You are assuming the limiting factor is whether the CofG of the whole stack goes outside the footprint sufficient to tip the stack, as if it were an unsupported pillar. Due to connections between the deck and the boxes, and between the boxes, actually the stack can (up to a point) lean past what would otherwise be the tipping point and not tip over or collapse. The limiting factor is whether the container structure on the lean-to side can cope with the loading it sustains in this circumstance when the stack tips far enough, or whether it buckles.
One could probably come up with a theoretical scenario where heavier boxes lower down help balance forces. Having seen quite a number of engineering analyses of such collapses, I’ve never seen it work out that way. As I said, it’s largely a myth that heavier over light matters.
And I think you’re overlooking that the forces loading the structure and trying to buckle it, will behave differently depending on whether the centre of mass of the stack is higher or lower and whether it has moved outside of the footprint.
A stack with a higher centre of mass is essentially a longer lever with respect to the forces it will apply to the fastenings at its base.
This also suggests that any container dumped into the water will almost immediately begin shipping water at some rate. Even an empty container will almost certainly have a draft greater than your e.g. 20cm when floating.
Obviously the less cargo aboard, and less dense the cargo is, the more slowly any given container’s not-quite-watertight seals will admit enough water to sink it. Perhaps a container filled entirely with loose ping pong balls could float indefinitely … barely.
Without doubt. But whether they sink in the short term depends on a few things. They can quite often contain cargo that provides flotation. They may also sink such that the door end is down, trapping an air pocket at the other end.
To sink a standard 20-foot intermodal container, you would need to add at least
55,000 to 60,000 pounds of weight (about 25,000 to 27,000 kg), in addition to the container’s own empty weight. The total combined weight of the container and its cargo must exceed the weight of the seawater it displaces.
This calculation is based on Archimedes’ principle, which states that an object will float as long as the upward buoyant force (the weight of the displaced water) is greater than or equal to its total weight.
Calculations for sinking a container
Here is a breakdown of the key factors involved:
** Seawater displacement: A standard 20-foot container has a volume of approximately 1,300 cubic feet (36.8 m³). Because seawater has a density of about 64 pounds per cubic foot (1027 kg/m³), the buoyant force from a fully submerged container is roughly 83,200 pounds.*
** Container weight: An empty 20-foot container, also known as the tare weight, weighs around 5,000 pounds (2,300 kg).*
** Calculation: To sink, the total weight must be greater than the buoyant force.*
Total weight to sink > Buoyant force*
Total weight to sink > 83,200 lbs*
Weight of cargo > Buoyant force - Weight of container*
Great AI answer. Net of its poor management of sigfigs.
Going one step further, Twenty-foot equivalent unit - Wikipedia says a 20-footer has a legal max gross weight (so tare plus cargo) of 53 K#. Given ~83K# of buoyancy, you can’t legally load one heavy enough to sink. That ratio says a max weight loaded container will initially float about 2/3rds submerged.
Like any cargo carrying mechanism, it has both a volume and a mass limit. The density of the cargo determines which limit you hit first.
That’s what AI does. It would be helpful to at least flag AI answers as such, but, preferably I think, to use them as a launching off point for one’s own analysis and fact-checking. As someone who uses AI almost daily, it’s good at producing impressive-sounding results that are utterly wrong. But there’s a slew of threads about that already.
Other than the sigfig quibble, there’s nothing “wrong” with what the AI did. I didn’t mean my comments to be critical of @Jasmine’s work or their decision to post that.
The answer was incomplete versus the question I was originally asking which was whether it was legal to load a container so heavy that it would sink immediately. The AI answered the more preliminary question of “How much weight would be needed to sink one?”
Without knowing the prompt, there’s not much more to be said about the AI’s workmanship.
Up thread, without using AI, I did a back of the envelope calculation and came up with the same answer. I used techniques I learned in freshman physics about 50 years ago.
But you’re not allocating the heavy containers to the top and then deciding whether the bottom should be heavy or light. You have some set of containers you need to load, with whatever weights they have. Given that set of containers, it’s only logical to put the heavy ones on the bottom, because they have to go somewhere, and if it’s not light on top of heavy, then it’s heavy on top.
If the ship is to be loaded from empty to full at Port A and then to be unloaded back to empty again at port B, then loading just for CG is easy.
Where it gets messy is when the reality is the ship takes on and drops off containers at a more or less endless series of ports. Yes, some segments are more more full of containers than other segments. But a modern container ship is fundamentally more like a railroad freight train than an airliner.
You’d like your loading arrangement here at Port C to make it as easy as possible to unload everything going to port D without needing to unstack stuff bound for ports E, F, & G first just to dig down to the D containers. Some of which D, E, F, & G containers are already aboard before the ship got to C, and some of which you’re going to add here at C. etc.
Lather rinse repeat at each port as the ship essentially makes a perpetual round robin circuit around them all.
Well, whatever container ends up on the bottom, it’s got to get unloaded (and hence also all of the ones above it) eventually. I’d think that the simplest solution would be to put all of the containers for a given destination in the same stack.
I’d have expected a stop in e.g. Vancouver Canada, then after LA/LB, a stop in e.g. Mexico or Panama, and maybe even a South American port before re-crossing the Pacific westbound. To be sure, if they had the itinerary I suggested, they’d probably offload 80% at LA/LB and the rest at the others.
The only reason I know this is that it comes up in every longshoreman’s strike. (I know a few of them). Management complains that they take longer to clear a ship than other ports, but that’s just because they have to unload the whole thing. Because we’re such a large market, mixed cargos aren’t necessary.