What is a Ton in Boat Weight

[boatpoker]

Originally Posted by jmschmidt

Simply put it is a fictitious formula that calculates the physical volume of a vessel's theoretical capacity to hold persons and or cargo which it further converts to "Tons."

There is nothing fictitious about it and there is not mention of "persons" in IMO measurement rules. There are a number of precise mathematical formulas involved including the likes of Simpsons Rule #3

The "simplified method" used for some small pleasure craft is pretty inaccurate as there is no measure for hull taper or curvature and no contra. The powers that be don't really care about small pleasurecraft. In many jurisdictions (including Canada) the simplified method is not permitted on some types and sizes of pleasurecraft but the methods and measurements for an Appointed or IMO recognised Measurer are very precise and standard around the world. The are some "optional" contra on pleasurecraft that can make a bit of a difference between measurers depending on what they note as contra.

[boatpoker]

I send this out with every pleasurecraft tonnage measurement I do as I got frequent complaints that my numbers did not jive with the sales brochure.

TUNNAGE - A VERY SIMPLIFIED EXPLANATION

First of all GROSS TONNAGE and NET TONNAGE have absolutely no relationship to displacement or weight of a vessel. The length of the vessel for tonnage measurement is not the actual length of the vessel. Tonnage is a measure of volume, not weight. Measurements that you will see on your tonnage certificate will bear little resemblance to the measurements on your sales brochure just one example .... Tonnage measurement length is from the inner part of the stem to the forward face of the rudder stock.

A little history …. In the days of Queen Anne all harbours in England were owned by the local aristocracy until Annie decided to take them over. Now that the Crown owned all the harbours a uniformed tax was needed to run them.
One of the most common imports into England at the time were casks of port wine from Portugal. These casks were a specific size and were known as “tuns”. In 1347 a tax of 3 shillings per tun was imposed in England and this was called "tunnage." A ship's size became known by the number of “tuns” (casks) it could carry, and the word tun eventually became “ton “ It was found that if you took the length x the breadth x the depth of the hold under the deck and divided by 100 it was close to the number of casks. That is where we get the "Measurement ton" of 100 cubic feet per ton and ships were measured and taxed according to how many “tuns” of port wine they could carry.

Tonnage measurements around the world are now performed in accordance with methods determined by the IMO (International Maritime Organization) which is the UN body which regulates ColRegs.

In Canada Tonnage Measurers such as myself hold government appointments for this task. Any appointed tonnage measurer should be accepted in any of the IMO signatory countries as we all work from the same IMO regulations.

GROSS TONNAGE - The entire internal cubic capacity of the ship expressed in tons of 100 cubic feet to the ton. This is a fairly complicated measurement involving hull block coefficients (to measure hull curvature) and Simpson’s Rule # 3. The dimensions used by tonnage measurers are not at all what you are used to.

NET TONNAGE – Is gross tonnage less contra (non cargo space) and is obtained by deducting from the gross tonnage, peak and other tanks for water ballast; spaces above the uppermost continuous deck, such as: open forecastle, bridge and poop, certain engine, light and air spaces, domes of skylights, condenser, anchor gear, steering gear, wheel house, galley and cabins for passengers crew and navigating spaces and an allowance for the space occupied by the propelling machinery. Gross less Contra = NET Tonnage and is used for the calculation of tonnage taxes and the assessment of charges for wharfage and other port dues for commercial vessels.

DISPLACEMENT, LIGHT - The weight of the ship excluding cargo, fuel, ballast, stores, passengers, crew, but with water in boilers to steaming level.

DISPLACEMENT, LOADED - The weight of the ship including cargo, passengers, fuel, water, stores, dunnage and such other items necessary for use on a voyage, which brings the ship down to her load draft.

[Wotname]

Originally Posted by Adelie

1 cubic yard = 27 cubic feet.
At 64lb/cubic foot for seawater that’s 1728lb.
Short ton is 2,000lb, metric tonne is 2200lb and a long ton is 2240lb.

Not quite - a tonne is 1000kg which is close to 2204.6226218lb.

[Moontide]

Have not read all the prior posts so guessing maybe this info already up, but if not - if talking about USCG documentation the "tons" have nothing to do with the weight of the boat. It is a calculation of the load carrying capacity.

[Cheechako]

Originally Posted by Orion Jim

It is a measurement of internal volume, nothing to do with the weight of the vessel. When you say “the documentation” I presume you are referring to your USCG document.

https://www.dco.uscg.mil/Our-Organiz...tation-Center/

https://en.wikipedia.org/wiki/Gross_tonnage

YEP. ^^
And it's a gross cubic measurement, not the amount of volume your boat would really hold....

[TrentePieds]

No, Moontide - it's not. I think you meant the right thing, but you misstated it :-)

"Load carrying capacity" of a vessel, big or small, is a function of its "tons per foot immersion" if it's a grown-up vessel, or "pounds per inch immersion" if it's a baby vessel like yours and mine.

both of those metrics are a function of "Displacement".

"Tonnage", of which there are many kinds, is a volume measure. It's all carefully explained up-thread. Obviously, a ship whose cargo space, measured in "tons" of 100 cubic feet, is filled with feathers will "immerse" less than the same ship with the same "tons" of cargo space filled with lead. That is why you see "load lines" painted prominently on ships' sides. The "load line" governs. If the ship has to sail with empty cargo space, so be it. That is why there are different "freight rates" for different cargos.

A bit oversimplified, but you get the drift :-)

All the best.

TrentePieds

[boatpoker]

Originally Posted by Moontide

Have not read all the prior posts so guessing maybe this info already up, but if not - if talking about USCG documentation the "tons" have nothing to do with the weight of the boat. It is a calculation of the load carrying capacity.

Not quite. It is a measure of volume. "load carrying capacity" is a distant cousin. i.e. Your boat may be able to carry a ton of lead but a ton of feathers would bury it.

See post #47

[boatpoker]

Originally Posted by TrentePieds

No, Moontide - it's not. I think you meant the right thing, but you misstated it :-)

"Load carrying capacity" of a vessel, big or small, is a function of its "tons per foot immersion" if it's a grown-up vessel, or "pounds per inch immersion" if it's a baby vessel like yours and mine.

both of those metrics are a function of "Displacement".

"Tonnage", of which there are many kinds, is a volume measure. It's all carefully explained up-thread. Obviously, a ship whose cargo space, measured in "tons" of 100 cubic feet, is filled with feathers will "immerse" less than the same ship with the same "tons" of cargo space filled with lead. That is why you see "load lines" painted prominently on ships' sides. The "load line" governs. If the ship has to sail with empty cargo space, so be it. That is why there are different "freight rates" for different cargos.

A bit oversimplified, but you get the drift :-)

All the best.

TrentePieds

Ya beat me to it by 1.25 seconds

[Cheechako]

This discussion was about documentation, , my understanding is in the context the USCG uses it ....it IS based on their measurement of load carrying capacity. Not a technically correct measure. Granted....It's way off the way they do it, they simply use a big rectangular cubic shape. But that's how USCG Documentation started for commercial carrying vessels. Then it got applied to pleasure craft.

[Bowdrie]

This has all been great.
I gotta dig out my document and find out how many "tuns" of Port Wine I can carry on my next voyage.
I suppose that the amount of Port Wine is based upon sailing on the Port Tack, perhaps it'll change on Starboard Tack?
It's all so confusing, probably better off drinking the Port Wine before I leave Port.

[boatpoker]

Originally Posted by Cheechako

This discussion was about documentation, , my understanding is in the context the USCG uses it ....it IS based on their measurement of load carrying capacity. Not a technically correct measure. Granted....It's way off the way they do it, they simply use a big rectangular cubic shape. But that's how USCG Documentation started for commercial carrying vessels. Then it got applied to pleasure craft.

You appear to be talking of the "Simplified Method". There is a bit more to it than that. Note the issue of foreign travel in section 5 of the USCG Tonnage guide below.

5. MEASUREMENT SYSTEM APPLICABILITY
A vessel is eligible to be measured under the Simplified measurement system if it is either: 1) under 79 feet in length; or 2) a non-self-propelled or recreational vessel. Some vessels that are 79 feet or over in length may also require measurement under the Convention measurement system. This includes vessels that engage on foreign voyages, as well as recreational vessels that engage on voyages outside the Great Lakes and have keel laid dates after December 31, 1985. The Convention measurement system does not apply to vessels less than 79 feet in length, whereas vessels of any length can be measured under the Standard and Dual systems. For complete requirements on tonnage measurement system applicability, refer to Navigation and Vessel Circular (NVIC) 11-93, Applicability of Tonnage Measurement Systems to U.S. Flag Vessels

[boatpoker]

For anyone interested. here is the USCG Simplified Tonnage Guide.
https://www.dco.uscg.mil/Portals/9/D...-09-123757-680

I can't attach the Formal method guide because it is huge and my Wifi is not up to it
and there is no online link I know of.

[StuM]

Originally Posted by Moontide

Have not read all the prior posts so guessing maybe this info already up, but if not - if talking about USCG documentation the "tons" have nothing to do with the weight of the boat. It is a calculation of the load carrying capacity.

If talking about USCG docuemtnation, there are no "tons" involved!
"Tonnage" has nothing to do with the word "ton", it's an entirely different thing - it just sounds similar (which is what causes a lot of the confusion including for the OP ).

[El Pinguino]

Originally Posted by StuM

Using the term "Displacement tonnage" is what causes a lot of the confusion. Use of the expression is a common error.

Tonnage is not weight or displacement. As others have said it is a notional volumetric measure and should not be confused with or used in conjunction with the words "Displacement" or Weight" . Gross/Net Tonnage etc should never have the word tons or tonnes attached to them. Tonnage is a dimensionless quantity - it does not have units.

And strictly speaking, displacement and weight are not synonymous either.
Displacement varies with temperature and salinity of the water that the vessel is in. Weight doesn't.

Well, this is what Bonwick and Steer have to say about it in ' Ship's Business' , 5th edition.
They devote 4 pages to assorted gross, net, Panama, and Suez tonnages and sure enough the words ton and tonne are never mentioned - only tonnage.

However they do have this to say
'So far tonnage has been considered in relation to the official measurement of ships for the purpose of registration and the assessment of dues, but this must not be confused with the real meaning of tonnage, expressing weight, which, it has been advocated from time to time, should be be employed for assessing dues since it is more closely associated with the earning capacity of ships. The two tonnages which are used to describe the weight of ships or the cargoes they can carry are displacement and deadweight, which are defined as follows:

Displacement. The unit of measurement is one long ton, or 2240 lb. avoirdupois....

And so on it goes. The unit of measurement has changed since 1963.

And there is also this. As you can see DWT means Dead Weight Tonnage.
https://www.handybulk.com/what-is-de...t-tonnage-dwt/

Not mentioned so far in this thread is revenue tons. This relates to how freight is charged for general cargoes. Heavy cargo - zinc ingots, steel, etc would be charged by the actual weight. Light cargo - esparto grass, crates of cane furniture, etc would be charged by volume.
A bit more here
https://www.globalnegotiator.com/int...evenue-ton-rt/

[El Pinguino]

Some may find this of interest.
As stated above in various posts all sorts of dues are charged on net tonnage. Effectively the gross tonnage less deductions or the enclosed volume of the ship below the maindeck.
Ignoring open and closed shelterdeckers that effectively the bit of the boat below the top deck.

Early on - about 50 years ago - 'Big' container ships had very high freeboard as you can see here https://www.shipspotting.com/photos/2434309

Then the beancounters got involved and worked out a handy way of saving money which is why container ships now look like this https://www.shipspotting.com/photos/3374238


Note that the newer ship, while having only about double the gross tonnage of the older ship, has three times the summer deadweight tonnage.