Study their behaviors. Observe their territorial boundaries. Leave their habitat as you found it. Report any signs of intelligence.

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Saturday, September 08, 2012

A Battleship Would Not Float In A Bathtub

The world thinks (e.g. here, here, here, here, here) that a battleship could float in form-fitting tub of water after it displaced all but a tiny fraction of the water in the tub.  These writers cite Archimedes' Principle that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid the body displaces. They ignore Archimedes' assumption that the body of water has enough water and enough unused volume to combine together to balance the weight of the immersed body.

A floating ship is hydraulically balanced against the mass of the top layer of water that the ship has displaced upward in the body of water on which the ship floats.  A ship can only float if the body of water can contain that top layer of water and that water has a mass equal to that of the ship. If that water escapes or is otherwise not present, the equilibrium fails and the ship sinks.

Another way to think about it is to ask whether the battleship in the empty bathtub could be floated simply by pouring in the water to fill the bathtub around it. Battleship floaters claim this would work with an arbitrarily small amount of water.  But there is no free lunch -- you can't do the enormous work of lifting a massive ship merely by balancing it against a small mass of water.

Some floaters point to canal locks (e.g. the Miraflores in Panama) that can float a ship with just a foot of clearance on the sides (and allegedly the bottom).  However, they ignore the clearance at the front and back. After a ship enters a canal lock, you can bet that there is a new top layer of water (relative to the prior water level) whose mass is equal to that of the ship.

Floaters tell naive skeptics that when an object is floated in a full tub, the system doesn't remember that some water overflowed when the object was added, and that it floats just as well when it is taken out and then added back to the tub -- which now will not overflow at all.  However, what the floaters don't notice is that the no-overflow system has just enough room at the top to hold the mass of water that balances the object.

Some floaters point out that large machines like telescopes are often "afloat" on a thin film of oil. However, these lubricants are kept pressurized in a sealed system, and the pool of oil is not open to the atmosphere. It's a safe bet that such a telescope cannot be levitated (i.e. lifted) by a thin film of oil unless there is some balancing mass of oil (or some other way of pressurizing the oil).

It seems the physics folks who promote the bathtub battleship meme need to talk to some hydraulic engineers.

3 comments:

Markkimarkkonnen said...

Hi Brian,

This blog post is not correct; the battleship can float. Please see http://physics.stackexchange.com/questions/304245/can-a-ship-float-in-a-big-bathtub for several correct arguments including a point-by-point rebuttal of your post here.

Anonymous said...

Markkimarkkonnen is correct: Brian's post is complete nonsense, as any real hydraulic engineer could tell him. The assertion that Archimedes' principle contains some sort of an assumption about some amount of water that must be present before the ship enters the water is pure fantasy.

richard sullivan said...

The problem reduces to a close fitting piston in a cylinder which has one end sealed and the other end open to atmosphere. The piston can indeed be floated with a VOLOME of water less than its displacement volume. The effective weight of water in the cylinder as the piston floats equals the wieght of the water it freely displaces. Elevation of the water column surrounding the piston generates one psi for every 27.7 inches of elevation increase. The required volume required to float the piston off the bottom can be a small percentage of the free displacement volume. This leads to an interesting possibility......