SCHRS is a measurement based system with tweaks. It is a complicated formula that predicts boat performance by physical dimensions and configurations. Technically speaking, as long as the F18 keeps the same foil type, mast type, hull dimensions, and sail area, it's SCHRS number should not change even if the sail shapes and batten technology make minor evolution changes and make the boat quicker.

The SCHRS folks have realized that there is more to it than this and they evolve their formula as things change. It's a lot of complex hard work and physically measuring boats has to happen.

The two systems are very different - Portsmouth attempts to calculate a handicap number based on actual race results. The weaknesses are that A) you have to get people to submit race results B) the software that was used to run the statistics was written in Fortran (which was brilliantly assembled but hard to run today...someone may have modernized it by now though) C) boat age and sailor skill level will affect the ratings. For instance on "C", suppose the Hobie 14 is mostly sailed by inexperienced sailors and it's rating creeps as a result. Then a skilled sailor makes a campaign on it and wins everything in sight. Is that fair?

SCHRS suffers from the rigidity of it's system and that it is somewhat uncoupled from many of the other hard to measure dynamics that make a boat fast or slow. This results in the need for complex formula tweaks to refine it.

Which one is more accurate? They're pretty different. I can't honestly say that I've ever felt like a handicap race result under DPN was unfair (although Nigel might say different from Wateree ;-). But, then again, I have a different expectation for handicap racing and don't take the result quite as seriously as I would an OD event. Overall, however, I think Portsmouth has a better opportunity to be more accurate but suffers from human nature.