VPRS
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VPRS - Velocity Prediction Rating System

This page gives some summary information on how VPRS works and how we run it, how it differs from other rating systems ... and what it offers. None of the information on this page is needed for a rating application; that is all covered on the Apply & Measurements pages.

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 captured on the application form. A velocity prediction program (VPP) then uses these 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 used to calculate a 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 data set - 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 of 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. In turn, 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 create 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 considerably shortens the calculation time by reducing the number of iterations needed for each run of the VPP.
     


    An alternative approach

    VPRS is distinct from most other rating systems, because it uses a true VPP to calculate the ratings without needing a hull lines diagram, or a large number of measurements. This is achieved by translating commonly available yacht measurement data into the parameters recognised by the VPP, and it allows the ratings to be calculated from measurements similar to those used by other rating systems. In addition, the complete independence of the set of measurements held for every boat allows significant changes made during a class production run to be taken into account - and new designs and custom builds can also be rated without delay.

    The calculated ratings do not have any allowances or adjustments applied to them. The set of VPP models take account of all the important performance characteristics which determine a rating, and they are completely configured by the measurement data. Formulae characteristic of traditional rating rules don't appear in the calculations, but they are used to seed the VPP to speed calculation.

    VPRS is a performance-based (rather than a concession driven) system and this can be seen by comparing 'no-spinnaker' ratings. When the aerodynamic models are reconfigured for sailing without a spinnaker, the ratings fall by about 7% - 10% and represent the true loss of averaged performance expected on the water. When boats compete in strict 'no-spinnaker' classes, these are the fairest ratings to use. When racing against fully canvassed boats however (a less than ideal compromise), only a small proportion of the reduction is appropriate. This concession is given as an additional 'mixed-fleet' rating.

    Yachts are not explicitly assigned a 'boat type' in order to classify the hull form. Whilst this appears to be an intuitive means of differentiation, it turns out to be a quite deceiving simplification, which is very difficult to apply consistently. The 'boat type' characteristics are instead derived from the set of measurements - and hence more rigorously accounted for.

    Many rating systems use simple formulae for calculation and are forced to rely on complicated measurement rules and intricate definitions 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. An alternative sees all the complexity transferred to the rating calculator, giving an opportunity to write simpler measurement rules and definitions; this is the approach taken by VPRS. The measurements become easier to understand and the rating system becomes more accessible. The measurement guide will of course be revised as necessary, but the drive to keep it simple will remain.
     


    Optimising for VPRS

    Optimising for a rating system is rather contrary to the spirit of fair racing as it seeks to convey advantage by exploiting system weaknesses. This is one of the reasons that VPRS isn't a simple rule-based calculator. It also accounts for the near absence of boundary cases and conditional clauses in the measurement definitions.

    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. 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.

    When evaluating a rating system, 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 effectively 'remove the boat from the race' - to yield results reflecting crew performance and tactical decisions. One-design class events illustrate this point perfectly - the racing is frequently seen to be anything but close.

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




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