The two systems in use in Europe (Texel and SCHRS) are both measurement based, which means that you can take any boat, measure it, and get a rating. There are ratings for the formula boats such as F16 and F18, but those are based on a fully optimised boat, i.e. minimum weight, maximum sail area, maximum length/beam etc. The Viper is not minimum weight, so can race off a slower rating in a mixed event. In an F16 event, all boats race on the water.

Paul