Experimental Analysis of Rudder Contribution to Roll Damping

Abstract

Measuring and modeling the forces on the appendages of surface ships is important for understanding roll-damping and validating numerical simulations. In recent years, Atsavapranee et al (2007) showed that the bilge keel damping component can be modeled using the flat plate theory established by Keulegan and Carpenter (1958). This model treats the bilge keels as a flat plate that generates viscous damping, as well as added mass. The model comes as an improvement to models used in computational codes used for predicting roll damping, due to the fact that the added mass component is significant. In this study, uncoupled roll motion is investigated to quantify the rudder forces on a fully appended DTMB model #5415 with instrumented appendages at Froude numbers of 0 and 0.138. The objective of the current effort is to decompose the rudder force into its steady, symmetric, and antisymmetric components using Fourier analysis. In the force analysis the rudders are treated as flat plates for the Fr = 0 tests, using the model described by Keulegan and Carpenter (1958). The drag and lift forces are consistent with the flat plate model. The anti-symmetric term, however, does not show a clear trend. For a flat plate model, the anti-symmetric term should resemble a negative sine wave with respect to roll. However, the rudders represent a higher aspect ratio flat plate, and thus require a modification to the added mass formulation. Furthermore, during a normal roll period they tend to interact with the free surface, which can lead to wave damping, which should resemble a positive sine wave with respect to roll. Thus, the two components of the anti-symmetric portion of the signal are superimposed upon one another. In an attempt to decouple these two components, the added mass was artificially removed from the antisymmetric component of the force. This paper will detail the methods used to model the rudder forces for both the standstill and positive Froude number cases.