46 CFR §171.073

1. (a)A main transverse watertight bulkhead may not be stepped unless additional watertight bulkheads are located as shown in Figure 171.067(a) so that the distances A, B, C, and D illustrated in Figure 171.067(a) comply with the following:
   1. (1)A and B must not exceed the maximum bulkhead spacing that permits compliance with § 171.070; and
   2. (2)C and D must not be less than the minimum spacing specified in § 171.070(e).
2. (b)A main transverse watertight bulkhead may not be recessed unless all parts of the recess are inboard from the shell of the vessel as illustrated in Figure 171.067(c).
3. (c)If a main transverse watertight bulkhead is recessed or stepped, an equivalent plane bulkhead must be used in the calculations required by § 171.070.