Saddle tank (submarine)

Saddle tanks are fitted in pairs external to the pressure hull, one on each side, in a similar manner to that of a horse's saddle-bags, the positioning of which they resemble in appearance.

The first effective submarines, those of World War I, had hulls that were broadly circular in cross-section, with a deck plate mounted midway.

With the tanks full of water the submarine was stable, but when emptied, this large buoyant volume low down led to excessive rolling.

Later designs, notably those of the US Fleet submarines and German U-boats, placed two separate ballast tanks lower down on the sides of the hull (diagram C).

[2] The tanks themselves are usually streamlined to reduce water resistance and are prominent on many submarine designs produced prior to the end of World War II.