Cargo Capacity and volume and mass

[Rupert]

That works, and gives roughly neutral buoyancy. It is way past overloaded for most wheeled and tracked ground vehicles in Mongoose Supplement 6 Military Vehicles, which cannot drive on normal roads past around 450kg/m^3, depending on the exact configuration.

1 tonne per m^3 will result in substantial empty space when carrying heavier things like mined ore, but will allow lighter things like manufactured goods to be stuffed in as tightly as can be managed. That is good information, thank you.

A historical rule of thumb for non-containerised general cargo was one ton per 100 cubic feet, or about a tonne per 3 m³.

For what it's worth, this is what GURPS Vehicles, and thus GURPS Traveller assumes. This gives about 5 tons (or tonnes, we're not being precise here) per displacement ton (which is also about 500 cubic feet, which is convenient given GURPS' insistence on using US customary measures).

I tend to look at the 1 tonne per cubic metre of Striker, MT, & TNE as a maximum safe load - more than that and the referee gets to start doing evil things to your ship. Likewise if a very high density load (gold, uranium, etc.) is stacked up in one spot, over-stressing the deck. Most cargo is going to be well under the density of water, just as most is today (note that a fully loaded 20-foot container is less dense than water).

 

[TamsinP]

The WW2 Liberty Ships had a cargo hold volume of 4380 NRT* (12,404 m^3) with a DWT of 10865 tons; if the hold is completely filled, that would be a maximum of 2.48 tons per NRT.

* 1 NRT = 100 cubic feet.

 

[whartung]

Well, according to this diagram:

Modern density is 336kg/m^3.

 

[Rupert]

The WW2 Liberty Ships had a cargo hold volume of 4380 NRT* (12,404 m^3) with a DWT of 10865 tons; if the hold is completely filled, that would be a maximum of 2.48 tons per NRT.

* 1 NRT = 100 cubic feet.

DWT includes fuel, water, etc. though (and they had a 1,700 ton fuel capacity). Also, their actual total cargo volume was 562,608 ft3 'grain' (i.e. for things like grain that flow into all the spaces between frames, etc.

 

[Rupert]

Well, according to this diagram:

Modern density is 336kg/m^3.

That's for a 40-foot container. The maximum gross weight for almost all ISO standard containers is about 30.5 tonnes, and the maximum per the ISO standard is 36 tonnes for all. Thus the smallest (the 20-foot) is the densest at ~940 kg/m^3 using exterior dimensions (which is what the carrier cares about).

 

[Condottiere]

 

[kilemall]

That's for a 40-foot container. The maximum gross weight for almost all ISO standard containers is about 30.5 tonnes, and the maximum per the ISO standard is 36 tonnes for all. Thus the smallest (the 20-foot) is the densest at ~940 kg/m^3 using exterior dimensions (which is what the carrier cares about).

So shipping black holes might be a problem.

 

[coliver988]

So shipping black holes might be a problem.

well, if they can open a wormhole they could be self-shipping!

 

[Badenov]

So, we have votes for 1000kg/m^3, 940kg/m^3, 336kg/m^3, 250kg/m^3, 100kg/m^3, and I think the liberty ship converted to about 1000kg/m^3 (unless I mathed wrong, a strong possibility). I think in space it doesn't matter and inertia has been handwaved. Because what you actually carry varies depending on what cargoes you roll, I guess there's no real way to plan ahead for the on-planet phase of cargo distribution. I had supposed it might be planned that structures are built to support 1000kg/m^3 even if the engine struggles to move it. But...

So, for ground vehicles, I used Mg1 Supp 6 Military vehicles to try to build an ordinary 40 ft cargo truck at TL7 (130m^3, box config). The ground pressure goes over 5 at about 466kg/m^3 (or about 45,600kg), Ground pressure 5 or above would require specially paved surfaces to drive on, so 465kg/m^3 is probably the max practical cargo density limit for that wheeled vehicle, though it's slowed significantly above 333kg/m^3, where the top speed is 59mph/94kph, and the cruising speed is 44mph/71kph, so it eats fuel at highway speeds. Noteworthy is that the engine is nearly half the size of the cargo bay, which we also know isn't at all normal, and as I say, is terrible on fuel at highway speed. Total cargo space is 98m^3, so about 7 dtons, and at 333kg/m^3, the cargo capacity is 32,634 kg.

So, I guess if you're carrying cargo to TL5-7 worlds, this might be how they carry off your cargo. At TL8, grav propulsion makes ground pressure a non-issue, but you still have the issue of power to mass ratio.

One more tangent I have explored is using a hybrid drive, adding a grav lifter plate to the wheeled chassis. This eliminates the speed penalty for overloading at a much cheaper price than full grav movement. And while you can't fly, you can get moving by wheels, then lift up and coast, which amuses me. Though I can totally see accidents caused by someone slamming on the brakes and forgetting they're 2 meters above the ground with no control. On the other hand, the ability to lift vertically gives a very clever way to avoid an accident on the ground. I would expect normal movement to be via road, with the lifter plates limited somehow to either only allow it to partially reduce the mass too save tire and road wear, or to allow a limited altitude for accident avoidance.

 

[TamsinP]

Liberty ships at 7176 GRT have a volume of 20,320 m^3 and a maximum (full) displacement of 14,474 tonnes, so that's 712kg/m^3. Empty, their displacement is 3605 tonnes, which is 178kg/m^3.

 

[infojunky]

Liberty ships at 7176 GRT have a volume of 20,320 m^3 and a maximum (full) displacement of 14,474 tonnes, so that's 712kg/m^3. Empty, their displacement is 3605 tonnes, which is 178kg/m^3.

Note I was going to say something along these lines as well.

Water at a density of 1000kg/m^3 is an incredibly heavy cargo, and needs special handling/containers. Flour and the like generally fall in the 500kg/m^3 range and also is one of the heavier palletized cargos to carry.

What I am saying is cargo Tonnage is generally limited by mass not volume. With 750kg/m^3 being the upper limit.

Note there are iron boats and tankers that can carry the same mass as the biggest Container ships, but whose gross volume is much less.

 

[TamsinP]

There are a lot of bulk cargoes with higher densities than water - ores, coal, gravel. Liquid cargoes ideally need to be split into several smaller tanks and/or have baffles installed to reduce free surface effect due to motion of the ship (note - solid cargoes can suffer free surface effect too, particularly if there is fine powder).

 

[Badenov]

There are a lot of bulk cargoes with higher densities than water - ores, coal, gravel. Liquid cargoes ideally need to be split into several smaller tanks and/or have baffles installed to reduce free surface effect due to motion of the ship (note - solid cargoes can suffer free surface effect too, particularly if there is fine powder).

I feel like that effect is minimized by whatever compensates for inertia in the ship as a whole. If a liquid or particulate solid felt no motion, there would be no free surface effect. I totally agree it would need to be taken care of in cargoes not propelled by grav/thrusters, but for spacecraft above TL9 and ground vehicles with grav propulsion (TL8+), inertia is someone else's problem.

 

[Rupert]

For what it's worth, here's a bit on cargo densities from Ship Construction and Calculations, 7th edition, by George Nicol (1942, Brown, Son & Ferguson):

Rates of Stowage

Cargo No. of Cubic Feet per Ton (Imperial - 2240 lbs)
Coal, Scotch 44
Coal, Welsh 40
Coal, Newcastle 44
Manchester Bales 50 (may go as high as 150)
Pig Iron 9
Alkali in Casks 47
Wheat 46 (varies from 40 to 52)
Flour 45
Maize 46
Barley 58
Oats 72
Tea 83-120
Raw Sugar in Baskets 50
Cotton, American 130
Cotton, Indian 60 (machine pressed)
Cotton, Egyptian 70-220
Jute 49-77 (the lower if strongly machine pressed)
Wool, undumped 235
Wool, washed & dumped 100
Wool, greasy & dumped 84
Potatoes 50
Bacon & Hams in cases 64
Peas and Beans 43-53
Beef, frozen & packed 90-95
Beef, chilled & hung 120
Mutton, New Zealand 105-110
Mutton, River Plate 115


For cubic metres per tonne, multiply cubic feet per ton by 0.0288.

Flipping these numbers to give density gives the following (in kg/m^3):

Coal, Scotch 817
Coal, Welsh 898
Coal, Newcastle 653
Manchester Bales 719
Pig Iron 3992
Alkali in Casks 764
Wheat 781
Flour 798
Maize 781
Barley 619
Oats 499
Tea 433 - 299
Raw Sugar in Baskets 719
Cotton, American 276
Cotton, Indian 599
Cotton, Egyptian 513 - 163
Jute 733 - 467
Wool, undumped 153
Wool, washed & dumped 359
Wool, greasy & dumped 428
Potatoes 719
Bacon & Hams in cases 561
Peas and Beans 836 - 678
Beef, frozen & packed 399 - 378
Beef, chilled & hung 299
Mutton, New Zealand 342 - 327
Mutton, River Plate 312


The standouts are uncompressed loose fibres like wool and cotton at the low density end, and pig iron at the high end. Even then the pig iron is only about half the actual density of iron, presumably due to air space between the ingots and lost volume in packing, etc.

This list suggests that the only things that are likely to be denser than water are bulk metals and compact machinery made of dense metals. If your ship isn't going to be carrying those assuming a density of 500-700 kg/m^3 should be fine.

 

[whartung]

For what it's worth, here's a bit on cargo densities from Ship Construction and Calculations, 7th edition, by George Nicol (1942, Brown, Son & Ferguson):

Had to double check this wasn't a @Timerover51 post. Right up his alley.

 

[kilemall]

Heres a more comprehensive list of commodities by cubic meter-

Mass, Weight, Density or Specific Gravity of Bulk Materials

Mass, or density, of over 80 different Bulk Materials from bauxite to zinc ore

www.simetric.co.uk

I’ve posted others that have converters built in, but darn if I can find it right now.

 

[Timerover51]

Had to double check this wasn't a @Timerover51 post. Right up his alley.

I will have to check on that, but you are correct. I just have not had a chance to weigh in on this as yet.

 

[coliver988]

I will have to check on that, but you are correct. I just have not had a chance to weigh in on this as yet.

I see what you did there...

 

[infojunky]

There are a lot of bulk cargoes with higher densities than water - ores, coal, gravel. Liquid cargoes ideally need to be split into several smaller tanks and/or have baffles installed to reduce free surface effect due to motion of the ship (note - solid cargoes can suffer free surface effect too, particularly if there is fine powder).

Yes and if you look at their main holds the are often less than half full volume-wise. Well sealed hatch covers are also standard, as the bulk of the cargo lays at or below the water line.... Draft is often the limiting size to how big they can be.

 

[TamsinP]

Yes and if you look at their main holds the are often less than half full volume-wise. Well sealed hatch covers are also standard, as the bulk of the cargo lays at or below the water line.... Draft is often the limiting size to how big they can be.

Routes and ports are the main determinant for the various ship dimensions, giving rise to many of the "Xxxxmax" ship classes (eg Panamax, Neopanamax, Chinamax). In most case that's draft, beam and length, but in a few cases (where they go under bridges) the height is also a factor.

For container ships, beam is a major limiting factor as the gantry crane needs to be able to reach containers on the offshore side; that's why you won't see the 24k TEU ships calling at many ports, as very few ports have cranes with enough reach.