# Ocean liner in bucket full of water

Can we float an ocean liner in bucket full of water spread to a large area?

## Answers and Replies

You can float anything which has a total density less than water, as long as the water is deep enough.

Depends on how big the bucket is?

I suppose in theory, if you have a 'bucket' shaped exactly like the hull of the ship with just a tiny additional tolerance, the ship could technically 'float' in just the amount of water required to fill the gap.

A vessel floats once it has displaced a mass of water (from the volume it is floating in) equal to its own 'displacement', hence the term.

rcgldr
Homework Helper
Maybe it would be easier to explain that it's the volume of the "hole in the water" that makes the boat float, not the volume of water the boat is floating in (as long as there's some minimal amount of water to form a "hole" in).

phinds
Gold Member
2021 Award
Depends on how big the bucket is?

I suppose in theory, if you have a 'bucket' shaped exactly like the hull of the ship with just a tiny additional tolerance, the ship could technically 'float' in just the amount of water required to fill the gap.

A vessel floats once it has displaced a mass of water (from the volume it is floating in) equal to its own 'displacement', hence the term.

rcgldr has correctly debunked your idea, which is one that would have little or no displacement of water and absolutely would not work. You clear understand the displacement (your second sentence) so I'm puzzled why you would offer your first sentence.

rcgldr has correctly debunked your idea, which is one that would have little or no displacement of water and absolutely would not work. You clear understand the displacement (your second sentence) so I'm puzzled why you would offer your first sentence.

both rcgldr and cmb are saying the same thing and both are right.

Suppose I have a boat with a hemispherical hull that displaces 10,000,000 pounds of water. I have a hemispherical bucket that containt 10,000,001 pounds of water. The radius of the bucket is very slightly larger then the radius of the hull. Now I put the boat in the bucket, 10,000,000 pounds of water is displaced and flows out over the edge leaving a 10,000,000 pound boat floating in 1 pound of water.

Now I put the boat in the bucket, 10,000,000 pounds of water is displaced and flows out over the edge leaving a 10,000,000 pound boat floating in 1 pound of water.

You sure about that? If the water flowed over the edge of the bucket then the ship is no longer displacing anything...

phinds
Gold Member
2021 Award
both rcgldr and cmb are saying the same thing and both are right.

Suppose I have a boat with a hemispherical hull that displaces 10,000,000 pounds of water. I have a hemispherical bucket that containt 10,000,001 pounds of water. The radius of the bucket is very slightly larger then the radius of the hull. Now I put the boat in the bucket, 10,000,000 pounds of water is displaced and flows out over the edge leaving a 10,000,000 pound boat floating in 1 pound of water.

Nope. I don't believe it. They are NOT the same. If your bucket RETAINS the water that is displaced then the ship will float. If the water flows over the side and is gone, there's nothing left holding up the boat.

You sure about that? If the water flowed over the edge of the bucket then the ship is no longer displacing anything...

It's displacing the water that would be there if the boat wasn't

Here's another way to think of it...

A boat floats on the Atlantic ocean. The same boat can float in the same way on lake Michigan. Ignore for the moment that one is salt and the other fresh water. The physics of floating is identical regardless of the size of the body of water that the boat is floating in. Reduce the size of the lake to just lbigger then the size of the boat and the boat desn't know any difference, it doesn't know or care if the edge of the lake is 1mm or 1000km from the hull of the boat.

Go look at one of the old fashioned floating compasses people used to mount on their dash boards. There is a sphere 2-3 inches in diameter floating in a spherical container only a few thousandths of an inch larger.

[PLAIN]http://images.onccc.com/images/bfgx/product/middle/288-1.jpg [Broken]

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Nope. I don't believe it. They are NOT the same. If your bucket RETAINS the water that is displaced then the ship will float. If the water flows over the side and is gone, there's nothing left holding up the boat.

Sure there is, 1 pound of water.

phinds
Gold Member
2021 Award
Nope, I'm still not buying it. It just not going to work the way you say it will. It is NOT displacing any water unless that water is still around to push on the bottom of the boat.

Borek
Mentor
Nope, I'm still not buying it. It just not going to work the way you say it will. It is NOT displacing any water unless that water is still around to push on the bottom of the boat.

The water is all around - in the form of a thin film. What is important is that water exerts pressure on the hull - and it exerts pressure everywhere on the hull surface, just like it would in the case of the boat in the ocean. Yes, in theory it is possible to float an air carrier in a gallon of water.

phinds
Gold Member
2021 Award
The water is all around - in the form of a thin film. What is important is that water exerts pressure on the hull - and it exerts pressure everywhere on the hull surface, just like it would in the case of the boat in the ocean. Yes, in theory it is possible to float an air carrier in a gallon of water.

Hm ... I just can't see it at all. What is supporting the boat? There certainly is not enough pressure from a gallon of water to support it, so why does it not just move down to the bottom of the container (and thus not be floating)?

EDIT: The more I think about this, the more I'm convinced that I have it right. Take the following thought experiment. We have a cylinder of wood that floats half-in/half-out of a lake and it is being held (magically, for this thought experiment) vertical. Now we put the same cylinder, again vertical, in a pan that has an some water in it. You can, if you wish, assume that the "pan" is the same shape as the cylinder but with enough gap to allow the water to flow up the sides. Now, take "some" to be an amount of water that is just over half of the weight of the wood. It will float, just missing the bottom of the container. Now take "some" as an amount less that half the weight of the wood. I really can't see how it would not just push down to the bottom of the container and not float.

See, here's the thing. You HAVE to start off with enough water to allow the boat to float, otherwise it will never displace enough water. I guess if you could magically make the water sit precisely along the sides of the container, defying gravity, as you lowered the boat in, the the boat would float. Yeah, THAT'S where I'm not seeing it. I just don't see how you can start off in the right condition for it to work, but I can see that if you COULD start off in that impossible way, it would work. I'm an engineer, not a theorist and I say it won't work.

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256bits
Gold Member
The water is all around - in the form of a thin film. What is important is that water exerts pressure on the hull - and it exerts pressure everywhere on the hull surface, just like it would in the case of the boat in the ocean. Yes, in theory it is possible to float an air carrier in a gallon of water.

Absolutely correct.
For a more technical and solvable approach, if the ship is floating in say the ocean or lake, one would just have to make their control surface a thin film's distance from the hull. Replace whatever is on the other side of the control surface ( which would be water of the lake or ocean ) with say a metal bucket of any thickness desired that follows the thin film surface and, presto, the ship is floating in a bucket of water.

The water is all around - in the form of a thin film. What is important is that water exerts pressure on the hull - and it exerts pressure everywhere on the hull surface, just like it would in the case of the boat in the ocean. Yes, in theory it is possible to float an air carrier in a gallon of water.

Hm ... I just can't see it at all. What is supporting the boat? There certainly is not enough pressure from a gallon of water to support it, so why does it not just move down to the bottom of the container (and thus not be floating)?

I agree with phinds. It is not possible for a boat to float in water that is less than its weight. In your scenario, Borek, you have a magical barrier that contains the water in its thin film, or else what stops the water from shooting out of this bucket under immense pressure? You cannot simply assume that the water begins in a compressed state.

Lets say we have a boat floating in the ocean. If that boat were to disappear then there would be its volume of water rushing in to fill that hole. The reason that it was floating to begin with is because that water was applying pressure to the boat, equal to the boats weight. In your scenario you have no source of pressure. (Other than the contrived compressed state of the water).

A.T.
I'm an engineer, not a theorist and I say it won't work.

Here is a nice book for you Mr.Engineer:

It even has the battle ship floating in a bathtub on the cover. It is a classic physics question.

256bits
Gold Member
The reason that it was floating to begin with is because that water was applying pressure to the boat, equal to the boats weight.In your scenario you have no source of pressure. (Other than the contrived compressed state of the water).

Pressure depends on height of the water (column) not the volume of water.

Pressure depends on height of the water (column) not the volume of water.

I meant force. Show me where your force comes from. Explain how a thin film of water (uncompressed) exerts 10 000 000 lbs worth of force in any scenario.

A.T.
I meant force.

force = pressure * area

Since neither pressure nor the area change, why should the force change?

The "weight of the water displaced" is an useful tool to calculate the buoyant force. It does not mean that the action of "displacing" really needs to take place.
If we have the floating body in an empty container and start pouring the water, it will float at some moment. But it did not physically displace any water that was already in place.
So "there is no water to displace" is nor really an argument. And the "displaced" water does not need to be in the container. If I put 1 cm^3 piece of wood in a bucket of water it will displace 1 cm^3 of water. And it will float. Now if I remove the 1 cm^3 of water from the bucket, would the wood sink because the water displaced is not really there?

You can just look at it in terms of pressure difference.
For the simple case of a rectangular box immersed in liquid, the floating condition is $$\rho_{liquid} h_{liquid} > \rho_{solid} h_{box}$$

where h_liquid is the height of the liquid above the bottom of the box.
So we need enough liquid just to satisfy the condition, for a given size and shape of the container.
It seems that is hard to believe that a thin column of water will have the same pressure as a thicker one.

I suppose if the layer of liquid is very thin (just few molecular layers), superficial effects may change the behavior.

force = pressure * area

Since neither pressure nor the area change, why should the force change?

I don't understand your question. Why is this a matter of whether force changes or not? Tell me where the 10 000 000 lbs worth of force comes from to begin with. Satisfy, as nasu mentioned, the buoyant force conditions for me.

It is not uncommon these days, with space being at a premium and particularly miltary vessels coming in all shapes and sizes, for a wet dock to be pretty much as I described it - more boat than water!

For sure, there may well be less water in the wet dock than there is displacement of the vessel. This is known.

Sorry - yet another occasion to put your 'intuition' aside and trust what the physics says.

It is not uncommon these days, with space being at a premium and particularly miltary vessels coming in all shapes and sizes, for a wet dock to be pretty much as I described it - more boat than water!

For sure, there may well be less water in the wet dock than there is displacement of the vessel. This is known.

Sorry - yet another occasion to put your 'intuition' aside and trust what the physics says.

I can't believe that this idea is being proposed without a source of the force to keep the boat afloat.

EDIT: Poetic structure unintended.

Borek
Mentor
Borek, you have a magical barrier that contains the water in its thin film, or else what stops the water from shooting out of this bucket under immense pressure?

You are almost right - you just forgot we don't care about the bucket, we care about water. Yes, if the bucket is not strong enough, it will break. But the hull doesn't touch the bucket, so the ship floats in the water.

phinds
Gold Member
2021 Award
Oh, I got it now as a theoretically correct statement, as you'll see if you read my post carefully. What I object to, as an engineer, is the practicality (or actually the LACK thereof) ... I'm no longer arguing w/ the theory, just whether or not you could actually DO it, which I don't think you can.

You are almost right - you just forgot we don't care about the bucket, we care about water. Yes, if the bucket is not strong enough, it will break. But the hull doesn't touch the bucket, so the ship floats in the water.

I never once suggested that the bucket wasn't strong enough. I suggested that you cannot compress the water to create the buoyant force required for floatation. I feel as though your answer is inappropriately theoretical, and is based on a loose definition of floatation.

If you were to put a "cap" on the bucket so the water is contained within the bucket, then it would be the cap that is applying the retarding force, not the water. And that doesn't meet any definition of floatation.

A.T.
I never once suggested that the bucket wasn't strong enough. I suggested that you cannot compress the water to create the buoyant force required for floatation.

If you were to put a "cap" on the bucket so the water is contained within the bucket, then it would be the cap that is applying the retarding force, not the water. And that doesn't meet any definition of floatation.

Do you need a cap, when the ship floats in a lake?

Build a second hull around the floating ship, at small distance (no contact). Still floating?

Fix the second hull to the ground and pump out the rest of the lake. Still floating?

Do you need a cap, when the ship floats in a lake?

Build a second hull around the floating ship, at small distance (no contact). Still floating?

Fix the second hull to the ground and pump out the rest of the lake. Still floating?

no it is not still floating. I pray for you to tell me where the force comes from. You don't need a cap when a ship floats in a lake because it is displacing its weight in water. The water is pushing the boat from all directions, keeping it afloat. If you have water that is free to move in a bucket, and you put thousands of pounds of force on that water, it will displace unless it has no where to displace to.

EDIT: If it has no where to displace to, then there is a mechanism that is holding the water in place. I called it a cap.

A.T.
Do you need a cap, when the ship floats in a lake?

Build a second hull around the floating ship, at small distance (no contact). Still floating?

Fix the second hull to the ground and pump out the rest of the lake. Still floating?

no it is not still floating.

At which point does it stop floating? When you build the second hull or when you remove the water outside the second hull?

At which point does it stop floating? When you build the second hull or when you remove the water outside the second hull?

It stops floating when you build the second hull. The second hull will still float, but there will be contact between the second hull and the ship.

EDIT: My point is this, and I'd like it to be addressed.

No matter what scenario it is, there is the weight of the ship. If the weight of the ship is mass * gravity, and the ship is floating (not accelerating), where is this force causing buoyancy coming from?

Borek
Mentor

Do you agree the first one is floating?

Do you agree the second one is floating?

Do you agree the third one is floating?

Do you agree amount of water in the third one can be just a bucket?

256bits
Gold Member
I meant force. Show me where your force comes from. Explain how a thin film of water (uncompressed) exerts 10 000 000 lbs worth of force in any scenario.

Roller bearings, ball bearings, journal bearings ride on a thin film of oil nanometeres ( micro-inches ) thick with pressures approaching the MPa range. Consider that your car is supported on a thin film on each wheel nanometers thick, millimetersr so in width and centimeters long.

Scale that up to the size of a ship and a thin layer of fluid can support the weight of a ship,

Do you agree amount of water in the third one can be just a bucket?

Unless the weight of the water along the sides of the bucket is equal to the weight of your block, then it will not float. the block will sink to the bottom of your bucket, and push the same volume of water that was sitting under it out of the bucket.

A "thin" film suggests that the water's thickness approaches zero, which means that the water will have negligible weight, and therefore there is no plausible way that the block could be floating.

If you use your logic and approach the limits of the problem you will see a discontinuity. Borek, from your diagram, you could theoretically make the walls of the bucket disappear. Then you would have a block on top of an infinitesimally thin layer of water.

EDIT: And if you have no space between the walls of the bucket and the block, then you have compression. Which is the only method of possibility to this problem.

A.T.