Vijayakumar Rajagopalan

Hydrodynamic Drag of Surface combatants pose significant challenges with regard to fuel efficiency and exhaust emissions. Stern flaps have been used widely as an energy saving device, particularly by the U S Navy (Hemanth et al. 2018a Hemanth Kumar and Vijayakumar 2018b) 2018b). In the present investigation the effect of flap turning angle on drag reduction is numerically and experimentally studied for a high speed d isplacement surface combatant fitted with a stern flap in the Froude number range of 0.17 0.48. Parametric investigations are undertaken for constant chord length & span and varying turning angles of 5°, 10° & 15°. Experimental resistance values in towing t ank tests were validated with CFD. Investigations revealed that pressure increased as the flow velocity decreased with an increase in flap turning angle which was due to the centrifugal action of the flow caused by the induced concave curvature under the f lap. There was no significant change in stern wave height but there was a gradual increase in the stern wave steepness with flap angle. Effective length of the vessel increased by lengthening of transom hollow. In low Froude number regime, flow was not inf luenced by flap curvature effects and pressure recovery was marginal. In the intermediate and high Froude number regimes pressure recovery increased with the flap turning angle and flow velocity.

The surge in maritime trade is leading to large scale deployment of high-speed displacement ships by all nations. Cargo vessels are designed for a voyage in pre-determined routes at consistent speeds. On the other hand, high-speed displacement vessel engines designed with a capability to cater for top speeds are under-utilised during their normal course of operation. This sub-optimal utilisation impacts efficiency and increases emissions. In this study, a most favourable stern flap is designed for reducing the energy efficiency design index of a typical high-speed displacement vessel with a slender hull. CFD simulations and experimental model testing were conducted for 12 different stern flap configurations for determining most favourable flap design in the Froude no of 0.17-0.48. Performance of the most favourable stern flap was established by calculating, energy efficiency design index (EEDI) and fuel consumption based on typical operating profile. NOx, VOC and PM emissions were estimated in with and without flap condition. Studies demonstrated that the stern flap reduced effective power demand, average fuel consumption and emissions by about 8 per cent, which when considered for the ship's operating life cycle, are significant. The most favourable stern flap reduced EEDI by 3.74 units and 1.98 units as compared to the bare hull condition and the required EEDI respectively, thereby demonstrating that EEDI could be used as an index to indicate stern flap efficiency.

Surface Combatants are highly dependent on fossil fuels for their propulsion. During the course of its voyage, these vessels experience considerable amounts of drag or resistance based on the operational environment and their hull form. Reduction of this drag would result in a corresponding reduction in fuel costs, exhaust emissions, and an increase in the vessel’s speed and range. The operational flexibility of the vessel is enhanced by an increase in the time between successive refuellings, as well as the distance over which the vessel can operate without the need for replenishment. Of the many energy saving devices, fitment of a stern flap on Surface combatants is a very popular cost-effective means for drag reduction. The U.S Navy has extensively installed stern flaps on their combatants and, through this experience, found that suitably designed stern flaps had reduced the power requirement of the vessel they were fitted on by about 4–19%, an amount that translates to significant fuel savings and reduction in emissions. This paper will discuss the concept of stern flaps, examine the benefits offered by this technology on U.S Naval platforms, and will present the scope of leveraging this technology in Indian Defence Shipbuilding and Ship repair which could lead to significant reductions in power and emissions without compromising the platform’s performance.

Hydrodynamic drag of ocean-going vessels imposes severe penalty with regard to exhaust emissions and fuel consumption. Stern flaps have been extensively used as a cost-effective energy saving device, largely by the US Navy. Experimental analysis to identify an optimum stern flap fitted at the transom stern end of a generic high-speed displacement vessel operating at Froude number regime of 0.17–0.48 is described. In this study, twelve different stern flap configurations were studied. Identification of optimum stern flap was based on systematic analysis of extensive model test resistance data of various stern flap configurations over twelve different Froude numbers.