SYNOPSIS.The understanding of fish maneuvering and its application to underwater rigid bodies are considered. The goal is to gain insight into stealth. The recent progress made in NUWC is reviewed. Fish morphology suggests that control fins for maneuverability have unique scalar relationships irrespective of their speed type. Maneuvering experiments are carried out with fish that are fast yet maneuverable. The gap in maneuverability between fish and small underwater vehicles is quantified. The hydrodynamics of a dorsal fin based brisk maneuvering device and a dual flapping foil device, as applied to rigid cylindrical bodies, are described. The role of pectoral wings in maneuvering and station keeping near surface waves is discussed. A pendulum model of dolphin swimming is presented to show that body length and tail flapping frequency are related. For nearly neutrally buoyant bodies, Froude number and maneuverability are related. Analysis of measurements indicates that the Strouhal number of dolphins is a constant. The mechanism of discrete and deterministic vortex shedding from oscillating control surfaces has the property of large amplitude unsteady forcing and an exquisite phase dependence, which makes it inherently amenable to active control for precision maneuvering. Theoretical control studies are carried out to demonstrate the feasibility of maneuverability of biologically inspired bodies under surface waves. The application of fish hydrodynamics to the silencing of propulsors is considered. Two strategies for the reduction of radiated noise are developed. The effects of a reduction of rotational rate are modeled. The active cambering of blades made of digitally programmable artificial muscles, and their thrust enhancement, are demonstrated. Next, wake momentum filling is carried out by artificial muscles at the trailing edge of a stator blade of an upstream stator propulsor, and articulating them like a fish tail. A reduction of radiated noise, called blade tonals, is demonstrated theoretically. FIG. 1. Definition of length scales of a fish. FIG. 2. Morphology of dorsal fins of fish families.

No abstract

Successful operation of high‐speed craft requires close attention to the bottom hull coating system. A new generation of smooth anti‐fouling paints, with no adverse environmental impact, are being introduced to replace the tri‐butal tin (TBT) based coatings, banned in 2003 by IMO convention. This paper presents the results of high speed 2.23 m (7.54 ft) air cushion catamaran model tests with three of these new bottom coatings. The resistance tests show the bottom coatings result in a 2–7% drag reduction over the speeds tested.

The Langley seawater tow tank, located on Langley Airbase in Hampton, Virginia, was designed and built by NACA in 1931 for seaplane research and has been in various states of use since then. Since 1983, NUWC has maintained stewardship of the tank and has conducted a large number of tests there. This paper will present an overview of the facility and its capabilities as well as discuss several recent test results. The tank is reinforced concrete, 2880 feet long, 12 feet deep, and 24 feet wide, and can be filled with 5.4 million gallons of either fresh or baywater. Baywater can be filtered to 25 microns and has a salinity about half that of seawater. The carriage is powered by eight 75 hp D.C. motors with trolley style cables and can attain speeds of up to 40 knots. Power is generated by a 850 kW motor generator. Several struts exist for model towing and the carriage bas 110/120 V.A.C. and 3 phase 208 A.C. volt available power. A workshop is in the building and a variety of instrumentation and data acquisition is on hand. Unique possibilities exist at the LTT due to the organic and physical properties of seawater. For example, one of the largest ever artificial colonies of plankton was grown there. Furthermore, because of the electrical conductivity of the water, research into the control of turbulent flows by magnetohydrodynamic forces can be, and bas been, performed there. To illustrate the variety of test configurations an capabilities, results are presented from three recent test programs performed at the tank. The first will be the set-up and test results of a fully submerged axisymmetric 180 inch long cylindrical sting mounted model. The test rig will be shown including the floating swing balance which is coupled to an axial load cell. Test results will be shown for hydrodynamic resistance versus speed at zero and non-zero attack angles. The second set of results shown will be those of a towed cylindrical body equipped with remotely actuated hydrodynamic drag brakes which was released in flight an allowed to come to rest Drag measurements made at different drag brake settings and speeds will be presented as well as photographic documentation. Finally, a recent set of test results will be presented; taken on a submerged cylindrical body, strut mounted to a 6 degree of freedom load balance. Results to be presented will include force and moment measurements as a function of speed and attack angle.