Polar Curve Scoring
   
 

Polar Curve Scoring is the most powerful engine of the ORC rating systems. It is this unique feature which makes this rule fundamentally different from any other handicap system, as it recognizes that yachts of varied design perform differently when conditions change.

This means that yachts of different designs will have different time allowances in each race depending on the weather conditions and the course configuration for that particular race. For example, heavy under-canvassed boats are slow in light airs but fast in strong winds, boats with deep keels go well to windward, and light boats with small keels will go fast downwind.
 
   
 

Where is it shown on the certificate?
   
 

   
 

An ORC certificate provides a range of ratings (time allowances expressed in secs/NM) for wind conditions in the range of 6 – 20 knots of true wind speed, and at angles varying from an optimum VMG beat to 52, 60, 75, 90, 110, 120, 135, 150 degrees of true wind angle, as well as the optimum VMG run angle. Additionally, two pre-selected course are available as:
  Windward/Leeward (up and down) is a conventional course around windward and leeward marks where the race course consists of 50% upwind and 50% downwind legs.
  All Purpose includes equal distribution of all wind directions as a hypothetical course type in which the boat circumnavigates a circular island with the true wind direction held constant. 
   
   
 

How is it calculated?

   
  Taking for example, two boats with time allowances for All Purpose course their handicaps may be calculated as follows:
   
 
  6 kt 8 kt 10 kt 12 kt 14 kt 16 kt 20 kt
Boat A 964.1 783.9 679.9 621.5 588.0 565.9 541.2
Boat B 881.3 731.7 679.9 658.8 644.4 630.5 610.7
Difference 82.8 52.2 0.0 -37.3 -56.4 -64.6 -69.5
   
  This would mean that Boat B will give 82.8 seconds per NM to Boat B in light winds, while in strong winds it will be opposite with Boat A giving 69.5 seconds per NM to Boat B.
  To score the race, race committee needs to select the wind strength to be used for the scoring. So called Soring Wind is calculated from the performance of the boats. Time allowances for 7 wind speeds may be presented as performance curve.
   
 

   
  In a typical Performance Curve plot, the vertical axis represents the average speed of the boat around the race course, expressed in seconds per mile. The horizontal axis represents the wind speed in knots. When the finishing time of Yacht A is known, its elapsed time is divided by the distance of the course to determine the average speed in seconds per mile.
  For example, if the elapsed time of the boat with the curve shown above is 1 hour 28 minutes 11 seconds and the total length of the course is 8.11 NM, the average s/NM for the boat on that course is:
   
  Elapsed time: 1:28:11 hours = 5291 s
Course length: 8.11 NM
Allowance = Elapsed Time / Course length = 5291/8.11 = 652.4 s/NM
   
  This value is then found on the vertical axis, and the software finds the point where it intersects the performance curve as shown below:
 

   
  The corresponding point on this curve on the horizontal axis is the so-called Scoring Wind. This means the yacht has completed the course “as if” it has encountered that wind speed. The faster the boat has sailed, the higher the Scoring Wind, which is the primary index used for Polar Curve Scoring: the yacht with the highest Scoring Wind wins the race.
  Scoring Wind is intended as an interpolation between time allowances, not an extrapolation. This means that when the Scoring Wind drops below 6 knots or raises above 20 knots, the time allowances used for calculating the corrected times will be those of 6 knots and 20 knots respectively. This does not mean that ORC races need to be stopped (or not started) with wind below 6 knots or above 20. When the Scoring Wind results calculate to be less than 6 knots or more than 20, the corrected time values at these wind speeds are then used.
  With the winner known, the remaining rankings in the race are determined as follows: the Scoring Wind of the winner is used as the true wind speed to then calculate the corrected times of the other entries. With that wind on the horizontal axis, the appropriate time allowances are determined on each boat’s curve on the vertical axis. Such a time allowance is then used as a single number Time on Distance coefficient.
   
   
 

Constructed course
   
  Further sophistication of the Polar Curve Scoring and use of full power of the ORC VPP may be achieved by defining the course when the course does not fit with one of the pre-defined course models i.e. any course different from Windward/Leeward course (50/50) or All Purpose with equal distribution of all wind directions.
 

The use of a constructed course is not as complicated as it may appear. It requires the Race Committee to provide only a little more data in addition to their usual work of setting up the course, following the wind changes, making starts and taking finishing times. ORC provides free PC-based ORC Scorer Software that will do all calculations that enable results to be ready as soon as the elapsed times of the race are entered.
  The course may be constructed with these parameters in the ORC Scorer software:
 
  • distance
  • course bearing
  • wind direction
 

Leg Distance Bearing Wind direction
Start - 1 2.09 NM 162 160
1 - 1a 0.06 NM 060 155
1a - Gate (2-2a) 1.91 NM 340 155
Gate (2-2a) - 1 1.89 NM 161 160
1 - 1a 0.06 NM 060 160
1a - Gate (2-2a) 1.91 NM 340 160
Gate (2-2a) - Finish 0.19 NM 316 160
       
Typical course definition. Distance and bearings of each leg are entered, as is the approximate wind direction. Note wind speed is not entered.Current velocity and direction can also be entered for each leg, if it is known.
 
 

From the course constructed as described above, the true wind angle (TWA) is calculated as being the difference between the wind direction and compass bearing of each leg. With this information, a table is made for each boat that describes the theoretical speed of that boat over that course for the range of seven true wind speeds (TWS). Calculated time allowances for these wind speeds are then used to calculate Scoring Wind and corrected times as explained above.
   
 


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