The compass is on of the most valuable navigational instruments on a sailboat! The require no electricity, they do not need software updates, they just work!
The compass lives on the top of the binacle, where it can always be clearly seen. In the compass are a series of lines that flank the compass dial, these are called "lubbers lines" and they are crucial to navigation under sail.
In this example, I am sailing south (180) and the line that points ahead is in line with the ship, and marks our heading as 180. This line is simple to understand, as it points in the direction you are going.
If you look around the compass some more, you will see several other lubbers lines.
These other lines are set at +/- 45 degrees and +/- 90 degrees to the heading line. In our case, with a heading of 180, the next lubbers line to port would be 135, followed by 90, and to 225 and 270 to starboard.
What do these lines mean and what is their purpose?
Let's start with the +/- 45 lines. These lines show the approximate wind direction. If you are sailing as close to the wind as possible on port tack, and your heading is 180, the wind is coming from the port 45 line: 135. This means that the wind is coming from 135.
If you were on starboard tack, then the wind would be coming from 225.
The next line is the +/-90. These lines indicate your new heading after you tack. So if you are sailing as close to the wind as possible with a heading of 180 on port tack, the wind would be coming from 135, and your heading after the tack would be 90.
If you were on starboard tack, the wind would be from 225 and your new heading would be 270.
This may sound like a fun academic exercise with no real world application, but in fact, this is vital information that is needed to quickly navigate to you windward target using the least amount of tacks.
When you are sailing to your windward destination on port tack, with your destination on your port side, knowing when to tack is crucial.
If your heading is 180, and you are sailing close to the wind which is coming from the -45 line (135), your beating to the destination needs to be at 90 or less or you will fall short of your mark.
Instead of getting out a hand held beating compass, taking a sight, and plotting a course to see if you can track yet, simply use the lubbers lines!
All you need to do is sit on the starboard side and look at your target over the ships compass.
If the line in the center of the compass and the port 90 lubbers line line up, you know you are looking at the lines correctly. Now look up into the distance and locate your target. Once it has moved in line or aft of the port 90 lubbers line, you know you will be able to reach your mark on the next tack.