Abstract: An inflatable air bag for filling and sealing the gap between the flat, underside of a model of a gravity-based, exploratory drilling structure, or the like, to improve the accuracy and reliability of data obtained from hydrodynamic forces acting on the model when the latter is under test. The bag is of substantially the same shape in plan as the underside of the model and is inflated with gas to a pressure sufficient to seal the gap and prevent hydrodynamic forces from acting on the underside of the model during testing. The material of the bag is of minimal thickness so as to minimize load transfer thereto. Variations in the internal pressure of the bag during a hydrodynamic test of the model is utilized to improve the accuracy and reliability of the data obtained from load cells which support the model in the test facility.