US Sailing, the Offshore Racing Council (ORC), the Glenn L. Martin Wind Tunnel (GLMWT), Quantum Sail Design Group (QSDG), the Wolfson Unit and North Sails have collaborated on a series of wind tunnel test programs to investigate the performance of both upwind and offwind sails. These programs were initiated in response to perceived inequities in the ratings of boats of various rig designs sailing under the International Measurement System (IMS).
Observations of on-the-water performance have lead to the conclusion that there are biases within the rule with respect to rig planform design. Specifically, it has been concluded that large spinnakers are penalized so that a fractional rig, with its small spinnaker, is favored when sailing offwind, that there are un-rated benefits to a masthead rig upwind, and that there are errors in the relative handicapping of overlapping and non-overlapping jibs. The IMS Rule uses a Velocity Prediction Program (VPP) in which sail forces are represented by algorithms that are based on a combination of science and reverse engineering from the measured sailing performance of real boats. The results of investigations at both GLMWT and Wolfson have been used to modify this IMS aerodynamic model, thereby reducing the pre-existing biases.