Floating a cruise ship in a bucket of water

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  • #26
DaveC426913
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What is the pressure in that thin film of water 'floating' in the mold as compared to the pressure in an equivalent film of water around the ship where the ship is in the ocean?
This is why the thought experiment of building up the wall of the bucket around the ship is so informative.
Just keep placing bricks closer and closer to the hull, and eventually, there will be no room for more than a bucketful of water. At no point does the water immediately around the ship change in any way, and the ship never "knows" anything about the existence of bricks, just water.
 
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  • #27
Charles Link
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Ok, I agree with this argument now. It floats. Here is an argument based on Pascal and the Hydrostatic Paradox.

https://gizmodo.com/the-paradox-that-lets-you-float-a-cruise-ship-in-a-buck-1460438986
This has come up before in a couple of Physics Forums discussions, and it really isn't a paradox. Archimedes principle could use a slight changing of the wording, where it is the weight of the "effective" volume of water displaced, (i.e. volume below the waterline), that is equal to the buoyant force. With that qualifier, Archimedes principle works as it should.
 
  • #28
bob012345
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This has come up before in a couple of Physics Forums discussions, and it really isn't a paradox. Archimedes principle could use a slight changing of the wording, where it is the weight of the "effective" volume of water displaced, (i.e. volume below the waterline), that is equal to the buoyant force. With that qualifier, Archimedes principle works as it should.
I was using the common name. it doesn't mean I believe its a paradox. That's just what its called.
 
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  • #29
italicus
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Maybe you don’t know that, in compliance with Classification Societies rules ( consider f. i. the American Bureau of Shipping) , all ships, not only cruise ships, are to be put in dry dock periodically, for inspection , cleaning, painting and repairing purposes.
what is a dry dock? Well, in short, it is a kind of parallelepiped space, in free communicaation with the sea, in a shipyard, where the ship is carefully carried inside by means of tugs and ropes…and other technical facilities . When the ship is completely inside, still freefloating, a watertight door is put in place at the entrance of the dock, and some powerful pumps start pumping out the water left inside the dock, so the ship slowly goes down, until the bottom rests on a series of very robust wooden blocks, which all toghether react to the ship ‘s weight! All the water is taken out by means of smaller pumps, so that a lot of people can go down in the dock and do their jobs. First of all, cleaning and inspection.
So the ship is free floating until she touches the blocks : i have described the dry-dockingdocking operation in a few words, but you can imagine the complexity and required operational awareness.
 
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  • #30
sophiecentaur
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all ships, not only cruise ships, are to be put in dry dock periodically,
If you were to measure the time taken to empty the dock to ground the ship, the difference between times for a large and a small ship would correspond to the water they displace from the dock. That could be about the only way to find the weight of a big ship.
 
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italicus
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Furthermore, i will add that a dry dock is usually the best place to carry out the so called “inclining test” , to determine the lightship weight and the coordinates of the center of gravity ( empty ship). But of course now the door of the dock isn’t closed, the ship must be free floating!
I can’t add technical details, have a look at this:
https://en.m.wikipedia.org/wiki/Inclining_test
 
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  • #32
italicus
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@sophiecentaur

well, your idea could be a way to determine the weight of a ship. But actually there is a simpler method: when you a have a ship in harbor, you just read drafts fore, middle and aft, and make a mean ; then you use diagrams or computerized data, which give the displacement by means of Archimede ‘s law.
I have worked for more than 40 years as a marine surveyor, that was my job!
Thank you for the attention!
 
  • #33
sophiecentaur
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@sophiecentaur

well, your idea could be a way to determine the weight of a ship. But actually there is a simpler method: when you a have a ship in harbor, you just read drafts fore, middle and aft, and make a mean ; then you use diagrams or computerized data, which give the displacement by means of Archimede ‘s law.
I have worked for more than 40 years as a marine surveyor, that was my job!
Thank you for the attention!
That makes a lot of sense in practice but the accuracy depends on knowing absolutely everything about the structure and contents of the ship. No one would be in a position to challenge your answers but would it matter?
As a matter of interest, does the inclining test result agree well with the calculations that surveyors do? There is a philosophy that tells us to be pessimistic in design ratings and that works well except when a spot of corruption affects construction methods and materials.
It's interesting that most disasters where structures are involved can always be put down to commercial (and even criminal) interests, rather than the good old Engineers and Surveyors. Good regulation is the key.
 
  • #34
bob012345
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@sophiecentaur

well, your idea could be a way to determine the weight of a ship. But actually there is a simpler method: when you a have a ship in harbor, you just read drafts fore, middle and aft, and make a mean ; then you use diagrams or computerized data, which give the displacement by means of Archimede ‘s law.
I have worked for more than 40 years as a marine surveyor, that was my job!
Thank you for the attention!
For us Landlubbers, what does that mean reading drafts? Thanks.
 
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sophiecentaur
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For us Landlubbers, what does that mean reading drafts? Thanks.
Draft means the shallowest water that the boat can float in. That may be the same all along the bottom or in one place - for a sailing boat with a fin keel. Hence you may need to know the draft at several points along the bottom of a ship.
Marine terms are about the worst possible for the outsider - in quantity and in oddness.
 
  • #36
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I would not think it necessary. Prove displacement of water.

But if you really wanted. Build a triangular pool with a known volume. Fill it to the brim. Put a triangular ship (known volume and known weight ... just 8.3 lbs less than the weight of the water in the pool) in and measure the height of the ship and the water displaced. Show the weight of the ship and water displaced is the same. Take out 1 gallon. Measure the height of the ship and the water displaced. Show that the weight of the ship and the water displaced differs by the weight of the 1 gallon of water removed prior to displacement. Take out another gallon and repeat.

When you take out the last gallon, the ship is marginally lower. And your equation for the weight of the ship vs the water displaced (or removed prior to displacement) is suddenly unbalanced.

Personally, the unbalanced equation at the end of the sequence is enough for me. But the reality is that the ship is now lower, and a careful measurement will show that.

You could also go the other direction. Start with a gallon of water in transparent container. Use a large styrofoam bucket as your ship. Keep adding weights and measuring the height of the water around the styrofoam. Again you prove displacement. Your "ship" is always floating on a gallon, with the height based on the weight of the water displaced

The fundamental principle is difficult to show at the extreme case, and easy to show for non-extreme cases. I don't quite understand the reason why the extreme case needs proof, when the principle is established and no one needs a ship in a tank that is form-fitting.
 
  • #37
DaveC426913
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When you take out the last gallon, the ship is marginally lower. And your equation for the weight of the ship vs the water displaced (or removed prior to displacement) is suddenly unbalanced.
Marginally being the operative word here.

At the scale of a cruise ship and a bucket of water, that 'marginally' may be measured in microns.

And how do you demonstrate to a skeptic that that counts as floating?
 
  • #38
anorlunda
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One way used to remove fouling from a boat hull, without slipping the vessel, is to wrap the hull in a big plastic bag, pump out the water from between the hull and the bag, then pour a bucket of bleach into the gap to kill the fouling organisms.
I think that's a clever way to de-foul. A simpler way, if you can arrange it, is to simply sail into fresh water. The salt water creatures die quickly. When you return to salt water, the fresh water creatures attached will die quickly.
 
  • #39
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I wondered about this. Yeah, I think it would work.
It will if you do it right. Capacitance can be measured to exquisite sensitivity by counting individual electrons with a SQUID (vide Rod Harris-Lowe).
 
  • #40
italicus
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That makes a lot of sense in practice but the accuracy depends on knowing absolutely everything about the structure and contents of the ship. No one would be in a position to challenge your answers but would it matter?
As a matter of interest, does the inclining test result agree well with the calculations that surveyors do? There is a philosophy that tells us to be pessimistic in design ratings and that works well except when a spot of corruption affects construction methods and materials.
It's interesting that most disasters where structures are involved can always be put down to commercial (and even criminal) interests, rather than the good old Engineers and Surveyors. Good regulation is the key.
You can easily understand that it is not possible to know exactly what is the weight and the G coordinates ( referred to an Oxyz ideal reference frame, have a look at my sketch), of every steel plate, every section frame, every engine, motor, pump, compressor, pipe, electrical device, crane, rope, and so on, including furniture, that are assembled together to make a ship, whichever kind of ship : passenger, general cargo, bulk carrier, tanker, even navy ships!
So, concerning commercial ships ( but navy ship too are subject to similar rules), it is compulsory that , at the end of construction ( or almost the end...I cannot enter into details here!) , a ship undergoes an inclining test. This test is simple to describe in line of principle, but not so easy to carry out, believe me! I have directed a lot of tests like this, and it takes first of all an accurate survey of the ship in construction , in order to unload as many unnecessary items as possible, f.i. welding machines that are still on board...and other! The masses that create moments are moved from port to starboard several times, and the relevant angles of inclination are measured, in several ways : a test like that can require a whole day !

The result of the inclining test, imposed by international safety rules ( e.g. , first of all, the rules issued by the IMO = International Maritime Organisation) are sufficiently reliable, to be the basis of all subsequent stability calculations that are to be carried out in every given loading condition : the empty ship weight, and the coordinates of its center of gravity, are the basis for calculations of the ship’s stability future condition of loading, and these conditions are to be in compliance with the international safety rules , first of all SOLAS ( safety of life at sea) rules, which are in a continuous evolution and improvement , since the first London convention , which followed the TITANIC sinking. Nowadays , we are no longer in the conditions of the TITANIC , there are mountains of rules issued by periodical conferences at the IMO. IF you wish , look for its site, to get a simple idea ! You will be astonished against the quantity of rules to be applied in this context ! The shipping industry is a very complicated world , I hope you trust me.

It's interesting that most disasters where structures are involved can always be put down to commercial (and even criminal) interests, rather than the good old Engineers and Surveyors. Good regulation is the key.

Well, dear Sophie, don’t be so extreme! It is out of doubt that disasters at sea occur , but not always for commercial or criminal interests. A ship cannot depart a harbour with more goods on board than allowed by its Load Line marks, that establish the maximum allowed draft (Plimsoll marks, never heard?) : no Port Authority will let she go. Good regulation are there, and of course must be respected. But disasters sometimes occur for unforeseen circumstances, f.e. technical failure, and of course human errors, which are always behind the corner! I have been nominated by italian courts as an expert in some ships disasters, and have learned a great lesson from my experiences : big disasters are often due to a lot of concurrent causes, some of which may be considered not significant , if taken alone ! In any case, the heavy work of surveyors isn’t to be underestimated .

One of the guy, Bob 012345, asked what “reading the drafts” means; Bob, have a look at the attached sketch :
draft marks.jpeg


You have to imagine that the still water surface ideally cuts the ship in two parts : only the portion under the surface displaces water, of course. The Archimede’s principle says that the weight of the whole ship ( or a floating body in general) equals the weight of the amount of water displaced, for it is in equilibrium under its weight and hydrostatic buoyancy.
On both sides of a ship, draft marks are welded, fore, middle and aft. The mean draft allows you to enter the ship’s hydrostatic curves, nowadays replaced by computer calculations, which give you the immersed volume, so S = dgV ( d= density of water) , and other geometrical characteristics of the ship. But the final weight and position of G, when the ship is loaded, cannot be read from geometrical characteristics, they depend strictly on the amount of goods loaded , and their position on board.

Excuse me for my bad English.
 
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Marginally being the operative word here.

At the scale of a cruise ship and a bucket of water, that 'marginally' may be measured in microns.

And how do you demonstrate to a skeptic that that counts as floating?
I wouldn't call it floating either. Since capillary forces matter, it is likely that the force of gravity and buoyancy are exceeded by surface tension effects from the water.

I would actually expect deviation form the buoyancy masses earlier. The normally negligible surface tension force would probably create a measurable deviation as the volume of water becomes small, and the surface area large.

If that is true, then it is not correct to say that a ship with a large surface area can be floated in a very small water volume, with a carefully constructed tank.

I had not spend a lot of time thinking about the boundary conditions of buoyancy. If one wanted to take it to the ridiculous extreme, hypothesize a volume of water spread such that the depth is 1 angstrom. Obviously water molecules no longer fit. At the boundary conditions of a large thing and a very perfectly conforming tank ... it is not possible to prove buoyancy. Surface tension of water would matter.

Take a small barge. 200 ft x 50 ft. Call it 50 meters by 20 meters. So about 1000 square meters. Call it 4 liters of water, so 4 cubic decimeter of water. 4x10^-3 cubic meters. The barge on top of 1 gallon would squash it to a thickness of 4x10^-6 meters, or 1 micrometer.

That is a measurable distance.

But consider if instead you had placed a 1-gallon ball of tortilla dough under the barge. How thin can you squash the dough before it resists and supports the barge? Are you "floating" the barge?

1. The skeptic might be right.
2. It doesn't matter. I'm not invested in proving the boundary conditions of buoyancy in form fitting boundary conditions. I'm quite willing to allow the skeptic his doubt towards the limiting conditions of buoyancy. As long as the skeptic does not deny the basic principles.
 
  • #42
hutchphd
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surface tension effects from the water.
The font of all knowledge says the surface Tension of water is 72 mN/m. Gonna need a pretty small Ark for that to matter
 
  • #43
DaveC426913
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I wouldn't call it floating either. Since capillary forces matter, it is likely that the force of gravity and buoyancy are exceeded by surface tension effects from the water.

I would actually expect deviation form the buoyancy masses earlier. The normally negligible surface tension force would probably create a measurable deviation as the volume of water becomes small, and the surface area large.
Surface tension is something to consider - although I'm not sure, in the end, if it matters. Just because surface tension applies does not necessarily mean it negates the meaning of floating.

I mean, I can float a pin in a small glass of water that has a noticeable meniscus (surface tension). Though the pin's centre of mass might not be the same height in a larger container, does that disqualify the pin's claim to be floating?

But consider if instead you had placed a 1-gallon ball of tortilla dough under the barge. How thin can you squash the dough before it resists and supports the barge? Are you "floating" the barge?
Tortilla dough is not a liquid.
 
  • #44
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The point remains that there is a limit if we took that 1000 square meter flat barge, we had a 1 micrometer depth with the 4 liter volume. So 4 mls would be one 1-thousandth that depth, or 1 nanometer. 0.4 mls gets you to 1 angstrom, smaller than a water molecule. Put 0.4 mls under that barge and it will be higher than the height expected for "floating".

Likewise, a cruise ship that is 300 meters by 50 meters with a depth of 10 meters has 20,000 square meters of surface area in the water. The 4 liter volume spreads to one-twentieth of a micron, 50 nanometers.

Floating has a common meaning of freedom of movement. Not just buoyant suspension. When we constrain the volume to so small an amount that movement is no longer possible ... we may demonstrate buoyancy, but are we demonstrating floating?

Every "ship" will have a surface area below the water. We express buoyancy based on the volume based mass of water, and in theory, that could be demonstrated with the smallest amount of volume of water and a perfect tank. In practice, there are properties of water that matter and the boundary condition will always break down at some point. 1 gallon may well be close to that limit for some surface areas.
 
  • #45
anorlunda
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Floating has a common meaning of freedom of movement. Not just buoyant suspension.
Others may disagree with your points because they disagree on the freedom of movement as a requirement.
 
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Others may disagree with your points because they disagree on the freedom of movement as a requirement.
True. But a puck floating on an air hockey table, is not buoyant. We do somewhat consider the word floating to have a meaning beyond buoyancy. That may not matter to this debate.

EDIT: I've seen cool fountains with a spherical rock "floating" on water pumped from below. Again, this use of the word is not necessarily important, but definitions can cinfuse the issue.
 
  • #47
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I am somewhat unwilling to argue with that skeptic. If I have a ship that displaces 1 million gallons, I would only say that it displaces 1 million gallons. I would not make a claim that I could build a 1 million-plus-1 gallon tank and float it.

Sure in theory ... But it is not a thing anyone should TRY to prove. Prove displacement and buoyancy. Those matter. The actual experiment is really difficult. So let the skeptic be skeptical. If he wants to prove people wrong by building a 1-million-plus-1 gallon tank ... go for it.

I would add the surface area component I mentioned matters. 1 million gallons can have many surface areas.
 
  • #48
hutchphd
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Is anyone else concerned that we have passed the silliness threshold here? Angels and pinheads come to mind.....
 
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  • #49
DaveC426913
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Sure in theory ... But it is not a thing anyone should TRY to prove. Prove displacement and buoyancy. Those matter.
I'm going to hazard you haven't watched Mythbusters...
 
  • #50
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I'm going to hazard you haven't watched Mythbusters...
No ... but then again, I watch about 20 sports games per year on TV and nothing else. So the set of TV shows I haven't seen since Seinfeld is about everything.

I'm still of the opinion that the thing to say is that a particular ship displaces a certain volume. Anyone that wants to make statements for or against conformational tanks is free to do so. The fact is that a ship that displaces 1 million gallons, displaces 1 million gallons. If you want to buy a ship and try to prove some other thing about tanks and boats ... have at it. If you want to say the ship that displaces 1 million gallons DOESN'T displace 1 million gallons ... then we have disagreement.

I guess I hold with a position analogous to a Copenhagen-interpretation. I will say the fact I know (the ship displaces 1 million gallons) and if you want to make an unfounded statement about ships and impractical tanks ... I guess I don't care. Build your impractical tank and run the experiment.

This does seem like an angels-on-the-head-of-a-pin debate.
 

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