how do battleships float

[glenis]

if you see an aircraft carrier or a battleship face on they look like upside down pyramids with the pointy end down in the water ,i've always wondered how they stay afloat,why don't they topple over?

[SteveD]

Because the hull is not "pyramid-shaped" throughout its length.

[Jay70]

Anything will float if it weighs less than the water it displaces.

[tomd]

Boats have ballast (rubble and concrete, just dead weight) along the bottom, to keep them upright. The fins cutting deep into the water stop it blowing sideways in the wind (like a flat-bottomed boat would) and help it go in a straight line

[bernardo]

A boat is more stable if the centre-of-mass is low down.  If the hape of the hull is an upside-down pyramid, then the centre of mass would be below the water-level, so it would be stable.

[Hippy]

If you look at the ships out of water you will see that there is a sort of flattened bulge under the water line. This is the bilge containing balast. The point of this shape is that where the bow meets the water line it forms a thin edge to "part the water" and allow the ship to pass through faster for the same power than if it had a blunt bow. The front of the bilge will be domed for the same reason.

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[Ralph]

Ship stability is all about how the centre of gravity and the centre of buoyancy interact. The centre of buoyancy is effectively the geometric centre of the hole the boat makes in the water. Think of a section through a square boat where the waterline is half way up the side. The underwater body is rectangular and the centre of buoyancy is half way across this section and a quarter way up the total ship depth. Now lets say the centre of gravity is near the top of the section, like it would be in a kids wooden block if you stuck a 2p to the top. To be in equilibrium, the centre of gravity needs to be directly over (or under) the centre of buoyancy, if it is not, the boat will move until it is. If the boat heels over, the total area under the water needs to remain the same, but the shape will change. This change will mean that the new centre of buoyancy will have moved outboard. The centre of gravity will also have moved outboard, but not as much. The difference is called the righting lever. The total weight of the boat pushes down at the CofG while the total displacement (equal to the weight) acts up at the CofB a little distance away. These forces produce a twisting action which bring the boat back upright, or if you've got it all wrong, a twisting action that capsizes you.

Regarding your aircraft carrier, they are top heavy but if you consider the ENORMOUS weight of one of these beasts, it doesn't need a big separation of the CofB & CofG to produce more than enough twist to keep them upright. As long as the CofB remains ever so slightly outboard of the CofG, you won't be hearing about it on the news!

[Ralph]

Oh yeah, as has been said before, they are not pyramid shaped, most large ships have a roughly rectangular cross section in order to get the most cargo carrying capacity in, whether that's passengers, oil or F15s.

The ballast that's mentioned is used to control the centre of gravity and move it up or down. You may want to raise the centre of gravity to slow the rolling of a boat. A boat with too much stability will roll really fast and everything and everyone inside will be tossed around, not good especially if they are paying to be there and can afford really expensive lawers.

[AllYourBase]

F15s are land based aircraft. Carriers use F14s, amongst others

[glenis]

thanks to you all for your explanations especially ralph for such a detailed reply

[brachiopod]

As ralph says in his excellent answer;

"The difference..(between centre of buoyancy and centre of mass) is called the righting lever.. and if you've got it all wrong.. produces ..a twisting action that capsizes you."

 

See 'Zeebrugge "Herald of Free Enterprise" 1987' for a graphic illustration of what happens when you have a catastrophic, dynamic fluid mass intake, that shifts the centre of gravity beyond the 'righting limit' .

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Nothing sells like disaster - Let it be.

[Ralph]

Herald of Free Enterprise is an excellent example of stability going pear shaped. It was caused by a thing called the Free Surface Effect. Try to pick up a big tray that has been half filled with water, free surface effect gives a virtual rise in the centre of gravity so it's hard to carry very far without slopping from side to side. It's called a virtual rise, but its effects are very real. With the Herald of Free Enterprise, just a few inches of water all across the width of the car deck caused a rise in the centre of gravity which caused it to capsize very quickly.

Stick "Free Surface Effect" into Google, most people explain things better than I do!