46 CFR §171.100

1. (a)Stern tubes in each of the following vessels must be enclosed in watertight spaces:
   1. (1)Each vessel of 100 gross tons or more on an international voyage.
   2. (2)Each other vessel over 150 gross tons in ocean or Great Lakes service.
   3. (3)Each vessel under 100 gross tons that carries more than 12 passengers on an international voyage.
2. (b)The watertight seal in the bulkhead between the stern tube space and the machinery space must be located in a watertight shaft tunnel. The vessel must be designed so that the margin line will not be submerged when the watertight shaft tunnel is flooded.
3. (c)If a vessel has two or more shaft tunnels, they must be connected by a watertight passageway.
4. (d)If a vessel has two or less shaft tunnels, only one door is permitted between them and the machinery space. If a vessel has more than two shaft tunnels, only two doors are permitted between them and the machinery space.