Performance curve scoring is the most powerful engine of ORC International. 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 will have a different time allowance in each race depending on the weather conditions and the course configuration for that particular race. This simply recognizes the fact that yachts can behave very differently according to their unique characteristics. 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 go fast downwind.

ORC International certificate is providing a range of ratings (time allowances expressed in s/NM) for different wind conditions in the range of 6 – 20 knots of true wind speed from optimum beat, over 52, 60, 75, 90, 110, 120, 135, 150 degrees of true wind angle to the optimum run.

When calculating corrected time by the Performance Curve Scoring, a course to be sailed shall be taken as one of the pre-selected courses for which time allowances are given on the certificate, or constructed from the data measured at the racing area. Pre-selected courses are: 

• 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.

• Circular Random is a hypothetical course type in which the boat circumnavigates a circular island with the true wind velocity held constant. 

• Ocean for PCS is a composite course, the content of which varies progressively with true wind velocity from 30% Windward/Leeward, 70% Circular Random at 6 knots to 100% Circular Random at 12 knots and 20% Circular Random, 80% reach at 20 knots

• Non Spinnaker is a circular random course type (see above), but calculated without the use of a spinnaker

The use of PCS is not as complicated as it may look. For any pre-selected courses, as well as any other type of course of a given race a set of 7 values is derived by the computer program constructing each of them as a function of length, bearing and wind direction for each leg of the course. The points so obtained are linked by a curve which represents the predicted optimum performance along a scale of wind speeds. This curve is called the Performance Curve (Figure 1), and for each yacht this curve is different for any different course sailed.

The vertical axis represents the speed achieved in the race, 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. This number is represented by point A on the vertical axis. The computer then finds the point on the horizontal axis that corresponds for that course to the average speed obtained.

This results in point Aw, the so called “Implied Wind”. This means that the yacht has completed the course “as if” it has encountered that wind speed. The faster the boat has sailed, the higher the Implied Wind, which is the primary index for scoring. The yacht with the highest Implied Wind wins the race. The Implied Wind is then transformed into a corrected time.

The Implied Wind is intended as an interpolation between time allowances, not an extrapolation. This means that when the Implied 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 IMS races need to be stopped (or not started) with wind below 6 knots or above 20. When the “implied wind” results to be lass than 6 knots or more than 20, the respective values are used, as using a “fixed wind” option.

In order to present the result of the race in a comprehensive format we use a “Scratch Boat” (Figure 2). In most cases this is the potentially fastest boat of the fleet, yacht B. Her Performance Curve is the lowest in the drawing. From the point where the vertical line yacht A intersects with the curve of the Scratch Boat, a horizontal line is drawn to the left towards the vertical axis. This point, Ac, produces the corrected time when the seconds per mile are multiplied by the distance of the course in miles. The corrected time of the Scratch Boat is, by definition, same as its elapsed time. This exercise produces corrected times, expressed in hours, minutes and seconds, a format most sailors are familiar with.

                    
            Figure 1 - Performance Curve                                             Figure 2 - Determining Corrected Time

“Implied Wind” for the winning boat is normally in the range of real wind strength dominating for the race. However, in case that “Implied Wind” does not represent fairly the real wind strength during a race, the Fixed Wind method may be used to enter the performance curve with predominant wind speed at the horizontal axis and getting  the appropriate Time allowance at the vertical axis. Such a time allowance is then used as single number Time-on-Distance coefficient.

A more detailed explanation of the Performance curve scoring can be downloaded as PDF file or Power Point presentation.
 

Corrected time is calculated by the time coefficient as follows:

Corrected time = TMF (Low, Medium or High) * Elapsed time

The Triple Number system provides a set of three time multiplying factors TMF’s (as described above for Time-on-Time) given for three wind ranges:

• Low Range (less or equal 9 knots)

• Medium Range (between 9 & 14 knots)

• High Range (greater or equal 14 knots)

The Race Committee shall signal before the start the wind range to be used for scoring, but it may change this in case of significant change in the weather conditions.