# Floating a cruise ship in a bucket of water

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TL;DR Summary
This myth has been around a long time. (I wish I'd sent it in to Mythbusters.) Do we all agree it is theoretically possible? Once we do, can we figure out a practical way of testing it?
OK. I'm sure we're all in agreement that it is theoretically possible to float a cruise ship "in" a bucket of water, right? If not, maybe we need to sort that out first.

A couple of practical provisos to start:
1. Allow some leeway on what constitutes a Cruise Ship for our purposes. I submit that it's going to have to be an ideal shape below the waterline - no props or any other shoes - just a straight-sided/bottomed brick, to a high degree of precision.
2. Allow some acceptable leeway on what minimum mass constitutes a valid test. A typical cruise ship may displace 100,000 tonnes. Is 1,000 tonnes enough? How about 1 tonne?
3. We are not floating it literally "in a bucket of water", we are floating it in an amount of water that a bucket can hold. (Say, 5 gallons?)
4. We come to an agreement on what constitutes "floating".
Eventually I'd like to figure out how this can be tested convincingly, and I foresee some practical challenges, such as:
• how you measure a layer of water just micrometers thick, and
• how you can convince someone that that is actually floating, and not just "a wet layer".

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how you measure a layer of water just micrometers thick
Electrical capacitance with water as the dielectric.

Imagine a cruise ship at sea. The hull surface is wet. All other water is the sea. The ship is floating in the wet layer, independent of the sea. The pressure applied to the hull through the wet layer is the hydrostatic pressure of the sea.

Lnewqban
Gold Member
To me, the greatest challenge would be to support the bottom of the bucket in such a way that the whole weight of the ship is transferred down to the ground, as well as to reinforce the sides of that bucket to resist the water pressure gradient without suffering deformations that make our ship "sink" due to insufficient height of fluid.

Homework Helper
First: I want to float Noah's Ark. Too many drunken souls on a cruise ship. So that's settled.
Here's my method:
1. You make a polyethylene film cover for the ark hull, but slightly larger
2. At the ark dock, measure the flotation height of the ark
3. Using divers surround the hull with the polyfilm hull and pump various amounts of water between the poly and the hull.
4. Check for variation in the flotation of the ark, now contained in a polyfilm container of variable size.
Hey, you're the one who asked. Be sure to feed the animals.

.

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The @hutchphd method is in use now.
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. That proves a boat can float in a bucket of bleach. Then replace the bleach with water, by removing the bag. QED.

russ_watters and hutchphd
...reinforce the sides of that bucket to resist the water pressure gradient without suffering deformations that make our ship "sink" due to insufficient height of fluid.
Deformation is a problem for the other side too. The deformation of the hull also asks for more water. So the size of the experiment is severely limited by the structural strength of the equipment/boat.

Regarding the proof of floating:
- adding more water should raise the ship deck level accordingly
- adding weight to the ship should displace water accordingly
...both proof is limited by the amount of water permitted. Needs quite the accuracy to get good results: above a size it's not convincing at all

Lnewqban
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I doubt you can do this experiment with a flat sided prism. I think you must use a spherical shell, ground against a diam/3 deep spherical socket. Equilibrate the temperatures, then float the shell in the socket.

diam/3
Just curious - why 3 particularly?

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Just curious - why 3 particularly?
Because it is about right, the sides are sufficiently steep but not vertical.
Maybe π would be a better choice for a mathematician.
But then, knowing the coefficient of friction an engineer could calculate the critical cone angle to avoid at which the sphere might lock in place and so not float.

jrmichler and Ibix
Gold Member
Electrical capacitance with water as the dielectric.

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I think I've got a good idea of how to show the floating.

Scrap the ship being a perfect brick shape, and sitting in a negative brick-shaped mold.

The ship can be any shape, but it has a cylindrical keel-like protruberance out the bottom.
It is only the keel that is immersed in water; the bulk of the ship is measurably above water level. (like a stationary hydrofoil with only one boom).

The volume of the keel alone displaces more water than the mass of the whole ship. The keel is, say, .5m in diameter and (r squared, carry the one) 5m tall will displace 1 ton of water.

It sits in a "bucket" that is 5m tall and .5m in diameter.

The singular advantage of this setup is the vertical motion of the ship will manifest as a vastly exaggerated rise of water in the "bucket".

This means that the 1 ton object could be shown to be floating because you could push down on it, and - while it would sink an almost unnoticeable amount - a peephole in the side of the bucket would allow you to see the water level rise dramatically.

Anyone who has cleaned up after a party, and stacked discarded cups together without fully emptying them of liquids knows how dramatically the water level can rise and gush out.

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Rive and jedishrfu
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The volume of the keel alone displaces more water than the mass of the whole ship. The keel is, say, .5m in diameter and (r squared, carry the one) 5m tall will displace 1 ton of water.

It sits in a "bucket" that is 5m tall and .5m in diameter.
The keel would need to be a conical right frustum to minimise the water volume requirement.

Back to capacitance and the cup and ball ...
The choice of material becomes important if both the cup and ball must be conductive. Surface oxidation of the metal will determine the radial clearance. "Grounding" of the hull needs to be immediately detected by conduction when attempting a capacitance measurement. That requires a gold plated sphere, but gold is not lubricated by water, so there will be surface welding issues.

The question is also one of how to generate conforming surfaces. A cone would be difficult, not just because of the point, but because the grinding cannot be done in more than one dimension. That keeps me returning to the sphere. Producing the spherical shell by grinding in the cup is an advantage and a disadvantage with glass. The problem is that glass tends to grind metals and other glass when water is present.

Going back to optical measurements with a glass cup and ball, it seems the friction coefficient of glass on glass in a water saturated atmosphere is very close to 1, which I guess is why grinding glass works so well. The coefficient is found in fig 2 at the end of of DH Buckley, 1973, "Friction behavior of glass and metals in contact with glass in various environments”.

DaveC426913
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Back to capacitance and the cup and ball ...
You are just so far ahead of my understanding I'm just going to give you an : informative :

Baluncore
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You are just so far ahead of my understanding I'm just going to give you an : informative :
Thanks, but I am just the translator.
Once I understand a problem my brain sub-conciously searches the maze of possible solutions without the handicap of language.

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I believe the large massive circular stabilized optical tables (think Michelson-Morley) were floated on a rather small volume of mercury in a somewhat conformal circular tub. In addition to reducing the amount of Hg required, making them conformal also makes their absolute level more stable against weight variations for the reasons given. Hadn't thought about that previously.

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"An object with a mass of 25.2 g will displace 25.2 cm3 of water. If the object has a volume greater than 25.2 cm3, it will stop sinking before it is completely submerged. In other words, it will float. If its volume is less than 25.2 cm3, it will not stop before its entire volume sinks below the surface."

I can't reconcile this to cruise ships and buckets. One metric ton of ship displaces one cubic meter of water. 100,000 metric ton ship displaces 100,000 cubic meters of water. If the ship has a larger volume it floats.

If the 100,000 ton ship were beaten into a 1 micron layer it would still displace 100,000 tons of water. The thickness of the ship would always be far thicker than the available water layer given 5 gallons spread out. There would not be enough water on an area basis to support the ship.

https://scienceprimer.com/buoyancy

Richard R Richard
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When a ship floats, the shape of the hole made in the water, is the same shape as the hull. The volume of water that the hull displaces does not need to be provided, because the hull fills that space.

If you make a mold to fit the boat hull exactly, and you then pour 1 bucket of water into the gap between the boat and the mold, the wet boat would float in the mold, on a 10 micron film of water.

Richard R Richard, bob012345 and Ibix
I can't reconcile this to cruise ships and buckets.
Think of building a ship and a socket for the ship that has the same exact shape up to the water line, but scaled up by some tiny amount. Fill the socket with water, then lower the ship in. The ship displaces its weight of water, leaving only the tiny extra amount as a thin layer between the hull and the socket.

But the displaced water has spilled over the edge of the socket, all over the lab floor, and down the drain. We didn't actually need it to start with - it's irrelevant. We could have started with the socket empty except for the amount of water that we expect to be left between the hull and the socket - one bucket full. This is the approach @Baluncore describes.

phinds, DaveC426913 and bob012345
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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?

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Summary:: This myth has been around a long time. (I wish I'd sent it into Mythbusters.) Do we all agree it is theoretically possible? Once we do, can we figure out a practical way of testing it?

OK. I'm sure we're all in agreement that it is theoretically possible
I remember, when I was at primary school in Plymouth (a famous naval port city in UK - Sir Francis Drake was a Devon man - "he sailed the seven seas" etc.) and we were taken on a trip to the breakwater which protects shipping in the bay. A great day trip for ten year old school kids. We were taken up the lighthouse that's one end of the breakwater and shown the rotating lamp mechanism. We were told that it floated in an annular (not the word they used for us) trough of mercury. That impressed us all and we couldn't actually imagine what that meant and Mercury was exotic and expensive, even in out small brains. I'm not sure why regular bearings wouldn't have done but the mechanism was clockwork and the mirror and shutter arrangement must have been a few tens of kg and they clearly needed a low power to drive it. This is a similar scenario to the cruiser and the bucket and we clearly understood that there was not a lot of actual mercury in the trough so the lighthouse contained floating proof of the application of Archimedes' Principle that does not involve the actual existence of the 'displaced fluid'.
But the practicalities were in favour of the mercury flotation; moderate load and good engineering tolerances. Distortion would not have been a huge issue as the 'keepers' had a blind to avoid solar heating effects.
The lighthouse was actually staffed during the fifties!

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Sounds to me like the concept being proposed here is like using a water layer as a lubricant. In the tight fitting mold discussed, is it sealed so the water cannot leak out like an air cushion designed to move around a heavy object? Do we say machine parts float on a layer of oil?

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Homework Helper
No the metal does not float on oil. A pressurized oil bearing relies on the the surface adhesion and viscosity of the oil to resist loading: too thin (or too hot) oil will not resist asymmetric load leading to catastrophic failure.

It is true that as the water film becomes thin the dynamics of motion of the system will change because water is viscous and there are surface interactions but the statics of Archimedes principle will hold.

bob012345
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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?
It is absolutely identical.

phinds and bob012345
Gold Member
Think of building a ship and a socket for the ship that has the same exact shape up to the water line, but scaled up by some tiny amount. Fill the socket with water, then lower the ship in. The ship displaces its weight of water, leaving only the tiny extra amount as a thin layer between the hull and the socket.

But the displaced water has spilled over the edge of the socket, all over the lab floor, and down the drain. We didn't actually need it to start with - it's irrelevant. We could have started with the socket empty except for the amount of water that we expect to be left between the hull and the socket - one bucket full. This is the approach @Baluncore describes.
Ok, I agree with this argument now. It floats. Here is an argument based on Pascal and the Hydrostatic Paradox.

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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.

sophiecentaur, bob012345 and Ibix
Homework Helper
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Ok, I agree with this argument now. It floats. Here is an argument based on Pascal and the Hydrostatic Paradox.

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.

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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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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.

sophiecentaur
Gold Member
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.

hutchphd
Gold Member
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

sophiecentaur
Gold Member
@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!

anorlunda
Gold Member
@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.

hutchphd
Gold Member
@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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