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The term saddle tank is most commonly associated with a fuel tank design in automobiles. One style typically found on trucks uses two fuel tanks, one on each side. In railroad terminology, saddle tank refers to a type of small tank locomotive. Prior to 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 on 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. Dual tanks are especially desirable on pick-up trucks, which get lower fuel mileage and are often used in rural areas some distance from fuel stations. In most designs, a switch is mounted on the dashboard which allows the driver to transfer fuel usage from one tank to another.
Custom saddle tank designs are frequently found on rear wheel drive and four wheel drive automobiles. These actually consist of a single fuel tank which has duel chambers. The tank is bolted to the rear of the vehicle, with the two halves straddling the rear drive shaft. While these tanks have traditionally been made of steel, many are now made from molded plastic. A single fuel tank that fits over the tube of a motorcycle frame is another saddle tank design.
Saddle tanks are not only used for fuel, but often for storing and transporting potable water or farm chemicals. Others have been designed to catch and hold rainwater from downspouts for future use. These are generally made of plastic and come in a variety of sizes and configurations. Trucks used for industrial cleaning can obtain a saddle tank for fresh water designed to fit over the wheel well of the truck, balancing the weight of the water evenly between the axles.
Saddle tank locomotives are small steam engines which carry their own water and fuel. The water tank is located over the boiler, like a saddle, and is popular for industrial use. It can carry a large water supply, but has a higher center of gravity, so 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 tender.
Early submarines, especially German U-Boats, used ballast saddle tanks. These were fitted as pairs on the external pressure hulls. Post-war subs moved the ballast tanks from the exterior to interior to reduce water resistance and increase speed.