Join Date: Apr 2006
Thanked 181 Times in 178 Posts
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It is not just by observation. There are PHRF measurement rules, and if you look at any application for a PHRF rating you will see them. The basic "first" rating is from observation, but that is modified locally based on local observations, and modified every few years based on results and experiences, and modified further for each boat based on any variations in equipment or sail plan, i.e. do you have a 3-blade fixed prop? Or a folding prop?
PHRF numbers aren't "fair" and everyone knows that. They are an attempt to be as fair as practical without placing great expenses on the measuring system and the owners. If you've got a fast boat but don't sail it well, your position in the fleet goes down and the boat might be re-rated to compensate. If someone else buys the same model or the same boat and sails it up to potential...that's not fair, but that's eventually supposed to be accomodated in the reratings.
If you look at ratings lists, sometimes you will see "Number of years in fleet" listed with each boat, for that reason.