46 CFR §45.61

1. (a)Where the effective length E of superstructures and trunks that meet the requirements of subpart D of this part is 1.0L, the minimum summer freeboard may be corrected by subtracting 1/2Hs.
2. (b)Where the effective length of superstructures and trunks is less than 1.0L the minimum summer freeboard may be corrected by subtracting a percentage of one-half of the standard superstructure height (Hs) determined by the formula:
3. (c)To be eligible for the correction a trunk must—
   1. (1)Be at least as strong and as stiff as a superstructure;
   2. (2)Have no opening in the freeboard deck in way of the trunk, except small access openings;
   3. (3)Have hatchway coamings and covers that meet §§ 45.143 through 45.147;
   4. (4)Provide a permanent working platform fore and aft with guardrails;
   5. (5)Provide fore and aft access between detached trunks and superstructures by permanent gangways;
   6. (6)Be at least 60 percent of the breadth of the ship in way of the trunk; and
   7. (7)Be at least 0.6 L in length, if no superstructure, is provided.