Stoneways VPRS | 2022 | VPP Yacht Rating
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VPRS - Velocity Prediction Rating System - overview

This page gives some summary information on how VPRS works, how we run it and how it differs from other rating systems. You don't need to read anything on this page to get a rating.

VPRS is a yacht rating system that uses a set of mathematical models, which draw on a range of well-established analytical and empirical techniques, to estimate the resistance of a hull and the power available from the sails. The models are configured using the hull and sail measurements stated on the rating certificate.

A velocity prediction program (VPP) then uses the models to find the velocity at which the forces balance and this represents the speed potential of the yacht. The calculations are performed for various points of sail including beating to windward, for a variety of weather conditions.

The VPP results comprise a set of polar performance data (boat speed vs wind speed / angle) from which a weighted average (simulating the conditions encountered in a race, or series) is taken to calculate the rating.

The rating is given in the familiar form of a Time Correction Coefficient (TCC).

Using a VPP to calculate performance-based ratings

The rating calculator is partitioned into the following main components:
  • aerodynamic models for the sails, hull and rigging
  • hydrodynamic models for the hull and appendages
  • a stability model which estimates the stiffness of the yacht
  • additional righting moment components for hiking crew and for moveable ballast
  • the measurement dataset - and a number of routines which pre-process the data
  • independent variables describing the sailing condition (speed, heeling, leeway, trim, etc)
  • the solution subroutine (the heart of the VPP), which is tasked by ...
  • a wind speed / angle matrix defining the polars (including best VMG)
  • a seed generator which sets the starting point for each VPP calculation
  • Every boat on the system has an independent data file, from which the measurements are read when a rating is required. Estimating and interpolating routines then compensate for the restricted number of measurements and any missing data, and generate a validated and coherent data set.

    The validated data is translated into a set of conventional parameters used in naval architecture and these are used to configure the behaviour of the aerodynamic, hydrodynamic and stability models. The stability model allows the heel angle to be calculated, and this is one of several parameters fed back into the aerodynamic and hydrodynamic models.

    The VPP subroutine runs once for each of the entries in the task matrix and works by iteratively adjusting the independent variables, until all of the governing equations show the aerodynamic, hydrodynamic and righting forces acting on the yacht to be in balance. This corresponds to the point at which the yacht settles for the prevailing conditions—and the speed is recorded in the task matrix to build a set of speed-polars.

    The seed generator uses simple performance measures to estimate the independent variables for each entry in the task matrix; these estimates give the solution subroutine a good place to start each speed-polar calculation. This has no bearing on the final solution, but it considerably shortens the calculation time by reducing the number of iterations for each run of the VPP.

    A new approach

    VPRS was developed to use a VPP to calculate yacht ratings without needing a hull lines diagram, or a large number of measurements. This was achieved by transforming commonly available yacht measurement data, like that used by other rating systems, into the model parameters recognised by the VPP. Only the measurement data is used to configure the models, which include all the important performance characteristics needed for a rating. The data is not adjusted and there are no allowances so that VPRS is a true performance-based (rather than a concession-driven) system.

    Separate independent measurements are held for every boat to allow changes made during a class production run to be captured. New designs and custom builds can also be rated without either delay or reference to similar designs, or to performance data from the race track.

    The use of a 'boat type' in order to classify a hull form is best left in the domain of traditional rating rules. Whilst this appears to be an intuitive means of differentiation, it turns out to be a quite deceiving simplification, introducing arbitrary boundaries. The sailing characteristics are instead all determined by the set of measurements.

    When boats are allowed to compete without spinnakers alongside fully canvassed boats (a less than ideal compromise), the reduced 'no-spinnaker' TCC may be used. This concession is provided for the clubs which arrange mixed fleet racing, and is regularly reviewed with them.

    Many rating systems use simple formulae for calculation and are forced to rely on complicated measurement rules, intricate definitions and concessions to compensate for their deficiencies; collectively these are known as rating rules. Unfortunately, this approach seems to encourage attempts to defeat the rules, which become ever more arcane. In VPRS, most of the complexity is transferred to the rating calculator and the measurement rules and definitions are kept as simple as possible. Formulae characteristic of traditional rating rules have no place in the VPP.

    Optimising for VPRS

    Optimising for a rating system is rather contrary to the spirit of fair racing as it seeks to gain advantage by exploiting system weaknesses. As VPRS isn't a simple rule-based calculator this could be rather more challenging. The near absence of boundary cases and conditional clauses in the measurement definitions should also discourage such activity.

    The aim in racing should always be to work the boat harder than the rating system!

    Annual system review

    Whilst the models have in part been designed to ensure that the ratings are in line with observed performance, no subjective adjustments are made to any calculated ratings—these depend on measurement alone. The models are however continuously under review and will be updated between seasons in the light of racing experience and the evaluation of new designs. The resulting changes will be applied to all yachts and new ratings will be generated in time for the next season. The design of the computer models and estimating algorithms will not be made public in order to discourage owners or designers from attempting to gain unfair advantage.

    It must be remembered that close racing results are the goal of handicap systems (these include rating systems which apply corrections—overtly or otherwise). This is not the case for pure rating systems (like VPRS) which even out the advantages and disadvantages of the different boat types, yielding results that reflect crew performance and tactical decisions. One-design class events frequently illustrate this point perfectly—the racing is often seen to be anything but close.

    You are welcome to give feedback on any aspect of the system: our contact details are here.

    Copyright 2024 VPRS