UBC Theses and Dissertations
A study of the hydrodynamic performance of Voith-Schneider propelled escort tugs Ratcliff, Arlon J.T.
The hydrodynamic performance of a typical modern escort tug, as it relates to indirectly applied ship handling force, is examined through an experimental and theoretical study. Experimental results are presented from a series of tow tank tests performed on a 1:18 scale model of the Voith-Schneider Propelled (VSP) escort tug, AJAX. Steady state flow patterns close to the hull are investigated and possible areas of focus for improvement are highlighted. Using this information, three alternative skeg configurations are proposed and tested. A semi-empirical numerical model, based on current and previous experimental data, is developed as a standard basis of comparison for the alternative configurations. The numerical model also allows prediction of performance for hypothetical tug designs. A sensitivity analysis is performed using the numerical model to determine areas of focus for design optimization. No significant difference in performance was found between the alternative and baseline skeg configurations, indicating more comprehensive design changes are required to effectively improve escort performance. However, an alternative configuration, consisting of a skeg similar to the baseline skeg with a gap at mid- chord, achieved this comparable performance with 14% less lateral area, which may be of benefit to other aspects of tug performance. The results of the flow visualization study and sensitivity analysis indicate location of the skeg has a greater influence on performance than shape. Stability and tow point location are also relatively important. However, no single design parameter was identified for focus. Instead, design optimization requires a balance of several parameters. The numerical model can be used for this purpose. The numerical model produces performance estimates with an uncertainty of 15% of the maximum steering force. This uncertainty is adequate for initial estimates at the early stages of escort tug development.
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