Garratt - Advantages of The Garratt Concept

Advantages of The Garratt Concept

The principal benefit of the Garratt design is that the boiler and firebox unit are slung between the two engine units. This frees the boiler and firebox from the size constraints imposed where they are placed over the frames and running gear, as in conventional designs and other articulated designs such as Mallets. Garratts can have a boiler with a greater diameter, which increases heating area and aids the production of steam. The boiler can also be shorter than other designs with the same heating area. In some loco designs, the boiler is so long almost no heating of the water occurs at the smokebox end of the boiler. A larger firebox promotes more efficient combustion of fuel and also increases the heat available to the boiler.

Garratts enjoy an advantage over the Mallet system, because of the geometry of the design. When swinging around curves the boiler and cab unit move inward like a bowstring in the bow of a curve and this reduces the centrifugal force that would overturn a normal locomotive and which in turn permits faster running. The Mallet's forward articulated unit tends to throw out as the loco rounds curves. While most Garratts were designed for freight or mixed traffic, there were a number of passenger Garratt classes. A Garratt holds the world speed record for an articulated locomotive.

Garratts have several advantages when used on light and narrow gauge railways. They are tank locomotives and can easily be run tender-first, thus eliminating the need for expensive turntables or wyes. They do not need to be run through to terminals, increasing operational flexibility. Because the engine units are separated by the boiler unit, the weight of the locomotive is split over the two units, and they can run over bridges or line sections that might not be able to support conventional or Mallet locomotives of similar weight.

While at the end of the steam locomotive era most conventional steam locomotives had reached their maximum in "critical dimensions", the Garratt still had some way to go, with larger driving wheels, larger boilers and greater output still achievable.