Length, Beam, Tonnage… And All That Stuff — Fall 2002
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The Great Lakes… 84% of the continent’s fresh water… a different story in every drop.
By Walter C. Cowles
Having had a lifelong interest in ships and maritime history and being a reader of numerous related publications, I’ve often noted that many writers of maritime history, in spite of having researched the particular subject, have very little understanding of the concepts of measurement and tonnage. Hopefully, the following will be of some help.
Dimensions
LENGTH:
Writers are often confused and assume that someone has erred because they find different values in different sources. There are, for example, at least five different values for length.
- Register Length: a measurement of internal length in accordance with whatever admeasurement regulations are governing.
- Length on Keel: probably a significant measurement in the days of wooden ships, and approximately the length of the keel timbers. Of little relevance for steel ships but used by some builders. Great Lakes Engineering Works customarily used keel length as a major measurement.
- Length between Perpendiculars: LBP — FP is the intersection of the fore side of the stem with the design, (or assigned) summer load waterline and AP is the centerline of the rudder stock.
- Waterline Length: obviously the length between the intersection of the waterline with the stem and stern. Not so obvious, it may be based on waterline at design draft, or the final assigned draft which may differ slightly from the original design.
- Length Overall: length of hull from the extreme forward end to the extreme after end.
BEAM:
Many writers appear to have a tendency to use rather exotic terminology in stating beam. The term, of course, originates from wooden ship practice, where length of beam was the length of the longest deck beam. Suffice to say that in the case of modern steel ships, this is irrelevant and the simple term beam, perhaps modified by the adjective “molded” or “extreme” as appropriate. Molded, as explained below, is to the inside of the plating and extreme to the outside. Molded is the norm because it is readily determined while extreme normally adds only the thickness of the shell plating. In the case of side-wheel ships, beam is the width of the hull and the width over the guards is probably most appropriately defined as extreme width.
DEPTH:
Depth can also be confusing. Again register depth is the depth measured internally in accordance with admeasurement rules and has
little significance describing the size or configuration of the ship. In modern terminology, what we’re interested in is the depth from the bottom of the keel to the deck at side and this is the value that will appear in most registers. Again, it may be either molded or extreme, the difference being only the thickness of the keel plate and the deck stringer plate.
MOLDED DIMENSIONS:
Perhaps by way of a bit of background, molded dimensions are those most readily available and most accurately defined. Construction of a vessel begins with the transfer from the hull lines as drawn to a full scale layout on the floor of the shipyard mold loft. From these molded lines, wood patterns are made for use in forming the frames and plating of the vessel. As these lines are the basis for shaping the frames, they will relate to the inside of the plating of the finished vessel. For the most part, shell plating will seldom be much over an inch thick so the difference between molded and extreme will be of little significance relative to the actual size of the vessel.
Tonnage
First, tonnage comes in two kinds: weight and internal volume. There is absolutely no connection.
REGISTER TONNAGE:
Register tonnage is a measure of internal volume and has absolutely no relation to weight of ship, cargo, or anything else. It is truly an archaic concept and exists only because it always has and because no one has apparently come up with an alternative … it’s readily available and “official.” The sole purpose seems to be to come up with a basis for charges. Register tonnage dates back to the days of wooden ships. In fact, it was “tuns” and perhaps life would be less confusing if it still were. It made sense when ships were pretty similar in construction and general layout and there was really no alternative to determining the relative carrying capacity of an existing vessel afloat. Tuns were the barrel like containers used for the storage and shipment of wine, and our current “ton” approximates the volume of these containers.
GROSS REGISTER TONS:
The internal volume measured from the inside of all framing members. One registered ton is 100 cubic feet of volume.
NET REGISTER TONS:
Gross tons less the volume of bunkers, ballast tanks, crew accommodations and anything else not available for carrying cargo.
A little background: When I was at American Ship Building during the 1950s, admeasurement was a function of the U.S. Department of Customs. When it came time to establish the admeasurement values, a Customs representative would come down to the office, sit down with one of our senior draftsmen, and they would discuss the criteria for preparing the admeasurement plan. While the Customs regulations are detailed and complex, there is always room for a degree of interpretation. Once the drawing was complete, it was forwarded to the Customs office and was the basis for calculating gross and net
tonnage. Subsequently, responsibility for admeasurement calculations became the responsibility of the Coast Guard and — I believe — is now delegated to the American Bureau of Shipping.
Note that we have gross and net tonnage, but not gross and net tons — they are all 100 cubic feet.
Until a few years ago, every nation had its own admeasurement rules as did the Suez and Panama Canals. Thus, if a vessel were transferred from U.S. to Canadian registry, there could be a change in tonnage. Any physical change, for example conversion to a self-unloader, would result in a significant change in tonnage.
In short, for very similar ships, admeasurement tonnage values have little relationship to size, and even ships that are structurally identical may have different tonnage values, particularly if they have come under different jurisdictions.
How Much Does a Ton Weigh?
Generally speaking, in the U.S. we have two kinds of tons: the common short ton of 2,000 pounds and in shipping, the long ton of 2,240 pounds. There is then, of course, the metric ton of 2,204 pounds, now official in Canada. Why the long ton? Because our ship design has an English background and 20 hundredweight equals 2,240 pounds.
DISPLACEMENT TONNAGE:
The weight of water displaced by a floating vessel is exactly the weight of the vessel.
LIGHT WEIGHT TONNAGE:
This is the actual weight of the vessel. With few exceptions, this is an approximation. In the case of ships where stability may be a factor, an inclining experiment will be required. In connection with the inclining experiment, a close approximation of the light weight will be made. This is done by reading the drafts, calculating the displacement and adding the weight of any items remaining to be placed on board and deducting weights to be removed, or not included in the light ships such as ballast, fuel, miscellaneous stores, etc. In other words, the structural weight of the ship with liquids at their normal level, not including fuel or water in their tanks. Lacking this, the builder, may perhaps, at the time of trials, determine the displacement and make a more or less detailed correction for weights to go on or come off.
The ship will never again be so light. She’ll always have residual ballast, mud in the ballast tanks, variable quantities of water, fuel, lubricants, stores, spares, etc.
DEADWEIGHT:
The difference between whatever has been determined as light ship and displacement at any given draft. None of these values are official. They are provided to the regulatory bodies and registers to whatever degree of accuracy and completeness suits the owner.
Displacement and related values may also change without any physical change in the ship, if, as has happened in the last few years, regulations change to permit reassignment of load lines.
Back to gross and net tons. The Great Lakes fraternity has introduced another anomaly. While — as described above — these are measurements of volume, it is Great Lakes practice to refer to long tons as gross tons and short tons as net tons. The former is customary for the measurement of iron ore, while coal cargos are measured in short tons. Neither has any relationship to register gross and net tons.
Here I digress. I started out to make a point and after a bit of investigation will end up making several. Taking two essentially sister ships, Armco and Reserve, built from the same structural plans but by two different yards and with some minor variations in accommodation arrangements (those on Armco are more extensive). Referring to two sets of ABS Registers as noted:
| ARMCO 1972 | 1982 | RESERVE 1972 | 1982 | |
| Draft | 25’0″ | 26.333’* | 25’6¼” | 26.333’* |
| Gross Tons | 11,654 | 13,887 | 11,601 | 13,752 |
| Net Tons | 8,075 | 10,847 | 8,601 | 10,727 |
| Deadweight | 20,138 | 26,350 | 20,752 | 26,355 |
| Displacement | 26,400 | 26,947 | 34,000 |
*Lengthened 120′
The 1972 draft listed for Armco is the design summer draft. That listed for Reserve is the draft assigned by ABS based on the completed vessel. The draft differences, of course, account for the differences in displacement.
References
Principles of Naval Architecture, 1939, p. 3.
Principles of Naval Architecture, 1967, Chapter VI.
Ship Design and Construction, 1980, Chapter V.
American Bureau of Shipping Registers.
About the Author: With a degree in Naval Architecture from the University of Michigan, Walter C. “Bud” Cowles rejoined the staff of The American Ship Building Company where he had worked the previous summer. Over the next 20 years — with time off for an 18-month stint in the U.S. Navy — he worked as Draftsman, Chief Hull Draftsman and Acting Naval Architect, primarily involved in the design of Great Lakes ore carriers. His career continued at Esso International Tanker Department and Exxon where he was involved in the design of ocean going tankers, semi-submersible drilling rigs and other aspects of petroleum transportation and production. Following retirement in 1985, Bud continues to maintain broad interests in maritime history and coordinated development of a book commemorating the Centennial of the Society of Naval Architects. He now lives in northern Michigan where he “spent a lifetime of summers.”
