The surge in liquefied natural gas (LNG) carrier newbuildings and the accompanying development of a new generation of these specialized ships has generated interest in alternative propulsion systems to the traditional gas fired steam plant. These alternatives are addressed in the new ABS Guide for Propulsion Systems for LNG Carriers, the first such comprehensive set of standards to have been issued by a classification society. Currently several type of propulsion configurations are being ordered with others yet being planned. At the heart of the differing propulsion systems debate is economics together with safety concerns over the manner in which boil off gas (BOG) is treated.