The National Research Council of Canada's Institute for Marine Dynamics in December 1992 undertook to design and commission a new generation model yacht dynamometer capable of testing one third scale models of International America's Cup Class yachts in waves, and half scale models in calm water. A performance specification was set, and it was decided to base the design on flexible link technology similar to that used in wind tunnels. Finite Element models of the design were constructed and theoretically loaded to determine natural frequencies in drag, lift, and roll, as well as system compliance under static loads. The dynamometer is a four-component balance (lift, drag, roll moment, and yaw moment) with a theoretical minimum crosstalk determined by the ratio of flexural to axial compliance of the flexible links, approximately 23,000: 1. In individual cells this reduces the moment delivered to the load cells. In the system, it reduces crosstalk, the theoretical minimum is approximately 1 :4600. In practice, however, the measured cross-talk did not approach the theoretical limit of the dynamometer and a calibration rig was designed to determine the actual crosstalk to be used in software correction of the data.
This paper covers the basic design of the dynamometer, error analysis, calibration and crosstalk determination, and repeatability of results. The conclusion is that it is now possible to obtain a quality of data from the tow tank previously obtainable only in wind tunnels. Not only has this made possible a better job of the types of experiments previously done in tow tanks, but has also made possible new experiments. In particular the evaluation of lifting surfaces in the presence of the free surface, and experiments on devices such as strakes to reduce interference drag due to viscous effects, are now possible.
