How do large ships survive being tossed and heaved by waves on a rough open sea without breaking apart?
In: Engineering
The Edmund Fitzgerald springs to mind. It broke in half when the bow of the ship was on a different wave than the stern. Snap… Gone.
Two answers:
1) They don’t always survive. Ships are lost every year to heavy seas. [And sometimes it turns out that a particular design or model of ship is particularly vulnerable.](https://en.wikipedia.org/wiki/Liberty_ship)
2) There are centuries of shipbuilding and sailing experience that inform ship design. Combined with modern design tools and materials, and ships can be designed to withstand huge loads from waves (like dynamic loads of up to 50+ atmospheres in severe conditions). [You can read more about how ship design in this paper.](http://shipstructure.org/pdf/2007symp09.pdf) Unfortunately, there aren’t global standards for ships and, as I mentioned, ships do in fact break up all the time.
They don’t always survive. Plenty of ships are lost at sea, although much less so in recent times. Modern ships, while massive, are also very well designed. And while the fluid dynamics at sea can be wild, they’re also decently well-understood, so ships can be designed to be strong enough not to collapse/snap in heavy seas.
Of course, this isn’t absolute, and ships can still sink from rough weather, poor nautical skills, bad luck, or any combination. But a modern vessel, properly crewed and maintained, and with a good captain who has good information (weather forecasts) will stand a very good chance of making it out of any given voyage safe and sound.
The ship is largely moving with the water. A wave lifts the ship, a trough lowers it. Imagine a ping pong ball in the water. No matter how bad the waves are, nothing is going to sink the ping pong ball.
Also, in heavy seas, the crew will point the ship so that the ways don’t hit it hard. A ship hit broadside by a wave might capsize, but a competent crew will turn the ship into the waves.
Where ships get into trouble is if they hit something that won’t move, like rocks or a reef. Then the ship can’t move with the water any more, and the water will start pounding on it. And, of course, pounding it into the thing that won’t move. Few ships founder in the open ocean compared to the ships that break up near the shore.
These ships survive because of their design and different systems on the ship that can work together to keep the ship stabilized.
The most common system onboard to stabilize the ships are their ballast tanks. These tanks generally hold seawater to create weight.
Because of this weight, the ship has a lower point of gravity thus giving it more stability.
Imagine a Formula 1 car. It is designed for high speed. One of the reasons it can go that fast is because it is build as low to the ground as possible. Because the F1 car is lower to the ground, it has a lot more stability to handle these high speeds. The stability comes from a lower point of gravity.
The placement and dimensions of these tanks are calculated before the ship is build. They come in many different shapes and sizes but generally have a squarish or rectangular shape.
The seawater is pumped into, or out of, the tank via pumps. These pumps are either operated by crew onboard (most common) with the use of Alarm and control systems, or by manual control on a switchboard. Depending on the design of the piping system that is installed they generally come with one or two valves on smaller ships but can have more depending on the use case of the ship and the tank.
The amount of seawater in the tank is managed by trained crew on board with the piping system. There is more to it than just filling the tanks. You have to take into account your current cargo on the ship as well as different types of liquids in different tanks located throughout the ship. For example: The more weight on the left side of the ship facing forward means you will have a bias towards the portside of the ship. The more weight on the right side facing forwards means you will have a bias towards the starboard side of the ship.
Other than the ballast system there will be electrical systems that will help stabilizing the ship. These systems can help with planning the route of the ship so they don’t catch bad weather or heavy winds. They can also take over steering to stabilize the ship. The captain will be in charge of this and will judge whether or not it’s safe to go through storms and what route to take.
All of the systems are driven by an onboard power plant which will generate power for the ship. If these power plants fail then the aforementioned systems don’t work.
The captain also has to go to manual steering when there’s no power. You can imagine on a large ship that the rudders will be driven by large electrical motors. In the case these don’t have power, you won’t be able to rotate the rudder. With all of these systems having failed, the chance of capsizing or splitting will increase tremendously.
Of course there is backup power after the power plant fails. This is only for a certain amount of time. The minimum amount of time you’ll have is based on regulation for different components on your ship.
I am a project engineer for a company that is specialized in Alarm and Control systems on different sized ships. For us the general minimum norm is 30 minutes after the power plant fails. This can differ for electrical motors on a rudder or pumps on a ship.
If you have any other questions about a ship or about it’s control and logic. Please reply or send me a DM!
There are other answers here explaining how they don’t always survive, but none giving you the actual reason why boats CAN handle this treatment and survive.
The answer to this is weight displacement and distribution and is basically the same reason the boat floats in the first place. Namely the boat displaces as much water equal to its weight. When the boat is getting tossed around, it is still displacing as much as it weighs, so it’s not going to sink. Add to this that boat designers design the boat so its balanced to always return to an upright position. When a boat gets tossed upwards and strikes the sea on the way back down, the impact force is evenly spread across the area that impacts the water, and as its displacing its own weight, its no big deal. Imagine throwing a rubber duck into your bath and see how it “bounces” off the water. Basically the same effect.
Likewise, when huge waves wash over the boat and spill all over the decks, the water isn’t actually doing anything other than washing over it and off. The boat is sealed and unless the water gets inside and starts to lower the boats attitude in the water, its not gonna do much. Imagine trying to sink a rubber duck bath toy by pouring water over it. You will never do it. Cut a hole in the top and pour the water inside the duck though and eventually it will sink. And while thousands and thousands of gallons of sea water might weigh a huge amount to me and you, comparitively to the ship, it’s nothing.
So basically, enclosed boat is full of air and displaces the water it sits on. Without being able to overcome that, waves can’t sink the boat. Waves don’t break the boat apart because the energy of the water is quite spead out across the ship.
Having said that, as others pointed out sometimes boats do go down due to severe waves and stormy seas. However this tends to be because the boat is either capsized (Rolled over and unable to right itself), or other mechanical defects that allows water to breach the boat and get inside it, thus making it heavier and heavier till it takes on too much water. In fact in most modern ships, this is 99% the cause of ships going down in rough weather, and it’s now rare when mechanical reasons aren’t counted. A mechanically intact and correctly run ship can take on some truly horrendous seas just fine.
[Here’s a famous example](https://www.youtube.com/watch?v=DMNhO8dKJjQ) of a ship being broken up by heavy (Actually not considered all that bad by most maritimers) seas. The cause? Poor maintenance and an even poorer inspection programme for the ship.
They are designed to resist the stresses.
Although, as with everything in nature, there is no upper limit on what those stresses can be, so there are cases when ships break apart but it doesn’t happen very often.
During WWII the US created a class of cargo ships that could be built very quickly, the [Liberty ships](https://en.wikipedia.org/wiki/Liberty_ship). They were rather flimsy in design, the idea was that the US industry could build them faster than the German subs could sink them. There were a few cases when Liberty ships broke in half and sank during storms, but that was an exception rather than the rule.
I saw a video of a container ship, filmed along the row of bulkhead doors below deck. In the row of maybe, 20 doors, it flexed far enough that you could see the end of the row, then only the next door along. Both ways.
Ever hear the term “batton the hatches” a ship in rough seas, like those Deadliest Catch crabbers in Bering sea are supposed to close the doors and hatch covers. This does more than prevent a lot of water from filling the boat, it is supposed to support the structure. Take an egg and put in the palm of your hand, wrap your finger around it completely as best you can, the theory is that if you apply equal pressure on the egg you can squeeze it rather hard and it won’t break.
The ship structure is not infinitely rigid, it has some flexibility and can bend up to a point.
When waves pass through/under a ship then different parts of the ship will experience different forces as some parts will be better supported compared to others.
As long as the difference in forces is below the ship structure’s limits the ship will survive.
If the differences become too large the ship will break apart.
In fact some of the anti ship torpedoes work by exploding under the ship and using this force difference to break the ship in half.
http://www.dynamicscience.com.au/tester/solutions1/war/torpedo.htm
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