About Carl Alexander Expertise I can answer questions concerning the operations, cargo, maintenance, construction and design theory of vessels used for supply and service in the offshore petroleum industry. Supply, crew, survey, diving, tow boats and barges but no questions about tanker and freighter operations. Family members own commercial shrimp boats and I have a working knowledge of their operations in the Gulf of Mexico. Although not commercial, I have been part of the crew on a full rigged sailing ship and am a devoted history buff of the ships and crews of that period.
Experience I have worked in the offshore drilling industry over 30 years and since I am the representative of the oil companies that charter the service vessels and the project superintendent on the scene, I am closely involved in the boats operations.
Organizations Society of Petroleum Engineers.
Offshore Operations Technical Interest Group of SPE
Expert: Carl Alexander Date: 9/4/2007 Subject: Maritime shipping
Question Is there a way of approximating a vessel's length, beam, and draft from its "tons burden" rating? Many old ship's sizes are given only in tonnage.
Answer Randall,
If there is I have never run across it. Tonnage refers to the ships volume, or cargo space, rather than its displacement in Long Tons. You may have found that ton originated as “tun”, referring to a type of barrel used to carry wine. Old ships were taxed different ways and some pretty wild length to width ratios developed in order to confuse the tax man. The first try at calculating tonnage is known as The Ancient Formula and it is simply length overall x width x depth of hold /100 = tonnage.
The warship Mary Rose was a 700 tonner at 126 feet long. The barque Elissa is 150 ft long with a displacement of 620 tons and a gross tonnage, the cargo capacity, of 431 tons. This illustrates the lighter hull weight for greater displacement of a steel ship.
There is another formula, the Displacement to Length Ratio, which is; DLR = Disp. In Long Tons / (waterline length/100)cubed. As you can see, this also cannot be used in the reciprocal to find a Length to Displacement Ratio. A marine architect might know of such a formula but I doubt it.
The reason I doubt it is because in modern ships, hull length, shape and gross tonnage can vary widely with the same displacement. A Great Lakes bulk carrier has width constraints because of the need to traverse locks. An ocean grain carrier does not and will have a different displacement to length ratio but can still have the same displacement as the bulk carrier. A drill ship can have the same displacement and look completely different from the others. Gross tonnage is also deceptive. A cruise ship with 50,000 tons displacement may have a Gross Rated Tonnage of 150,000 tons but an aircraft carrier has a closer ratio.
The same conditions can apply to old ships. Clipper ships were about 180 ft long with a displacement of 2000 to 2500 tons and a 74 gun Ship of the Line was about the same length with a displacement of 3000 tons.
So, I don’t know but perhaps I have enlightened you a little. I hope so.
