Appendix IV RULES FOR TONNAGE MEASUREMENTS

THE English system of measuring the tonnage of vessels in the eighteenth century is given in Falconer’s Marine Dictionary, 1780, as follows:

“To determine the burden, or, in other words, the tonage, of a ship, it is usual to multiply the length of keel into the extreme breadth of the ship within board, taken along the midship beam, and multiplying the product by the depth in the hold from the plank joining to the keelson upwards to the main-deck, and divide the last product by 94; then will the quotient be the burden required, in tons.”

This rule continued in force till 1819, when it was changed by the Lords Commissioners of the Admiralty as follows:

“Multiply the length of the keel by the breadth of beam, and that product by half the breadth of beam, and divide the last product by 94, and the quotient will be the tonnage” (Marine Dictionary, William Burney, LL.D., 1830). Dr. Burney remarks: “It appears from the general construction of merchant ships, that more attention is paid to evade the tax on tonnage than to their sailing well with the wind in different directions; and if the real tonnage of ships were taken, an alteration would soon be made in the construction for the better.”

This form of the rule continued until 1842, when by Act of Parliament the following method was adopted:

“Divide the length of the upper deck between the after part of the stem and the fore part of the stern-post into six equal parts. Depths: at the foremost, the middle, and the aftermost of these points of 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. Breadths: 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 fore part of the stem-post; then, to twice the midship depth add the foremost and the aftermost depths; add together the upper and lower breadths at the foremost division, three times the upper breadth, and the lower breadth, at the midship division and the upper and twice the lower breadth at the after division, for the sum of the breadths; then multiply the sum of the depths by the sum of the breadths, 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” (Young’s Marine Dictionary, 1846).

In 1854 this rule was changed by the Merchant Shipping Act, which provided that the actual cubic contents of a vessel’s hull should be measured, a registered ton being reckoned as 100 cubic feet. This is known as the Moorsom system, and is still in use and likely to continue. It was adopted by the United States in 1865; Denmark, 1867; Austria, 1871; Germany, France, and Italy, 1873; Spain, 1874; and Sweden, 1875.

The old practice of calculating tonnage in the United States was adapted from the English, and the mode of measurement was as follows:

The length was measured on deck from the fore part of the stem to the after part of the stern-post; the breadth from outside to outside planking at the broadest part of the vessel; the depth of the hold from the plank on deck to the ceiling of the hold. This last measurement was not used, the depth of a vessel for tonnage purposes being assumed to be one half of her breadth. In order to find the tonnage, three fifths of the breadth were deducted from the length and the remainder multiplied by the breadth, and this product multiplied by one half the breadth, or the assumed depth, the last product was then divided by 95, giving the formula:

(L - ⅗ B) × B × ½ B

95

Thus in a vessel measuring 100 ft. x 20 ft. x 18 ft.:

Length of vessel100

Subtract ⅗ breadth12

Length for measurement88

Multiply by the breadth20

1760

Multiply by half breadth10

17,600

Divide 17,600 by 95 and

the result is185 + ¹²/₁₉

Total tonnage185 + ¹²/₁₉

This mode of measurement continued from colonial times until the Moorsom system was adopted in 1865.

The dimensions of ten representative American and British clippers were as follows:

Length Breadth

Nightingale (1851) 178“36“

AmericanOriental (1849)183 ft. 36 ft.

Celestial (1850)158“ 34“ 6 in.

Stag-Hound (1850)209“ 39“

Flying Dutchman (1852)187“ 38“ 6 in.

British Falcon (1859)191“ 4 in.32“ 2 in.

Taitsing (1865)192“ 31“ 5 in.

Titania (1866)200“ 35 “

Spindrift (1867)219“ 4 in.35 “ 6 in.

Thermopylæ (1868)210“ 36 “

Although these British ships show less breadth than the American, yet they have more breadth in proportion to length than the earlier British clippers, such as the Stornoway (1850), Lord of the Isles (1855), etc.