The main role of the tug is to assist vessels in the river or within the dock system by moving them by pushing or towing. They are also used to tow barges or platforms which have no engines or methods of propulsion. Tugs are designed to be highly manoeuvrable and powerful as they normally work with large vessels in restricted spaces. The main towing companies operating on the Mersey in the twentieth century were J. & J. H. Rea Limited, Liverpool Screw Towing Company and the Alexandra Towing Company Limited. Mersey tugs were employed to work with passenger liners, cargo vessels, oil tankers and other vessels working on the river.

The modern tug is equipped with azimuthing thrusters or Voith Schneider Vertical propellers which enable them to generate the thrust required for towing the larger vessels which are now being built. In this book, Ian Collard charts the development of the Mersey tug from the late nineteenth century to the present day.