An Assessment of Naval Hydromechanics Science and Technology (2000)

In this report, naval hydromechanics is defined as the study of the hydrodynamic and hydroacoustic performance of naval ships, submarines, underwater vehicles, and weapons. The importance, value, and contributions of naval hydromechanics science and technology (S&T) to the success of naval forces can best be understood from a historical perspective. The era most relevant to the purpose of this study extends from the formation of the Office of Naval Research (ONR) shortly after World War II to the present. During that period, the technical accomplishments of naval hydromechanics are epitomized by those of the David W. Taylor Model Basin (now the Naval Surface Warfare Center, Carderock Division (NSWCCD)). Some examples of its accomplishments, along with other examples from two white papers on naval hydromechanics written by Marshall P. Tulin¹ and Thomas T. Huang,² are described here.

• After World War II, basic hydromechanics research was conducted to support submarine construction and operation. A series of 24 body-of-revolution hulls (DTMB Series 58) were tested in a towing tank to determine their resistance, motion stability, depth and course-keeping control, and ocean surface effects at high speeds. An optimal axisymmetric hull shape had minimum resistance and a mild pressure gradient enabling the development of a hull boundary layer suitable for placing control surfaces upstream of a single-screw propeller. This basic research provided the Navy with a concept for a superior submarine that had reduced flow resistance, more effective control, and highly efficient propulsion. This submarine concept could improve not only the speed but also the stealth performance. A 20 percent gain in propulsion efficiency could be achieved by using the wake-adapted single-screw propeller instead of twin-screw propellers. The axisymmetric hull provided the minimum circumferential inflow variation to the propeller, which drastically reduced propeller-induced noise and cavitation.

• The Navy's first research submarine, the USS Albacore (SS 569), was built to evaluate at sea the innovative ideas of control and propulsion that had been derived from the basic research program, and it provided firm support for these ideas. With this submarine, the Navy, the science and technology community, and the shipbuilding industry stepped outside the traditional technology box of the fleet submarine. The fundamental data obtained on a new hydrodynamic hull, control surfaces, and propulsion, along with the utility of low-carbon, high-yield-80 structural steel, became the foundation of U.S. submarine design and construction for the next half century. The development of the high-speed submarine hull form is a prime example of a technological breakthrough. It enabled a submerged submarine to travel well in excess of 30 knots. More importantly, when combined with the parallel development of nuclear propulsion, it resulted in the U.S. Navy' s first truly high-speed submarine. The research foundation and technical expertise made possible by sustained investments in Navy S&T substantially enabled this revolutionary advance in naval warfare capability.

• Equally important to the continued superiority of U.S. submarines have been the sustained improvements in submarine stealth. The sudden increase in submarine speed and endurance produced an urgent need for quiet propulsion for stealth and for effective control for submarine safety. This drove the hydromechanics S&T community to continue to improve the stealth and hydromechanics performance of the submarine fleet. A long-term national S&T research program was implemented to solve the acoustic side effects of sustained submerged high speed and to meet the threat of the Soviet submarine fleet during the Cold War period. Fundamental and applied stealth and hydromechanics research was vigorously pursued in the Navy's laboratories and in universities, under the sponsorship of the ONR. Hydromechanics innovations ranging from advanced propeller designs to reduced hull acoustic radiation have enabled a large reduction in submarine signatures. As a result of a broad range of technological developments, U.S. attack and ballistic submarines have maintained an underwater acoustic advantage over the submarines of all other navies.

• The Small Waterplane Area Twin Hull (SWATH) ship concept was developed from the technology base and design methods established by sustained investments from Navy 6.1, 6.2, and 6.3. This concept permits greatly improved seakeeping and seaway performance, particularly in small and medium-sized ships. Innovative design configuration capabilities were also developed, including the unique steering system embodied on the TAGOS 19 and a number of semiactive and active control system concepts. SWATH technology has been applied commercially to a large (12,000-ton) passenger/cruise ship and to all-weather ferries and hydrographic and survey ships. At present, about 40 naval and commercial SWATH ships have been built worldwide.

• Surface ship hull form technology and design methods have been applied to recent classes of surface combatants, resulting in superior seakeeping, powering, and acoustic performance. This major performance advance is a direct result of years of investment in hull form technology R&D.

• Continued compilation of the variability of sea conditions and their statistics has improved the seakeeping design specification for surface combatants, and satellite ocean wave observations have provided timely guidance for ship operations. The basic understanding of ship response to the ocean waves associated with different sea states has improved the ability to design surface combatants with better seakeeping characteristics, less deck wetness, cost-effective shell plating and hull girders, and improved helicopter landing and takeoff operations.

• The sustained development and implementation of numerous innovations in the fleet have reduced energy consumption and operating costs for U.S. Navy ships. Innovations include new, environmentally acceptable, effective hull antifouling coatings; improved hull and propeller cleaning and maintenance programs; and stern modifications that permit fuel savings of 3 to 10 percent for several

classes of surface ships. All of these advances are supported or enabled by a sustained capability in hydromechanics research and design.

• In the late 1970s, the Navy needed to improve the target acquisition range of the Mk 48 torpedo. A limiting factor in the performance of the acoustic array was a basic hydrodynamic phenomenon, the noise caused by the transition from laminar to turbulent flow. The Naval Undersea Warfare Center (NUWC) developed the methodology to optimize array diameter, acoustic window thickness, transition location, and cavitation index and to resolve the key issue of window deformation under hydrodynamic loading. Experiments determined the location and intensity of the transition region, so that techniques to predict transition location could be validated. These advances in technology capabilities led to a substantial reduction in self-noise and a major improvement in torpedo performance.

• Hydrodynamic modeling based on theoretical and experimental research has played a critical role in the development and improvement of fleet weapons by providing estimates of forces and moments experienced by these vehicles during launch and maneuvers. Hydrodynamic force and moment predictions generated through this research were used as inputs to vehicle launch and trajectory simulations and throughout the development and design process. This process was instrumental in the development of Mk 46 and Mk 48 torpedo hardware and software and to a succession of advanced weapons such as the advanced capability and Mk 50 torpedoes.

• Basic research in hydromechanics and naval technical expertise have enabled advances in propulsor design through enhanced simulation and experimental methods that directly and indirectly reduced the noise signatures of Navy submarines, weapons, and tactical-scale vehicles. Substituting a single rotation propulsor for the traditional counterrotating propellers has meant indirect noise reduction due to machinery simplification while maintaining high efficiency and off-design performance. Using alternatives to traditional propulsor design reduces propulsor-radiated noise.