Saddle tanks are commonly used in automobiles, with some designs featuring two external fuel tanks, while others have a single tank with dual chambers. They are also used for storing and transporting water or chemicals, and in small steam engines for industrial use. Early submarines used external ballast tanks, but post-war designs moved them inside for increased speed.
The term saddle tank is most commonly associated with a fuel tank design in automobiles. A style usually found on trucks uses two fuel tanks, one on each side. In railway terminology, the saddle tank refers to a type of small tank locomotive. Before the end of World War II, submarines were designed with external ballast tanks also known as saddle tanks.
A saddle tank fuel system is currently used in a number of automobiles. Some of these first appeared on trucks as two external fuel tanks, but the vehicles were later redesigned to place the tanks inside for safety purposes. Double tanks are especially desirable in pickup trucks, which get lower fuel economy and are often used in rural areas some distance from fuel stations. In most designs, a switch is mounted on the dash that allows the driver to transfer fuel usage from one tank to another.
Custom saddle tank designs are frequently found on four-wheel drive and rear-wheel drive cars. These actually consist of a single fuel tank that has dueling chambers. The tank is bolted to the rear of the vehicle, with the two halves straddling the rear driveshaft. While these tanks have traditionally been made of steel, many are now made of molded plastic. A single fuel tank that fits over a motorcycle frame tube is another saddle tank design.
Saddle tanks are not only used for fuel, but are often used to store and transport drinking water or agricultural chemicals. Others have been designed to capture and retain rainwater from downspouts for future use. They are generally made of plastic and come in a variety of sizes and configurations. Trucks used for industrial cleaning can get a freshwater saddle tank designed to fit snugly over the truck wheel, balancing the weight of the water evenly between the axles.
Saddle tank locomotives are small steam engines that carry their own water and fuel. The water tank is located above the boiler, like a saddle, and is popular for industrial use. It can carry a large supply of water, but it has a higher center of gravity, so it must be operated at lower speeds. Historically, saddle tank engines were very popular in mines, lumber mills, manufacturing plants, and construction sites, often eliminating the need for a bid.
Early submarines, especially German submarines, used ballast tanks. These were installed as pairs on the external pressure hulls. Post-war submarines moved the ballast tanks from the outside to the inside to reduce water resistance and increase speed.
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