Effect of Stern Flaps on Powering Performance of the FFG-7 Class

Abstract

The effect of a stern flap on the powering performance and annual fuel consumption of an FFG-7 Oliver Hazard Perry Class Guided Missile Frigate is presented. Analytical analysis using free surface potential flow theory was conducted to evaluate the flow on the afterbody of the ship. Model tests were conducted to determine the optimum geometrical characteristics of the stern flap. Based on the analytical and model test results, a stern flap was retrofitted to an FFG-7 Class frigate, and full-scale results on powering performance were collected. Results of model powering experiments showed that the flap, set at 10 deg trailing edge down, decreased the delivered power by 8.4 percent at 26 knots, and provided for a 3.8 percent decrease in annual fuel consumption. This decrease in annual fuel consumption places the payback period on the cost of the flap installation at only 10 months. The full-scale comparison of the flap's effectiveness was marred by adverse weather conditions and differences in the pre-and post-flap ship configurations. However, observations of the full-scale stern wave system, before and after the installation of the stern flap, lead the authors to conclude that the flap will produce a larger decrease in delivered power at full scale than shown by the model experiments.