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: 9/5/2007 Subject: Maritime shipping
Question 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.
---------- FOLLOW-UP ----------
QUESTION: My question was too broad, so your answer is well put. I was focusing primarily on merchant ships of the 17th century, which I hoped might share similarities in length/beam/draft ratios. When, for example, it is noted that the Frontenac, one of the first sailing vessels built on the Great Lakes, was of ten tons burden, it means little to me. I can figure out a volume of ten tons displacement (at 64 lbs per cubic foot), but the burden doesn't include the weight of the ship, which also must be displaced. But you are, no doubt, correct that there is no way to approximate a vessel's size by its tonnage rating. Perhaps if I knew the volume of a tun of wine... (Thank you very much for your excellent answer.)
Answer Randall,
Your question was not too broad; I recognized that you were referring to old ships. I think my answer was too broad. You see, I have the same problem but it may be some milder than yours as I have crewed on sailing ships and have some insight as to what they can hold. In my previous answer, I mentioned the tonnage of the Elissa in tons but I also know that her hold is large enough to hold 6 of the large ocean containers. I too have difficulty with such things as your ten tons displacement. These Great Lake boats were about 65 feet long.
A tun is a wine barrel that contains 252 gallons and barrel plus contents weighed 2,240 pounds which probably the origin of the long ton, which is used in ocean commerce, as opposed to the ton of 2,000 pounds.
Let’s take this part, “a volume of ten tons displacement”, and this part, “doesn't include the weight of the ship, which also must be displaced”. You are obviously aware that in order to float, an object must displace more weight of water than its’ own weight, and a ship must displace more weight of water than its’ own weight plus the weight of the cargo. A barrel filled with fresh water will float in salt water but fill it with sea water and it will sink. That, by the way, is how sailing ships replenished their water during their voyages. The barrels were rolled to the beach and towed to the ship.
Even though displacement of water (Eureka!) had been discovered and utilized for centuries I don’t think it was completely understood. All references to ships before the late 19th century referred to tonnage as cargo weight, or as a measure of volume, rather than the weight of displacement. As I mention, it was understood but not measured.
In my library are these figures;
Hanse Cog, 14th century: tonnage 36, displacement 91, length 60 ft
Elizabethan merchant ship, tonnage 300, displacement 800, length 120 ft
This is an estimate as the ship is still being restored
Mary Rose 17th century warship: tonnage 300, displacement 700, length 125
Elissa, 19th century merchant ship: tonnage 430, displacement 620, length 150
This looks to me as if ship design and/or construction were getting better as the ratio of cargo tonnage to displacement is rising. Since ships were described by what they could carry rather than what they displaced it may be that your Great Lake 10 Tonner displaced some 1.5 times its’ tonnage. And since it would be shaped like a schooner rather than a Cog that would put it in the 55 to 65 foot range. If you have access to an interlibrary loan, here is a book on the subject.
The Birth of Naval Architecture in the Scientific Revolution, 1600-1800 by Larrie D. Ferreiro http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=11020&mode=toc
I have read it, however, and while it goes into stability it, strangely, does not discuss the progress of thinking on ship design.
Once again, no help!
Randall,
I have looked in a reference book on old gaff rigged vessels and have come up with some numbers that seem to point to displacement as 1.5 times gross tonnage, or put another way, tonnage is 70% of displacement.
The boats I found are:
Cutter rigged fruit hauler Margraret, 1870
Disp 360, Tonnage 265, WL length 120 ft, Beam 24 ft, Draft 10 ft. Said to be a very wet sailer.
Cargo cutter J.N.R. , 1893
Disp 60, Tonnage 41, Length 58, Beam 18, Draft 6.5 ft. Loaded to 70 tons, draft 8 ft. Said to have only 1 ft free board at 70 tons.
Cargo cutter Eagle, 1703
Disp 90, tonnage 77, W.L.Length 53, Beam 18, Draft ?
By comparision, the huge racing cutters were 95 to 100 ft long and displaced 140 to 160 tons. They would have had a smaller ratio and most of the weight is in the keel. One, with 140 tons displacement had 80 tons of it in her keel!
More and more it looks to me as if the ratio of displacement to gross tonnages is wider now than in sailing days due to ligher hull weight allowing the displacement to be accomplished by hull volume as the ship is loaded. This would account for a modern ship having a net tonnage of three times its displacement.
Regards,
Carl
