Breitling EXOSPACE B55 YACHTING Manual De Instrucciones página 69

Example:
Follow the line of the starboard tack start line, starting at the committee boat
and aiming for the buoy at the end of the line; note the number on the vessel
compass. On your watch, compare this number with the one on the bezel in the
line axis (9 o'clock).
For a 90° head wind, a neutral start line towards the buoy must be at 0°. If your
heading on the boat is at 15°, your number will be in the + zone, so the start will
be favorable at 15° left. If your heading on the boat was at 345°, your number will
be in the - zone, so the start is therefore unfavorable at 15° to the left, and you
therefore need to start from the committee boat.
NB: This measurement can also be done on the port tack by following the line, but
in this case, you will not have priority.
The same procedure can be used by noting the heading of the first course marker
(windward buoy) if it isn't in the same axis as the wind.
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