How do I estimate the capacity of a cargo ship?
September 27, 2019 2:49 PM   Subscribe

I’m looking for the answer to what I thought would be a simple question: approximately how many tons of Product X can Ship Y carry? I can look up any vessel on various public websites and find out its gross tonnage and deadweight tonnage. Both of those are used to refer to the ship’s capacity. Great! But the better I understand them, the less helpful they seem.

Here’s my understanding: gross tonnage, confusingly, is a number that does not appear to be expressed in tons and actually refers to the volume of cargo space available. It is calculated based on a formula.

Deadweight tonnage is expressed in tons and is the maximum weight the ship can safely carry (i.e. the weight that would make it sit at its plimsoll line in the water). BUT that number includes not only the weight of the cargo but also crew, provisions, fuel, and ballast. It excludes only the weight of the ship. That is not helpful to me because I have no way of knowing how much all that other stuff weighs.

There MUST be some rough measure or shorthand that industry people use. Right? I’m sure fancy software is used to calculate how to ship cargo most efficiently, but surely there is a way for a knowledgeable industry person to make an educated guess. Or for a prosecutor or compliance person to look at a bill of lading and judge whether something seems fishy. It’s got to be a thing in trade-based money laundering for example.

Am I overthinking this? This is driving me insane.

If it matters, all the vessels I am interested in are container ships or cargo ships.
posted by leeloo minai to Travel & Transportation (11 answers total)
For container ships, the rough measure is twenty-foot equivalent units.
posted by djb at 2:59 PM on September 27, 2019 [5 favorites]

You need to know the density of Product X. Is (density times max volume ) smaller than the net deadweight tonnage for this trip? Then that’s what the ship can carry. If not, you can only carry up to that weight limit.

The trip matters because the route and season and so forth are going to determine staffing, food, fuel.

I know nothing about ships actually, but I have repacked much less exciting transport after filling it with dense cargo. There’s only so many rocks you can backpack if you’re carrying the day’s water.
posted by clew at 3:50 PM on September 27, 2019

I think the OP is asking if a number like "net deadweight tonnage" exists and how it can be found.
posted by amtho at 4:37 PM on September 27, 2019 [1 favorite]

That’s why I gave an example of why the route matters as well as the ship.
posted by clew at 4:53 PM on September 27, 2019

Also, if you’re moving more mass you need to carry more fuel which also has mass. But sometimes ships can refuel partway.
posted by clew at 4:56 PM on September 27, 2019

Capacity is expressed in TEU because the limiting capacity is generally not weight, it is space to put the containers in (and how high you can stack them).

But if you are looking at bulk cargo ships, they likely will indicate what their capacity is for whatever commodity they carry in tonnes (which may be really a volume limit as well).
posted by ssg at 5:40 PM on September 27, 2019 [1 favorite]

Yeah, you have to stuff your product in a shipping container. Shipping containers come in sizes. Each size and strength-of-container has a maximum weight gross and net. You need to figure out how much product you can shove into a shipping container that's below it's maximum weight capacity, and how many shipping containers the ship can carry. You also might get pinged on weight distribution. Maybe ships can carry max containers or max weight. Or maybe they sorta rely on some difference in containers and weight in order to keep the balance of the ship within limits. Maybe they put heavy containers towards the center and lighter containers toward the fore or aft just to keep things balanced. It could get complicated unless you know the exact design of the placement of the containers on the ship. At least in regards to packing each container to its max weight. They could technically carry that max weight, but they would have to stack them so high near the center of the ship that would make it easy to tip over. It's complicated.
posted by zengargoyle at 7:05 PM on September 27, 2019

On top of what everyone else has said, the shallowest point of the route may either determine total loading capacity or force you to partially unload just before you reach it (and oops, let's just check if your previous port of call has the right equipment and ability to get your cargo to the destination landside), or if it's your destination, maybe you unload in the nearest deepwater port and use a feeder ship or three. And then maybe your container-based calculations are off because someone just commissioned your ship to ferry over giant parts for wind power plants.

I do calculations like these for (among other things) port-side investments and basically what you do is estimate your average cargo tonnage from whatever's passed through the port (or through a nearby port with similar parameters). There's too much variability in any particular ship's cargo load.
posted by I claim sanctuary at 11:17 PM on September 27, 2019

Finally most boats are set up to carry specific types of cargo which have very different requirements for on and off loading, storage, contamination prevention, fire suppression, timeliness etc. So a grain ship will express its capacity differently than a tanker or a frozen fish tramper or a container ship. Gross tonnage is usually used for bulk carriers, containers ships use number of containers, tankers use a volumetric measure etc

The deadweight tonnage is useful because it tells you if you take the boat and retrofit it what you'll be able to carry. Its like a baseline for the vessel. Some vessels are constantly being tweaked or have been fully retrofitted or eveb built out and youll often hear deadweight tonnage in relation to those Some are truly one purpose and people tend to use the cargo or passenger capacity referring to those. Fishing boats are categorized by length usually, to be extra confusing.
posted by fshgrl at 10:13 AM on September 28, 2019

Okay, so there are broadly three main groups of cargo ships out there, although there are billions of specialized versions. Container ships carry entirely shipping containers. Bulk carrier ships basically carry a big heap of a single commodity, like a ship piled full of coal, or wood pulp; I'm including oil and other chemical tankers in here, as well. And then general cargo ships -- this is the old school type of cargo ship -- carry random amounts of stuff, a pallet of this, a piece of machinery, what have you. General cargo ships are common, but tend to be for "last mile" (or hundred miles) trips from a real port to a dock that doesn't have full facilities. While these are common, they don't carry a large share of the actual world cargo ton-miles; the land equivalents of the first two categories is a full freight train, the land equivalent of a general cargo ship is a UPS truck or cube van.

Container ship capacity is defined in terms of TEU, as djb said above. A TEU is both a volumetric and a weight measure; it's a box that has certain physical dimensions but also a certain maximum weight. A standard TEU is a twenty foot container that has a volume of 33.2 cubic metres and a maximum weight for cargo (subtracting the weight of the container) of 21,600 kg. That means the average density of what's in a container can't be more than about 650 kg/m^3. Water is 1000, and lots of things are denser - sugar is 1600, porcelain is 2400. So if you're a porcelain tile factory, you can only ship containers that are about 1/4 full of tiles. On the other hand, roasted coffee beans are about 360, so if you're shipping a container of those, you're only going to be able to ship half of the maximum weight. There's software in the logistics industry that can combine multiple products in a shipment to optimize for both volume and weight (ship a load of both pillows and bricks combined), but that depends on you actually being able to put both of these in the same shipment; tough luck if you're only a pillow factory. So the actual capacity is in TEU; there is a theoretical maximum weight that's available if every container is full to maximum, but they aren't in practice.

And then bulk carrier ships, who use deadweight tonnage. (Or for tankers, volume, but you can convert volume to weight as long as you know the density of the thing they're carrying -- crude oil is about 6.5 barrels per ton. Although I'll add a note -- just like a container full of pillows, a bulk carrier carrying something relatively low density, like grain or even coal can "cube out", that is, hit the maximum volume before it hits max weight. As you said: BUT that number includes not only the weight of the cargo but also crew, provisions, fuel, and ballast. It excludes only the weight of the ship. That is not helpful to me because I have no way of knowing how much all that other stuff weighs.

Here's my estimate. I'll start with the fuel. According to this link, a 700 foot bulk carrier holds up to 800k gallons of diesel; that's about 3,000 tons. According to this link, that's at the high end of Handymax, which has a DWT of 50,000 tons. So 6% maximum of the DWT is in the fuel tanks.

On to the other three things we don't know about: The weight of the ballast is zero. Ballast is something added to the ship so that it handles properly when it's less than fully loaded and doesn't bob around like a cork. Therefore, if the ship is loaded to weight capacity, the ballast tanks are empty. A big ship will have a crew of 20 or so, so we know how much 20 people weigh -- for all intents and purposes, nothing. (20 people at 200 pounds is 2 tons, the fuel is 1,500 times heavier.) The same for their provisions; it's like asking how much I weigh, and then noting that when I step on the scale there are also skin mites and germs crawling on me and worrying about subtracting their weight. I'd use 94-95% of the DWT as a pretty reasonable estimate.
posted by Homeboy Trouble at 6:40 PM on September 28, 2019 [6 favorites]

Homeboy Trouble, you glorious, shimmering, beautiful human, thank you. Exactly what I needed.
posted by leeloo minai at 10:46 AM on September 29, 2019

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