What is the Wind Rose?
The Wind Rose is a comprehensive visual summary of wind speed and direction at a specific place and time. See the photo below to understand how this appears in the Orca app.
On Wind Rose, the blue triangle with an A is Apparent Wind Angle (AWA); the green triangle with a T is the True Wind Angle (TWA) and the smaller black triangle with a line is the indication of Course Over Ground (COG).
When your boat speed drops below 0.3 m/s, the black COG triangle will disappear, this is because Course Over Ground cannot be accurately measured below this speed. When the boat speed is detected to be over 0.3 m/s, it will appear.
There is also another blue triangle with a number inside it, which is the water current vector indicator. In the image, this is the triangle at the bottom of the circle.
The arrow shows the direction of the water current relative to the boat.
The number is the water current speed in knots.
How is the vector calculated?
The current vector is calculated from your boat’s motion through the water compared with its motion over ground from GPS. The system also estimates and corrects for leeway (sideways slip due to wind) based on heel angle, so the result is intended to show water current rather than leeway, though a small influence may still remain.
What do the lines mean?
The red and green lines on the wind rose are laylines. They show you the two optimal courses you can sail to make progress upwind. Because a sailboat can’t sail straight into the wind, you have to zig-zag (tack) back and forth.
The green line marks the best course on port tack.
The red line marks the best course on starboard tack.
The black line shows heading, which can be either Course Over Ground (where you are physically travelling through the water) , or where the front of your boat is heading.
This can be modified by yourself, or automatically selected depending on speed. See this article for more information
Together, these lines form the “corridor” you need to stay within to eventually reach a destination that lies upwind. They save you from guessing when to tack and help you steer more efficiently toward your goal.