Just as a town or city has a system using street signs and addresses to help you find your way around, so does a Navy ship. Each space in a Navy vessel has a unique identifier that consists of a yellow rectangle with black letters and numbers, known as the bullseye. It will have several lines of information, with the topmost line, made up of numbers and letters, providing location information. For example: 4–95–3–M.
For each compartment, the numbers and letters will differ, but the format will not. That format tells you the following:
- First number—deck or level number
- Second number—ship frame number
- Third number—compartment’s position relative to the centerline
- Letter(s)—code conveying the purpose of the compartment
Ships have a main deck, which generally is defined as the uppermost deck that extends from the stem (the very forward-most point of the ship) to the stern (the aftermost point). The main deck is one of the references used in the compartment numbering system, and all the “floors” above the main deck are called levels and all below are called decks. These are indicated by the first number of the compartment identifier on the bullseye. Some important clues to deciphering this number system:
- The main deck is always numbered “1.”
- All decks below the main deck are numbered higher than “1” (2, 3, etc.).
- All levels above the main deck are preceded by a zero (01, 02, etc.).
These numbers increase as you move away from the main deck. The first deck below the main deck is numbered “2,” the next one down is “3,” and so forth. One level above the main deck is the 01 level. The next one up is the 02 level, and so on. If a compartment extends through more than one deck (such as an engineering space that must be large enough to hold a reactor or huge turbines), its deck number refers to the space’s bottommost deck.
Shipbuilders start construction of a vessel with its virtual backbone, called a keel, that essentially is a very large I-beam running from stem to stern. Other beams, called frames, are attached to that backbone at regular intervals—roughly perpendicular (athwartships) to it—like ribs to the human spine. Plating is attached to these ribs to form the ship’s hull. Frames are numbered, starting at the bow and increasing toward the stern—the farther aft you are, the higher the number of the frame. If you know how many frames the ship has in total (say 300) and you are at frame 100, you know you are about one-third of the way aft in the ship (or two-thirds of the way forward). Frame 150 in this case would be exactly amidships.
The third number of the bullseye is referenced to an imaginary line called the centerline, which runs from stem to stern, bisecting the ship into two long halves. The number tells you where you are in relation to that centerline. The bigger the number, the farther away from the centerline (outboard). Compartments with even numbers are on the port side (left as you look forward) of the centerline; odd numbers are on the starboard (right, looking forward) of the centerline. This means that a compartment with the number “3” in this position on the bullseye would be the second compartment outboard from the centerline (“1” being the first) on the starboard side. Similarly, the second compartment on the port side would have the number “4,” the third compartment on the port side would be “6,” and so on. If a space straddles the centerline, it has a zero as its third number.
The last part of the bullseye is a letter, or pair of letters, that tells you what the space is used for.
In the example at the beginning (4–95–3–M), the compartment identified is on the fourth deck, is at the 95th frame, is the second compartment outboard from the ship’s centerline on the starboard side, and is a magazine or some other ammunition-related space.
Compartment Identifiers
A Supply and stowage
AA Cargo holds
C Control centers (such as the combat information center)
E Engineering (machinery)
F Fuel stowage (for use by the ship; that is, not as cargo)
FF Fuel stowage compartments (when cargo)
J Jet (aviation) fuel for use by embarked aircraft
JJ Jet (aviation) fuel as cargo
K Chemicals and dangerous materials
L Living spaces
M Ammunition spaces
Q Miscellaneous spaces
T Vertical access or escape trunks
V Void (spaces that are normally empty)
W Water Stowage
