Uncertainty assessment for free-running model cases at the IIHR wave basin

Uncertainty analysis is performed to analyze the motions and results of maneuvering characteristics of a 1/49 scale surface combatant model during free-running maneuverability testing. The model is designed with a twin rudder and twin propeller rotating inwards. Calm water and wave testing is completed with an initial ship speed corresponding to a Froude number of 0.20 while the wave cases have wavelength to ship length ratio of 1.0 and wave height to wavelength ratio of 0.02. These conditions were tested for course keeping, turning circle, and zig zag maneuvers. The turning circles were completed to both port and starboard side. Tracking of the model is completed with an overhead carriage design with a mounted camera to record the motions of the ship and convert these motion to six degree of freedom motions. The combination of the tracking systems are analyzed to find the systematic standard uncertainty of the system.

Uncertainty was performed in accordance with the performance test codes written by ASME during 2013 to find the systematic standard and random uncertainty of measurements. The random uncertainty is found based on the standard deviation of repeated measurements, while the systematic standard uncertainty is found based on the bias of the measurement system and the sensitivity coefficients found from the data reduction equations. The data reduction equations are used to non-dimensionalize the measured values to compare to CFD results as well as results from other model scales. From the data reduction equations partial derivatives are taken to determine how the uncertainty propagates throughout the sensitivity coefficients. After the uncertainties are calculated the results were compared to other facilities to evaluate the method used and gauge the quality of the repeatability of the measurements. Few other facilities have analyzed the uncertainty during free running tests past looking at the random error based on repeated tests. The comparison with these facilities displayed that the uncertainty process and measurement repeatability used by IIHR at the wave basin produce consistent results with limited uncertainties when the end results of maneuvering characteristics are observed. Large uncertainties occur for some of the measured variables during the full scale of the testing time when the uncertainties are reported as a percentage of the harmonic amplitudes and the reported harmonic amplitude are near zero with a small uncertainty.