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April 2011www.sname.org/sname/mt uctuations. ese load cycles can accu- mulate quickly, making fatigue life an important consideration as well. Impact of material properties Material selection can also set limits on achievable power density levels. e elasticity modulus and tensile prop- erties will influence the structural integrity of a given design, again limit- ing the region of the design space that is realizable. Dynamic properties such as endurance strength will set fatigue life limits. Additionally, impact toughness properties and abrasive erosion prop- erties will influence the ability of the waterjet to withstand potential damage incurred by the transport of higher- mass foreign objects, which may be entrained by higher velocity ows and shallow water operations. Finally, cavitation erosion properties may set limits on the allowable power densities, depending on the type and extent of cavitation present at various points across the operating range. With the use of steel, aluminum, or composite hull structures, the selection of a suitable material to minimize corros ion concerns is a sizeable challenge. Crevice corrosion, pitting corrosion, and other galvanic potential-related corrosion must all be taken into account when establishing the design margins in order to provide ade- quate protection against material loss, stress risers, and the possibility of crack initiation in specic locations. As a nal factor in improving power density, one must consider the quality of the manufacturing process. is aspect cannot be overlooked, as poor manufac- turing will jeopardize any anticipated gains made in power density based on structural and hydrodynamic design improvements. For example, excessive porosity in cast components and defects in welds will, at best, result in increased design margins on strength or life; at worst, these issues can cause life-limit- ing failures of components. Even apparently minor issues such as inattention to detail in blade machin- ing or grinding may result in poor hydrodynamic performance, earlier onset of cavitation, or an increase in the magnitude and severity of cavitation. It is therefore critical that manufacturing take place at a facility with expertise not only in hydrodynamics, structural design, and materials, but also in manufacturing processes. Most impor- tantly, the manufacturer must have the experience to understand the critical relationships among these factors. FNC program In the United States, the Navy has recently placed greater focus on the development of high power density propulsion through a Future Naval Capabilities (FNC) program overseen by the Oce of Naval Research (ONR). is program has focused recent atten- tion on compact high power density waterjet propulsion as an important technological advance. ONR has man- aged the execution of a multi-phase development eort to demonstrate and transition advanced waterjet technol- ogy for use by the U.S. Navy in future naval combatant vessels. In the earli- est phase of the program (initiated in 2006), two industry leaders with exper- tise in waterjet propulsion systems were selected to develop a compact, high power density waterjet hydrodynamic design. ese competing designs were evaluated at model scale by the Naval Surface Warfare Center experts in a bat- tery of performance tests. Specifically, pump loop tests were conducted at the Navys 36-inch variable pressure water tunnel in Bethesda, Maryland to establish fundamental head, efficiency, and cavitation versus flow rate charac- teristic curves for the pumps. ese data are a prerequisite for determin- ing performance in a ship application over a range of size and power absorp- tion. In addition, waterjet system tests were conducted on both models at the Navys large cavitation channel in Memphis, Tennessee. In this test- ing environment, the waterjet pump was combined with simulated hull, inlet, and discharge configurations analogous to real-world shipboard installation. e results from the large cavitation channel testing provide valuable insights into the interactions between the waterjet pump and the rest of the propulsion system. In par- ticular, these data reveal the impacts of hull boundary layer and inlet ow eld non-uniformity on delivered e- ciency and cavitation performance. In 2008, a design from naval pro- pulsion experts at Rolls-Royce Naval Marine Inc. was selected for the sec- ond phase of the FNC program. is design was executed in collabora- tion with Rolls-Royces Waterjet Center of Excellence in Sweden. In this phase, Rolls-Royce will oversee the design, construction, and at-sea Poor manufacturing will jeopardize any anticipated gains made in power density based on structural and hydrodynamic design improvements. Waterjet MODEL THE