Ocean liner in bucket full of water

[A.T.]

No. If we view the volume it takes up in the water as 'water-equivalent' (because the ship displaces the water, and only the water, equivalent to its mass), and also that the water level remains the same, so you can see there is no change.

Correct

If the vessel went fast enough and the sensors in your bridge were sensitive enough, you'd see an increase (above the normal) in load as the bow overpasses. A decrease (below the normal) as the stern follows is self-evident because there will have to be an 'average-normal' over the time interval that the vessel passes overhead.

Interesting effect when you go with a ship through a narrow channel, with reed at the bank. The water in front of the ship seems calm, but the reed bends towards the ship, long before the bug wave reaches it.

 

[A.T.]

volume displaced = volume of the ship below the water surface

I know that.

Then stick to it and don't confuse it with the volume of water outside of the ship.

 

[gulfcoastfella]

View the film of water holding the boat as being surrounded by boundary conditions. It doesn't matter what is holding the water in place, as long as the water experiences pressure on it's outside surface. The boundary condition means it can be water, steel, concrete, anything, as long as it is fortified enough to withstand the pressure.

 

[jmmccain]

boats aren't that deep.

Only as deep as they need to be... A boat doesn't care how close to shore it is - only that it has enough draft (or head, or whatever else floats your boat).

 

[DaveC426913]

View the film of water holding the boat as being surrounded by boundary conditions. It doesn't matter what is holding the water in place, as long as the water experiences pressure on it's outside surface. The boundary condition means it can be water, steel, concrete, anything, as long as it is fortified enough to withstand the pressure.

Yes. This is important. The .1mm layer of water next to the the ship needs to be held in place by something. In the open ocean it's more water; in a tank it's the walls. This is what ultimately holds up the 100,000 tonne vessel.

 

[dacruick]

Chris, you are going through EXACTLY the same thought process that I did. I vehemently opposed the correctness of the concept of floating a battleship in a bucket of water, but when you have thought it all through, you will realize, as I did, that it does indeed work.

The graphics, by the way, are selfcontained and have all the information you need to figure this out.

I'm acutally going back over the thread now with a pen and paper. ;D

Chris, I as well went through the exact same thing (as I'm sure you are finding out as you read over the thread), I realized I was wrong but I'm still not sure why. If I think about the concept of floating and extend it to this thought experiment, I show up with misconceptions, an intuitive crux. But if I start thinking about the water holding the ship in place in this bucket (pretty much the same thing) it helped me think in a less limited way about the situation. I'm still ironing it out so I'd like to hear what conclusion you come to.

 

[nasu]

Think about those nested glasses again. When the inner glass is empty, it will float very high. To get it to sink manually, you have to apply pressure to push it down. This will exert pressure on the walls. Enough pressure and you could break the outer glass.

Maybe if you do it very fast and the water has no time to escape, you may create a large pressure for a while and break the glass. I think this is what you have in mind, right?
Otherwise, at equilibrium, the pressure on the walls of the outer glass should not be more than the pressure when you have a glass full of water.

 

[mrspeedybob]

Some real calculations might be helpful to this thread.

Suppose I have a rectangular boat that is 100 meters wide and 1,000 meters long. It weighs 1,000,000 metric tons. The density of water is about 1 metric ton per cubic meter, for the sake of this calculation we will assume it is exactly that. Since the boat weighs 1,000,000 metric tons it will displace 1,000,000 cubic meters of water. We know the width and length of the boat multiply to 100,000 square meters so the depth of the hull in the water must be 10 meters. 10x100x1000=1,000,000. Since the sides of our boat are vertical there is no upward thrust on them, the only pressure that is relevant is the pressure at the bottom of the hull. The only surface it acts on is the flat surface at the bottom. As established earlier the density of water is 1 metric ton per cubic meter so the pressure at 1 meter of depth is 1 metric ton per square meter, this pressure arises from nothing more then the weight of the water pushing down from above. At a depth of 10 meters the pressure is 10 tons per square meter. Now suppose the column of water is not 1 square meter but 1 square cm. A column of water 1 cm x 1 cm x 1000 cm weighs 1 kg. The pressure at the bottom is 1 kg per square cm. There are 10,000 square cm in a square m so that is equivalent to a pressure of 10,000 kg per square meter, 10 tons per square meter. So the pressure is the same regardless of how wide or long the column of water is.

Now let's try to float our boat in a container that is 100.02 meters wide, 1000.02 meters long, and 10.01 meters deep. Each side of the boat is up against a column of water that is 1000.01 meters long, 10 meters deep, and .01 meters thick. That is 100.001 cubic meters of water on each side. Each end is a column of water 100x10x.01= 10 cubic meters. Since the boat has 2 sides and 2 ends the total volume of water (excluding the water that is under the hull) is 220.002 cubic meters. The surface area of this column of water is (1000.01x.01x2)+(100x.01x2)=22.0002 square meters. 220.002 metric tons per 22.0002 square meters = 10 tons per square meter.

10 tons per square meter of pressure applied to the 100,000 square meters of the bottom of the boats hull = 1,000,000 tons of upward force. This exactly equals the weight of the boat so it moves neither up, nor down. It floats with 1 cm of water beneath it.

That 1 cm layer of water is a total volume of 1000.01x100.01x.01=1000.110001 cubic meters. So our 1,000,000 ton ship is floating in only 1220.112001 tons of water.

Narrowing the margins from 1 cm to 1 mm or .1 mm does not change the physics.

 

[cmb]

Some real calculations might be helpful to this thread.

D'you mean like in #61?

 

[cmb]

the only pressure that is relevant is the pressure at the bottom of the hull.

No air pressure on the top?

 

[jmmccain]

Take all of the air away and the boat would still float.

 

[cmb]

Take all of the air away and the boat would still float.

I thought you'd have explained where/how the air pressure acts, if you wanted to give a thorough calculation.

The pressure in the water is the water column height and the air pressure above it, and on top of the ship is just the air pressure.

(I suspect that some of those new super-cruise ships, the air pressure is significantly higher in the water (it will carry sea level pressure) than that on top of its 40m height!)

 

[logics]

Fact that the water overflowed and is no longer present in the bucket doesn't matter. Hull occupies the place water should occupy, so it displaces the water..

There is no arguing: the principle is true and anyone can verify it, on a smaller scale.
If you put a heavy steel pan in any slightly-bigger vessel you'll see it floating, as long as water surrounds it up to *its waterline. [*where weight of submerged volume of water (displaced water)= its total weight]
But you get a most striking demonstration when you put it on the bottom of the empty vessel: you won't believe your eyes seeing it rise as you pour just a little water.

btw: probably it takes at least two buckets to float a liner

 

[DaveC426913]

Yeah, I'd love to have someone build a model and verify this for all. I know it works but it is extremely hard to explain to someone who can't see it.

Too small scale doesn't work. A teaspoon of water in a tumbler won't prove anything to a skeptic.

Question: how big a scale do you think a demo would need to be to prove the point?

I wonder what Mythbusters is doing these days?

 

[phinds]

Yeah, I'd love to have someone build a model and verify this for all. I know it works but it is extremely hard to explain to someone who can't see it.

Too small scale doesn't work. A teaspoon of water in a tumbler won't prove anything to a skeptic.

Question: how big a scale do you think a demo would need to be to prove the point?

I wonder what Mythbusters is doing these days?

Dave, I think an example with just a couple of drinking glasses would do the trick. I think this was mentioned somewhere above. Fill one with water and put the other one in, weighing it down with coins or something until it sinks down almost to the bottom. Now THAT is not going to prove it to a skeptic, but you then take the upper glass out, note that there is VERY little water in the lower glass and then put the upper one in again and note that it DOES float. If that won't convince'm probably nothing would. Of course, when this thread started, *I* probably would not have been convinced by that. I just was not willing to "get" it at first.

I love your idea of getting the mythbusters to do it with something REALLY large.

 

[DaveC426913]

Dave, I think an example with just a couple of drinking glasses would do the trick. I think this was mentioned somewhere above. Fill one with water and put the other one in, weighing it down with coins or something until it sinks down almost to the bottom. Now THAT is not going to prove it to a skeptic, but you then take the upper glass out, note that there is VERY little water in the lower glass and then put the upper one in again and note that it DOES float. If that won't convince'm probably nothing would. Of course, when this thread started, *I* probably would not have been convinced by that. I just was not willing to "get" it at first.

I love your idea of getting the mythbusters to do it with something REALLY large.

I can see skeptics believing a 150g glass can float on a teaspoon of water while still not believing that a 100,000 tonne ocean liner can float on a bucket of water. A teaspoon of water compared to the weight of the glass is just just the right amount. It just doesn't scale down in a way that's incontrovertably convincing.

It wouldn't have to be anything near full scale (that could be a bit impractical ), but you'd want it large enough that the weight of the object is orders of magnitude greater than the weight of the water.

 

[phinds]

... It just doesn't scale down in a way that's incontrovertably convincing.

It wouldn't have to be anything near full scale (that could be a bit impractical ), but you'd want it large enough that the weight of the object is orders of magnitude greater than the weight of the water.

Yeah, I can't argue with that.

I logged onto the Myth Buster's site to see about suggesting it, but it seems to require setting up a password with them, and I don't do that. It just gets you spam.

 

[DaveC426913]

Yeah, I can't argue with that.

I logged onto the Myth Buster's site to see about suggesting it, but it seems to require setting up a password with them, and I don't do that. It just gets you spam.

You've got a password here.


I'll write to them.

 

[phinds]

You've got a password here.

Yes but I CARE about being here (and the one other forum I'm on) ... don't care about being there.

I'll write to them.

Excellent

 

[jmmccain]

Perhaps a pair of nestable garbage cans would be large enough? That's a fair amount of water to be displaced and it's still cheap. Except for the coins...

 

[OmCheeto]

This thread reminds me of the only question I missed one day on an exam, about 32 years ago:

Regarding the red dots above, labeled A, B, and C, which one experiences the the highest pressure?

A. ____
B. ____
C. ____
D. None of the above ____

I felt so stupid.

And if Dave answers "C!", because Om's geriatric shaking, placed it one pixel lower than A & B, I will come and kill him...

 

[NascentOxygen]

The ship would be floating on water in the same way as the load in your old wheelbarrow floats on oil.
(EDIT)

 

[NascentOxygen]

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.

With no part of the ship's hull being in direct contact with (say) the wet-dock walls, then what is supporting the ship's weight?

 

[phinds]

The ship would be floating on water in the same way as your old wheelbarrow floats on oil.

Huh ?

 

[phinds]

With no part of the ship's hull being in direct contact with (say) the wet-dock walls, then what is supporting the ship's weight?

That has been COMPLETELY discussed. Please read the thread.

 

[DaveC426913]

The ship would be floating on water in the same way as the load in your old wheelbarrow floats on oil.
(EDIT)

He's talking about the oiled bearings.

But I don't think that's applicable. It uses viscous properties, not bouyancy.

 

[jmmccain]

NascentOxygen's comment was meant to point out that there are other places where a small amount of fluid supports a large force. The bearings (oil) supporting an automobile's piston rod is another.

 

[DaveC426913]

NascentOxygen's comment was meant to point out that there are other places where a small amount of fluid supports a large force. The bearings (oil) supporting an automobile's piston rod is another.

Yes but it is irrelevant.

 

[jmmccain]

Is it?

A fluid supporting an aircraft carrrier and a piston is that different? In a general sense?

The idea may be horrid, but even the piston is floating.

 

[logics]

it is extremely hard to explain to someone who can't see it.

Anyone can see it clearly in the picture if he is willing to see it: in the bottom picture...

...in the bottom picture...

..skeptics not believing that a 100,000 tonne ocean liner can float on a bucket of water... It just doesn't scale down in a way that's incontrovertably convincing..

....you see "a bucket" of water supporting a liner. is that impossible?
now take the liner away and put back in the same place/volume the missing water.
...you see same "bucket" of water is supporting 100,000 tonne of water. is that impossible?
can we "scale up" to the Mariana Trench, or to the Atlantic Ocean? I suppose we can. Is this controvertible?

 

[mushinskull]

This reminds me of the question:
There is a boat floating in a tub, the boat is full of stones. If you throw the stones out of the boat and they sink to the bottom of the tub, does the water level in the tub go up, down, or stay the same?

 

[cmb]

There is a boat floating in a tub, the boat is full of stones. If you throw the stones out of the boat and they sink to the bottom of the tub, does the water level in the tub go up, down, or stay the same?

An amusing addendum, indeed!

When in the boat, the stone displaces its mass of water.

Once in the water, it displaces only its volume.

(Ultimately, its mass will be supported by the bottom, rather than by bouyancy.)

As it is more dense than water (because the condition is that it sinks), therefore less water will now be displace by the stone. The water level will sink, as the boat rises.

 

[DaveC426913]

An amusing addendum, indeed!

When in the boat, the stone displaces its mass of water.

Once in the water, it displaces only its volume.

(Ultimately, its mass will be supported by the bottom, rather than by bouyancy.)

As it is more dense than water (because the condition is that it sinks), therefore less water will now be displace by the stone. The water level will sink, as the boat rises.

Nicely solved.

 

[McQueen]

Dave,
I think even the mythbusters couldn't solve this, because volume comes into the equation! Buoyancy depends on the object displacing an equal volume of water, right. There is no way that you can compare the volume of an Ocean liner with the volume of water in a bucket !

 

[Doc Al]

Dave,
I think even the mythbusters couldn't solve this, because volume comes into the equation! Buoyancy depends on the object displacing an equal volume of water, right. There is no way that you can compare the volume of an Ocean liner with the volume of water in a bucket !

It's the volume of water displaced that matters. Not the amount of water.