LATEST SK2 POLAR DIAGRAMS – FOR INFORMATION ONLY
Polar diagrams are a convenient way to display the speed prediction data generated by the VPP, and are
extremely useful in understanding both the general and specific relationship between the three most
important factors in a boat’s performance: wind speed, wind angle, and boat speed.
In the attached polar diagrams, True and Apparent Wind directions are indicated by the drawn arrows
and increase radially from 0° at the top to 180° at the bottom. Each radial line extending from the centre
represents a sailing angle relative to the indicated True or Apparent Wind Angle (TWA or AWA). Note
that it is AWA that is the wind as felt or observed on the boat or as seen aloft by the masthead fly.
Each radial line shown is graduated into one-knot increments by tick marks with smaller ticks on tenths
of knots. These are scales of the predicted boat speed: the farther from the centre the higher the boat
speed. The corresponding scale of speeds in knots are shown along the 90° line.
The plotted curves represent the boat’s speed at six different True Wind Speeds (TWS): 6, 8, 10, 12, 16
and 20 knots. The inner curve nearest the centre presents the boat speeds at the six-knot TWS and the
curve farthest from the centre presents boat speeds at 20-knot TWS.
The shapes of these curves give a qualitative as well as quantitative notion of performance: notice for
example that at close-hauled sailing angles, near 45° TWA, the boat speeds do not increase very much
with stronger TWS, but in reaching conditions they do increase greatly with wind strength.
Colour coding is used to indicate the boat’s performance under headsail and spinnaker sail
combinations, showing the effects of these sails on performance. The optimum beat and run angles and
speeds are also indicated for racing on windward-leeward courses.