A quasi-steady method for efficiently conducting open water model tests

Open water tests of propellers, pod units and thruster units are usually performed in a steady manner where the forward velocity and propeller rotation rate are constant during each measurement. It can require a considerable amount of time in the towing tank to complete an open water diagram with this method. Therefore, a faster method is proposed which applies a quasisteady approach; the forward velocity of the towing carriage is varied while the thrust and torque are measured continuously. With this method, the normal open water diagram (J=0 to K_T = 0) can be determined in one single run through the towing tank instead of multiple runs. This quasi-steady open water (QSO) procedure greatly reduces the time required to perform open water model tests. The method has been tested at MARIN. During those tests, it was seen that there are hysteresis effects on the raw data, but those can be negated by taking the average of the accelerating and the decelerating halves of the measurement. Test results of four different types of propulsors are shown: an open propeller, a ducted propeller, a pod unit and a thruster unit. The results show a very good agreement between the results of the standard, steady procedure and of the QSO procedure.