46 C.F.R. § 171.112 – Watertight door openings.

(a) The opening for a watertight door must be located as high in the bulkhead and as far inboard as practicable.
(b) No more than one door, other than a door to a bunker or shaft alley, may be fitted in a main transverse watertight bulkhead within spaces containing the following:
1. (1) Main and auxiliary propulsion machinery.
2. (2) Propulsion boilers.
3. (3) Permanent bunkers.