Cruise ships, which can weigh hundreds of thousands of tons, remain afloat due to their U-shaped hull that displaces water outwards and downwards. This law of physics is known as the Archimedes principle, which states that a body immersed in a fluid is subjected to an upward force. Cruise ships are massive vessels that can weigh hundreds of thousands of tons, yet they remain afloat effortlessly on the water.
The Archimedes principle states that a body immersed in a fluid is subjected to an upward force. Cruise ships, such as Royal Caribbean, operate the largest liner on the open ocean, and even a ship twice as high as the Washington Monument might leave you amazed. In this episode of Explainers, we explore the science behind how cruise ships stay afloat by focusing on buoyancy, which is upthrust, and the density of the water they displace.
Buoyancy is upthrust, and ships are extremely buoyant. It is dependent on several factors, but the most important one is density. As a cruise ship rests in water, it provides room for its mass by displacing water outward and downward. The opposing forces of the ship’s weight and the seawater on the outside help maintain buoyancy.
Engineers play a crucial role in maintaining the buoyancy of cruise ships. They are located near the bottom of the ship, usually towards the aft of the ship, and the hull must be designed to resist obstacles such as concrete piers, rocks, and other obstacles. By understanding the principles behind cruise ship floatability, we can better understand the challenges faced by these massive vessels and their ability to navigate the ocean.
📹 How Do Massive Cruise Ships Float? | Explainers | Travel + Leisure
We capture the pure joy of discovering the pleasures the world has to offer—from art and design to shopping and style to food and …
📹 WHY DON’T CRUISE SHIPS TIP OVER? | An introduction to ship stability
What keeps ships from toppling over? I explain how Cruise ships stay upright and what can cause them to lean over. This video …
I am a retired UK Class 1 Master Mariner and I like this article. It’s not meant for mariners as a teaching aid. It’s meant for ordinary people and it uses non specialist language. Great stuff and I am going to recommend it to my MSc Maritime Studies class in Singapore. Some of them are mariners. Most of them are not and this article will be ideal learning for them. Well done sir!!
The quality of this article tutorial has been recognised by many of its viewers, but one aspect has not been commented on so far: there is no ‘music’ track added behind the speaker. I am so happy about that because many other YT articles have intrusive, and very often highly repetitive, ‘musical’ phrases added; they distract and irritate me enormously. So well done and thanks.
The key part that isn’t described, which is that the shape the hull is designed so that a tilt causes the centre of boyency to move, it doesn’t happen by chance. If the hull was semi circular the ship would fall over unless the c of g was lower. So it’s the shape of the hull that allows this top heavy object to stay upright.
Great article big guy. I did smash the like button. I am an engineer and you explain science and physics and engineering very well to the non science minded and or the interested who have a natural ability to grasp concepts but have not learned by experiment experiance or someone like yourself. You have a great teaching ability and that is rare, Thank you.
I did a five year indentured apprenticeship in marine engineering + college day-release.I worked on the Queen Mary,Queen Elizabeth 1 and many other large liners.Some of these large liners had stabilisers on each side of them.If the weather was rough,the stabilisers were extended into water. This stabilised the liner. My apprenticeship,covered Fitting Shop,Fitting on Liners,Machine Work and the Drawing Office.This also included Dry-Dock Work. I enjoyed perusal the article.
Thanks for sharing this simple infos. And i think, aside from sort of liquids and heavy machines underneath as with pulling gravity and its proper arragements of cargoes, etc, aside from bouyancy above… they could control it thru a certain built-in gadget at the Captain’s deck area to check the ship’s balance situation in 24/7 monitoring.
I know the title of your article, along with the visual effect is intended to capture interest. It’s doing the job brilliantly. In addition to the wonderful content and comments, i’ll add my piece, because nobody has bothered to in the past year, hopefully it might generate more interest for you. It’s my understanding fall means to lose one’s balance and collapse to the ground. Falling/tip/tipping from an upright position. To fall over oneself or an object. The glass of liquid tipped over. Capsize is defined as a boat rolling over onto its side or completely over. I guess that’s defined as flooding, as long as the ship maintains some boyancy. If a ship was unfortunate to end up on the bottom of the ocean, because it capsiized, then it didn’t fall there, because it sank, after it capsiized.⛴⚓️🛳
i feel like this does a good job of explaining what it explains.the thing is this is just the explanation of why ships stay up in general. most people clicking on this would be looking at cruise ships in particular, and an answer that would address the fact forces applied higher up on the ships would have a greater impact on pushing the ship over, than those near the waterline. also the higher. also the higher the center of the mass the less of a tilt is required to push the ship past the point of no return.
Thank you. Surprised you used the Queen Mary 2 as an example here, which as you know is not a cruise ship but a true ocean liner. The QM2 never looks like she’s going to topple over, unlike those ghastly floating Chuck-e-Cheese-type vessels – the “… of the Seas” variety. Now they really DO look top-heavy, as well as being grotesquely ugly.
I saw a vid of a rather large boat being launched in another country. According to the narrator of the vid, for some reason they couldn’t install the engine until after the boat was launched. An engine-less boat wasn’t planned for by the engineers that designed the boat. The boat tipped over immediately without the mass of the engine giving ballast.
Coming from a pilot perspective, I’m making comparisons to weight and balance computations for airplanes. (Center of gravity and center of lift in that example.) I’m enough of a nerd that I want to see what weight and balance calculations for ships look like. I’m curious how similar or unique those processes are.
I have never worked on building cruise ship but I’m sure that the same rules for building them that apply to building ships for the US Navy. I realize the LSD’s that I worked on probably 1/4th the size of a cruise ship. I worked on the USS Harpers Ferry (LSD 49) as well as the USS Carter Hall (LSD 50). The is a stabilizer that runs almost the entire bottom of the ship. This stabilizer prevents the ships from rolling very much on it’s side. There is no doubt in my mind that there are more than one stabilizer on each side of the ship. They will catch a lot more water went the ship starts rolling to one side.
Cruise ships also have stabilising fins that can be used to correct for different weight distribution and steady state wind at a cost of efficiency moving through the water. I sat through a presentation that suggested that in by gone days cruise ships tolerated a 30deg tilt before falling over and that modern ones only tolerate around 15 deg. This is of course due to when the centre of gravity moves outside the effective “footprint” of the ship. It might be valuable to add this criteria to the mix.
Not a mariner but..suggestion: an arcing arrow depicting the twisting moment would really clarify your explanation of the “twisting” comment. Not criticizing. Thought this was an excellent article about the subject. As previously noted, I wish you could have been my physics teacher. I would’ve been at the head of my class and I’m no genius. Thanks..
Very helpful; a blinding flash of the obvious of course, but no, I didn’t know it before I watched this brief very helpful article. It is obvious to everybody that the current generation of cruise ships appear worryingy top-heavy., Let us hope that sustained massive seas and high winds never overwhelm any of them., Thank you for taking the trouble. A brilliant short article.
Excellent article, as is your second article about capsizing the vessel. I had always wondered how a cruise ship could be stable if its CG was above the waterline. Your article explains it very clearly. I also pondered the fate of a round-hulled ship as it heeled over. I suppose that the centre of buoyancy remains on the centreline, the twisting moment is in the wrong direction, and the ship rolls over. Is that correct?
Point of question: are cruise ship hulls catatonically squared, or slightly oblique away from center at the deepest part of the chine? A “spreading” of the hull’s base by a meter or more in scale to height might presumably give the center of buoyancy an advantageous and supportive force that may cancel a wandering c.o.g. (?) Only a thought. I have dozens of hull line drawings from the 1840’s for tall masted Clipper ships that then depended upon deep hull cargos for balance. Today, a cruise hull must constantly be rebalanced owing to the weight of fuel, liquid waste, and such being in an elastic state. Thank you… am looking forward to your next presentation.
So this is why you want a flat or pontoon-type of bottom to your boat, and not a more V-shaped cross section, as if you perturb a more V shaped cross section, the center of buoyancy moves the wrong direction and tips the boat over (or if the boat tips too much, which will kick the center of buoyancy out to the wrong side of the CG.
One important factor was not taken into account: wind. A category 5 hurricane can, by my calculations cause a moment that will incline a cruiseship enough to incline it around 45 degrees, and I guess that at that angle, water will invade the ship initiating disaster, because it is not a submarine. I only can not be sure about those 45 degrees, without exact data of a cruiseship. But what I am quite sure is that a category 6 hurricane can do the trick, and they are coming! Another aspect is at what windvelocity windows will start breaking and let water in from waves and water.
You need more than the center of the waterplane area at 1:58 to determine the centre of buoyancy. The centre of buoyancy is basically the centre of gravity of the displaced volume so you need to know the complete form of the displaced volume to be able to obtain the coordinates for the center of buoyancy.
I do believe what you call the center of gravity, is actually the center of mass, which is more telling of where in the ship the weight is most dominant. Where as the center of gravity is based on the balance of weight distribution through out the entire mass of the ship. Sometimes it is hard to determine the difference between the two for, not only in the case of ships but for any physical object, can be physically in close relation to each other. The center of gravity can be any point along the Z axis determined by the center of mass. Example the center of mass on a ship can be and usually is lower in the physical form of the entire ship for stability reasons, and would make sense to maintain this center of mass in the center of the lower portion of the ship. However if the center of mass is skewed on either the X or Y axis, this could change your center of gravity without changing the center of mass given this change happens in the lower part of the ship… if that makes sense. My two cents
Ships have ballast tanks which can keep the ship level, but on one occasion while a ship was up on drydock and ready for launching, but when the ship hit the water it leaned far over on its side, the ballast tanks on one side were full of water but the other side the tanks were empty, the ship was straightened up by pumping sea water over to the other side to balance her upright On a rare occasion a sea valve might leak and so then the ship is pilled back up to fix the problem usually a gasket or packing problem
I think consideration of metacentric height (MH) would be a better way to tackle this subject: (MH) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.
there is a slight glitch there buoyant force comes from the weight of the water (or any fluid) displaced by and object partially or completely immersed in it therefore its not possible for buoyant force to be greater than the weight of the object. It can be either equal which is the equilibrium that holds the object partially submerged (floating) or weight can be higher than the buoyant force causing the object to sink.
People are still not understanding. Buoyancy is an upwards force and the center of gravity is a downwards force. As a ship rolls the center of buoyancy shifts so that a righting arm is created between the two forces, righting the ship. A ship capsizes when that righting arm goes to zero or becomes negative. This is called a negative metacentric height.
This explained how it stabilises and corrects within certain limits. It did not explain why it does not tip over if the limits are exceeded e.g if a very large wave tips the ship over to a greater angle. Are ships built so that these limits are never exceeded and can never tip over like a toy duck in a bath? Or could one tip over and stay over if the right force is applied? I watched the article but still don’t really know the answer to the question posed in the title of the article. Maybe they can tip over. The question is when? That is what a lot of cruise shop passengers probably want to know in very bad weather.
Great article the only thing I’d say is you used the queen elizibath 2 is an ocean liner not a cruise ship ocean liners are designed to take people from 1 location to their destination but cruise ships take you a lot of different places and then back home but apart from that great article I enjoyed it and can tell my parents about it 🙂
That’s fine in fair seas when the engine is running. The issue here is the margin of stability, and modern cruise ships have a very low margin of stability. Lose the engines in a storm and they’ll go right over. The real reason they don’t tip over is because they dash into harbor whenever a storm is coming.
All well and good however you didn’t actually answer the question of why they don’t capsize/ tip over . Do they have ballast tanks, separated fuel cells ? Would be good to know. Your Costa Concordia clip explains the title of this clip far better with various tanks and cross flow valves that large ships have, excellent 👌🏾
This article is not complete however. Since the figure shows a ship where the CG is above the CB the presented ship is definitely not a stable vessel. If the listing continues (e.g. for stronger wind and/or for big waves) the horizontal distance (leverage) between CG and CB shall shorten meaning that the sabilizing torque weakens to a point where it is zero. From that point any additional listing leads to the capsizing of the ship. Beacuse of that modern sailboats are built that CG is always below of CB by means of several types of keels. In fact cruise ships also could be built that way but it is not deemed to be economical in the light of the low possibility of such rude conditions that may lead to the capsizing of the ship.
This is lesson 1 in series of buoyancy, right? We know there are more. Say, for example, if that’s the situation, all container cargo ship should stalk their containers in pyramid shape, but in reality they don’t. They stalk it rectangle shape 8~10 storeys high. So we know there are more in this issue of calculating center of gravity.
Ok, nice article so far as it goes, but kind of avoids the point – most peoples intuition is that these cruise ships just ‘look’ top heavy, and it’s the superstructure we are looking at. I just looked at MS Oasis of the seas. This seems to have an estimated diplacement of 100,000 tons, and a deadweight of 15000 tons. Deadweight is all the stuff you put in colour in your diagram, oil, stores etc etc. That gives and approximate lightweight (the total weight of the ship minus the deadweight) of 85000 tons, 85% of the total. So the mystery for me is not that the deadweight is low down, but how on earth 85000 tons is distributed in such a way that most of that, much bigger number, is towards the bottom. Clearly the answer lies in the materials used, but I still find it astonishing that they don’t roll much more than they do – some of that is achieved by widening the submerged section to provide a better righting moment, I know, but most must simply be that in 85000 tons of steel and alloy, most of it is in the bottom 25% of the structure.
Does anyone know what kind of response time these forces have in reality? with respect to the how the material (likely 1140 or some other structural grade steel) and the actual internal load bearing structure that you would need for these types of forces at that scale? It would be interesting to see a stress map Great article!
No matter how the center of gravity or bouyancy sits, if an idiotic captain in heavy seas fails to turn head on into the oncoming swells, the ship is going to tip, if not turn turtle and SINK! For a ship to remain stable, it must attack at a 90 degree angle to the seas! See Poseidon Adventure and how it turned turtle when it got broad sided! DOC Dead On Contact! One day my ship a 1936, 327 foot Coast Guard Cutter was Steaming north, in 75 knot /86.3 MPH winds, and 40/50 foot seas off the Oregon Coast! Now that is Rockin’ and Rollin’, when you take on swells that go over the bridge which is 38 feet above the water line! Now that was FUN! Arrrrgh Maytee!
Ok, and what about water surface waves? I mean the water surface don’t remain flat when it’s windy. Especially when the wind is strong enough to push vessel to the side. Let’s imagine a situation when the wind pushes ship to the right like it showed on the article, and at the same time there is a wave that make it’s way through the ship. And it is only left side of an underwater part of the ship actually remain underwater, and the right part is over the water surface. In this case we have a centre of buoyancy on the left side, because only left side is still underwater, and centre of gravity on the right side, because of wind that pushed ship to the right. And as a result of this two forces interacting – there appear a torque wich makes vessel to tip over. What about that?
Very useful article thank you… I have a quiestion, i like to make model ships. I do that. But in calculation time i face some problem. I can not calculate maximum how much hieght i can take for good stability.. i cannot identify this. Can you explain the relation among ship’s super structure’s hight, hull’s length and width and draft?? Please..
What if that corrective “twisting moment” is coupled with a sudden change in wind direction and/or swells in water level? Could that cause an overcorrection of sorts resulting in a capsize? Also, how many degrees can one of these ships list before it capsizes? Anyone educated on these things to answer these questions? Thanks!
Do these ships have a form of weight shifting in their lower compartments? The crew want to allow passengers free reign of the ship, but that means there’s less control over those two factors. (Center of gravity and center of buoyancy.) If they want to keep the ship stable, they’d need a way of counteracting this in at least a general sense.
I am an engineer working in cruise ships for more than a decade. My opinion is that if the center of buoyance would be below the center of gravity, a cruise ship would be very sensitive to wind and even passengers crowding on one side. Such a ship would never be upright, because at very small angles of listing, the buoyance and weight would form a couple of forces that would increase the listing, maybe not untill capsize but for sure increasing it. Sorry but you are wrong.
Fine and good but still doesn’t explain why it doesn’t tip over. Yes, you’re shifting forces and weight but how and with what? Now to further to answer that question, buoyancy is a constant, doesn’t change but gravity in relations to weight does. To account for shifts related to the positions of the shift, we have counter weights that are automatically geared to the shifting of the gravity and stability lines of the ship and move accordingly to keep both the gravitational pull and buoyancy equal. It is the same effect, just opposite with tall high rise buildings and wind. Gravity is pulling it straight down, the ground pushing up but have the constant force of the wind around it after a certain height. To prevent major swaying (and major motion sickness for those inside), counter weights are up on the top floors if not roof moving in the opposite direction to limit any motion.
They do tip if the gyros fail If the gyro stabilizers quit the ships are doomed. No built-in stability without them. They’ll cut all other power on the ship before they let those things stop spinning. Modern cruise ships are obscene, top-heavy hazards to navigation and fire traps. Every ship should be required to self-right WITHOUT ANY POWER AVAILABLE to run gyros from a 30 degree list. Cruise ships would flop on over from less than 10 degrees without the gyros.
Actually, I’ll disagree with a few here and say that ‘more proper’ terms probably should have been explained relative to ‘yaw, roll and pitch’, and how they affect the dynamic focii of ‘buoyancy’ and ‘cg’. Also, a better explanation of ‘static and dynamic masses’ relative to ‘what moves, what doesn’t, and why this affects your overall buoyancy’. It is hard to ‘unlearn’ bad introductions to a topic, and basically, if someone sits through this information, then they are probably seeking it to start with. This information is the basics to not only ships, but to aircraft and general loadmaster work – see it for the larger topic that it actually is.
There is a fire in the engine room and all power is lost. The stabilisers will not operate and the ship is in the path of a tropical revolving storm with Beaufort force 12 winds. The ship, out of control, naturally turns it’s weather side at right angles to the wind, swell and waves. The ship heels so far over from the wind impacting the enormous area of the cruise ship and rolls from the waves. The centre of gravity moves further from the centre line than the centre of bouyancy and the righting moment becomes a capsizing moment. The vessel temporarily has negative stability and capsizes. 5,000 passengers and crew drown. John Sampson Extra Master