Saddle tanks are a type of

ballast tank A ballast tank is a Compartment (ship), compartment within a boat, ship or other floating structure that holds water, which is used as ballast to provide hydrostatic stability for a vessel, to reduce or control buoyancy, as in a submarine, to co ...

configuration fitted to

submarines A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or info ...

. Saddle tanks are fitted in pairs external to the

pressure hull A submarine hull has two major components, the ''superstructure'' and the ''pressure hull''. The external portion of a submarine’s hull—that part that does not resist sea pressure and is free-flooding—is known as the “superstructure” i ...

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

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...

saddle A saddle is a supportive structure for a rider of an animal, fastened to an animal's back by a girth. The most common type is equestrian. However, specialized saddles have been created for oxen, camels and other animals. It is not know ...

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

Development

The first effective submarines, those of

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

, 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

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 valves 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 a particular problem because early submarines spent almost all of their time surfaced and had little

freeboard In sailing and boating, a vessel's freeboard is the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship. In commercial vessels, the latter criterion measured relativ ...

to begin with. The saddle tank was formed by moving the ballast tanks outside 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 A fleet submarine is a submarine with the speed, range, and endurance to operate as part of a navy's battle fleet. Examples of fleet submarines are the British First World War era K class and the American World War II era ''Gato'' class. The ...

s and German

U-boat U-boats are Submarine#Military, naval submarines operated by Germany, including during the World War I, First and Second World Wars. The term is an Anglicization#Loanwords, anglicized form of the German word , a shortening of (), though the G ...

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 A tank locomotive is a steam locomotive which carries its water in one or more on-board water tanks, instead of a more traditional tender (rail), tender. Most tank engines also have Fuel bunker, bunkers (or fuel tanks) to hold fuel; in a #Tender ...

", but for submarines the "saddle" label remained.A source of some confusion to railway aficionados since. 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 submarines, this tank wall was at least curved to resist pressure.

Development after World War II

The tanks themselves are usually streamlined to reduce

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

resistance and are prominent on many submarine designs produced prior to the end of

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

. Saddle tanks are not fitted to current

naval A navy, naval force, military maritime fleet, war navy, or maritime force is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operatio ...

submarine designs, as external fittings are discouraged in order to improve underwater speed and reduce

hydrodynamic In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in moti ...

noise detectable by an enemy's passive

sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects o ...

. For this reason, post-1945 designs, i.e. post ''Albacore'', usually use internal ballast tanks, which are fitted inside the

teardrop hull A teardrop hull is a submarine hull design which emphasizes submerged performance over surfaced performance. It was somewhat commonly used in the early stages of submarine development, but was gradually abandoned in the early 20th century in favo ...

References

{{Reflist Submarine design Submarine components