Stern Flap System Dubbed Technical Breakthrough

Technical innovation in the name of saving money and maintenance has become somewhat of a cliche in many industries, leaving many with the impression that the message is more marketing hype than real world results.

An innovation which is solidly in the latter category was recently honored by the U.S. Environmental Protection Agency for its role in conserving fuel, thus reducing pollutants.

Specifically, the U.S. Navy's CG-47/DD-963 Stern Flap R&D Team has received one of the EPA's 1998 Climate Protection Awards, as it was singled out for "Technical Breakthroughs in Ship Energy Efficiency" demonstrating powering improvements and significant fuel savings deriving from installation of new stern flaps on Navy ships.

The modification is an extension of a hull bottom surface at the rear of a vessel. The Carderock Division team determined the appropriate shape via model ship tests in the David Taylor Model Basin. Trials on USS Arthur W. Radford (DD- 968) (pictured), a Spruance Class destroyer, have shown a reduction in required power up to 14 percent resulting in a projected annual fuel savings of 4,400 barrels per ship. This equates to approximately $240,000 per year in fuel savings, as well as a .75 knot increase in speed. Radford is the destroyer also being used to demonstrate the new composite mast recently announced.

While the flap has been trialed on a number of different ship types, both real and model simulated, and the performance of the stern flap varies based on hull form and to propulsion power and efficiency. An Evolving Revolution The advent of the stern flap solution to fuel savings and performance enhancing can hardly be termed a new development, at least in terms of publicity surrounding the system. For the past decade, the U.S. Navy have been investigating, at model scale, the potential powering improvements due to stern flaps, as low cost retrofits on many ship designs. It is interesting to note here that flap performance is optimized where the ship expends the most power.

While the fuel savings and power enhancing aspects of the stern flaps are truly amazing, from a business stand point they are outstanding performers as well, as predicted annual fuel savings will repay the flap retrofit costs in less than a year.

A stern flap is simply an appendage which reduces the power required to propel a ship through the water. It is a relatively small extension of the hull bottom surface aft of the transom.

The critical parameters for stern flap design geometry are the flap angle, referenced to an extension of the local centerline buttock angle, flap chord length and flap span across the transom.

Flap design is dictated, ultimately, by the hullform design and the mission requirements of the ship. The stern flap's critical parameters, which are optimized through model testing, greatly influence the performance potential. Compromises must be reached between high speed and low speed stern flap performance, the relative importance of which is indicated by the particular ship's speed/time/operations profile.

In terms of propulsion interaction, in general, delivered power reductions averaged a few percent greater than resistance reductions during all model tests with stern flaps. An examination of model data from the CG 47 and DD 963 experiments showed an improvement in propulsive efficiency on average between 2 to 2.5 percent, and a reduction in wake factor of as much as 1.5 percent at certain speeds.

The increased pressure under the hull, due to the stern flap, can also serve to suppress propeller cavitation and reduce thrust breakdown losses at higher speeds.

The stern flap effect, combined with the reduced propeller loading, can provide additional powered improvement by way of increased propeller efficiency.

Stern flaps have been retrofit to two O.H. Perry (FFG 7) class frigates; two Cyclone (PC 1) class patrol coastals; as well as the A.W.

Radford. All ships have experienced improved powering performance and fuel savings, and have reported not adverse impacts due to the stern flaps.

EPA presented the award during the Earth Technologies Forum in Washington, D.C. Winners came from eight countries and one international environmental leadership partnership. The Carderock Team was headed by Dominic Cusanelli (pictured) of the Hydromechanics Directorate, and included Lowry Hundley, who was in charge of the ship performance trials; Gabor Karafiath, who provided technical direction; and Steven Chun, who provided technical support. The Carderock Division provides research, development, test and evaluation for U.S. Navy ships and submarines. With its unique facilities and advanced computer design, it has become known as the place "Where the Fleet Begins.

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