Eli5 how do cargo ships not tip over? Aren’t they super top heavy?
In: 195
They can in fact, tip over, but a lot of time and planning is spent so that they don’t. Cargo ships are not just platforms with cargo on top, there are giant engines underneath, the enormous prop shaft, fuel tanks, the bilge, and often more storage. Pains are taken in assuring that the ship isn’t unstable in any way, often with the assistance of automated systems which use ballast to enhance lateral balance. The real risk isn’t with containerized shipping though, it’s with bulk dry goods, especially certain cargoes like bentonite which are notorious for suddenly shifting and causing a boat to capsize.
Yes, they are top-heavy, in the sense that the center of mass (all that cargo) is above the center of buoyancy (that hole in the water).
Cargo ships are stable in the upright position because the increase in buoyancy as they tilt to one side (called “listing”) more than makes up for how much the weight has shifted to that same side.
If a ship is listing to the left (“port”), the center of buoyancy moves faster to the left than the center of mass. This makes the ship want to return to an upright position.
This will be true until the center of mass is more to the left than the center of buoyancy, at which time it will flip over (“capsize”).
Imagine a panel of styrofoam floating in water, not about to tip over eh? That’s how barges and flat bottomed ships work, just that the proportions are less extreme than that, but they are all similarly “top heavy”.
They also have ballast tanks, commonly along the bottom and/or sides, depending on the ship’s design. These ballast tanks are computer-controlled to pump water in and out depending on what’s needed at any given time to keep the vessel upright and stable.
Once in a while, the computer system fails, and the vessel tips in unexpectedly. Google the “Westwood Rainier” and it’ll be the first thing to pop up. The computer system failed and it listed unexpectedly causing damage to both the vessel and to the shoreside crane.
The hull is designed so that if it lists (tips to the side) the centre buoyancy (the up force) moves sideways more than the centre of gravity (the down force). As long as the centre of gravity remains closer to the ship’s centreline than the centre of buoyancy, the ship will right itself.
Beyond a certain list angle (between 40-60 degrees, depending on the ship) the centre of gravity moves outside the centre of buoyancy and the ship capsizes.
They are not as top heavy as you might think. You are absolutely correct that the higher the centre of gravity is on the ship, the closer that ship becomes to unstable. If a ship is unstable in a storm or something, the procedure is to find a way to make the ship heavier at the bottom or lighter at the top.
However, most ships have a low enough centre of gravity, but it may not look like it. When you see a ship floating on the water, remember there is a good chance that more of the ship is underwater than above water. Ships might have a draft of several meters. *If* the ship was barely on the water, the part of the ship you see would probably raise the centre of gravity too high. However, since ships are maybe 10 metres underwater, their centre of gravity if much lower.
Also, and this depends on the cargo, but the weight of the cargo is oftentimes lower than higher. If carrying denser cargo, ships will not fill their cargo holds; the weight of their cargo will put the ship at its maximum draft without taking up much room. This means that if you look at the whole cargo hold, the centre of gravity is not at the centre of the cargo hold, but closer to the centre of the cargo near the bottom. This lowers the centre to a safe level. Similarly, when a ship is in ballast (no cargo, the ballast tanks are full of water), the larger tanks are often at the very bottom of the ship. Again, this put more weight near to keel of the ship, lowering the centre of gravity.
They extend a lot farther below the surface of the water than you realize, those ships have a 45+ foot draft when fully loaded. Add a few hundred tons of ballast to that and the result is quite stable under most conditions.
I’ve worked for numerous import/export brokers. Part of the fees in the insurance is knowing that if needed your container may be thrown overboard if they need to redistribute weight or incase of emergency
The shops are quite stable, whilst there’s ballasting and load calculations that I’m sure have to be done, the common misconception is that a cargo ship just has a flat deck with containers sitting on top which makes it too heavy.
The simple fact is, the ship is very heavy at the bottom too, as there is an internal cargo deck.
Here’s a simple diagram: https://media.istockphoto.com/id/479586322/vector/container-ship-section.webp?s=612×612&w=is&k=20&c=j7adXTP_LKTUu-4farzQVf9iWM1TErJmO46Y_ZPrcYo=
The biggest risk is if the front falls off, but that very rarely happens.
[I don’t think it can get more ELI5 than this youtube video](https://www.youtube.com/watch?v=DPV_VjzU9kE)
Basically. The weight of the ship has a center of gravity (the force that pushes the ship down) which is balanced against the center of bouyancy (the part of the ship that displaces water, which creates a force of lift). The ships hull is designed so that when center of gravity shifts in one direction (and makes the ship lean in that direction) the center of bouancy shifts in that direction as well, creating a counterforce that pushes the ship upright again.
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