Measurement Based Handicapping

Some excellent comments here, esp. from Jim C and Stephan as to why you can't have a 'number for the day'; my view entirely, and that sets the limits on the usefulness of a predictor. The predictor mainly allows you to see why you did well/badly on a given day - or at least gives you some idea. It also is pretty good at predicting what happens when you make changes within a given class, e.g. by taking out weight or adding sail area.

As Daniel Holman says (my approach was pretty similar to his by the sound of it), the big problem in predicting performance is determining waterline beam and wetted area. I just guessed at w/l beam from the o/a beam and my recollection of how flared a classes topsides were. The wetted area I then calculated using a very crude formula. Waterline length is also important and is a guesstimate in many classes.

I also tried an alternative 'statistical' approach, plotting all classes' known PY's against a simple formula combining all the parameters and looking for a 'best fit', playing with the constants. For displacement sailing that came out pretty well and would be good for a 'first shot' for a new class. I found it very hard to come up with a good fit for planing performance, however; maybe hull shape detail is more important for that than it appears to be for displacment sailing.

Of course, the relative performances can best be estimated for like-classes - skiff, keelboat, or whatever, though for mixed handicap racing that doesn't get you very far. For my comparison, I used the Wayfarer as my yardstick class (I was mostly racing against them at the time in a Laser 2). The real problem then, though, was planing - it's very hard to predict when it will happen and for what percentage of the time. What was clear though, was that barring accidents or pure luck, they were going to win in displacement conditions and we could hardly lose once planing.

As to Rick's perennial plug for class racing, I'm afraid that for me what's most important is to have a good sail in the right boat for me, but I can't expect most of my club-mates to want the same class as me, so handicap racing is inevitable. As for results, people barely seem to remember them, so are they so important? - I once asked a bunch of sailors at a nationals 'who won last year?' - nobody could remember for sure!