The Carpenter's Measurement was an unofficial measurement of cargo capacity which was popular in the earlier period. This simple formula intended to give an estimate of the cubic contents of a ship.
CM = (L×B×D) / 95
The CM normally resulted in more inflated figures than the Old Custom House method. W.H. Webb, the builder of the ship Guy Mannering, always gave the size of his ships in round figures of CM.
The Old Custom House Measurement, which was in use in the United States until 1865, tended to underestimate the cargo capacity for all but the sharpest vessels.
Old CHM = ((L - 3/5×B)×B×1/2×B) / 95
L = the length from the fore part of the stem to the after part of the stern-post, measured on deck.
B = the breadth from outside to outside of planking at the broadest part of the vessel.
D = the depth of hold from the plank of the deck to the ceiling of the hold (assumed to be ½ B). For single-decked vessels the actual depth was taken, instead of considering one-half as the depth.
The New Customs House Measurement system, which was based on the British system devised by Moorsom, was introduced in the United States in 1865. This system which used Simpson's rule to compute the volume of the the ship is still, with changes, the basis for the calculation of the gross tonnage.
If we study the different tonnage measurements for a couple of packet ships built by W.H. Webb we find that he tended to give fairly round figures for the CM.
| Old | New | ||||
|---|---|---|---|---|---|
| Year | CHM | CHM | CM | Builder | |
| 1830 | Hibernia | 551 | 601 | 664 | Brown & Bell |
| 1845 | Havre | 870 | 928 | 1000 | W.H. Webb |
| 1847 | Yorktown | 1150 | 1341 | 1300 | W.H. Webb |
| 1849 | Albert Gallatin | 1435 | 1533 | 2000 | W.H. Webb |
| 1850 | Isaac Webb | 1359 | 1497 | 2000 | W.H. Webb |
| 1860 | Alexander Marshall | 1232 | 1507 | 1450 | W.H. Webb |
These figures are based on American formulas for tonnage measurement. Great Britain used slightly different formulas and also changed from the Old to the New method eleven years before the United States. Other countries not only used variations of these formulas or completely different ways to compute the tonnage, but also used units of measurement of length which were different.
Updated 1995-10-14 by Lars Bruzelius
Sjöhistoriska Samfundet | The Maritime History Virtual Archives.
Copyright © 1998 Lars Bruzelius.