A superstructure is a decked structure on the freeboard deck, extending from side to side of the ship or with the side plating not being inboard of the shell plating more than 4% of the breadth (B).
A raised quarter-deck is regarded as a superstructure. (Raised quarter-decks are often associated with smaller ships. With the machinery space sited aft and being proportionally larger in smaller ships there is a tendency for the ship to be trimmed by the head when fully loaded. To prevent this, the height of the aftermost holds may be increased to increase deadweight aft; this is achieved by means of a raised quarter-deck.)
An enclosed superstructure is a superstructure with:
- enclosing bulkheads of efficient construction;
- access openings, if any, in these bulkheads fitted with doors complying with the requirements of regulation 12 .
- all other openings in sides or ends of the superstructure fitted with efficient weathertight means of closing.
A bridge or poop shall not be regarded as enclosed unless access is provided for the crew to reach machinery and other working spaces inside these superstructures by alternative means which are available at all times when bulkhead openings are closed.
The height of a superstructure is the least vertical height measured at side from the top of the superstructure deck beams to the top of the freeboard deck beams.
The length of a superstructure is the mean length of the part of the superstructure which lies within the length (L).