Optimal propeller selection when accounting for a ship's manoeuvring response due to environmental loading

Abstract

Carbon Dioxide (CO₂) is considered to be the most detrimental of all the Green House Gases on global warming (IMO, 2009). In an attempt to reduce the amount of CO₂ emissions from ships, this research approaches the problem from the perspective of more efficient design through superior estimation of design points. Conventionally, a propeller is selected from the viewpoint that a ship travels at a constant design speed, with zero drift angle. However, a ship is subjected to the motions imposed on her from the environment. These motions tend to push a ship off her intended course, resulting in helm correction, speed correction (if the ship is to arrive at her intended destination on time) and consequently, altered inflow velocity to the propeller. It is the novel aim of this research to determine if accounting for a ship’s manoeuvring motion will result in a propeller selection that has an overall higher efficiency, compared to one selected which neglects the manoeuvring motion. To achieve this aim, a ship manoeuvring simulator has been developed which incorporates a modified mathematical propeller model that accounts for the unsteady manoeuvring response of a ship subjected to an environment in which she is expected to sail. The developed simulator has an iterative routine which enables it to select a propeller from a standard series that has the highest efficiency for the route in question. Case studies are constructed which highlight how the efficiency of a propeller fairs when using the newly proposed propeller selection method, compared to the conventional propeller selection perspective. The newly proposed propeller selection method is most suited to ships which are susceptible to relatively large drift angles and/or relatively high installed power requirements. Results from the case studies are encouraging, with a gain of 2.34% in open water propeller efficiency for a 3600 Twenty foot Equivalent Unit container ship, equating to a saving of 3.22% in Carbon Dioxide emissions.