A review of the British Laws for the Admeasurement of the Tonnage of Merchant Shipping
By G. Moorsom, Esq., Of Her Majesty's Customs, London. Member of the late School of Naval Architecture, and Member and Honorary Secretary of the late Commission for the Revision of the Laws of Tonnage. Treatise on Tonnage, 1853
The term "Tonnage" as originally applied to mercantile vessels, whether intended to express the burthen that a ship can safely carry, or whether to convey an idea of the relative sizes of vessels, as indicated by the cubical contents of the hold, or space for the stowage of cargo, is by no means unequivocally set forth in the earlier parliamentary documents relating to the subject.
Whatever was originally intended, "tonnage" has been, and still is, the only term by which we form an idea of the magnitude of vessels. And, although when we speak of a ship of 1000 tons burthen, we are quite at a loss to judge, in consequence of the inadequacy of our Admeasurement Laws, whether she will carry a cargo of 1,500 tons, or one of little more than 1,000 tons weight; yet, from our experience and general knowledge of the various proportions of vessels as associated with the results of official measurement, we think we form a pretty good idea of her principal dimensions; and we, therefore, always averse to the inconveniences of change, have appeared almost inclined to be satisfied with this imperfect criterion of size. The establishment of the "New Law," so-called, greatly improved our means of arriving at a more definite idea of the capacity of shipping, but still left this important problem, so intimately connected with the best interests of shipping, in a state of solution highly to be deprecated, and generally conceded to be much behind the intelligence and improvements of the age, which are to be perceived everywhere displaying themselves in all that regards the material connected with far less important interests.
But the evil of such loose and imperfect mensuration as here alluded to, and as has hitherto obtained, does not rest here. If it were limited simply to depriving us of a just conception of the relative magnitude of vessels, no very serious harm or inconvenience might arise from such deficiency. But when we consider that not only are all dues to which ships are liable levied according to the tonnage, but vessels are also built, bought and sold for a price per ton of their admeasurement, and frequently freighted on the same basis; and by the regulations of the "Society of Lloyd's Register of British and Foreign Shipping," they must also be timbered and fastened, and have their anchors, cables and boats all in proportion to this same regulating standard; when all these things, added also to the great encouragement given to evasion by imperfect measurement, are brought into review, the importance of the highest degree of truth and perfection in such a standard is made manifest. And, therefore, the apathy which has been evinced on this cardinal point by many of our intelligent ship owners, and more particularly by our merchant ship builders, is truly astonishing.
Erroneous and empirical measurement, which cannot possibly be upheld for any other reason than its brevity and ease of application, has been found, even when established on the most proper basis, to be much more injurious in its practical working than the greatest inequalities which are known to be possible to result from the selection of the most ineligible basis, in connection with a mode of correct mensuration. And there are those who go so far as to maintain that tonnage may be founded on the principle either of external or internal measurements, on displacement or internal capacity; and, in either case, may be a sufficiently fair criterion of the burthen or size of a vessel, by which to levy the dues, provided the mensuration of each is correct; because, it is maintained, all that is required on the part of the interests concerned is, that the duties should be levied in equal proportions on all vessels. But this, though it may appear, prima facie, to be a very reasonable argument, must be confessed to be fundamentally erroneous, (the condition of its truth, that of the dead weight of the cargoes of vessels being in proportion to their internal capacities for stowage, being wanting) particularly under the present circumstances of an accelerated increase in the construction of iron vessels.
This will appear sufficiently evident, from the consideration that the weight of cargo which a vessel can carry depends entirely, as we know from the principles of hydrostatics, upon the external magnitude of that part of the hull which is immersed by it, and must therefore be quite independent of the cubical contents of the internal capacity of the hold; for the sides or shell of the vessel may be made of any thickness, at pleasure, without the least alteration in the external magnitude, though greatly diminishing the internal capacity. And consequently, as the thickness of the shells of vessels in the present construction of ships — some built of oak, others of fir, and others of iron — is continually varying, (in some instances to the amount of nearly 30 per cent, of the capacity) and therefore bears no constant ratio to the external magnitude, there can be no definite relation between the external bulks and internal capacities; and hence the dead weight carried, dependent on the former, can be no proportionate measure of the internal capacity for stowage. Consequently, if it be just to levy dues in proportion to the dead weight of cargoes which vessels can carry, it cannot also be just to levy them in proportion to their internal capacities for stowage.  With the various commissions appointed by the British Government at different periods, for the revision of the law, it has always been a problem whether internal or external measurement should constitute the basis of tonnage; and from these authorities it has received different solutions. But it is evident, from what has been above stated, that one of these two principles, to the exclusion of the other, must constitute, for general commercial purposes, the most fitting basis for admeasurement; and in which of them the eligibility lies, it will be my aim to endeavour to develop.
In the earlier annals of the British Mercantile Navy, when voyages were almost entirely confined to the crews navigating their vessels, and consequently passengers constituted but little, if any, of the profits of the owner; when steam vessels, built mainly for passenger traffic, and, therefore, as to their profits, comparatively independent of cargo, were not thought of; when vessels built of iron were equally unknown, and few, probably, were built of anything but oak, this being the indigenous timber of the country; and when, therefore, all vessels were comparatively equally buoyant; — under such a retrospect of the earlier nature and constitution of our commercial shipping, it is only reasonable to suppose that a law to be established for tonnage admeasurement would have reference only to cargo, and that too in its simplest consideration, namely, the greatest weight which a vessel could safely carry. Whatever, therefore, now (the above circumstances of commercial navigation being entirely altered) may be the conviction as to whether displacement or internal capacity be the most eligible basis, there is much reason for supposing that the former principle was entertained by the earlier projectors of the law.
Having made these preliminary observations, with the hope of assisting on these general grounds in removing the uncertainty in which, as at first stated, we have been left by the projectors of the original general law, as to what was intended to be understood by the term "tonnage," whether to express a measure of weight or of space, we pass on to the official admeasurement of vessels in earlier times.
There appears to have been a very partial legislative interference with the operation for determining the tonnage of shipping. It related, primarily, according to our earlier records, only to vessels engaged in some particular branches of trade; and in course of succeeding ages it was extended, prior to its general establishment, partially to others under certain circumstances of suspected evasion of the public revenues. As early as the year 1422, the ninth year of the reign of King Henry the Fifth, (which appears to be the era having claim to the origin of the legislative admeasurement of British shipping,) we find from existing parliamentary records of that period, this simple enactment: "That keels that carry coals at Newcastle, shall be measured and marked." Also, in the year 1648, in an act of the thirteenth year of the reign of King Charles the Second, for the remedy of "Deceits" therein complained of, we read "That Commissioners should, from time to time, be appointed by his Majesty, his heirs and successors, for the admeasuring and marking all and every the keels and other boats," &c, "to be used for the carriage of coals in the port of Newcastle, and all other places within the counties of Northumberland and Durham."
Progressing with our Parliamentary Annals, we arrive at the year 1694, the sixth of the reign of William and Mary, and learn, that whereas the legislature hitherto had been contented simply to direct that the above admeasurement of keels, &c. should be made by Government Commissioners, it was now determined, in consequence of "divers new frauds, deceits, and abuses," that the method of admeasurement should be distinctly defined and prescribed. And, accordingly, it was enacted, that the "said admeasurement shall be by a dead weight of lead or iron, or otherwise, as shall seem meet to the said Commissioners, allowing three and fifty hundred weight to every chaldron of coals," &c.; "and cause the said keels and boats so admeasured, to be marked and nailed on each side of the stem and stern and midships thereof," &c, and "provided that no such keel or boat shall be admeasured, marked or nailed, to carry more than ten such chaldrons at any one time."
This method of admeasuring keels, (limited, at first, to the ports of Northumberland and Durham,) was afterwards extended by Act 15 George III., to vessels used in loading coals at "all other ports of Great Britain." The vessels, as before, to "be admeasured by a dead weight of lead or iron, allowing twenty hundred weight avoirdupois to the ton, and marked and nailed as aforesaid, to denote what quantity of coals each will carry up to the mark so set thereon."
No further progress appears to have been made as regards official admeasurement of coal vessels, (and the laws so far applied to no other,) till in the year 1720, the sixth year of the reign of King George the First, when we find that the attention of Parliament was directed to the necessity of arresting the clandestine trade then carrying on in the importation of foreign spirits, which was found to be facilitated by the smallness of the vessels allowed by law to be employed in the trade.
It was, therefore, enacted, that no spirits should be allowed to be imported in vessels of "thirty tons burthen and under;" and, "for the preventing disputes that may arise concerning the admeasurement of ships laden with brandy and other spirits," it was also enacted "That the following rule shall be observed : Take the length of the keel within-board, (so much as she treads on the ground,) and the breadth within-board by the mid-ship beam, from plank to plank, and half the breadth for the depth, then multiply the length by the breadth, and that product by the depth, and divide the whole by ninety-four, the quotient will give the true contents of the tonnage." These partial enactments extending only to coal and spirit-vessels, and, of course, only to a very small section of the whole mercantile navy, continued to comprise the entire law of admeasurement until the year 1773, when the legislature of that day, called to the consideration of the subject by the "disputes" that were continually arising on various occasions where the tonnage of vessels, trading to and from Great Britain, was necessary to be known and ascertained, deemed "it expedient that one certain rule for this purpose should be settled and established in all cases." And in accordance with this enlarged view of the question, an act was passed, establishing a general rule for the admeasurement of all vessels in every branch of the commercial navy; excepting only those vessels carrying coals, and those employed in the British white herring fisheries.
The first general rule for the admeasurement of vessels in all cases, as above, is briefly as follows :- "The length shall be taken on a straight line along the rabbet of the keel of the ship, from the back of the main stern-post to a perpendicular line from the fore part of the main stem under the bowsprit, from which subtracting three-fifths of the breadth, the remainder shall be esteemed the just length of the keel to find the tonnage; and the breadth shall be taken from the outside of the outside plank in the broadest place in the ship, be it either above or below the main wales, exclusive of all manner of doubling planks that may be wrought upon the sides of the ship; then multiplying the length of the keel by the breadth so taken, and that product by half the breadth, and dividing the whole by ninety-four, the quotient shall be deemed the true contents of the tonnage.
Subsequent to the establishment of the above, it having been found necessary, occasionally, to measure vessels afloat when the length could not be taken as by this rule directed, an additional act was passed: describing that in the case of measuring vessels afloat, the length shall be taken at the load water-line, from the back of the stern-post to the front of the stem," subtracting therefrom three inches, for every foot of the load draught of water for the rake abaft, and three-fifths of the ship's breadth for the rake forward, the remainder being the length of the keel for tonnage." The subsequent introduction of steam navigation seemed to require a modification of the above rules, to ascertain the true tonnage of steam vessels.
In the year 1819, it was enacted, that for the purpose of measuring the tonnage of any vessel propelled by steam, the length of the engine-room shall be deducted from the length of the keel for tonnage as therein prescribed, the remainder being esteemed the just length of the keel to find the tonnage.
The Commissioners reported on the 24th of May, 1821, "that there are sufficient reasons for being dissatisfied with the mode of admeasurement now legally employed," &c, adding, "they would have been desirous of removing all doubt upon the subject, by proposing the admeasurement of that portion of the ship which is included between the light and heavy water-lines," (or in other words, the displacement occasioned by the cargo,) "but this method has been considered as liable to insuperable objections, on account of the impossibilities of ascertaining the position of these lines in a satisfactory manner." The report concluded by recommending a "simpler method of measurement and computation, "which consisted only of a few internal measurements, proved to be but an inadequate and distant approximation to true mensuration, and being also greatly open to evasion, no legislative results accrued from it, and the "Old Law," as it is designated to this day, being also called the "Builder's Measurement" in Great Britain, was still left, year after year, to its injurious operations; the several acts relating to it being finally consolidated into one during the reign of William the IVth.
End part 1, p. 172-179 and start part 2, p. 372-378
In the preceding article [part 1] a simple historical view of the British laws concerning the admeasurement of merchant shipping has been briefly presented. The more complex and disputative consideration of the subject will now be entered into, with a view to showing the effects of the operations of these different enactments in regard to influencing the dimensions and forms of the vessels of the merchant navy. Taking, seriatum, the several enactments relating to tonnage, we have first that of the year 1694, for the "admeasuring and marking all and every the keels that carry coals in the port of Newcastle," which was established to protect the public revenue from divers frauds, deceits, and abuses, and to secure the due shipment in these vessels of ten chaldrons of coal of fifty-three hundred cwt. each, consisted in simply loading the vessel to be measured with lead or iron weights, to the extent of the above weight, or twenty-six and a half tons; and then marking by nails at the stern, and midships, the draught of water occasioned thereby. So that in loading these vessels at the staiths with coals, for conveyance to ships in the offing, it was only necessary to bring them down to the line of flotation indicated by the nails, and the exact weight of ten chaldrons, without the possibility of fraud, was secured.
This being a measure of pure and unerring displacement, free from the possibility of evasion, and giving the exact dead weight of the cargo shipped, whatever may be the form or construction of the vessel, it offers, therefore, no inducement to the building of one kind of form more than another; and the consequence is, that many of these peculiar vessels are remarkable for their sailing capabilities.
Although the process of "admeasuring and marking," to which these vessels are still legally directed to be submitted, involve neither the taking of measurements nor computation, and can therefore, it may be supposed, scarcely be termed a mode of admeasurement, in the usual acceptation of the operation; yet it is, nevertheless, essentially and absolutely the most correct measurement possible of the displacement, or external cubature [sic. math: the determination of the cubic contents. Ed.] of that part of the vessel which lies, or is contained between the light and load lines of flotation, that is, of the cargo shipped; and is, therefore, not only an assurance of the security of the public revenue to be derived from the export of the coals, but is found, also, to tend greatly to the general accommodation of the trade in which these vessels are engaged; and from what has been already predicted of their sailing capabilities, the process is, moreover, an eminent and satisfactory, though it may appear a humble example, that an operation founded on truth, without the possibility of evasion, is an operation without influence in the production of ill-formed vessels.
Next in succession is the rule enacted in the year 1720, for the prohibition of the importation of foreign brandy and other spirits in vessels of "thirty tons burthen and under, and for the preventing disputes that may arise concerning the admeasurement of them." In this partial act we have the first process of admeasurement in which the taking of defined measurements is established. The rule is set forth as follows: The length of the keel, as much as she treads on, is to be multiplied by the inside midship breadth taken from plank to plank, and the product of the half breadth and the whole being divided by ninety-four, the quotient is the true contents of the tonnage."
That it was the dead weight tonnage which was here intended to be obtained, appears evident when we consider that the dead weight known to be carried by our small coasting vessels, in which the depth is about half the breadth, agrees very closely as appears in the example below, with the results given by this rule. Take the example of the coasting sloop Ann, usually employed in carrying stones, whose main inside half breadth is 6.41 feet, and depth from upper side of beam to ceiling 6.37 feet. Length of keel, "as she treads on the ground," equal to 35.2 feet, multiplied by main breadth inside, equal to 12.82 feet, multiplied by half main breadth, equal to 6.41 feet, and the production of these three factors divided by 94 is equal to 30.8 tons, per tonnage. — The weight of actual cargo usually carried is equal to 31 tons of stone. The depth and half breadth being practically the same, strongly verifies the inference, that the intention of the law was to express the weight of cargo in tonnage. It also shows, that when the law was framed, the usual depth of vessels was equal to the half breadth. But, being of limited application, this rule had no deleterious influence upon the construction of vessels in general.
Next in chronological order, we find the general, or "Old Law," introduced in the year 1773. This rule will be found to be briefly as follows: The length is to be taken on the rabbet of the keel, from the front of the main stem under the bowsprit, from which, subtracting three-fifths of the main breadth, taken to the outside of the plank of the bottom, the remainder is the length for tonnage, which length, multiplied by the above breadth, and the product by half the breadth, and the whole being divided by ninety-four, the quotient is deemed the true contents of the tonnage.
This rule was similar to its predecessor, except that the measurements were internal, and when applied to vessels whose depths are half their breadths, the same results are found to accrue from both rules, and these results are equal to the weights of the actual cargoes, found by experience, to be carried by such vessels.
It will at once be observed, by inspecting the rule, that neither any variation in the absolute depth, nor the form of the vessel both being absent from the formula, can have any influence whatever on the register tonnage; though the least variation in either must, necessarily, have its due effect on the real burthen or capacity.
At the time of the introduction of the "Old Law," the depths of ships were not far distant from half their breadths, and the register tonnage represented the weight of the cargo which could be safely carried. Shortly after the establishment of this rule, however, this proportion of depth to breadth was continually increased and carried to the most injurious extent — amounting in many instances, to more than three-quarters of the breadth. Of course it followed, that the evasion of dues was in exact ratio to the increase of depth, — so that the tonnage given by the rule bears the same proportion, practically speaking, to the actual tonnage or weight of cargo carried, as the depth assumed by the rule, (namely, half breadth,) bears to the actual depth of the vessel.
Thus, the "Free Trader," length 123 feet on deck, deducting three-fifths of main breadth, (equal to 19.05 feet,) for the length for tonnage, which is equal to 103.95 feet, being multiplied by the main breadth, (equal to 31.75 feet,) and this product multiplied by the main half breadth, (equal to 15.87 feet,) which product divided by 94, gives the result 557.2 tons for tonnage, while the actual cargo carried varies from 850 to 900 tons, the average of which may be taken at 875 tons. From the example of the coasting sloop Ann, whose depth agreed with the main half breadth, already given, we may now make a comparison of the utter inaccuracy of the rule in its application to disproportioned vessels. The depth of the "Free Trader," from upper side of beam to outside plank at the rabbet of keel is equal to 25.2 feet, while the half main breadth, which the law assumes to be equal to the foregoing depth, is only equal to 15.87 feet — being a difference of 9.33 feet between the actual and assumed depth. It is easy to discover, by a simple proportion, as given above, if we only know the weight which a given vessel really carries, what she would also really carry if her depth were equal only to half her breadth. Example :
which is within six tons of the tonnage given by the rule. It would seem, that af the time of framing this "Old Law" rule, the depth of vessels were so uniformly about equal to their half breadths, that the one dimension was substituted for the other. The selection of the divisor 94, in preference to a more convenient factor, must have arisen simply because it was found to bring out, from involution of the principal dimensions as prescribed by the rule, a near approximation to the weight of the actual cargoes carried at that period by merchant vessels of the usual form.
Having considered the "Old Law" only in its influence on the depth of ships, let us next investigate its operation in influencing the relative proportion of the breadth. Suppose a vessel of the form of a rectangular parallelopiped, or oblong box, having a length of 12 feet, a breadth of 19.9 feet, and a depth equal to half the breadth, or 9.95 feet; computing the tonnage by the process of the "Old Law," according to these dimensions, the result will be found .126, or a little more than one-tenth of a ton.
Now, suppose the breadth of this oblong box, or vessel, to be diminished by one-half, or to 9.95 feet, then the same process of computation would give a result of 3.17 tons, register tonnage' The absurdity of these results speak for themselves. In the first case the register tonnage is about one-tenth of a ton; and in the second, by diminishing the breadth one-half, the register tonnage is increased to upwards of three tons. Or, in other words, an alteration, which ought to have diminished the register tonnage one-half, has absolutely increased it upwards of two thousand four hundred per cent! Can it be believed that a people claiming pretensions to scientific intelligence — this first maritime nation of the world — can have been subject, for three-quarters of a century, to a law involving such monstrous absurdities? 
We may learn from this analysis, that under any scheme of dimensions whatever, the more the breadth is increased in relation to the length, the more the real capacity is increased in proportion to the increase of the register tonnage, and holds out a premium for an undue increase of breadth.
A single practical example will more clearly illustrate this important injurious property of the system. Suppose a vessel of principal dimensions according to the usual proportions, as follows:- Length 40 feet, breadth 10 feet and depth 7 feet. Computing the tonnage by the old rule, we have the result equal to 18 tons; and the true geometrical measurement in cubic feet is 40 X 10 — 7=2800 cubic feet. Now let the breadth of this vessel be increased to 11.4 feet, the depth following in proportion, which will give about the proportions of length, breadth, and depth to be found in some of our worst proportioned ships. The registered tonnage will be found to have been increased to 22.92, or nearly 23 tons; and the cubic capa- city of the latter dimensions will be 40 X 11.4 X 7.98 = 3833.9 cubic feet.
From these results, we see that by increasing the breadth from 10 feet to 11.4 feet (preserving at the same time the relative proportions between breadth and depth), the register tonnage is increased from 18 to 22.92 tons, or 27 per cent only; while the real capacity is increased from 2800 cubic feet to 3638.9 cubic feet, or 30 per cent.  Is it, then, to be wondered at that owners, bent on immediate and positive gains rather than on prospective advantages, should avail themselves of these legal means to increase the carrying capabilities of their ships, free from any additional taxation? The undue increase of breadth in proportion to length would naturally follow, and as the first analysis showed the influence of the "Old Law" in begetting deep ships, so this exposition exhibits the mischievous encouragement it has held forth for the construction of short ships.
We will now briefly consider the influence of the "Old Law' in regard to its effects on the forms or models of vessels. If we revert to the computation by the process described in the Rule, we shall perceive that the form or shape of the body is totally disregarded in it, and that, therefore, the most bulky form is encouraged; inasmuch as, under the same principal dimensions of length and breadth, the register tonnage will remain unaltered, whether the vessel be of the fullest or sharpest and finest model- Reviewing, then, the results of these various inquiries, it will be observed that the tendencies of the "Old Law"  are briefly as follows :
- To increase unduly the depth of vessels.
- To decrease their length bj increasing their proportionate breadth.
- To give vessels the fullest form possible.