What to expect from the hydrodynamic energy saving devices

Many retrofitting technologies have been proposed to improve the hydrodynamic performance of existing fleets with the aim of reducing the fuel consumption and consequently CO2 emission. The magnitudes of savings predicted by manufacturers are very promising however ship owners are often still doubtful whether they can achieve what is claimed in operations. This study evaluates the performance of four energy saving devices (ESDs) at ship scale with the aim of assisting ship owners with the decision of selecting suitable devices for their ships. Due to the uncertainties associated with extrapolation of viscous flows from model to full scale it is proposed that investigations must be carried out at full scale; hence a full-scale com putational model was adopted as the only feasible method at the design stage. Two vessels representing di fferent types of ship were selected: a gas carrier and a container ship. Various retrofitting technologies to reduce resistance or to improve the propulsive efficiency were considered. The latter group is subdivided into devices located before, at and aft of the propeller. The resistance induced by large openings on the hull, such as a bow thruster tunnel was quantified and several devices designed to streamline the flow in this region were evaluated. Pre-swirl fins technology was the ESD investigated from preswirl devices. The existing propeller of the gas carrier was replaced with a new type profile propeller which improved the propulsive efficiency. Twisted rudder was the technology investigated from post-swirl ESDs. The level of savings obtained from these technologies was generally less than the values published in the literature. It was concluded that this discrepancy arose for one of three reasons: either the metric used to evaluate the savings was inappropriate, or that the method used to quantify the measure was in accurate, or finally, because the designs examined in the case studies were not suitable optimised. However if some of these devices did not deliver the expected savings because the designs considered in this study were not sufficiently optimised, then the question arises as to whether these devices must be optimised for a specific operational conditions and how well these ESDs behave when the vessel is not operating in the design conditions