Ton types

by Greg Walsh

When stating the measurements of a boat, it is common for most mariners to describe its length. However, there are many ways of measuring the size of a vessel. Each method looks at size from a different angle. These measurements can be somewhat confusing to the average boatowner.

There is length, of course. The longer one's boat, the larger is one's invoice for dockage, for instance. Applicability of regulations for horns and lights is determined by a vessel's length. Applicability of certain state and federal regulations is also sometimes determined by length.

In addition to overall length, there is waterline length, length between perpendiculars, registered length, on-deck length, and sparred length. Generally speaking, however, when a yacht owner refers, ever so casually, to his 68-foot yacht, he is referring to its overall length. That is the entire length of the vessel from bowsprit to boomkin.


What really counts, however, especially in the eyes of officialdom, is the size of a vessel as measured in tons—tonnage measures are the universal language of boats and ships of all sizes. To talk shop with the shipping magnate who lives down the street, one must be conversant with the parlance of tonnage. To apply for documentation or licensing from the U.S. Coast Guard, one must be familiar with the tonnage of vessels involved. To pass through the Panama Canal one must pay a toll based on tonnage. To arrange for a haul-out at a marina, one must often be familiar with the tonnage of a vessel, just as a trucker would need to know the tonnage of a vessel being shipped by highway.Displacement tonnage


The most basic measure of tonnage is displacement tonnage. This is a direct measure of the actual weight of a vessel. Displacement tonnage is the weight of water displaced by the hull of a boat. A vessel floating in fluid will always displace an amount of fluid equal to its own weight.

A cubic foot of salt water weights 64 pounds. If a vessel displaces an amount of salt water equal to 1,000 cubic feet, its displacement weight would thus be equal to the weight of 1,000 cubic feet of salt water or 64,000 pounds.

Fresh water is known to be somewhat lighter than salt, weighing 62.5 pounds per cubic foot. Thus, the same boat would displace 1,024 cubic feet of fresh water. The same vessel would float more deeply in the fresh water.

It is customary, however, for displacement tonnage to be expressed in long tons, which are rated at 2,240 pounds. The vessel would be rated at 28.6 tons displacement.

In real life, the displacement weight of a vessel is always changing, however. As a vessel takes on a load of fuel, food, cargo, or people its draft increases and its displacement thus increases. Diesel fuel, for example, weighs about seven pounds per gallon. If a vessel were to take on a load of 1,000 pounds of fuel, its displacement weight would increase by a like amount.

Most yachts go through their lives with one displacement weight which is provided by a yacht designer before the vessel is launched. The designed waterline of a yacht is calculated to include an average load of people, stores, fuel, water, etc. A happy yacht designer is one who sees his new creation floating well over her designed (and painted) waterline when she is first launched and still empty of equipment and stores. At that time the vessel is considered to be in light-displacement mode, just like a cargo ship devoid of cargo.

Considering that a yacht's waterline remains relatively stable, regardless of her load of fuel, water, or stores, a single displacement value is generally sufficient. Things are dramatically different on a cargo-carrying ship, however. A tanker's draft, and thus her displacement, may change by 20 feet or more depending on her weight of cargo. Ship's officers, forever processing paperwork, need to know their vessel's displacement tonnage at any time.

To calculate his ship's displacement, a captain need only check the draft of his vessel (by reading those numbers painted on its bow) and then refer to a table which provides displacement for every given draft. The table, called a deadweight table, is provided by the ship's designer.

A ship's deadweight table simply cross-references three factors: cargo, draft, and displacement. An empty ship has light displacement. Add cargo and displacement increases. Add a full load of cargo and displacement increases to its respective maximum, as does the ship's draft. In terms of displacement, the difference between an empty ship and a full ship is weight of cargo. Weight of cargo is called deadweight tonnage, which is why the table mentioned above is called a deadweight table.

Now some examples: A 24-foot Flicka sailboat, produced by Pacific Seacraft, has a displacement of about 5,700 pounds, according to its builder. And a Grand Banks 42 power yacht has a displacement of 35,000 pounds according to that company. A Hinckley 52 sailboat has a displacement of 39,000 pounds, according to the Hinckley Co. (Remember that these figures could be converted to long tons by dividing pounds by 2,240).

Moving up a bit in size, the famous Alden-designed steel ketch, Minot's Light, measuring 58 feet LOA, had a displacement weight of 28.5 tons. The schooner, Pride of Baltimore II, with an overall length of 173 feet, has a displacement of 185.5 tons.

The sailing frigate Constitution, measuring 204 feet, and the oldest commissioned U.S. Navy ship, carries a displacement of 2,200 tons. The U.S. nuclear attack submarine, Seawolf, 350 feet, currently under construction at New London, CT, will have a displacement of 9,300 tons when submerged, but her displacement would be considerably less when surfaced. By comparison, the famous Swedish freighter, Stockholm, 524 feet in length, carried a displacement weight of 13,000 tons just before she collided with Andrea Doria. The U.S. aircraft carrier, Saratoga, measuring 1,040 feet in length, is rated at 61,000 tons displacement when light, and 81,500 tons displacement when fully loaded with 80 aircraft, 5,000 men and a tremendous weight of supplies and fuel. Deadweight tonnage


It might be tempting to think of the difference between Saratoga's light displacement and heavy displacement as her deadweight tonnage (about 20,500 tons). But such consideration would not be appropriate, since, as a cargo carrier, the naval vessel would be highly inefficient. (A similarly-sized tanker, by comparison, would likely have a deadweight tonnage of closer to 100,000 tons.) Deadweight tonnage is a description generally only employed in the measurement of cargo ships which carry heavy cargo: tankers, bulk carriers, and some others. Such a measurement would clearly not be appropriate for hospital ships, cable-laying ships, research vessels or submarines. The measure also does not work for yachts, fishing vessels, gambling vessels, cruise ships, or tugboats.

Many ships are casually referred to by their deadweight tonnage (abbreviated dwt). This is especially true of tankers. If a tanker is not initially described in terms of length, then it will most likely be described by its deadweight tonnage.

In 1953, Tina Onassis was one of the largest tankers in the world. It was a 45,700 dwt petroleum carrier. Twenty years later one of the largest ships in the world was Globtik Tokyo, which measured at 483,660 dwt.

Gross tonnage

Knowing the deadweight tonnage of a vessel is one method of measuring it cargo-carrying capacity. However, gross tonnage is almost always required. Gross tonnage is the interior volume of a vessel as measured in units of 100 cubic feet. (Each such unit equals a gross ton). Gross tonnage is a measure of all enclosed interior space of a vessel, including all compartments, machinery spaces and most enclosed deckhouses. It has little to do with the actual weight of a vessel.

Anyone handy with a tape measure could easily measure the interior volume of a compartment. Indeed, if it comes right down to it, Coast Guard officials or surveyors from organizations like the American Bureau of Shipping are fully prepared to do just that. Short of physically measuring each compartment, however, a surveyor might also determine gross tonnage by taking measures of length, breadth and depth from each station of a vessel as shown on its design plans and doing his calculations by section.

There is an even easier way to compute gross tonnage, however, using the simple formula:where: length is the length overall, not including bowsprits, boomkins, figureheads, etc.; breadth is a vessel's maximum beam; and depth is the vertical distance from gunwale to bottom of keel.

This is the formula used by Coast Guard documentation offices to calculate gross tonnages for yacht owners who do not, typically, know this measure of their vessels. This is a thumbnail technique for determining gross tonnage but it is generally accepted as accurate enough for most record-keeping purposes.

Here are some more examples of gross tonnage. The 52-foot Hinckley sailboat mentioned above measures about 45 gross tons. A typical 35-foot light displacement fiberglass sailboat measures in at 12 to 15 tons. A Mason 44 has a gross tonnage of 36.85. A Nordhavn 62 trawler yacht has a gross tonnage of 101.9. The 125-foot LOA wooden schooner Spirit of Massachusetts is rated at about 90 tons.

The somewhat simplistic formula for calculating gross tonnage is sometimes used by yacht designers and Coast Guard inspectors to get a preliminary estimate of a vessel's gross tonnage, often with a more formal measurement process to follow. A preliminary measure of tonnage, however, can be useful in determining how a vessel will fall into various systems of regulations, rules, and requirements for crewing, licensing, safety equipment, and inspections. In another situation, a yacht designer may wish to comply with his client's wish that a vessel being designed and built not exceed, say, 100 gross tons.

For most of this century the Coast Guard has provided vessel documentation service for yachts of more than five gross tons. (The minimum tonnage was raised somewhat for a number of years but was subsequently lowered again and remains today at five tons.) The majority of yachts of 25 feet or more measure at least five gross tons. In addition, all fishing vessels and all commercial vessels are required to be documented, including hundreds of U.S.-registered ocean-going tankers and freighters.

In theory, a government provides this form of federal registration of privately-owned vessels so as to have a readily-available supply of auxiliary craft in time of war. During the Falklands War between England and Argentina, for instance, the government of Britain exercised its right to commandeer the liner Queen Elizabeth 2 as a troop transport.

However, the national-service aspect of documentation may be a bit outdated today. "The probability of this happening today in the United States is absurdly small", said Anne Wokanovicz, of Atlantic Maritime Services, a documentation service agent working out of Westport, CT. In actuality, the Coast Guard provides documentation service today, and thus keeps track of the measurements of all documented vessels, first and foremost because of the lobby of the boating industry and the lending industry.

Lenders have always had trouble maintaining security for financed vessels. It is all too easy for the owner of a vessel to literally sail away, taking his collateral with him, leaving a lender with little likelihood of collecting on his loans. Nowadays there are complex state and local barriers which may make it difficult for a lender in Boston, say, to collect what he is owed, on a fishing vessel which has mysteriously been relocated to Mobile, AL. With federal documentation, however, lenders have a nationally-recognized legal record of every vessel afloat, no matter where it is located. Federal documentation provides an avenue for collection and reclamation that transcends all state and local barriers. Today it is virtually impossible to finance the purchase of a yacht greater than five or 10 tons without federal documentation.

Wokanovicz said the Coast Guard was recently developing a plan to do away with federal documentation of yachts by tying together state registration systems with computers and shifting much of the record-keeping tasks to state governments. That plan, however, was quickly hog-tied and buried through the combined complaints of state governments, lenders, and yacht manufacturers.

"Periodically, there are outcries to do away with documentation simply as a cost-cutting measure", said Wokanovicz, "but I think that most people realize that documentation is a very fee-oriented service, and the service is recognized as one which truly does pay for itself while providing a valuable service to the U.S. maritime community."

It is not always easy to equate gross tonnage to the actual size or carrying capacity of a vessel. Consider the following figures on tankers, for instance. In 1992, the average U.S.-registered, privately owned tanker (petroleum and non-petroleum) was sized at 38,367 gross tons, according to Merchant Fleets of the World, published by the U.S. Maritime Administration. Yet the average deadweight tonnage of that same fleet was 70,598 tons.

One area where gross tonnage is an all-important measure is with individual licensing of professional and amateur mariners. All licenses are categorized by gross tonnage. Licensed yachtsmen generally earn licenses of 100 gross tons or less. Many professional mariners have licenses of 1,600 tons or more, extending up to "unlimited" tonnage licenses. There are other size categories in between: notably 200 tons and 500 tons. The 88-foot steel schooner Ocean Star, which this company operates as a navigation training vessel, is rated at less than 100 gross tons, but the captains and first mates who operate the vessel all have licenses of 100 tons or more. A yachtsman applying to the Coast Guard for a license based on sea days of service aboard various yachts will need to provide the gross tonnage of each vessel for which time is submitted. Net tonnage


Knowledge of the total interior volume of a vessel is useful enough for all of these reasons, but there is another measure of volume equally valuable: net tonnage.

This is a measure of interior volume which does not include certain compartments and spaces that do not carry cargo, notably engine and machinery spaces, chain lockers, crew quarters, charthouse, supply compartments, some tankage, etc. A vessel's net tonnage will always be less than its gross tonnage. For documentation purposes, the Coast Guard's Tonnage Survey Branch estimates that a vessel's net tonnage is typically about 80 percent of its gross tonnage. Net tonnage is always listed on a vessel's documentation certificate, along with gross tonnage.

Some types of taxes, fees and tolls are based on net tonnage. For instance, a vessel passing through the Panama Canal pays a toll based on a measure of net tonnage equal to 100 cubic feet of cargo space. A fully-laden cargo ship pays a toll of $2.21 (U.S.) per net ton. A vessel in ballast (empty), or a passenger vessel without passengers pays a toll of $1.74 per net ton, according to the Panama Canal Commission. In reality the canal commission uses its own formula to figure a vessel's net tonnage. Vessels passing through are given a certificate which indicates the vessel's measurement in Panama Canal tons, and this certificate is kept on record for all future passages through the canal. Such a measure would not be suitable for warships, hospital ships, yachts, and the like, however. These pay a toll of $1.23 per displacement ton. The Suez Canal also has its own unique measure of a vessel's interior cargo space, known as the Suez Canal ton.

It's easy to remember the meaning of net tonnage if the reader draws upon the meaning of any type of net measure. Just as a net profit is considered to be free and clear of all costs and expenses, and net freight is considered to be free and clear of waste, packaging, and dunnage, so too, net tonnage is considered free and clear of auxiliary or non-revenue spaces. Net tonnage has no relevance at all to a typical yacht. For a large cruise ship, however, all spaces normally used to house or accommodate passengers would be included in the net tonnage measurement.

These measures of interior space are sometimes difficult to conceptualize. A space that is 10 feet long, 10 feet wide, and one foot high represents 100 cubic feet of space and constitutes a gross (or net) ton. In teaching classes about cargo ships at Maine Maritime Academy, Sam Teel, an instructor in the Nautical Science Department, likens these types of space-tons to the equivalent of three average-sized household refrigerators. Thus, according to Teel, a modern containership with a gross tonnage of 16,000 would have space for about 48,000 refrigerators.

"Another way to get a feeling for the difference between weight and space is to visualize a football field (300 feet by 160 feet) completely covered with ping pong balls to a depth of one foot", says instructor Teel. "That would be the equivalent of 480 gross tons of ping pong balls. And it would be the exact same gross tonnage even if the field was covered with cobblestones."

Although displacement tonnage can change depending on the extent to which a vessel is loaded, and depending on whether it is submerged or running on the surface, the two measures of internal volume, gross and net tonnage, never change unless a vessel is reconstructed or redesigned in some way.

By Ocean Navigator