Classification of Underwater Vehicles and Submarines

Underwater vehicles are deployed across diverse domains, from defense to scientific research and extreme-environment exploration. They vary in size and type, capable of diving to profound ocean depths and remaining submerged for hours or even days. These vehicles can be manned or unmanned and may operate via self-contained power systems or other propulsion modes.

Submarines, the most significant category of underwater vehicles, are large, self-propelled vessels composed of one or multiple pressure hulls, equipped with dedicated propulsion and supply systems. However, underwater vehicles extend beyond submarines and include other types.

Types of Underwater Vehicles
Based on crew presence, underwater vehicles are categorized as follows:

Submarine

Submersible

Autonomous Underwater Vehicles (AUVs)

Submersibles are compact underwater vehicles designed for specialized tasks. Most are non-self-propelled, relying entirely or partially on external operators, facilities, support vessels, or submarines. However, some submersibles are self-propelled. Primarily used for research, exploration, and occasionally commercial ventures, these vehicles are precursors to modern submarines, with human exploration of the ocean spanning centuries.

Manned vs. Unmanned Submersibles

Manned submersibles accommodate one or more crew members.

Unmanned submersibles operate without onboard personnel.

The first manned submersible, built in 1620 by a Dutch scientist, marked the beginning of human experimentation. In 1775, the revolutionary "Bushnell’s Turtle" became the first operational manned submersible. Its hull consisted of a wooden and brass elliptical pressure chamber. Ballast operations for submersion and surfacing were managed via a manual pump and small tanks, while two hand cranks provided propulsion. Illuminated instruments and small windows enabled visibility, and the vessel was designed to attach explosives to enemy ships—a pioneering concept in underwater warfare.

Modern Manned Submersibles
Modern manned submersibles are termed Human-Occupied Vehicles (HOVs). Key subtypes include:

Deep-Submergence Vehicles (DSVs): Engineered for critical deep-sea operations, military missions, and research, these self-propelled vehicles withstand extreme pressures at great depths. Notable examples include the U.S. Navy’s DSV Alvin, operational for nearly 60 years and renowned for locating the Titanic wreck. DSVs feature titanium alloy hulls and high endurance.

Bathyscaphes: Free-diving deep-sea vehicles with a large upper hull (flotation chamber) and a domed pressure capsule for crew. Ballast water and gasoline-filled tanks regulate buoyancy, while limited lateral mobility is achieved via small propellers. Bathyscaphes represent significant advancements over early bathyspheres.

Unmanned Underwater Vehicles
Unmanned vehicles are smaller due to the absence of crew accommodations. Common types include:

Remotely Operated Vehicles (ROVs): Tethered to surface or shore-based control stations, these vehicles receive power and commands via cables. They perform diverse tasks across varying depths with full maneuverability.

Autonomous Underwater Vehicles (AUVs): Untethered, self-propelled robots operating via pre-programmed instructions. Ranging from portable units to large vessels (10–20 meters), AUVs are breakthroughs in scientific and military applications. Their payload capacity scales with size, analogous to submarines.

Submarine Types
Submarines, the largest and most advanced underwater vehicles, are classified as follows:

By Design

U-boats: Asymmetric, ship-like submarines pivotal in World Wars I and II, capable of surface and submerged operations.

Midget Submarines: Sub-150-ton vessels carrying 5–6 crew members, designed for torpedo/mine deployment and reliant on support ships. Exclusively military.

Human Torpedoes: Compact, crewed torpedo launchers, phased out post-World War II.

Conventional Submarines: Cylindrical, multi-role vessels for military, commercial, or civilian use.

By Propulsion

Diesel-electric

Hybrid (diesel + electric)

Nuclear or hybrid (nuclear + diesel/electric)

Air-Independent Propulsion (AIP)

Steam/mechanical (obsolete)

By Application

Defense

Commercial

Civilian

Scientific Research

Pressure Hull Design Classifications

Single-Atmosphere Submersibles: Maintain internal atmospheric pressure, requiring hulls to withstand external hydrostatic pressure. Complex and costly.

Isobaric Submersibles: Equalize internal and external pressures, simplifying design.

Wet Submersibles: Partially exposed to water, with crew using diving gear. Depth-limited but offering immersive experiences.