An Engineering Method to Predict Propeller Loads on Maneuvering Underwater Vehicles

Abstract

An engineering method is presented for calculating propeller shaft loads on a maneuvering underwater vehicle. The method predicts time-averaged loads on apropeller in nonuniform, inclined flow. The computational affordability of the method makes it ideal for use in preliminary vehicle design. Comparisons of measured and predicted data for propeller alone and propeller-hull configurations show that the method accounts for the effect of nonuniform and inclined flow on shaft loads. INTRODUCTION A propeller influences the hydrodynamic performance of an underwater vehicle through direct and indirect loads. Direct loads, consisting of thrust, torque, lift, and side forces are transmitted to the hull through the shaft. Indirect loads are caused by the propeller-induced flow on the stem and tail fins. Because the propeller operates in the nonuniform flow field of the vehicle, propeller loads typically contain time-averaged and harmonic components. This paper considers direct, time-averaged loads only. Methods to predict direct propeller loads range from empirical to advanced numerical methods. The Hydrodynamic Analysis Techniques (HYDAT) program is a reliminary design tool for towed and free-swimming submersibles? HYDAT predicts propeller loads using an empirical method based on open water performance data for a particular propeller series. Further development of this and other empirical methods has been hampered by the lack of data. Potential flow-ba~edmethods~~~ are capable ofpredictingpropellerloads, given the geometry and inflow conditions. These methods have been successfully used to design propellers, but they are large and difficult to run on a production basis. Semi-empirical methods are available to predict propeller loads, given the open water performance characteristics of the propeller. McCarthy4 developed a method to predict the thrust and torque of an arbitrary propeller operating in a nonuniform but noninclined flow. McCarthy's method accounts for the substantial effect of the axisymmetric hull boundary layer on propeller thrust and torque. G~tsche~developed amethodto predictthe thrust, torque, andinplane normal force of a propeller in a uniform, inclined flow. A method developed by Perkins and Mendenhal16 combines the McCarthy and Gutsche methods, providing a means to compute direct loads in a nonuniform, inclined flow. This method is capable of predicting the three forces and three moments acting on a propeller attached to a maneuvering vehicle. For use in preliminary vehicle design, semi-empirical methods offer the most flexible and affordable approach for propeller load prediction. This paper presents a semi-empirical method based on the work of Perkins and MendenhalL6 The following sections describe the procedure for calculating the propeller inflow and load components. Predictions are compared with measured shaft loads on propeller alone and propeller-hull configurations. TECHNICAL APPROACH The propeller model, designated PROPLD, computes direct, timeaveraged propeller loads on a maneuvering submersible. Typical execution times on a VAX 8810 computer are less than 10 seconds. Propeller input for PROPLD consists of the noninclined open water curves for thrust and torque and geometrical data consisting of the propeller diameter, hub radius, and the number of blades. The method is currently restricted to non-skewed propellers but is capable of modeling skew. Steady maneuver conditions may consist of combinedangle of attack and rotation. Figure 1 defines the coordinate system and sign conventions. The technical approach is summarized below. Changes made to the propeller inflow model of Pe rk id are described in detail. Other aspects of the propeller model, including the mathematical expressions for the load calculations, may be found and are not repeated here.