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Net Tonnage of a Vessel and Its Computation (1920)

Thirteen Countries Have Adopted the American System of Measuring Ship’s Commercial Capacity

By Robert Phillips

In a recent issue, we took up the measurement of the ship for gross tonnage. We now come to net tonnage. In a general way, net tonnage may be described as that portion of the ship's internal capacity which may be devoted to commercial uses—the carriage of passengers and freight—and is the tonnage left after certain deductions have been made from the gross tonnage.

The deductions allowed by our laws are as follows: Crew's quarters, master’s cabin, navigation spaces, donkey boiler and engine, sail locker and machinery space.

It must be borne in mind, however, that no space may be deducted unless it has been measured and included in the gross tonnage. When bounded by flat surfaces, the deducted spaces may be measured by taking the product of the three dimensions; but when it happens that a space is bounded by one or more curved sides, the cubical contents of such space is to be measured in the same manner as deck erections—by using the Moorsom system.

Crew’s Quarters

In crew spaces may be included the necessary passageways, provided such passages are exclusively for the use of the crew—and the necessary mess rooms, bath rooms and toilet rooms, for the exclusive use of the officers and crew.

In addition, there may also be included spaces reasonable in extent, necessary to shelter the cook when employed in the preparation of food, the engineer when employed in condensing water for the crew, the chief engineer’s office, and the wireless operator when engaged in his work or where he may be berthed.

A hospital space for the crew, which shall be separated from other spaces, is allowed as a deduction. No deduction may be made unless there is permanently cut into a beam and over the doorway of every such space the number of men it is to accommodate, or purpose for which it is to be used.

Navigation Spaces

The spaces allowed as deductions under this head include any space reasonable in extent exclusively used for the working of the helm, the capstan, and the anchor gear (which includes the chain locker), when below decks, or for keeping charts, signals, lamps, and other instruments of navigation and boatswain's stores. Words indicating the use of any of the deducted spaces must be cut in a beam and over the door of eachspace.

Diagram showing spaces to be included in the machinery space and points from which measurements are taken.

Diagram showing spaces to be included in the machinery space and points from which measurements are taken.

Donkey Engine and Boiler

Where the donkey engine and boiler are situated within the machinery space, it should not be the subject of a separate allowance, as it would then be included in the machinery space deduction. When it is in a house above the upper deck, and therefore not measured in ascertaining the gross tonnage, it will not be allowed as a deduction. In all other cases the space occupied by the donkey engine and boiler, if connected with the main pumps of the ship, is to be allowed as a deduction, provided the place is reasonable in extent and properly constructed. It is to be marked as other deducted space.

Sail Locker

In a vessel propelled wholly by sails, a deduction is allowed for necessary space to store sails, not exceeding two and one-half per cent, of the gross tonnage. The space must be reasonable in extent, and properly constructed.

Machinery Space

A deduction for the space occupied by the machinery must be made for vessels propelled by steam or other machinery. This deduction is not to be taken as the actual tonnage of the machinery space, but is determined in the following manner:

  • In paddle steamers, where the tonnage of the space occupied by the machinery and necessary for its proper working is above 20 per cent, and under 30 per cent, of the gross tonnage, the deduction shall be 37 per cent, of the gross tonnage.
  • In screw vessels where the tonnage of the machinery space is above 13 per cent, and under 20 per cent, of the gross tonnage, the deduction shall be 32 per cent, of the gross tonnage.
  • In measuring machinery spaces, shaft trunks shall be included as a part thereof.
  • In paddle vessels where the tonnage of the space occupied by the machinery is 20 per cent, or under of the gross tonnage, the deduction shall be one and one-half times the actual tonnage of the machinery space.
  • In the case of screw vessels, when the machinery space is 13 per cent, or less of the gross tonnage, the deduction shall be one and three-fourths times the actual tonnage of the machinery space.
  • But if the actual machinery space is so large as to amount in the case of paddle vessels to 30 per cent, or above, and in the case of screw vessels to 20 per cent, or above, of the gross tonnage, the deduction shall consist of 37 per cent, of the gross tonnage in the case of a paddle vessel and 32 per cent, of the gross tonnage in the case of a screw vessel.
  • Or, if the owner prefers, there shall be deducted from the gross tonnage the tonnage of the space actually occupied by or required to be enclosed for the proper working of the machinery, with the addition in the case of vessels propelled by paddles of 50 per cent, and in the case of screw vessels of 75 per cent.

Spaces in Engine Room

Within the meaning of our measurement laws, only those spaces actually occupied by the engine and boiler rooms, together with the space necessary for the working of the machinery, may be deducted, and are as follows:

  • Space below the crown of the engine room.
  • Light and air spaces for the machinery, framed in between the crown of the engine room and the upper deck.
  • Light and air spaces for the machinery, framed in above the upper deck.
  • Shaft alleys or trunks and escape trunks.

After all the allowed deductions have been made, the amount of tonnage remaining shall be deemed to be the net, or register tonnage, which shall be permanently marked on the main beam of the vessel.

A method of measurement similar to ours having been adopted by Great Britain, Belgium, Denmark, Italy, Sweden, Norway, Spain, Holland, Russia, Finland, Portugal, Japan and France, the tonnage of their vessels in our ports must be taken to be the tonnage expressed in their documents, as a like courtesy has been extended to our vessels in their porta. Vessels of other foreign countries are to be measured according to our laws.

Phillips, Robert, “Net Tonnage of a Vessel and Its Computation,” in The Nautical Gazette: An International Weekly Chronicle of Shipping, Volume 99, No. 15, Whole No. 2565, New York, Saturday, 9 October 1920, P. 456-457

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