# Ocean liner in bucket full of water

1. Oct 27, 2011

### Himal kharel

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

2. Oct 27, 2011

### dacruick

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

3. Oct 27, 2011

### cmb

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.

4. Oct 27, 2011

### rcgldr

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).

5. Oct 27, 2011

### phinds

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.

6. Oct 27, 2011

### mrspeedybob

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.

7. Oct 27, 2011

### dacruick

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

8. Oct 27, 2011

### phinds

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.

9. Oct 27, 2011

### mrspeedybob

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]

Last edited by a moderator: May 5, 2017
10. Oct 27, 2011

### mrspeedybob

Sure there is, 1 pound of water.

11. Oct 27, 2011

### phinds

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.

12. Oct 27, 2011

### Staff: Mentor

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.

13. Oct 27, 2011

### phinds

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.

Last edited: Oct 27, 2011
14. Oct 27, 2011

### 256bits

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.

15. Oct 27, 2011

### dacruick

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).

16. Oct 27, 2011

### A.T.

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.

17. Oct 27, 2011

### 256bits

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

18. Oct 27, 2011

### dacruick

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.

19. Oct 27, 2011

### A.T.

force = pressure * area

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

20. Oct 27, 2011

### nasu

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.