Read the article How do Polar Plots Work for updated and additional explanations.
The polar plot is the navigator’s friend. Below is an example for one particular boat for one particular true wind speed. They are published by each manufacturer for each model of boat with their various sail plans. It shows the theoretical attainable speed for a specific sailboat at various wind speeds.
It’s easy to read – move your finger radially along any true wind angle. Stop when you reach the boat performance line associated with the true wind speed you are experiencing. Now, read back around the concentric boat speed circles to the vertical axis and read off the polar boat speed for that given true wind angle and that given true wind speed. This is the speed at which your boat should be sailing and is the polar boat speed to give your sail trimmers to try to achieve.
Observe the example animation of a polar plot for an example sailboat for true wind at 12 knots. Click on any TWA (true wind angle) on the right bar and watch:
a. the boat icon move to that position on the polar plot. Then read around the polar plot to find the boat speed
b. the wind meter boat speed and TWA. They should match the polar plot if the sail trimmers are doing their job properly.
On a real polar plot, you would see other true wind speeds shown on the plot. Left off here so as not to clutter the image.
NauticEd sailing courses are jammed with information like this to make you a better sailor and give you a recognized sailing certification by yacht charter companies worldwide.