Saddle tanks are a type of ballast tank

Ballast tank

A ballast tank is a compartment within a boat, ship or other floating structure that holds water.-History:The basic concept behind the ballast tank can be seen in many forms of aquatic life, such as the blowfish or argonaut octopus, and the concept has been invented and reinvented many times by...

 configuration fitted to mid-era submarines, those of World War II.

Saddle tanks are fitted in pairs external to the pressure hull

Submarine hull

The term light hull is used to describe the outer hull of a submarine, which houses the pressure hull, providing hydrodynamically efficient shape, but not holding pressure difference...

, one on each side, in a similar manner to that of a horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

's saddle

Saddle

A saddle is a supportive structure for a rider or other load, fastened to an animal's back by a girth. The most common type is the equestrian saddle designed for a horse, but specialized saddles have been created for camels and other creatures...

-bags, the positioning of which they resemble in appearance.

Development

The first effective submarines, those of World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, had hulls that were broadly circular in cross-section, with a deck plate mounted midway. Their heavy battery tanks were mounted beneath this deck, for stability. The ballast tank

Ballast tank

A ballast tank is a compartment within a boat, ship or other floating structure that holds water.-History:The basic concept behind the ballast tank can be seen in many forms of aquatic life, such as the blowfish or argonaut octopus, and the concept has been invented and reinvented many times by...

s were mounted inside the pressure hull. For compactness the ballast tanks were wrapped around the batteries, low down and sharing the flat surfaces of the battery tank. The Kingston valve

Kingston valve

A Kingston valve is a type of valve fitted in the bottom of a ship's fuel, water and ballast tanks. Named after its inventor John Kingston , an English engineer....

s linking the ballast tanks to the sea could be left open, a practice known as "riding the valves", and the water level in the tanks controlled solely by the vent and blowing air valves. The drawback was that the ballast tanks, open to sea pressure, had a flat surface to the crew compartment and were thus restricted in strength to the bursting strength of this flat plate. In particular this led to accidents where the boat bottomed safely after an accident, but the excess pressure of 'blowing out' was sufficient to cause the internal tank wall to fail.

A secondary drawback of these internal ballast tanks could be poor sea-keeping when surfaced. With the tanks full of water the submarine was stable, but when emptied, this large buoyant volume low-down led to excessive rolling. This was made worse as early submarines also spent almost all of their time surfaced, and had little freeboard

Freeboard

Freeboard may refer to:* Freeboard , the height of a ship's deck above the water level.* Freeboard , a six-wheeled skateboard designed to act like a snowboard....

 to begin with.
The saddle tank was formed by moving the ballast tanks outside of the pressure hull. Early designs raised the weather deck and casing above the pressure hull, producing a ballast tank that wrapped around the upper part of the pressure hull (diagrams A & B). Following the terminology of steam railway locomotives, this was termed a "saddle tank". Later designs, notably those of the US Fleet submarine

United States Porpoise class submarine

The Porpoise class were submarines built for the United States Navy in the late 1930s, and incorporated a number of modern features that would make them the basis for subsequent classes such as the Salmon, Tambor, Gato, Balao, and Tench classes...

s and German U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

s, placed two separate ballast tanks lower down on the sides of the hull (diagram C). In railway practice this would have been a "pannier tank", but for submarines the "saddle" label remained.

For some time it remained practice to place the main ballast tanks outside the pressure hull, but to keep the smaller auxiliary tanks inside the hull and below the deck. In the US Navy T class submarine

AA-1 class submarine

The AA-1 class was a class of three experimental submarines of the United States Navy, built toward the end of World War I, between 1916 and 1919. The design was not a success and none of the submarines saw active service...

s, this tank wall was at least curved to resist pressure.

Development after World War II

The tanks themselves are usually streamlined

Streamliner

A streamliner is a vehicle incorporating streamlining in a shape providing reduced air resistance. The term is applied to high-speed railway trainsets of the 1930s to 1950s, and to their successor "bullet trains". Less commonly, the term is applied to fully faired recumbent bicycles...

 to reduce water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 resistance

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 and are prominent on many submarine designs produced prior to the end of World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

.

Saddle tanks are not fitted to current naval submarine designs as external fittings are discouraged in order to improve underwater speed and reduce hydrodynamic noise detectable by an enemy's passive-sonar

Sonar

Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...

. For this reason, post-1945 designs, i.e. post Albacore

USS Albacore (AGSS-569)

USS Albacore was a unique research submarine that pioneered the American version of the teardrop hull form of modern submarines. The revolutionary design was derived from extensive hydrodynamic and wind tunnel testing, with an emphasis on underwater speed and maneuverability...

, usually use internal ballast tanks, which are fitted inside the teardrop pressure hull

Teardrop hull

A teardrop hull is a submarine hull design which emphasizes hydrodynamic flow above all other factors. Benefits over previous types include increased underwater speed and a smaller acoustic signature, making detection by sonar more difficult...

.