German Type 212A), Stirling engines (Swedish Gotland class), and MESMA closed-cycle steam turbines (French Scorpene class). AIP extends submerged endurance from days to weeks.
Anechoic Tiles
Rubber or synthetic tiles bonded to a submarine's outer hull to absorb active sonar pulses and reduce the return echo, making the submarine harder to detect. Also helps dampen machinery noise radiating from inside the hull. First developed by Germany in WWII (Alberich coating) and now used on virtually all modern military submarines.
B
Ballast Tank
Tanks located between the inner pressure hull and outer hull (in double-hull designs) or within the hull structure (in single-hull designs) that are flooded with seawater to submerge the submarine and blown with compressed air to surface it. Main ballast tanks control overall buoyancy; trim tanks fine-tune the submarine's attitude.
Beam
The width of a submarine at its widest point, measured from the outer hull. For reference, a Virginia-class submarine has a beam of 10.4 meters (34 feet), while the massive Typhoon class has a beam of 23 meters (75 feet).
Bollard Pull
A measurement of the pulling (towing) power of a vessel, expressed in tons-force. While primarily used for tugs, bollard pull ratings are relevant to submarine rescue vessels and the ships that tow deep submergence rescue vehicles to a submarine's distress location.
Bridge (Flying Bridge)
The open-air platform atop the submarine's sail (conning tower) used for navigation and lookout when operating on the surface. The bridge is exposed to the elements and waves. It is occupied only when the submarine is surfaced or transiting at periscope depth in restricted waters.
C
Cavitation
The formation and collapse of vapor bubbles in water, caused by a propeller spinning fast enough to create areas of very low pressure. Cavitation produces a distinctive broadband noise that is easily detected by passive sonar. Avoiding cavitation is critical to submarine stealth — speed, depth, and propeller design all affect the cavitation threshold.
CIWS (Close-In Weapon System)
An automated point-defense weapon system (such as Phalanx or Goalkeeper) mounted on surface warships to destroy incoming anti-ship missiles and aircraft. While submarines do not carry CIWS, understanding these systems is relevant to submarine warfare because they are part of the defensive screen that a submarine must penetrate to attack a surface warship.
Conning Tower
Historically, the armored tower from which a submarine was commanded during surface operations and attacks. In modern submarines, the term is often used interchangeably (though incorrectly) with "sail" — the streamlined vertical structure on top of the hull that houses periscopes, masts, and the bridge. Modern submarines are commanded from the control room inside the pressure hull.
Control Room
The nerve center of the submarine, located inside the pressure hull. Contains the ship control station (helm and diving controls), fire control systems, navigation equipment, periscope stations, and the officer of the deck's position. All tactical decisions and submarine operations are directed from the control room.
Crush Depth
The depth at which a submarine's pressure hull will catastrophically fail (implode) due to hydrostatic pressure. Crush depth is typically 1.5 to 2.0 times the submarine's rated test depth. For example, a submarine with a 300-meter test depth might not collapse until 450-600 meters. Exact crush depths are classified for military submarines.
D
DISSUB (Disabled Submarine)
The NATO designation for a submarine that has sunk or is disabled on the seabed and cannot surface under its own power. A DISSUB event triggers international submarine rescue protocols. The crew may attempt to escape using escape suits or wait for rescue by a deep submergence rescue vehicle.
Diving Planes (Hydroplanes)
Movable control surfaces mounted on the submarine's hull — either on the sail (fairwater planes) or the bow (bow planes), and on the stern. Diving planes control the submarine's angle of ascent or descent (pitch) and help maintain depth. They work like an aircraft's elevators but in water.
Draft
The vertical distance between the waterline and the deepest point of the submarine's keel when on the surface. Draft determines the minimum water depth in which a submarine can safely operate on the surface and is critical for entering ports and navigating shallow channels.
E
ELF (Extremely Low Frequency)
Radio communications using frequencies between 3 and 30 Hz. ELF signals can penetrate seawater to depths of hundreds of meters, making them the only way to communicate with deeply submerged submarines. However, the data rate is extremely low — a single three-letter code group can take 15 minutes to transmit. Used to send emergency action messages to ballistic missile submarines.
F
Fairwater
The streamlined housing around the submarine's sail structure that reduces hydrodynamic drag. Also refers to the sail itself in some navies. The fairwater planes are the diving planes mounted on the sail. Modern designs use retractable fairwater planes to reduce flow noise.
G
Galley
The submarine's kitchen. Despite extremely limited space, submarine galleys produce four meals a day for the entire crew. Submarine food is widely regarded as the best in the military. The galley operates around the clock to serve the rotating watch sections.
H
Head
Naval terminology for a toilet or bathroom. Submarine heads operate using a sanitary tank system that must be periodically "blown" to the sea using compressed air. Flushing a submarine head incorrectly — particularly when the sanitary tank is being blown overboard — can result in a spectacular and deeply unpleasant backflow. Operating the head correctly is one of the first skills new submariners learn.
Hull (Pressure Hull / Outer Hull)
The pressure hull is the inner hull that withstands ocean pressure and contains the crew, machinery, and weapons. It is constructed from high-strength steel (HY-80, HY-100) or titanium. The outer hull (on double-hull submarines) is a lighter structure that provides hydrodynamic shaping and contains ballast tanks, sonar arrays, and missile tubes.
Hydrophone
An underwater microphone used to detect and analyze sound in the ocean. Arrays of hydrophones form the basis of both submarine-mounted passive sonar systems and fixed seabed surveillance systems like SOSUS. Modern towed-array hydrophone systems can be thousands of meters long.
K
Keel
The bottommost structural element of the submarine's hull, running along its length. The keel provides structural strength and is the reference point for draft measurements. Keel laying is the traditional first milestone in submarine construction.
Knot
A unit of speed equal to one nautical mile (1.852 km / 1.151 miles) per hour. Most military submarines have a maximum submerged speed of 25-33 knots. The fastest submarine ever built — the Soviet Alfa class — could reportedly reach 41 knots submerged.
P
Periscope
An optical instrument that allows a submerged submarine to observe the surface and sky from below the waterline. Traditional periscopes use mirrors and prisms in a tube that penetrates the hull. Modern submarines increasingly use photonic masts — non-hull-penetrating camera systems mounted on a mast that transmit digital images to screens in the control room.
Ping
The sound pulse emitted by active sonar. The ping travels through the water and bounces off objects (submarines, ships, the seafloor), returning as an echo that reveals the target's range and bearing. Using active sonar ("going active") reveals the searching vessel's position and is only done when passive detection has failed or when prosecution of a contact requires precise range data.
Port
The left side of the submarine when facing forward (toward the bow). Port is indicated by red navigation lights. The opposite side is starboard.
Pressure Hull
The watertight inner hull of a submarine, designed to withstand the hydrostatic pressure of the ocean at operating depth. Constructed from high-yield steel alloys (HY-80 to HY-130) welded into a cylindrical or conical shape. Every penetration (for pipes, cables, shafts, and hatches) is a potential failure point and receives extreme scrutiny during construction and inspection.
R
Reactor (Nuclear)
A pressurized water reactor (PWR) that provides propulsion power and electricity aboard nuclear submarines. The reactor heats pressurized water in a primary loop, which transfers heat to a secondary loop that produces steam to drive turbines. Modern submarine reactors can operate for 25-33 years without refueling (life-of-ship cores). The reactor compartment is heavily shielded and is a restricted area even aboard the submarine.
S
Sail
The vertical structure rising from the top of the submarine's hull. Houses periscopes, communication masts, snorkel mast (on conventional submarines), and the bridge. In older terminology, this was called the conning tower, though modern sails serve a different function. The sail's shape is designed to minimize hydrodynamic drag and flow noise.
Screw (Propeller)
The submarine's propeller, which converts shaft power into thrust. Propeller design is one of the most closely guarded secrets in submarine technology — blade shape, number, skew angle, and tip design all affect noise generation. Many modern submarines use pump-jet propulsors instead of open screws for reduced acoustic signature.
Snorkel (Schnorchel)
A retractable tube that allows a diesel-electric submarine to run its diesel engines while submerged at shallow depth. The snorkel draws in air for the engines and expels exhaust. Invented by the Dutch Navy and perfected by the German Kriegsmarine in WWII. Still used on all conventional submarines today.
Sonar (SOund NAvigation and Ranging)
The primary sensor system used to detect objects underwater. Passive sonar listens for sounds (engine noise, propeller cavitation, biological sounds). Active sonar emits sound pulses and analyzes the returning echoes. Modern submarine sonar suites include bow-mounted spherical arrays, flank arrays, towed arrays, and mine-avoidance sonar.
Starboard
The right side of the submarine when facing forward (toward the bow). Starboard is indicated by green navigation lights. The opposite side is port.
Stern
The rear end of the submarine. The stern houses the propulsion machinery (propeller or pump-jet), stern diving planes, and rudder. The stern section of the pressure hull typically contains the engineering spaces, turbines, and reduction gears.
Submarine
A watercraft capable of independent operation underwater. Distinguished from a submersible by its ability to operate fully independently for extended periods. Modern military submarines are either nuclear-powered (SSN, SSBN, SSGN) or diesel-electric with AIP. The term comes from the Latin "sub" (under) and "marinus" (of the sea).
Submersible
An underwater vehicle with limited independent capability, typically requiring a support ship. Submersibles include deep-sea research vehicles (DSV Alvin), tourist submarines, and remotely operated vehicles (ROVs). The key distinction from a submarine is limited endurance and operational independence. The OceanGate Titan was a submersible, not a submarine.
SUBSAFE
The US Navy's quality assurance program for submarine hull integrity, established after the loss of USS Thresher in 1963. SUBSAFE certifies every component in systems that interface with the sea — from pipe fittings to hull welds. Every piece of material is traceable, every weld is X-rayed, every test is documented. No SUBSAFE-certified submarine has ever been lost.
T
Test Depth
The maximum depth to which a submarine is certified to operate under normal conditions. Test depth has a built-in safety margin — crush depth is typically 1.5 to 2.0 times test depth. Submarines routinely operate near test depth during sea trials and may approach it during tactical operations. Exceeding test depth is an emergency.
Torpedo Tube
A cylindrical launching device used to fire torpedoes (and sometimes cruise missiles, mines, or unmanned vehicles) from the submarine. Modern torpedo tubes are typically 533mm (21 inches) in diameter, though some Russian submarines use 650mm tubes for larger weapons. A submarine usually carries 4-8 torpedo tubes.
Trim
The process of adjusting a submarine's weight distribution to achieve neutral buoyancy and a level attitude (neither bow-up nor bow-down). Trim is controlled by pumping water between trim tanks fore and aft. Proper trim is essential for silent operation — an out-of-trim submarine requires constant use of diving planes, which generates noise.
V
VLF (Very Low Frequency)
Radio communications using frequencies between 3 and 30 kHz. VLF signals penetrate seawater to moderate depths (approximately 20 meters), allowing communication with submarines at periscope depth or trailing a VLF antenna near the surface. Higher data rates than ELF but less penetration. The primary method of broadcasting routine messages to submarines at sea.
W
Wolfpack (Rudeltaktik)
A submarine warfare tactic developed by German Admiral Karl Donitz in WWII. Multiple U-boats would spread across convoy routes; when one detected a convoy, it would shadow it while others converged. The pack would then attack simultaneously, overwhelming the escorts. The tactic was devastatingly effective until Allied countermeasures (radar, Ultra intelligence, escort carriers) neutralized the advantage.
Frequently Asked Questions
What is the difference between a submarine and a submersible?
A submarine is a self-sufficient vessel capable of independent operations — it can deploy from port, transit to its operating area, conduct its mission, and return under its own power. A submersible requires a surface support vessel, has limited range and endurance, and typically serves a specific purpose like research or rescue. Military vessels are submarines; research craft like Alvin are submersibles.
Why is a submarine called a "boat" and not a "ship"?
By naval tradition, submarines are called "boats" regardless of size. This dates back to the earliest submarines, which were indeed small craft. Even modern nuclear submarines displacing 18,000+ tons are referred to as boats. Calling a submarine a "ship" marks you as an outsider to the submarine community.
What does SSBN stand for?
SSBN stands for Ship Submersible Ballistic Nuclear: SS = Ship Submersible (submarine), B = Ballistic missile, N = Nuclear-powered. Similarly, SSN = nuclear attack submarine, SSGN = nuclear guided missile submarine, SSK = diesel-electric hunter-killer submarine.
What is the difference between active and passive sonar?
Active sonar sends a sound pulse ("ping") and listens for echoes. It gives precise range and bearing but reveals the operator's position. Passive sonar only listens for sounds from other vessels. It is stealthy but cannot determine range directly. Submarines almost always use passive sonar to remain undetected.
What is crush depth?
Crush depth is the depth at which a submarine's pressure hull will catastrophically fail (implode) due to water pressure. It is typically 1.5-2x the submarine's rated test depth. For example, a submarine rated to 300 meters might implode between 450-600 meters. Exact crush depths are classified for military submarines.
Dive Deeper
Now that you know the terminology, explore the technology, history, and operations that bring these terms to life.