Saddle tanks are a type of ballast tank configuration fitted to

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

. Saddle tanks are fitted in pairs external to the

pressure hull A submarine hull has two major components, the ''light hull'' and the ''pressure hull''. The light hull (''casing'' in British usage) of a submarine is the outer non-watertight hull which provides a hydrodynamically efficient shape. The pressur ...

, 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 million ...

saddle The 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 kno ...

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

Development

Submarine ballast tanks, before saddle tanks (Warships To-day, 1936)

The first effective submarines, those of

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

, 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 tanks 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 to begin with. Submarine saddle tanks (Warships To-day, 1936)

Submarine saddle tanks (Warships To-day, 1936)

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 submarines and German

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare ro ...

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.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 (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

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

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

. Saddle tanks are not fitted to current naval submarine designs, as external fittings are discouraged in order to improve underwater speed and reduce

hydrodynamic In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) and ...

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 on ...

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

References

{{Reflist Submarine design Submarine components