William Robertson Shipowners Ltd, a private limited company, was formed in 1949 to take over the assets of the shipping business, William Robertson, including the ownership of its fleet of steamers and motor ships. The management of the fleet, however, was left in the hands of the old William Robertson partnership.
In 1952, Kneeshaw Lupton & Company Ltd, a quarrying business which for many years had been closely associated with the business, was made a wholly owned subsidiary of William Robertson Shipowners Ltd.
In the 1950s, the company invested in the Norwegian shipping line, Thor Thoresen Linje.
In 1958, the company changed its name to Gem Line Ltd. At the same time, the old William Robertson partnership was wound up and a new company, called William Robertson Shipowners Ltd, was formed to continue the management of the fleet.
The business diversified into geological research and consultancy, forming the Robertson Research Company Ltd in 1961. In the same year, it also formed the Ailsa Craig Quarrying Company Ltd.
In the 1950s and 1960s, the company invested in new and larger ships, but by 1970 the coastal trade had declined, with the advent of vast containers using large, deep water ports. As a result, in 1970, the company was taken over by Powell Duffryn Ltd, the owners of Stephenson Clarke Shipping Ltd. In 1971, following the takeover, the company reverted to its former name, William Robertson Shipowners Ltd. In 1976, the company was formally made a subsidiary of Stephenson Clarke Shipping Ltd, although it continued to trade under its own name. In 1987, it changed its name to Gem Line (Management) Ltd. The company was dissolved in 1997.