Yarrow boilers are an important class of high-pressure water-tube boilers. They were developed by Yarrow & Co. (London), Shipbuilders and Engineers and were widely used on ships, particularly warships.

The Yarrow boiler design is characteristic of the three-drum boiler: two banks of straight water-tubes are arranged in a triangular row with a single furnace between them. A single steam drum is mounted at the top between them, with smaller water drums at the base of each bank. Circulation, both upwards and downwards, occurs within this same tube bank. The Yarrow's distinctive features were the use of straight tubes and also circulation in both directions taking place within the tube bank, rather than using external downcomers.

Early use of the water-tube boiler within the Royal Navy was controversial at times, giving rise to the 'Battle of the Boilers' around 1900. These first boilers, such as the Belleville and Niclausse, were large-tube designs, with simple straight tubes of around 4" diameter, at a shallow angle to the horizontal. These tubes were jointed into cast iron headers and gave much trouble with leakage at these joints. At the time, an assumption was that thermal expansion in these straight tubes was straining the joints. These boilers were also large, and although fitted to many pre-dreadnought battleships, could not be fitted to the small torpedo boats and the early destroyers then under very active development.

To provide a lighter boiler for smaller vessels, the 'Express' types were developed. These used smaller water-tubes of around 2" diameter, giving a greater ratio of heating area to volume (and weight). Most of these were of the three-drum pattern, particularly of the Du Temple and Normand designs. This gave a more vertical arrangement of the water-tubes, thus encouraging thermosyphon circulation in these narrow tubes. The previous problems of tube expansion were still a theoretical concern and so the tubes were either curved, or even convoluted into hairpins and S shapes, so as to increase heating area. In practice these shapes gave rise to two more practical problems: difficulty in cleaning the tubes and also difficulty in forming a reliable joint into the water drums, particularly where tubes entered the drum at a variety of angles.

Alfred Yarrow developed his boiler as a response to others that had already developed water-tube boilers. This was a long process based on theoretical experiment rather than evolution of practical boilers. Work began in 1877 and the first commercial boiler was not supplied until 10 years later, a torpedo boat of 1887.

Despite this long gestation, the boiler's origins appear to have been most direct. Yarrow's initial conversation with William Crush, head of the boiler department, is recorded to have included a rather direct approach and Yarrow's statements, "We must wake-up about water-tube boilers", "Why not a boiler like this?" (placing his fingers together as if praying), and "Straight tubes?" already expressed two of the boiler's three basic design principles.

Early water-tube designers had been concerned with the expansion of the boiler's tubes when heated. Efforts were made to permit them to expand freely, particularly so that those closest to the furnace might expand relatively more than those further away. Typically this was done by arranging the tubes in large looping curves, as for the Thornycroft boiler. These had difficulties in manufacturing and required support in use.

Yarrow recognised that the temperature of a water-filled tube was held relatively low and was consistent amongst them, provided that they remained full of water and boiling was not allowed to occur within the tubes themselves. High temperatures and variations only arose when tubes became steam filled, which also disrupted circulation.