46 CFR §171.116

1. (a)A vessel may have port lights below the bulkhead deck if—
   1. (1)It is greater than 150 gross tons; and
   2. (2)It is in ocean service.
2. (b)All port lights in a space must be non-opening if the sill of any port light in that space is below a line that—
   1. (1)Is drawn parallel to the line formed by the intersection of the bulkhead deck and the shell of the vessel; and
   2. (2)Has its lowest point 2 1/2 percent of the beam of the vessel above the deepest subdivision load line.
3. (c)For the purpose of paragraph (b) of this section, the beam of the vessel is measured at or below the deepest subdivision load line.
4. (d)Except as provided in paragraph (e) of this section, no port light may be located in a space that is used exclusively for the carriage of cargo, stores, or coal.
5. (e)A port light may be located in a space used alternately for the carriage of cargo or passengers.
6. (f)Each port light installed below the bulkhead deck must conform to the following requirements:
   1. (1)The design of each port light must be approved by the Commanding Officer, Marine Safety Center.
   2. (2)Each non-opening port light must be watertight.
   3. (3)Each opening port light must be constructed so that it can be secured watertight.
   4. (4)Each opening port light must be installed with at least one bolt that is secured by a round slotted or recessed nut that requires a special wrench to remove. The nut must be protected by a sleeve or guard to prevent it from being removed with ordinary tools.