(a) Defined. “Between-deck tonnage” means the tonnage of the space above the line of the tonnage deck and below the line of the uppermost complete deck.

(b) Identifying the line of the uppermost complete deck.

(1) If the uppermost complete deck runs in a continuous line from stem to stern, the line of the uppermost complete deck is the longitudinal line of the underside of the uppermost complete deck.

(2) If the uppermost complete deck is stepped, the line of the uppermost complete deck is the longitudinal line of the underside of the lowest portion of that deck parallel with the upper portions of that deck. Steps that do not extend from side to side or are less than three feet in length are ignored when establishing the line of the uppermost complete deck. Spaces between the line of the uppermost complete deck and the higher portions of the deck are included in superstructure tonnage.

(c) Method for calculating tonnage. The tonnage of each level of the between-deck space is calculated separately, as follows:

(1) The length of each level is measured at the mid-height between the line of the deck above and the line of the deck below. Measure from the point forward where the continuation of the line of the normal frames intersects the center line of the vessel aft to the forward face of the normal transom framing.

(2) Divide the length under paragraph (c)(1) of this section into the same number of equal parts into which the tonnage length is divided under §69.109(g)(1).

(3) Measure at mid-height the inside breadth of the space to the line of the normal frames at each end and at each point of division of the length. Number the breadths successively “1”, “2”, and so forth beginning at the bow.

(4) Multiply the even numbered breadths by four and the odd numbered breadths by two, except the first and last, which are multiplied by one.

(5) Add together the products under paragraph (c)(4) of this section and multiply the sum by one-third of the interval between the points at which the breadths are taken. The product is the square foot area of the space at mid-height.

(6) Multiply the area of the space at mid-height by the average of the heights taken each point of division of the space. The product divided by 100 is the tonnage of that space.

(7) The between-deck tonnage is the sum of the tonnage of each level within the between-deck space.

[CGD 87-015b, 54 FR 37657, Sept. 12, 1989, as amended by CGD 97-057, 62 FR 51045, Sept. 30, 1997; USCG-2011-0522, 81 FR 18726, Mar. 31, 2016]