46 CFR §171.106

1. (a)This section applies to each vessel that has a well installed in a double bottom required by § 171.105.
2. (b)Except as provided in paragraph (c) of this section—
   1. (1)The depth of a well must be at least 18 inches (45.7 cm) less than the depth of the double bottom at the centerline; and
   2. (2)The well may not extend below the horizontal plane C illustrated in Figure 171.105.
3. (c)A well may extend to the outer bottom of a double bottom at the after end of a shaft tunnel.