Floating an ocean liner - Mythbusters submission

  • #1
DaveC426913
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Main Question or Discussion Point

Okay, I've composed a letter to Mythbusters regarding the https://www.physicsforums.com/showthread.php?t=544619" thread.

(I started a separate thread here to avoid any weird recursivity issues in case the Mythbusters ever follow up on my link).


Before I submit it, can you guys offer any improvements to my text? (Such as how I can work in a date with nerdy-yet-ultra-hottie Carrie?)

On a physics forum (https://www.physicsforums.com/showthread.php?p=3583247#post3583247) we have been discussing the plausibility of floating an extremely large vessel in an extremely small amount of water. Ideally, an ocean liner in a bucket of water.

It is totally counter-intuitive - how can 40lbs. of water support a 100,000 tonne vessel? The best physicists and engineers on our forum rage against the idea - yet ultimately they come around to the realization that it is perfectly doable - at least in principle.

The idea is that the container holding the water is perfectly form-fitting around the ship's hull, with nary a millimeter of clearance. As the boat is lowered into the container, it will ultimately displace the small amoniut of water, which will rise up the hull until it reaches the waterline of the boat, at which point the boat will stop sinking before touching bottom, i.e. floating.

I won't go into the details of proving it is so, but it is most undeniably so. (You can always follow the thread above to hear all the arguments and counter-arguments.)

I think this needs to be seen to be believed!

In my opinion, the experiment would not be effective unless done at a relatively large scale. It's one thing to float a 10 gallon boat on a cup of water, but that's only a few orders of magnitude between mass doing the displacing and mass of remaining water. I think people would need to see it in terms of several hundred pounds - enough to make a many-orders-of-magnitude discrepancy between the mass of the vessel and the mass of the supporting water.

I envision a plexiglass box nested inside another plexiglass box, with only a millimeter clearance. (It needs to be transparent so you can see the water rising around the inner vessel.) The whole apparatus being at least as big as a refrigerator.

You'd need to demonstrate unequivocally that the bottom of the boat is not simply resting on the bottom of the container. We've thought of some ways to do this.
1] Electrodes. if the inner and outer vessels touch, an alarm goes off. Very tricky and error-prone.
2] Coloured water. If the water were deeply dyed with ink, you could see up through the bottom of the outer vessel and see that there is a thin coloured film of water all across the bottom. If it were touching, you'd see no inky colouring and could see right through to the inner vessel.

What think?
 
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Answers and Replies

  • #2
Simon Bridge
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Another test - put a metal plate on the bottom of the "boat" and the bottom of the container so they form a capacitor. The capacitance depends on the separation of the plates.

You could also use contact to set off an explosive - I know that's the same as the alarm thing but it is more likely to make it on to the show :)
 
  • #3
davenn
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Looks cool Dave

I would love to see them so some experiments along those lines. They have the financial ability to really go for broke.

as far as your text goes.... just do some proof reading ;) "small amoniut of " LOL

cheers
Dave
 
  • #4
DaveC426913
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as far as your text goes.... just do some proof reading ;) "small amoniut of " LOL

cheers
Dave
Yes. Content editing first, then proofing.
 
  • #5
sophiecentaur
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On the same subject, I remember when I was a kid having been taken on a School trip on to the breakwater in Plymouth Sound (UK). We were taken up in the lighthouse and told that the whole of the massive rotating lens / reflector mechanism was floating in a trough of mercury, as a bearing. Anything like that is conceivable as long as you get the tolerances tight enough between the floating object and the container.

I did worry, subsequently, about problems with the mercury dissolving the metal (brass?) of the unit.
 
  • #6
I just went through the other thread and I find this astonishing. It took me a while to realize why the ship could float but now that I know it, its so (no other word for it) cool! I do hope mythbusters takes this up
 
  • #7
sophiecentaur
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The water that Archimedes displaced was on the floor when he was floating. It need never have been in the bath in the first place.
 
  • #8
Borek
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This is a thread about submission of the problem to Mythbusters, not about the validity of the statement. If you want to discuss the statement, do it in the thread Dave linked to in his first post. That means HERE.
 
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  • #9
Q_Goest
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Interesting proposal! The only issue I see is to give Mythbusters something they can work with. Being this is a physics board, we should be able to hand them not just the myth but the method to test the myth also.

Let's model the boat as a brick to make it easy. A brick shaped boat the size of a row boat might be 10 ft long by 3 ft wide and have a depth of 1 ft fully loaded. This boat displaces about 1900 pounds. If the film of water on the outside of the boat is 1 mm thick, then the water will weigh 12 pounds or about 22 cups of water. Even on this small scale, the water is still much more than a cup. And manufacturing 2 row boat sized objects that have a uniform 1 mm of water film between them would be difficult. I certainly wouldn't cut a PO to Mythbusters to manufacture a boat and a tub that had exactly 1 mm between the two. Not that they aren't resourceful, but let's face it. They don't have high precision manufacturing technology at their disposal.

So the problem I think Mythbusters will have is just in manufacturability. We might suggest however, they use a boat and a 'mold' around the outside of the boat and inside a second, structural tub. The structural tub would have silly putty or clay or simillar material* on the inside, and the boat would be placed into this mold so the putty could conform to the shape of the boat. Under pressure like this, the putty will act like a fluid, filling in gaps and displacing the air (done properly). Once you have the mold made, you could then 'inject' water from a central location in the bottom of the mold, squeezing a film of water between the boat and mold of uniform thickness. Not much pressure is needed for this, just a few feet of head is all that's needed. So basically you just pour the water in. I think that would be something Adam and Jamie could manufacture without too much trouble.

As a side note, adding water would be kinda interesting too. You could measure how much water was added by having a graduated water bottle and a valve held a few feet above the contraption to provide the head pressure. Just opening the valve would allow water to flow into the gap and lift the boat. Then you could measure how much water came out of the graduated bottle.

*Cement might work well. Something that hardened to take the shape of the boat.
 
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  • #10
DaveC426913
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Hm. Does this discussion belong in this thread or the other thread?

The other thread should contain every thing we want the Mythbusters to read. This thread is for stuff we don't want to confuse them with.


And manufacturing 2 row boat sized objects that have a uniform 1 mm of water film between them would be difficult. I certainly wouldn't cut a PO to Mythbusters to manufacture a boat and a tub that had exactly 1 mm between the two.
I thought about this. What you do is a standard moulding(molding) technique. Make the boat. Apply a 1mm layer of wax or other removable material, then apply a hard coating on the outside of that to make the outer hull. You then build out a support structure for the outer hull (say, layer it out with concrete by a foot to keep it rigid).

Now peel the two away from each other and remove the wax.

Regardless of the precision of the manufacturing of the boat, your outer and inner hulls are perfectly form-fitting with a 1mm gap.

Once you have the mold made, you could then 'inject' water from a central location in the bottom of the mold, squeezing a film of water between the boat and mold of uniform thickness. Not much pressure is needed for this, just a few feet of head is all that's needed. So basically you just pour the water in. I think that would be something Adam and Jamie could manufacture without too much trouble.
I was thinking the other way around. Pour the bucket of water into the outer hull, then drop the boat into it.
 
  • #11
Q_Goest
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Well, the other thread is really a discussion of whether it will work or not. I have no doubt it will work. In fact, I think it's pretty obvious that it will work. My concern is with building something and not just show it works but make it easy enough to build. If it can't be built, it won't be put on TV.

How would the wax be put on so that it's uniform? Maybe spray it on?

Still, the wax is an extra step and probably not needed. Imagine a V shaped boat hull - if you didn't have the wax, you could still easily get a 1 mm gap on each side as it rises up. The gap will be slightly larger than 1 mm along the ridge at the bottom of the V shape, but it won't be significant.

Sounds like you would agree that a mold of some kind would be best for manufacturing, so I think a mention of it in your letter would be appropriate.
 
  • #12
AlephZero
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We did a similar experiment "by accident" a few years ago where I work, and it certainly "worked".

The basic setup was a thick cylindrical steel tank about 5m diameter and 10m high, in a hole in the ground. It was nominally sealed in place by concrete filling the (small) gap between the tank and the hole, but after years of being used to contain large amounts of energy released when components being tested (intentionally) failed, the concrete had become seriously cracked. The cracking had no effect on the functionality of the system, so nothing was done about it.

One day, there was an exceptional rainstorm which caused a flash flood. The flood water getting into the cracks and under the 100-ton tank lifted it about a foot out of the hole, and it then got stuck so it didn't sink back into place when the water drained away. There was nothing "forcing" the water underneath the tank, except the weight of the water.

Just to be clear, the tank was empty, and it was covered by a lid that stopped any flood-water getting inside it.
 
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  • #13
DaveC426913
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One day, there was an exceptional rainstorm which caused a flash flood. The flood water getting into the cracks and under the 100-ton tank lifted it about a foot out of the hole, and it then got stuck so it didn't sink back into place when the water drained away. There was nothing "forcing" the water underneath the tank, except the weight of the water.

Just to be clear, the tank was empty, and it was covered by a lid that stopped any flood-water getting inside it.
An excellent example, showing how something can be very heavy, while at the same time low density - less than water.
 
  • #14
DaveC426913
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Well, the other thread is really a discussion of whether it will work or not. I have no doubt it will work. In fact, I think it's pretty obvious that it will work. My concern is with building something and not just show it works but make it easy enough to build. If it can't be built, it won't be put on TV.
The other thread is really meant to include anything that the Mythbusters might find useful in reading up on the subject. I think our ideas will be a big help.
 

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