Investigation Of “Unsteady” Method For Model Calmwater Performance

The purpose of this study is to investigate and validate “unsteady” model testing method for calm water performance evaluation. Model tests for calm water resistance are typically performed at a set of discrete constant speed runs where the relevant parameters are measured and averaged over a steady speed data window of each run. However, a test matrix can become substantially large when required number of speeds, loads and hull configurations increase. In this study, an “unsteady” method was investigated where the model was towed accelerating slowly and covering the full speed range of interest in a single run. The forces due to inertial mass and added mass were subtracted from the measured unsteady resistance data and compared with data from standard calm water constant speed experiments. Steady constant speed runs were conducted in calm water over a range of Froude numbers between 0.08 and 0.55. The “unsteady” tests were conducted in calm water over the speed range using four different accelerations of 0.1, 0.15, 0.2 and 0.3 ft/s2. Runs were conducted in air at these accelerations to determine inertial forces, and a value of 0.1 was assumed for surge added mass coefficient. The results showed that the unsteady method compares well with the standard constant speed method.