Probably a stupid question but for some reason I've started wondering about displacement versus boat weight. Checked the archives and couldn't find the answer to the specific question so here it is. Is the displacement of the boat and the weight of the boat the same thing? I don't think so because as I recall displacement refers to the amount of water displaced by the boat's hull in the water and therefore the bouyancy and it doesn't seem that would be the same as the weight of the boat, but I could and probably am wrong. Also, my boats listed weight is 13,000lbs. but the CG documentation list it a 13 tons? What's up with that? Again, probably a stupid question and the answer doesn't really matter I just like to fill my head with useless minutia sometime.
Displacement Question
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That number on the Documentation is not weight
The weight of a volume of water equal to the amount of water you displace is the weight of the boat. Salt water and fresh water have a different weight per cubic foot so your waterline changes when you move from the sea into a fresh water river for instance. Seawater is 64 pounds a cubic foot and fresh water is 62.4 pounds. When you design a boat you calculate the volume under water and multiply that times the weight of water to figure the displacement. After adding up the weight of everything you are designing into the boat you subtract that from the displacement and what’s leftover is the amount of ballast you need to add for the boat to float on her lines. The number on your documentation is not the weight of the boat but instead a number leftover from the old day and a rule for determining volume of a commercial boat for tax reasons. The number really represents how many turns (barrels) will fit in the hold and it’s a measure of volume with the name for barrels getting corrupted into tons over time. As boats evolved over time a lot of measurement rules have come and gone for taxing and racing purposes and they have greatly influenced the design of boat. You often hear of CCA, MORC, IOR or Tames Measurement Tonnage and these are just a few of the rules developed over the years and each one has a unique style of boat associated with. All the best, Robert Gainer
The weight of a volume of water equal to the amount of water you displace is the weight of the boat. Salt water and fresh water have a different weight per cubic foot so your waterline changes when you move from the sea into a fresh water river for instance. Seawater is 64 pounds a cubic foot and fresh water is 62.4 pounds. When you design a boat you calculate the volume under water and multiply that times the weight of water to figure the displacement. After adding up the weight of everything you are designing into the boat you subtract that from the displacement and what’s leftover is the amount of ballast you need to add for the boat to float on her lines. The number on your documentation is not the weight of the boat but instead a number leftover from the old day and a rule for determining volume of a commercial boat for tax reasons. The number really represents how many turns (barrels) will fit in the hold and it’s a measure of volume with the name for barrels getting corrupted into tons over time. As boats evolved over time a lot of measurement rules have come and gone for taxing and racing purposes and they have greatly influenced the design of boat. You often hear of CCA, MORC, IOR or Tames Measurement Tonnage and these are just a few of the rules developed over the years and each one has a unique style of boat associated with. All the best, Robert Gainer
No, you are correct. The displacement hull will always displace (push away) water equal to its total weight. If its total weight is equal to the weight of the water that it's displacing, the boat will float. If, on the other hand, its weight is more than the weight of the water that it displaces, the boat will sink. In other words, if the average density of the total volume of the boat is greater than one, it will sink and vise versa. Weight is the measure of mass the object has, density is how spread out that weight is. An aircraft carrier floats because its total weight is equal to the total weight of the water that it displaces. CG documentation uses some rediculuos formula for figuring the tonnage of a vessel. It has little to do with your boats actual weight in tons.
A minor point of no importance
Alan, A minor point of no importance I am sure but both a displacement hull and a planing hull will displace an amount of water equal to their weight while at rest. A planning hull unlike a displacement hull will use dynamic forces to replace some of buoyancy’s force while moving. Not sure if I phrased that right but you get the idea. All the best, Robert Gainer
Alan, A minor point of no importance I am sure but both a displacement hull and a planing hull will displace an amount of water equal to their weight while at rest. A planning hull unlike a displacement hull will use dynamic forces to replace some of buoyancy’s force while moving. Not sure if I phrased that right but you get the idea. All the best, Robert Gainer
tonnage....(useless, but interesting)
from Ocean Navigator.....link is to full story 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.
from Ocean Navigator.....link is to full story 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.
Steve thanks but.............
One little clarification on the link. ......."The nautical mile is one minute of arc on a great circle of the earth". But only on the lattitude but not the longitude. All lattitude lines are parrallel and therefore 'the same'. The longitudeinal lines converge at both poles and fatten up at the equator and therefore are not the same distance at different lattitudes. This is very important when using calipers to mark or measure distances on a chart. You set your calipers on the vertical longitudinal 'minutes of arc' on the left or right hand side of your chart. Tony B
One little clarification on the link. ......."The nautical mile is one minute of arc on a great circle of the earth". But only on the lattitude but not the longitude. All lattitude lines are parrallel and therefore 'the same'. The longitudeinal lines converge at both poles and fatten up at the equator and therefore are not the same distance at different lattitudes. This is very important when using calipers to mark or measure distances on a chart. You set your calipers on the vertical longitudinal 'minutes of arc' on the left or right hand side of your chart. Tony B
Another interesting fact...
Everywhere you read you come up with "The total internal volume of a ship is its gross tonnage, and if we subtract all the volume not used for cargo, we get the net tonnage." For some reason unbeknownst to be, this does not apply to the oil fields of the Gulf of Mexico. They seem to have their own little Kingdom. If your look at every 'crew boat' and 'utility boat' in the oil industry in the Gulf, from 86' LOA to 135' LOA with all varying beams, they are all documented as 99.8 Gross Tons so as to allow a 100 Ton Master (Capt. License) to run the boat. I KNOW THIS FOR FACT. Crew Boats and Utility Boats, both carry passengers and cargo as well as cargo fuel and cargo water. Somehow thay have been able to manipulate the inside cargo space and passenger space to come up with the Tonnage. What we are referring to here as Net Tons, the oil industry has been able to get classified as Gross Tons. BTW, For all of you Capts and wanna be's, the shortage of 100 Ton Masters has driven the wages up to $500/Day
Everywhere you read you come up with "The total internal volume of a ship is its gross tonnage, and if we subtract all the volume not used for cargo, we get the net tonnage." For some reason unbeknownst to be, this does not apply to the oil fields of the Gulf of Mexico. They seem to have their own little Kingdom. If your look at every 'crew boat' and 'utility boat' in the oil industry in the Gulf, from 86' LOA to 135' LOA with all varying beams, they are all documented as 99.8 Gross Tons so as to allow a 100 Ton Master (Capt. License) to run the boat. I KNOW THIS FOR FACT. Crew Boats and Utility Boats, both carry passengers and cargo as well as cargo fuel and cargo water. Somehow thay have been able to manipulate the inside cargo space and passenger space to come up with the Tonnage. What we are referring to here as Net Tons, the oil industry has been able to get classified as Gross Tons. BTW, For all of you Capts and wanna be's, the shortage of 100 Ton Masters has driven the wages up to $500/Day
Think what it would cost if...
the industry couldn't use "100-ton" masters to run their boats, but needed a higher grade of license? I bet the 100-ton guys are 10 times more abundant than the higher grades (just a guess), and they are in short supply? BEO
the industry couldn't use "100-ton" masters to run their boats, but needed a higher grade of license? I bet the 100-ton guys are 10 times more abundant than the higher grades (just a guess), and they are in short supply? BEO
Bavaria
Bavaria: Yes, the 100-ton guys are 10 times more abundant than the higher grades and they are in short supply. There is also a demand for 500 Ton tickets also, but not as much as the 100 Ton because the 100 ton vessel are the most common and the most versatile. Now, I'm not trying to start a war here or intentionally belittle the occupation, but truth be known, most 100 masters in the gulf are functional illiterates with maybe an 8th grade education. The initial shortage came many years back when the boat companies sent and paid for their deckhands to go to sea schools and get their license. Most of them, by a vast majority, failed to get their license. Now that the CG no longer administers the tests, the 'sea schools' do, and advertise to the boat companies they guarantee everyone passes. Well, thats like hiring the fox to guard the hen house. Anyway, the large amount of construction and reconstruction after hurricanes Katrina and Rita created a new shortage. Tony B
Bavaria: Yes, the 100-ton guys are 10 times more abundant than the higher grades and they are in short supply. There is also a demand for 500 Ton tickets also, but not as much as the 100 Ton because the 100 ton vessel are the most common and the most versatile. Now, I'm not trying to start a war here or intentionally belittle the occupation, but truth be known, most 100 masters in the gulf are functional illiterates with maybe an 8th grade education. The initial shortage came many years back when the boat companies sent and paid for their deckhands to go to sea schools and get their license. Most of them, by a vast majority, failed to get their license. Now that the CG no longer administers the tests, the 'sea schools' do, and advertise to the boat companies they guarantee everyone passes. Well, thats like hiring the fox to guard the hen house. Anyway, the large amount of construction and reconstruction after hurricanes Katrina and Rita created a new shortage. Tony B
I guess it's just a different type...
of "Master's" Degree, one that doesn't require a great deal of higher education, or even high school. Sounds like it pays well, though. BEO
of "Master's" Degree, one that doesn't require a great deal of higher education, or even high school. Sounds like it pays well, though. BEO
More web trivia on tonnage..
From a web site: enjoy! "Tun barrel" is redundant. By definition, a "tun" is a barrel to hold liquids and appears in Old English as "tonne." There are related words in the various languages over the Celtic range, so it is believed the origin may be Celtic (defined in this context as a group of related cultures and languages spread from Asia Minor to Ireland). The tun of 252 U.S. gallons (the same as the Elizabethean wine gallon) is an English measure of approximately 2 butts or 4 hogsheads or 8 barrels (Elizabethean wine measure). The Danish "tonde" is about 139 liters, which makes it roughly equivalent to the English wine barrel (119.24 liters). It is related to one of the meanings of the German "Tonne" meaning "barrel." The "tonelada" is a traditional Spanish and Portuguese weight measure standardized from their "tuns." The Spanish "tonelada" is 2000 Spanish "libras" (about 919.9 kg). The Portuguese "tonelada" is 1728 Portuguese "libras" or "arratels" (about 793.15 kg). Variations between Spanish, Portuguese, Genoese (there were several colonies in Spain and engaged in trade) and Moorish measures may be part of the problem with the wide variation in Spanish measures of the period. A number of Spanish measures(especially land measures) were standardized in 1568, but the old measures continued to be used.
From a web site: enjoy! "Tun barrel" is redundant. By definition, a "tun" is a barrel to hold liquids and appears in Old English as "tonne." There are related words in the various languages over the Celtic range, so it is believed the origin may be Celtic (defined in this context as a group of related cultures and languages spread from Asia Minor to Ireland). The tun of 252 U.S. gallons (the same as the Elizabethean wine gallon) is an English measure of approximately 2 butts or 4 hogsheads or 8 barrels (Elizabethean wine measure). The Danish "tonde" is about 139 liters, which makes it roughly equivalent to the English wine barrel (119.24 liters). It is related to one of the meanings of the German "Tonne" meaning "barrel." The "tonelada" is a traditional Spanish and Portuguese weight measure standardized from their "tuns." The Spanish "tonelada" is 2000 Spanish "libras" (about 919.9 kg). The Portuguese "tonelada" is 1728 Portuguese "libras" or "arratels" (about 793.15 kg). Variations between Spanish, Portuguese, Genoese (there were several colonies in Spain and engaged in trade) and Moorish measures may be part of the problem with the wide variation in Spanish measures of the period. A number of Spanish measures(especially land measures) were standardized in 1568, but the old measures continued to be used.
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