Seakeeping Characteristics of the Amphibious Assault Landing Craft

Abstract

The predicted response characteristics of the JEFF Amphibious Assault Landing Craft while running on cushion in waves are presented. The vertical plane motion responses were calculated using model towing tank data obtained from oscillator tests and captive model wave excitation tests. Results are presented in terms of frequency response functions, response amplitude operators and impulse response functions. Results of a validation study are presented where model motion data in random waves was compared to predicted motion levels. The dependence of the craft's natural frequencies and damping characteristics on the overall dynamic performance of the craft in waves is discussed. S part of the Amphibious Assault Landing Craft (AALC) Program Office's technology development effort, the David W. Taylor Naval Ship Research and Development Center (DTNSRDC) has been constructing a Six Degree of Freedom (6DOF) motion simulations program for the two JEFF craft in waves. The simulations are based on empirical data: i.e., the model data taken in the towing tank and wind tunnel. The calm water maneuvering and control program for both craft was completed in 1974. 1'2 The pur- pose of this paper is to describe the method used to predict the pitch and heave motion in waves, and to present and discuss the on-cushion dynamic performance characteristics of the craft while operating in head waves. Description of Craft and Model The AALC is an air cushion vehicle (ACV) designed primarily to transport war material from off-shore stationed ships to a beach or staging area in unfavorable environmental conditions at high speeds. Principal characteristics of the two prototype craft are given in Ref. 3. The skirt system of the JEFF (B) craft is of the bag and finger type and the air cushion is subdivided into four compartments. The JEFF (A) craft has a peripheral cell skirt/stabilization system and an undivided air cushion. Photographs of models of both craft are shown in Fig. 1. The models used to obtain data on the dynamics of both craft were equipped with lift fans, ducting, and a flexible skirt system so that on-cushion overwater performance was simulated in model towing tank experiments. Experimental Data Used in the Analysis (6.35, 12.7 and 25.4 mm) were used. The resulting forces and moments on the models were measured using block gage assemblies. The fundamental harmonics of the unsteady forces and moments were resolved into components in-phase and out-of-phase with the oscillatory motion. The force data was reduced by subtracting inertial and friction tares and the resulting coefficients were nondimensionalized using con- ventional stability and control techniques.4 The oscillation experiments were conducted at Froude- scaled model speeds corresponding to 35 and 50 knots full scale. The nondimensional derivatives were found to be highly dependent upon Froude number. Since the motion prediction was to be correlated to motion data at 30 and 40 knots, it was necessary to linearly interpolate and extrapolate the derivatives to these speeds. The resulting nondimensional stability derivatives are listed in Table 1. Another series of experiments was conducted to obtain the wave excitation forces and moments. These experiments used essentially the same test setup as the oscillation experiments. The craft height was held fixed at the design equilibrium height, and with attitude and cushion pressure held constant, the model was run through regular head waves produced by a pneumatic wavemaker located at the end of the towing basin. The resulting heaving forces and pitching moments were obtained at 35 and 50 knots (full scale) for various wavelengths.