The tonnage of a ship is the capacity which the body has for carrying a cargo or weights, or the burthen which is computed according to any established rule.

The common rule for finding the burthen of ships, or what is called the builders' tonnage, is to multiply the length by the extreme breadth, and that product by half the extreme breadth, and divide the product by 94; thus:--

length × extreme breadth × half the extreme breadth
= the builder's tonnage

The Length, or the length for tonnage, is determined by making certain deductions, in proportion to the breadth and height of the wing transom, for the rake of the stem and stern-post; that is, from the distance between two perpendiculars to the keel, one drawn passing through the fore part of the stem at the height of the hawse holes, except in flush-decked vessels, when it is to pass through at the height of the upper deck; and the other through the after part of the stern-post at the height of the wing transom, subtract three-fifths of the extreme breadth for the rake of the stem; and, as many feet as the upper side of the wing transom, at the middle line, is above the upper edge of the keel, deduct so many two inches and a half for the rake of the post, and the reminder is the length of the keel for tonnage. But if the stern-post rakes more than two inches and a half in a foot, the perpendiculars are then drawn for the first length, through the fore part of the stem at the height of the upper deck and the after side of the main post at the upper edge of the keel, and a deduction is made of only three-fifths if the extreme breadth, for the rake of the stem, neglecting the rake of the post. This last rule for determining the length of the keel for tonnage is observed, likewise, in measuring all ships for tonnage duties, without regarding the rake of the post. The Breadth is the breadth to the outside of the timber, with the thickness of the plank of the bottom added on each side.

The foregoing rule gives to all bodies, whatever their capacity, while their length and simple breadth are equal, the same tonnage;- to the fullest merchant ship the same as to the sharpest cutter.

An exact rule for determining the capacity of a ship, can be only such as will give the difference of the load and light displacements, or the capacity of the body between the load and light water lines; but there would be considerable difficulty in determining this; whilst any rule formed for this purpose would not be suited for general application, since in foreign ships and in ships in the merchants' service, the light water line is seldom known, and the load water line is not generally determinate.

if the bodies of ships were regular solids, or varied according to some general law, there would be but little difficulty in forming a common rule, that would answer correctly for general purposes; but since this is not the case, and the only means by which the tonnage can be ascertained with accuracy is the regular cubing of the body, all that can be done is, from a few simple elements. to give a rule, of easy application, which shall give a result sufficiently free from errors to answer the purpose; and since the measurement must take place under different circumstances, the rule should be such as can be applied with the lading in as well as out.

To form a rule such, that when it is applied to differently formed bodies, the computed tonnage will vary in some degree with the capacity of the vessels, requires some measurement to be taken, as an element, that changes the most with the form of the body, according to its sharpness or fullness, since the principal dimensions may be the same in bodies of very different capacities.

the following rule, for computing the tonnage, has been framed to approximate more nearly to the actual burthen or capacity of the ship; it is called the Parliamentary or new rule. It will be seen that a portion of the arbitrary character of the old rule is retained:--

Divide the length of the upper deck between the after part of the stem and the foremost part of the stern-post into six equal parts. Depths: at the foremost, middle, and aftermost of these points division, measure in feet and decimal parts of a foot, the depths from the under side of the upper deck to the ceiling at the limber-strake. In the case of a break in the upper deck, the depths are to be measured from a line stretched in a continuation of the deck. Breadth: Divide each of those three depths into five equal parts, and measure the inside breadths at the following points, viz.: at one-fifth and at four-fifths from the upper deck of the foremost and aftermost depths, and at two-fifths and four-fifths from the upper deck of the midship depth. Length: At half the midship depth, measure the length of the vessel from the after part of the stem to the foremost part of the stern-post; then to twice the midship depth add the foremost and aftermost depths for the sum of the depths: add together the upper and lower breadths at the foremost division, three times the upper breadths, and the lower breadth at the midship division, and the upper and twice the lower breadth at the after division, for the sum of breadths; then multiply the sum of the breadths by the sum of the depths, and this product by the length, and divide the final product by three thousand five hundred, which will give the number of tons for register. If the vessel have a poop or half-deck, or a break in the upper deck, measure the inside mean length, breadth, and height of such part thereof as may be included within the bulkhead; multiply these three measurements together, and dividing the product by 92.4, the quotient will be the number of tons to be added to the results as above found. In order to ascertain the tonnage of open vessels, the depths are to be measured from the upper edge of the upper strake.

That each of the several rules hereinbefore prescribed, when applied for the purpose of ascertained the tonnage of any ship or vessel propelled by steam, the tonnage due to the cubical contents of the engine-room, shall be deducted from the total tonnage of the vessel as determined by either of the rules aforesaid, and the remainder shall be deemed the true register tonnage of the ship or vessel. The tonnage due to the cubical contents of the engine-room shall be determined in the following manner: that is to say, measure the inside length of the engine-room in feet and decimal parts of a foot from the foremost to the aftermost bulkhead, then multiply the said length by the depth of the ship or vessel at the midship division aforesaid, and the product by the inside breadth at the same division at two-fifths of the depth from the deck taken as aforesaid, and divide the last product by 92.4, and the quotient will be deemed the tonnage due to the cubical contents of the engine-room.

John Fincham: Outline of Ship-Building in Four Parts.
Whittaker and Co., London, 1952. Part IV, pp 114-116.

Transcribed by Lars Bruzelius

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Copyright © 1996 Lars Bruzelius.