How A Ship’s Gross Tonnage Is Computated


This Knowledge Is Necessary to Understand Thoroughly Relation Between Cubical and Carrying Capacity



All of us who are concerned with shipping are more or less familiar with tonnage terms. However, we may not understand just how tonnage measurements are taken. This is important, especially to those about to construct or otherwise acquire ships, as a clear understanding of our measurement rules will lead to a more thorough knowledge of the relation between tonnage and carrying capacity.

Before any vessel may be registered, enrolled, or licensed, she must be measured by a customs officer, who ascertains, among other things, her gross and net tonnage. The unit of measurement is 100 cubic feet and is known as the ton, which is expressed as the internal capacity of the measured spaces of vessels. By our laws, specific spaces are exempt from measurement, as described further on.


Gross Tonnage

The gross tonnage of a vessel consists of the sum of the following items:

  • The cubical capacity below the tonnage deck.
  • The cubical capacity of each space between decks above the tonnage deck.
  • The cubical capacity of the permanent closed-in spaces on the upper deck is available for cargo or stores or the berthing or accommodation of passengers or crew.
  • The excess of hatchways.
  • All permanent closed-in spaces situated elsewhere available for cargo or stores, for the crew's accommodation, or for the charts of navigating instruments (except cabins or staterooms for passengers constructed entirely above the first deck, which is not a deck to the hull).


Under-Deck Tonnage

The first step is to ascertain the "tonnage length." The points between which the tonnage length is measured differ from those used to determine the register length. The forward point is at the underside of the tonnage deck plank (or plate in a steel vessel), where the inside of the inner planking meets the side of the stem.


Tonnage Length Diagram


In steel ships with no inner planking, a fair line is drawn around the inside of the frames to meet the side of the stem. The most convenient way to measure the tonnage length is on the top side of the tonnage deck. But as the object of the measurement is to ascertain the cubical capacity under the tonnage deck, and as there may be some rake to the stem, the increased length caused thereby must be deducted.


How this is done is shown in an accompanying sketch.

Diagram for Mesuring Gross Tonnage

The after point is where the underside of the tonnage deck planking meets the inside of the inner planking on the stern timbers or frames (or the inside of angle irons or frames in a steel vessel), less the rake of the stem frame to the extent of one-third of the round of the beam at that point. If the length is measured on top of the deck plank, then the rake of the stem timber or frame in the deck's thickness must also be deducted.

The after point is also shown in the sketch. In the case of iron or steel vessels with steel decks, the bow and stern rakes in the deck's thickness are negligible. They may be disregarded altogether, but it must be considered with wooden ships, where the decks are pretty thick.


Tonnage Length

The tonnage length so ascertained is then divided into equal parts as follows:

Under 50 feet, six parts; 50 to 100 feet, 8 parts; 100 to 150 feet, 10 parts; 150 to 200 feet, 12 parts; 200 to 250 feet, 14 parts; above 250 feet, 16 parts.

The points dividing the tonnage length according to the above table are to be marked with chalk and numbered consecutively from bow to stern, No. 1 being the extreme forward point. Should a vessel have a break in the line of her double bottom, her tonnage length is to be divided into longitudinal sections by erecting transverse vertical planes at such intervals.

The length of each section will then be divided into equal parts according to the table, just as if it were the length of a separate ship. The tonnage length, having thus been determined and divided into the required number of equal parts, the points of division are transferred to the keelson. This is done by first locating the position of the midship section on the keelson, from which the common interval between the points of division is measured.


Ascertaining Depth

The next step is to ascertain the tonnage depth of the midship section and each other section. The tonnage depth is the distance between a point located one-third of the round of the deck beam below the tonnage dock plank and a point on the upper side of the ceiling, if any, on the floor timbers or frames—otherwise to the upper side of the floor timber or frame—at the inside of the limber-strake.

In case the vessel is constructed with a double bottom for water ballast only, the tonnage depth is to be taken to the upper side of the inner plating of the double bottom, allowing for ceiling, if any, and where vessels have deep double bottoms or deep tanks at the bottom having ordinary floors therein, used other than for water ballast, the depth shall be taken to the upper side of the floors.

If the depth of the midship section is less than sixteen feet, the depth of each section is to be divided into four equal parts—and if it exceeds sixteen feet, into six parts. After the divisions of the tonnage depth are located at each section, the inside horizontal breadth at each point of division (including the upper and lower points) is measured to the inside of the ceiling, average thickness, or, if the ship is not ceiled, to the inside faces of the frames.


Transverse Sections

Now, we are ready to ascertain the area of each transverse section. Where the tonnage depth is less than sixteen feet, and each section is therefore divided into four equal parts, the area is obtained in the following manner.

The breadths are numbered from above. No. 1 is at the top, and No. 5 is at the bottom. Multiply the second and fourth by 4, and the third by 2, add the products, and to the sum, add the first and fifth breadths; multiply the quantity thus obtained by one-third of the common interval between breadths and the product will be the transverse area of the section.

As all measurements are taken in feet and decimals of feet, the areas are in square feet. Where the midship tonnage depth exceeds sixteen feet or where the vessel has a double bottom, there is a slight change in the formula, but the principle is the same.

The hull has been divided into a number of equal sections, and the area of each section is ascertained. To obtain the cubical capacity under the tonnage deck, the areas are combined as follows: The sections are numbered consecutively, beginning at the bow. Multiply the area of the even-numbered sections by 4 and the odd-numbered sections, except the first and last, by 2; these products will be added to the first and last areas.

The sum thus obtained is then multiplied by one-third of the common interval between the sections. The product will be the number of cubic feet below the tonnage deck, which, when divided by 100, will give the under-deck tonnage.

The between-deck tonnage is determined upon much the same principle and then divided by 100.


Spaces on Upper Deck

If there be a break, poop, or any permanent closed-in space on or above the upper or spar deck available for cargo or stores or for the berthing or accommodation of crew or passengers (except the passenger accommodations on a deck not to the hull, exempted by law), the tonnage of such space must be ascertained.


Space on Upper Deck


It is found like the method employed in finding the other tonnage, except that the space is divided longitudinally into an even number of equal parts, which must be most nearly equal to the length of the sections of the tonnage deck. The breadths must be taken at the middle of its height.

The cubical contents of the hatchway must be obtained by multiplying the length by the breadth; the product is then multiplied by the mean depth taken from the top of the beam to the underside of the hatch. The whole amount of the hatchway tonnage is not to be added, but only that part of the hatchway tonnage that exceeds one-half of one percent of the ship's gross tonnage, exclusive of the tonnage of the hatchways.


Spaces Exempted

Specific spaces are exempt from measurement but must differ from the deductions to ascertain the net tonnage. Double bottoms for water ballast are not measured; side water ballast tanks extending up from the top of the double bottom wholly or partly to the deck above are likewise exempt.

Spaces under the shelter deck, in the way of the shelter deck opening, are not measured if such spaces in all respects comply with the requirements of the laws. When on or above the upper deck, any space fitted with machinery, the wheelhouse, the galley, bakery, condenser space, toilets, skylights, and companions will not be measured, provided the spaces are no larger than required for the purposes named.


Phillips, Robert, “How A Ship’s Gross Tonnage Is Computed,” in The Nautical Gazette: An International Weekly Chronicle of Shipping, Volume 99, No. 13, Whole No. 2563, New York, Saturday, 25 September 1920, P. 392-393


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