A stem flap is an extension of the hull bottom surface which extends aft of the transom. It is a relatively small appendage (typically constructed so as to include internal metal bracing beams and external metal plate material) which is fitted to the ship's transom. Critical stern flap geometry parameters include: (i) chord length, (ii) span across the transom; and, (ii) an angle denoted as "trailing edge down" (TED), referenced to the local buttock slope (run) at the transom. The main purpose of a stern flap device is to reduce the shaft power required to propel a ship through the water, thereby reducing the engine's fuel consumption and increasing the ship's top speed and range.
The stern flap originated from stern, or transom, wedge research conducted in the 1980s. Stern wedges or flaps have been installed on naval destroyers to create a vertical lift at the transom and to modify the distribution of pressure on the after portion of the hull. The Navy reports better fuel efficiency, higher top speed, and reduced emissions. The cost of implementation, $170,000, can be recouped within approximately one to two years.
Stern flaps create a vertical lift force at the transom modifying the pressure distribution on the aft portion of the hull. This reduces the drag on the ship, which modifies the wave resistance of the ship, therefore reducing the propulsion power required to achieve a given speed. This, in turn, reduces fuel consumption and provides commensurate cost savings and environmental benefits. By maintaining ship speed using less power and a lower shaft speed, the cost of maintenance is reduced and the life of the propulsion plant machinery is extended. Other benefits to using stern flaps include reductions in propeller load, cavitation, vibration, and noise.
The application of stern flaps to large displacement vessels is a fairly recent innovation. The US Navy has been investigating the use of stern flaps on many different hull types. Stern flaps have now been proven by the US Navy to reduce the requisite amount of propulsive power during navigation, with several concomitant advantages. Stern flaps: foster reductions in operating and life-cycle costs through fuel savings; increase both ship speed and range; decrease the amount of pollutants released by ships into the atmosphere; and, reduce propeller loading, cavitation, vibration and noise tendencies.
The standard (traditional or conventional) stern flap is designed with parallel, linear (straight) leading and trailing edges for orientation of these linear edges perpendicular to the ship centerline. The standard stern flap had its limitations and would be disadvantageous for the task at hand. A configuration involving a standard stem flap and a curved transom would present various practical problems and would not be propitious.
To elaborate, installation of a standard stern flap on a highly curved transom would necessitate recession of the leading edge of the standard stern flap, at its centerline, under the transom. Full-scale installation and implementation would be difficult, particularly with regard to the arrangement and attachment of the partially recessed appendage to the curved transom. Moreover, such application of a standard stern flap with respect to a curved transom would inherently fail to fully utilize the entire stern flap chord length. In principle, a stern flap itself produces drag, and the stem flap's interactions with the hull, wave systems and propulsor produce the net decrease in required power. Generally, chord length is one of the parameters to be optimized; increase in effective ship length enhances reduction in ship wave resistance, and increase in stern flap total surface area increases the associated drag (resistance). The partial recess of the installed standard flap would directly limit the increased effective ship length associated with the stern flap installation. Furthermore, the partial recess of the flap would not make full use of the flap surface area.
A contour stern flap on U.S. Coast Guard Hamilton Class (WHEC) high endurance cutters marked initial use of this new geometry. Model tests in the David Taylor Model Basin here showed performance exceeded traditional designs. During full-scale at-sea trials on the **WHEC 722 Morgenthau, it reduced power up to 15% and overall fuel consumption 9.5%, an annual savings of $33,000 per ship. In addition, the contour flap increases the ship's attainable speed on both diesel and gas turbine engines. Morgenthau not only represented the first use of the contour stern flap, but also first usage of a low-angle flap optimized for low speed. Stern flaps had previously been shown to work superbly on high-speed naval combatants. This new flap, however, exhibited peak performance at 16 knots yet still provided an increase in top speed.
In recent years, a stern flap modification has been installed on Navy cruisers, destroyers, frigates and patrol craft, as well as Coast Guard cutters and patrol boats. All exhibited significant power and fossil fuel emission reductions, fuel savings and top-end speed increases during at-sea trials. But ships designed with older-style, highly-curved transoms challenged designers. The shape of a traditional flap would require a partial recess under the transom, directly limiting the increase in effective ship length, and failing to make full use of the flap's surface area. The contour stern flap, best described as 'boomerang' shaped, was designed to intersect with the transom along its radius of curvature. The flap's trailing edge is also matched to this curvature. This way, a constant chord length measured perpendicular to the transom is maintained.
In order to increase helicopter flight-deck area, the stern of the initial DDG 51 was lengthened by 5 ft (1.52 m), increasing the ship LWL to 471 ft (143.6 m). A stern flap was developed in parallel with the lengthened transom, and the resultant design is referred to as the DDG 51 Flight IIA sub-class. The stern flap was designed specifically for performance enhancement. It is projected to decrease annual propulsion fuel consumption resulting in significant cost savings, and as an added benefit, the stern flap will also increase the maximum ship speed by 0.3 knots.
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