The physical draft of keels has long been known to be critical for the upwind performance of sailboats and has long been an element of handicap rules. The IMS rule uses a relatively sophisticated algorithm for assessing keel draft yet, as is the case with other rules, has never included any influence of rudder span. However, there is considerable empirical evidence that rudders can make a significant contribution to performance by increasing the total effective draft. Tank test research was conducted to explore rudder span and quantify performance differences. The results have been incorporated into the IMS 2002 rule in a way that is intended to give some rating relief to boats with shallow rudders yet not drive optimised design in that direction.
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