(a) Defined. “Under-deck tonnage” means the tonnage of the space below the line of the tonnage deck, as that volume is calculated under this section.

(b) Method of calculating tonnage. Under-deck tonnage is calculated by applying Simpson's first rule using the tonnage length and the areas of the transverse sections prescribed by this section.

(c) Identifying the tonnage deck. In vessels with two or less enumerated decks, the tonnage deck is the uppermost complete deck. In vessels with more than two enumerated decks, the tonnage deck is the second enumerated deck from the keel as determined in paragraph (d) of this section.

(d) Enumerating the decks to identify the second deck from the keel. The uppermost complete deck is an enumerated deck. Decks below the uppermost complete deck that extend from stem to stern and side to side at all points along their lengths are also enumerated, provided they are not disqualified by either of the following deck discontinuities:

(1) A through-deck opening that is not fitted with a cover (or equivalent) and whose area is more than 10 percent of the total deck area, as viewed from above.

(2) A deck recess that is not through-hull for which the depth at its deepest point is more than five feet below adjacent portions of the deck and whose area as viewed from above is more than 10 percent of the total deck area from stem to stern, as viewed from above.

(e) Identifying the line of the tonnage deck.

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

(2) If the tonnage deck is stepped, the line of the tonnage 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 tonnage deck. (See §69.123, figures 1 and 2.) Spaces between the line of the tonnage deck and the higher portions of that deck are not included in under-deck tonnage.

(f) Tonnage length.

(1) “Tonnage length” means the length of a horizontal straight line measured at the centerline of the vessel from the point forward where the line of the tonnage deck intersects the line of the inboard faces of the ordinary side frames to the point aft where the line of the tonnage deck intersects the inboard face of the ordinary transom frames or cant frames. (See §69.123, figure 3.)

(2) For a vessel having a headblock or square end with framing which extends from the tonnage deck to the bottom of the vessel, the tonnage length terminates on the inboard face of the headblock or ordinary end frames. (See §69.123, figure 4.)

(3) For a vessel having a square bow or stern and tonnage deck with camber, the effect of the camber on the tonnage length must be considered. The tonnage length must be measured below the tonnage deck at a distance equal to one-third of round camber and one-half of straight pitch camber.

(4) The forward and after termini of the tonnage length must be a distance of no more than eight and one-half feet from the associated inboard surface of the skin of the hull at the bow and stern as measured at the centerline of the vessel, and the after terminus must not be forward of the centerline of the rudderstock.

(g) Division of vessel into transverse sections.

(1) Except as under paragraph (m)(1)(iii) of this section, the tonnage length is divided into an even number of equal parts as indicated in the following table:

(2) Transverse sections are cut at each end of the tonnage length and at each point of division of the tonnage length, whose location is referred to as a tonnage station, and assigned sequential tonnage station numbers, beginning at the stem. Intervals and one-third intervals between the points of division are measured to the nearest thousandth of a foot. (See §69.123 figures 5 and 6.)

(h) Depths of transverse sections.

(1) Transverse section depths are measured at each point of division of the tonnage length at the centerline of the vessel from a point below the line of the tonnage deck equal to one-third of the camber or to one-half of the pitch of the beam down to the upper side of the ordinary frames, floors, longitudinals, or tank top of a double bottom for water ballast, as the case may be.

(2) When a depth falls at a point where the tank top of a double bottom for water ballast has a straight fall from centerline to the wings, the depth terminates at one-half of the height of fall. (See §69.123 figure 8.)

(3) When a depth falls at a point where the tank top of a double bottom for water ballast rises from the centerline to the wings, the depth terminates at one-half the dead rise. (See §69.123, figure 9.)

(4) The depth at the midpoint of the tonnage length or, when a vessel is measured in parts, the depth at the midpoint of each part determines the number of equal parts into which each depth is divided, as follows:

(i) If the midpoint depth is 16 feet or less, each depth is divided into four equal parts. If the midpoint depth exceeds 16 feet, each depth is divided into six equal parts. (See §69.123, figure 7.)

(ii) The interval between the points of division of a depth and one-third intervals are carried to the nearest hundredth of a foot.

(i) Breadths of transverse sections.

(1) Transverse section breadths are measured horizontally at each point of division of each depth and also at the upper and lower points of each depth. Breadths are measured to the inboard face of the ordinary frames or to the line of the ordinary frames. Breadths are measured parallel to each other and at right angle to the vessel's centerline. (See §69.123, figure 7.)

(2) Upper breadths are not reduced by measuring to deck-beam brackets. In cases of camber when an upper breadth passes through the deck (see §69.123, figure 7), the breadth is measured to the line of the side frames at the under side of the deck projected vertically up to the height of the upper breadth.

(3) Bottom breadths are measured only as far as the flat of the floor extends. (See §69.123, figures 7 and 10.) When bottom frames rise immediately from the flat keel, bottom breadths are equal to the breadth of the flat keel. Where there is no double bottom for water ballast and where there is dead rise of the bottom out to the sides of the vessel, bottom breadths are equal to the part of the bottom plating not affected by dead rise.

(4) Bottom breadths falling in way of a double bottom, the top of which rises or falls from centerline to the wings, are measured between the inboard faces of the frame brackets which connect the double bottom with the frames. (See §69.123, figures 8 and 9.)

(j) Measuring spaces having ceiling. The maximum allowance for terminating measurements on ceiling is three inches on the bottom frames or tank top and three inches on each side frame. When ceiling is less than three inches thick, only the actual thickness is allowed. When ceiling is fitted on a platform directly above the bottom frames, depths are measured down through the platform to the upper side of the frames and the allowable ceiling on the platform is then deducted.

(k) Area of transverse sections.

(1) A transverse section at an end of the tonnage length may not yield area, except in vessels (such as barges) with an upright bow or stern.

(2) The breadths of each transverse section are numbered from above, the upper being “1”, the second down being “2”, and so on to the lowest.

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

(4) Add together the products from paragraph (k)(3) of this section.

(5) Multiply the sum from paragraph (k)(4) of this section by one-third of the interval between the breadths. The product is the area of the transverse section.

(l) Tonnage.

(1) Number the transverse sections successively “1”, “2”, and so forth, beginning at the bow.

(2) Multiply the area of the even numbered sections by four and the area of the odd numbered sections by two, except the first and last sections, which are multiplied by one.

(3) Add together the products from paragraph (l)(2) of this section and multiply the sum by one-third of the interval between the sections. The product is the volume under-deck.

(4) The volume under-deck is divided by 100 and is, subject to exemptions, the under-deck tonnage.

(m) Steps in double bottom for water ballast.

(1) The tonnage length of a vessel having a step exceeding six inches in height in its double bottom for water ballast is divided into longitudinal parts at the step. Each part is subdivided as follows to determine the number of transverse sections:

(i) Parts 20 feet or under in length are divided into two equal parts.

(ii) Parts over 20 feet and under 40 feet in length are divided into four equal parts.

(iii) Parts 40 feet or over are divided as provided in paragraph (g)(1) of this section.

(2) The tonnage of each part is calculated separately. The sum of the tonnages of the parts is the under-deck tonnage.

(n) Spaces open to the sea. In calculating the tonnage of spaces below the uppermost complete deck, subtract from each breadth measurement the portion of that measurement that spans a space, or a portion thereof, that is open to the sea.

(o) Open vessels.

(1) An open vessel is a vessel without an uppermost complete deck.

(2) The line of the tonnage deck for an open vessel is the upper edge of the upper strake. Depths of transverse sections are taken from this line.

(3) Any vessel, other than one having a mechanically refrigerated hold, that is not an open vessel and that has a tonnage length of less than 50 feet is measured as an open vessel, if the distance between the line of its tonnage deck and the upper edge of the upper strake is more than one-sixth of the midship depth. “Midship depth” means the depth measured from the line of the upper edge of the upper strake to the point in the bottom used for measuring tonnage depths.

(p) General requirements on ordinary frames—

(1) Construction. An ordinary frame must not be penetrated by an intersecting frame used to strengthen the vessel's hull, except in a vessel of wooden construction. Ordinary frames must be of the same material, or have the same material properties, as the adjacent hull, and attach to the adjacent hull to at least the same extent as adjacent ordinary and normal frames. If comprised of different elements, the elements must be joined to each other to the same extent that the frame is joined to the hull. The frame, or portions thereof, not meeting these requirements must be treated as if not there when establishing the line of the ordinary frames.

(2) Frame spacing and extension. Ordinary frames used to establish the line of the ordinary frames must be spaced on centers that are a maximum of four feet apart. These frames must extend for a length of at least one tonnage interval that begins at, ends at, or crosses the associated tonnage station. For a longitudinally-framed vessel, the frames must begin and end at a transverse ordinary frame or at the vessel's hull.

(3) Different sized framing. When an ordinary frame has a different depth of frame than an adjacent ordinary frame, the line of the ordinary frames is established using the set of alternating frames that yields the smallest sectional area at the associated tonnage station, with the sectional area based on the frame with the smallest depth of frame in the chosen alternating set.

(4) Frame openings. If an opening in an ordinary frame is oversized, or is penetrated by a frame other than an ordinary frame, the line of the ordinary frames is established as if the frame material above and inboard of the opening is not there. Similarly, frame material separating adjacent openings that are within the longest linear dimension of either opening must be treated as if not there when establishing the line of the ordinary frames. An opening is oversized if the opening is:

(i) Circular in shape with a diameter exceeding 18 inches;

(ii) Oval in shape of a size greater than 15 × 23 inches (i.e., either the minor axis exceeds 15 inches or the major axis exceeds 23 inches, and the oval's area exceeds 255 square inches (345 square inches in a fuel tank)); or

(iii) Any shape other than circular or oval, whose area exceeds 255 square inches (345 square inches in a fuel tank).

(5) Asymmetrical framing. Where ordinary frames are configured such that the line of the ordinary frames would be asymmetrical about the centerline of the vessel, breadth measurements are determined by taking half-breadths on the side of the vessel that yields the greatest sectional area at the associated tonnage station, and multiplying those half-breadths by a factor of two to yield the full breadths.

[CGD 87-015b, 54 FR 37657, Sept. 12, 1989; 54 FR 40240, Sept. 29, 1989; USCG-2014-0688, 79 FR 58281, Sept. 29, 2014; USCG-2011-0522, 81 FR 18725, Mar. 31, 2016]