Proposed National Keelboat Rating System – How it Works, Part 3
Following on from comments in the S/A Forum.
The system is not a simple regression formula, but a very involved model which looks at every major feature, and which are then inputted into a multi layer structure which allocates a value (weighting) to each component, and the final result is a rating #. There are in excess of 20 material inputs, and these interact against each other in multiple calculations with a check at each level to ensure makes sense……
The initial speed generation model is similar to IMS/ORC where works out a dynamic windward hull speed and downwind VMG hull speed in 20 kts TWS. The course weighting is a combination of a windward/leeward course and an upwind beat/offwind fetch to factor in a portion of offwind usage. Net effect (by distance) is 55% upwind and 45% downwind & offwind. What one would expect to generate from a circular random profile, and to relate to general club racing.
The basic speed numbers are 99% of final model and rest of mix averages out to less than 1% for performance cruisers and club level cruiser/racers, about 2% for racer/cruisers and about 4% for carbon and kevlar kitted out racer/cruisers. Many of the features compensate or offset each other, such as accelerative ability and low DLR offsets stability factor. But if tick all the performance boxes for a racier rig, then expect to pay in rating #.
The question as to asymmetrical’s refers. If run a spinnaker and asymmetrical, the system charges you for the largest sail and longer of SPL/STL. If run an asymmetrical only, the yacht is rated on the area of sail, the STL measure, and then an adjustment off the total rating for lack of downwind VMG ability. This is adjusted and based on a combination of inputs by a single formula in the model. If put this into a sail area comparative, would allow an average extra 20% more sail area (without rating penalty) for asymmetrical on sprit only, compared to a spinnaker only(no penalty pole). This example would be for a J-120. The sail area calculations are measured at the IRC 0.83 and not IMS/Sailmaker 0.95 factor.