A polar diagram shows the speed (potential
) on various points of sail.
speed is set or known, as well as the sail combination.
The circles (radius going from the center point) are how one gauges the speed of the boat. The lines radiating out from the center point show the point of sail. The 12:00 position shows as if you were trying to sail directly into the wind
. This can't happen, so the curved line starts some degrees off. These degrees show from close hauled (1:00 or so) to dead down wind... 6:00. The degrees of radiating lines show the true wind angle. Doubling the degrees from where the curved line starts, gives a good idea of how many degrees the boat "Tacks through" and gives data which you can compare to other boats.
The curved line shows the speed of the boat, at that point of sail (see where it crosses the % lines. Look to the nearest circle and calculate out actual boat speed from there.
Multiple curved lines will show various sail combinations and or wind speeds. Sometimes but not always, you will have heeling angles posted as well, which can be helpful to show wind speed and sail combinations... for when its time to reef.
It is a visual way to show a lot of data on one page, instead of having to write it all out!
Hope this helps!
Edit: What they don't really help interpret is leeway. The closest you can sail to the wind isn't necessarily ideal when it comes to getting there fast... the Velocity Made Good. (VMG) Yes, some show the boat speed as VMG... but some folks argue the accuracy when out on the water