A buoy or mark used to indicate something notable in the area it is located in. It has no lateral significance or directional meaning.
The yellow color of the buoy is a reminder that caution should be exercised until you have determined the reason that any “Special Purpose Buoy or Mark” has been placed in a particular location. If you cannot determine the reason the mark is placed in its location, it should be given a wide berth where possible or the area should be transited with caution.
As defined by the International Association of Marine Aids and Lighthouse Authorities (IALA), the Special Purpose Mark is authorized for usage in both IALA Region A and IALA Region B.
Below is a list of where you may encounter the Special Purpose Buoy:
The color of the Special Purpose Buoy consist of the solid color Yellow, making it easy to recognize during daylight hours.
The Special Purpose Mark can be found in any one of the (5) basic buoy configurations: A Can Buoy, a Nun Buoy, a Pillar Buoy (Combination Buoy), a Spherical Buoy, or a Spar Buoy.
However, the most common shapes found will be the Can, Nun, and Pillar (Combination Buoy) shown below.
Markings - If marked, Special Purpose Buoys will only be lettered. Numbers are not authorized on other than ODAS weather buoys.
If a topmark is fitted, the Special Purpose Mark will only display the shape of a “St. Andrews Cross.” It is easily identifiable as it closely resembles an "X."
The St. Andrews Cross topmark will always be Yellow.
If the Special Purpose Mark is lighted, it will display the following characteristics:
It will only show a "Yellow" light.
Any light phase can be used with the exception of those prescribed for cardinal marks (combinations of Quick Flashing or Very Quick Flashing), Isolated Danger Marks (Flashing (2), or Safe Water Marks (Isophase, Occulting, Long Flash 10 seconds, or Morse “A”).
The period for the light is optional; meaning any period may be used.
Below is a chart excerpt showing (2) Special Purpose Buoys at the beginning of the Traffic Separation Scheme at the mouth of the Delaware Bay in the U.S.
The southeastern most buoy - Y “D” Fl Y 6s RACON (▬ ● ▬) surrounded by a Magenta Circle. and marks the actual beginning of the Traffic Separation Scheme.
The chart notation indicates that this is a yellow “Y” buoy marked with the letter “D” displaying a flashing yellow light every 6 seconds “Fl Y 6s.” It is equipped with a “RACON” displaying the Morse code equivalent of the letter “K” (▬ ● ▬) dash-dot-dash and is also equipped with an AIS transponder (indicated by the magenta circle around the mark.
The second mark, located just to the northwest, Y ODAS “44009” Fl (4) Y 20s is a Weather Buoy.
(Y ODAS “44009” Fl (4) Y 20s)
The chart notation indicates that this is a yellow “Y” Ocean Data Acquisition Systems “ODAS” buoy marked with the numbers “44009” displaying a group flashing (4) yellow light every 20 seconds “Fl (4) Y 20s.”