Recently, the naval community worldwide felt the flame in their hearts because of the KRI Nanggala incident. After the search and rescue efforts lasted a few days, debris of the Indonesian submarine was found in 838 meters depth. As far as we’ve learned, the submarine had a significant defect in the diving and propulsion system and couldn’t handle the boat in a safe depth. This incident raised the curiosity of people about the submarines’ diving limits. How deep can they dive into the water? What is the maximum depth can a submarine endure? Why do they have such a limit?

## Structure of a submarine hull

The accurate information of submarines’ endurance to the ocean’s pressure is generally classified—however, we have some open-source information about design, test and maximum operational depth of different types of subs.

A submarine’s endurance to the water pressure mainly depends on the material used to build the subs and the hull capacity. A sub cannot dive as deep as an underwater research vehicle. While the design purpose of an underwater research vehicle is to navigate underwater and search, there are different purposes in designing submarines, including stealth, sound-absorbing, silence, manoeuvrability etc. And the cost is also is a critical concern.

A submarine hull has two major components, the light hull and the pressure hull. Hulls are unique structures in a submarine to hold water in them, which help the sub to maintain buoyancy underwater. The light hull of a sub is the outer non-watertight hull which provides a hydrodynamically efficient shape. The light hull is usually made of a thin steel plate, as it has the same pressure on both sides. Inside the outer hull, there is a strong hull, or pressure hull, which withstands the outside pressure and has normal atmospheric pressure inside. The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve.

Carbon steel or Titanium is also used to build subs. Titanium is better for stealth but cannot perform many multiple dives as the metal tends to get brittle.

## Depth Terms

Before talking about the depth limits, it is a “must” to learn the relevant terms.

**Depth ratings** are primary design parameters and measures of a submarine’s ability to operate underwater. The strength of their hulls limits the depths to which subs can dive. It is essential to realize that there is a limit to how high the pressure can build inside the sub as problems develop. For example, oxygen becomes toxic at high pressures; thus, the pressure cannot equalize.

**Design depth** is the nominal depth listed in the submarine’s specifications. The designers calculate the thickness of the hull metal, the boat’s displacement and many other related factors. Since the designers incorporate a margin of error in their calculations, the crush depth of an actual vessel should be slightly deeper than its design depth.

**Test depth** is the maximum depth at which a sub is permitted to operate under normal peacetime circumstances and is tested during sea trials. The test depth is set at two-thirds of the design depth for U.S.Navy subs, while the Royal Navy sets test depth slightly deeper than half (4/7ths) of the design depth, and the German Navy sets it at exactly one-half of design depth.

**The maximum operating depth** (popularly called the never-exceed depth) is the maximum depth at which a sub is allowed to operate under any (e.g. battle) conditions.

**Crush depth**, officially called collapse depth, is the depth at which a submarine’s hull will collapse due to pressure.

## How deep can a submarine dive?

There is a safe diving depth for any sub, and this is regularly achieved for exercise purposes. The theoretical maximum diving depth is then calculated by the engineers involved in the construction and design of the boat. This can, theoretically, be exceeded as it has an element of caution built in. Beyond this is the crush depth of the sub, if exceeded, will lead to the boat imploding. These limits can be worked out, but unless there is an accident, you will never know what the exact crush depth will be.

The age of the submarine’s hull and how much the sub exceeded its max depth are other factors that affect the submarine’s endurance. A submarine’s hull life is estimated by submergence cycles, each time a submarine submerges and goes deep, and it shortens its hull life. Exceeding test depth reduces hull life. It then becomes a matter of it being economical to keep the sub active. Submarines have several significant depth numbers, test depth, and sub expected to achieve without any ill effects operationally. When testing and certifying submarine systems exposed to sea pressure, it is to this pressure that systems are tested to.

Each submarine has its own specified max depths of X feet gained during the initial testing of the submarine class. For military subs, the “test depth” (the deepest a sub can repeatedly go) is classified. It’s generally accepted that the maximum depth (depth of implosion or collapse) is about 1.5 or 2 times deeper.

The latest open literature says that a **US Los Angeles-class test depth** is 450m (1,500 ft), suggesting a maximum depth of 675–900m (2,250–3,000 ft). This is a submarine with a pressure hull made of HY-80 high-tensile steel. The latest American sub is said to be constructed of HY-100, so they can certainly go deeper. Some Soviet/Russian subs use Titanium (stronger but more brittle than steel). The first one, Project 705 (Лира/Lira, “Lyre”) class (NATO “Alfa”) is known to have gone to 1,000m (3,300 ft), suggesting a maximum depth of 1,500–2,000m (5,000–6,600 ft). Later classes (such as Project 945B Kondor — NATO Sierra II) can probably go at least that deep.

The depth limits of the most known nuclear powered submarines’ depth limits, as follows;

- Typhoon-class: Test depth 900 m (3,000 ft)
- Astute-class: Over 300 m (984 ft 3 in)
- Akula-class: 480 m (1,570 ft) test depth for Akula I and Akula I Improved, 520 m (1,710 ft) for Akula II and III, 600 m (2,000 ft) maximum operating depth
- Ohio-class: Test depth >240 m (800+ ft)
- Virginia-class: Test depth >240 m (800+ ft)
- Borei-class: Test depth 950 m
- Rubis-class: Test depth 350 m
- Barracuda-class: Test depth >350 m

To make a little comparison with diesel-electric subs (SSK), the revealed depth limits of SSKs are given below;

- Scorpene-class: >350 m (1,150 ft)
- Type 209-class: 500 m (1,600 ft)
- Type 212-class: Test depth is 250 m (820 ft), crush depth over 700 m (2,296 ft)
- Type 214-class: Operating depth is more than 250 m (820 feet) officially, 400 m estimated (1312 feet), test depth is nearly 400 m (1,300 ft)
- Kilo-class: Operational depth is 240 m (790 ft), maximum is 300 m (980 ft)
- Yuan-class: Test depth is 550 m

The deepest-diving large, military-style submarine was the **Soviet submarine K-278 Komsomolets**, with a hull made of, Titanium making it very expensive but able to withstand significantly deeper dives than the best subs made of high-grade steel, like American nuclear subs. The Komsomolets was a nuclear-powered submarine specially designed to make trips as far down as 1300 meters (4265 feet) below sea level.

Special-purposed subs can submerge much deeper. The US DSRV (Deep Submergence Rescue Vehicle), built to rescue the survivors of a sunken submarine, has a test depth of 1,500m (5,000 ft), suggesting a maximum depth of 2,250–3,000m (7,500–10,000 ft).

A small sub, the **bathyscaphe Trieste**, made it to 10,916 meters (35,813 feet) below sea level in the deepest point in the ocean, the Challenger Deep in the Marianas Trench, a few hundred miles east of the Philippines. This part of the ocean is 11,034 m (36,200 ft.) deep, so it seems that a sub can make it as deep as it’s theoretically possible to go.

**Check out Naval Library App to find out the specifications of submarines.**