SailGrib WR’s polar library includes more than 400 boats. Most of these polar are theoretical and come from software called VPP (Velocity Prediction Program).
They are often optimistic and must be adjusted according to the way you sail.
To display a polar, go to the left menu and choose “polar”.
Take a good look at the speeds indicated by these polar diagrams at different true wind angles and ask yourself the question: is this reasonable for a routing?
Here, for example, for a Sun Fast 3200 in 12 knots of wind.
- upwind, a boat surface speed or speed through water (STW) of 6.8 knots with a true wind angle (TWA) of 40°, is this reasonable for a routing? My answer is No, most likely I will sail around 6.2 knots with a TWA of 45°. The polar speed for a TWA of 45° is 7.1 knots. I will therefore sail at 87% of the polar with a 45° limit.
- crosswind, a STW of 7.8 knots for a 90° TWA, is this reasonable for a routing? My answer is No, I will sail at around 7.2 knots or 92% of the polar.
- downwind, a STW of 6.2 knots for a 160° TWA, is this reasonable for a routing? My answer is No, I will sail at around 5.8 knots or 93% of the polar.
With this quick approach, I believe I should sail at around 90% of the boat polar at 12 knots with a 45° upwind limit.
In the app, you can easily modify a polar in the routing menu using the following settings:
a minimum upwind angle: if, as in the example above, the theoretical polar indicates an upwind angle of 40° in 12 knots of true wind, this is probably impracticable when sailing. Enter 45° for this parameter.
a maximum downwind angle: this is more for safety reasons. If the theoretical polar indicates a downwind angle of 175° for 12 knots, it is probably not a good idea when sailing and you don’t want the routing to propose that. For example, enter 165° for this parameter. An autopilot will probably be able to steer in true wind mode at 165° without risking gybing. In the above case, the best downwind angle at 12 knots is 160°, the polar at 12 knots will not be affected by this parameter.
2 polar efficiency coefficients. One below 10 knots of true wind, the other above 20 knots. To ensure continuity, the app will linearly interpolate the coefficient between 10 and 20 nodes. If cruising, you should notice that you sail between 70% and 75% of the theoretical polar in light winds and between 75% and 80% above 20 knots. For racing yachts we can see that we are generally around 90% of the theoretical polar and sometimes around 85% in light airs.
If you have connected the app to the boat’s NMEA data, this output can be displayed permanently. Even better, you can display the graph of this performance with the Histograms module. Use these indications to adjust the coefficients in the routing menu.
Here, we sail at 91% of the Sun Fast 3200 polar by 16 knots across, the sea is flat.
The efficiency percentage is very stable around 90%. We take a reef to see how the boat behaves. No significant change in performance. The reefing lasted 2 minutes and the efficiency percentage fell to 72% during this time. In a race, at 7.7 knots, we would have lost around 85 meters or 8 boat lengths…
