General Math Sucks: Cargo Containers

[Rigel Stardin]

Need some to check my math here.

Ok, the standard Cargo Container is 3m by 1.5m by 3m. Which because of the deck height of 3m, I shrank to 2.7m based off real world examples so you could get it into a cargo bay without raising the roof. So a Cargo Container should be 3m by 1.5m by 2.7m that gives you 30 cm of clearance. So here comes the math:

3*1.5*2.7= 12.15 cubic meters.
Go to the interweb look up what is the weight of 1 cubic meter is kilograms. It turns out to be 1,000 Kg
Times 1000 by 12.2 (rounded up) 12,200 kilograms
Go back to the interwebs to check what a kilogram is in pounds. It 2.2 pounds which I knew but had to double check myself.
Fancy online caculator says it's 26896.396 pounds.
(BLINK, BLINK, muttering to myself 'that can't be right') that would mean a 1 Ton Cargo Container could carry 13.5 tons of cargo (Math: 26896.396 divded by 2,000 pounds)

So I double check myself again.
A single square is 1.5m by 1.5m by 3m =6.75m or 6.75 cubic meters.
Times that by 2 an it 13.5 cubic meters.
Times that by 1,000 kg and it becomes 13,500 kg .
Use fancy Caulator again, it says 29762.405 Pounds
Divide 29762.405 by 2,000 = 14.88 Tons

So, I go metric. Set to Pounds to Metric tons
Fancy Caculator: 12.2 Tons
13.5 Tons

So the math is telling me a 1 Ton Cargo Container can carry between 12.2 and 13.5 Metric tons of cargo?

SO either I'm wrong or the game system is? Because, isn't a Ton in game 13.5 Cubic Meters?

 

[Condottiere]

Containerization is likely to be standardized, though you might have a variety of regional, megacorporation, and Imperium standards.

For a one tonner, maybe 2.41 metres cubed?

 

[tjoneslo]

You are confusing displacement tons ( a measurement of volume) with mass tons (a measurement of mass).

 

[Rigel Stardin]

I've been looking at shipping containers online. An this is a quote from Goo:

"When stacking shipping containers, it's important to know how much weight they can withstand and how many containers can be stacked on top of each other. In general, a 20' container may weigh up to 5000 pounds and hold up to 60,000 pounds. A 40' container may weigh up to 10,000 pounds and hold up to 120,000 pounds. Jun 23, 2021"

A 20' container is about 6 meters. 1 ton container is ruffly 1/6 the weight. So math again:

5000 pounds divide by 6 = 833.33 pounds
60,000 pounds divide by 6 = 10,000 pounds

That's +/- 5 tons per container.

I've done the math using ruffly the same scale as a modern container and got 4.3 tons cargo for a 1 Ton Container. That's still a big discrepancy between the game and the math.

 

[MichaelSTee]

>> what is the weight of 1 cubic meter is kilograms. It turns out to be 1,000 Kg
1,000 kg would be the weight of 1 cubic meter of water, not necessarily of other materials. (That's how the metric system gets its weights, by using the weight of water for standard volumes.) As tjoneslo says, you are confusing units of volume with units of mass.

 

[AnotherDilbert]

So the math is telling me a 1 Ton Cargo Container can carry between 12.2 and 13.5 Metric tons of cargo?

SO either I'm wrong or the game system is? Because, isn't a Ton in game 13.5 Cubic Meters?

You are completely correct; a "displacement ton" is ~14 m³, and can contain in the region of 14 tonnes (i.e. metric) of stuff. Or more, metals are often in the region of 5-10 tonnes per m³. Machinery, such as starship drives, are often in the 25-30 tonnes per displacement ton range.

One ton (displacement) isn't at all the same as one ton (mass). Welcome to Traveller!

[Except for fuel; one ton (displacement) of fuel (liquid hydrogen) has a mass of one ton (mass). This is kind of the definition of the displacement ton.]

 

[Rigel Stardin]

I'm look for a realistic value for a one ton cargo container. Just looking at the real world container, the inside of a container is .2m smaller that outside. 3m by 1.5 by 2.7m Container inside would be 2.8m by 1.3m by 2.5 or 9.1 cubic meters of interior space. Let's say, I have a 100 kilogram box that is 1 cubic meter, that's 900 Kg. Double that, 1,800 . Triple that 2,700 and so on. It all depends on how much the container can handle before it breaks (crushes in this case).

I've worked around large object for all my life being a welder. I know the difference between practical space and space by volume. You can fit 9,000 lbs cylinder ( 4090.9 kg) into standard box truck and not much else. If you're using a tractor trailer you fit three, and have room for more. It all depends on how much a vehicle can carry or pull.

Using the Goo Statement: the ratio between cargo and container is 12 to 1. If I were to use that as cargo capacity for 1 ton Container that would be 12 Tons. So, it's close to the math I am using which is causing the confusion, I'm having. Realistic 4.3 Tons sound about right and I can even explain why it only 1,000 credits is the standard rate.

By the way, in my studies of this little container thing, the 20' container can carry more than the 40' for reasons, mostly likely length and structural details.

 

[Rigel Stardin]

Here is a little visual aide to show the problem in my head.

​

 

[willtron3030]

I'm trying to find the original thread, but someone once quoted RAW saying that cargoes (breakbulk or containerized) were 1 ton mass per dT. That implied a lot of wasted volume. I remember making a conscious choice to ignore it IMTU.

 

[Rigel Stardin]

I always thought cargo was sold by floorspace? So it didn't matter if you had something 3 meter tall or not.

 

[tjoneslo]

According to GURPS Far Trader (p56), and several online sources, a standard 2 dton container (being about 3m x 3m x 3m), is rated to carry 12 metric tons of mass. A 4 dton container (3m x 3m x 6m) is rated for 24 metric tons of mass. An 8 dton container (3m x 3m x 12m) is rated to hold 48 metric tons.

The largest size is about 40 feet long, and matches the size of the container you see on the large container ships or being loaded/unloaded at sea ports in any number of pictures and videos.

 

[Dragoner]

Relevant wiki for cargo containers:

Twenty-foot equivalent unit - Wikipedia

en.wikipedia.org

So regular shipping container is 2.45 dtons, game wise, a 2 teu, bigger intermodal container, like seen on tractor trailers is 5 dtons. Weight or mass is another issue, generally with spacecraft I assign 0.25 to 0.33 metric tonnes to a cubic meter, which comes out to 3.375/4.455 metric tonnes mass, per game dton for a long ton, a US short ton is 0.91 of a metric tonne (3.71/4.90).

Now to find material mass, one has to look at specific gravity:

Specific Gravity materials, gasses and liquids Table Chart and Equation

Specific gravity of various materials, gasses and liquids. Specific gravity is a measure of the relative density of a substance as compared to the density of water at a standard temperature.

www.engineersedge.com

Just get your metric tonnes per cubic meter from the table as with si, one cubic meter of water is one tonne.

 

[mike wightman]

RAW in CT - one cargo ton can be a maximum of 1000kg but he space allocated is 14 cubic metres.

So the shipping container itself can be a bit less than 14 cubic metres but hold a maximum of 1000kg.

Note that this 1000kg should include the mass of the container and the goods inside said container.

 

[AnotherDilbert]

CT Striker, Book 3, p10:
A. Weight: Vehicle weight is determined by adding together the weights of all the components. The vehicle is assumed to be carrying a full load of ammunition and a full load of cargo, at 1 ton per m³.

Later used explicitly for starships in MT and TNE.

MT RM, p85:
To compute the average weight of a full cargo hold, multiply the volume of the cargo hold in kiloliters by 1000 kg (one metric ton).

1 kilolitre = 1 m³, so 1 tonne/m³.


Cargo is limited by volume and mass; You can't push in more volume than the size of the cargo bay, and not more mass than the design constraints allow.

Look at a van, it has a max payload size and a max cargo mass.
Look at a railway car, it has a max physical size and a max cargo mass.
Look at a container, it has a standardised size and a max cargo mass.

If you are shipping feathers, the max mass rating does not matter, only the max volume rating.
If you are shipping lead, the max volume rating does not matter, only the max mass rating.
Does that make sense?

Traveller uses 1 tonne per m³ for simplicity. We could use complicated calculations and arrive at, say, π/3.5 tonnes per m³ but the difference is small and not all that relevant to a 10 000 tonne starship.

 

[AnotherDilbert]

So the shipping container itself can be a bit less than 14 cubic metres but hold a maximum of 1000kg.

I hope you mean a "1 displacement ton" container can be a max of 14 tonnes = 14 000 kg?

No, sorry, I remember we have had this discussion before, devolving into how we evaluate an example far from the starship design chapter, that was presumably left unconsidered and unchanged between the '77 edition and '81 edition.

 

[Rigel Stardin]

@tjoneslo Thanks, that helped allot, that easy my mind allot, I thought the Math was screwing with my head.
@Dragoner Gravitronic wipes out concerns about weight on ship.

 

[Spinward Flow]

Cargo Container (travellerwiki link)

 

[Condottiere]

There are two ways to look at this:

1. Does propulsion depend on weight or volume?

2. Can the floor handle the heavier interpretation?

 

[Rigel Stardin]

The only reason I'm looking into this is because, I need to know the actual weight a container can carry for a story I'm writing. Saying, a ship has a 60 ton cargo bay by floor space does not equate to the maximum load. Nor would I consider it a game change either, since ship architects would have a chart that tell them what sort of mass could be carried in such a space. Thus, the can calculate that into the design of the engine and ship structural element.

I use displacement to determine how many light years one of my ship travel. in the case of cargo, I use floor space in determining the added mass to the ship. The same goes for fuel, I'm not worried about the volume of liquid hydrogen when it turns into a gas just the weigh,. each of these two elements. I figure or explain it away by saying "the naval architects have consider these matters and installed the proper equipment and component necessary to run a 400 tons starship.'

I'm just looking a quasi-realistic reason to keep 100 crewmen/women to keep 180 day in space with only 60 tons of floor space. Knowing a cargo container carries more than what is it's volume on the floor space, helps explain that.

 

[Condottiere]

In theory, the gravitation can be varied on the floor, so the weight the floor has to bear can vary.

And what happens with the shipboard swimming pool?


Or the entire crew is aquatic?