46 C.F.R. § 171.106 – Wells in double bottoms.

(a) This section applies to each vessel that has a well installed in a double bottom required by § 171.105.
(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.
(c) A well may extend to the outer bottom of a double bottom at the after end of a shaft tunnel.