I'm still not making my point very well, so let me try another approach.

Shouldn't you be able to predict mathematically how a boat will perform? If so, you predict an outcome and compare that to an individual's actual performance.

For instanced if our model predicts a boat should attain ten knots on a certain course but a sailor only attains eight his performanc could be expressed as .8 or, .800.

Carry this a little further. Let's assume, arbitrarily, that a tack should take ten seconds, and that a weather leg of 100 meters should be sailed in six minutes. If a sailor is required to make four tacks while sailing the 100 meters, his best possible time would be six minutes forty seconds and score 100%. If in fact he sails the course in ten minutes, his score would be .667 (6.67/10= .667) And that would be his rating- .667.

Obviously we would need to add an adjustment for a start and for a down wind leg.