Whyt do aircraft carriers float?

Question

I know it has something to do with the amount of water they displace and bouyance but can anyone give me a better idea as to why it is they float?

Answer 1

So they can trap and launch aircraft...that's why they float.;)

Sorry, as a retired Naval Aviator who logged a lot of arrested landings on half a dozen or so aircraft carriers, I couldn't resist.

Anyhow, besides its anchor, an aircraft carrier weighs W tons. That means it'll sink because W = mg; where m is the ship's mass and g is the acceleration due to gravity. Thus, unless something neutralizes that weight, the carrier will disappear below the sea.

As it does not sink, as you noted, there must be that something to offset it's weight. And that something is called buoyancy B. Thus we have f = W - B = ma; where f is the net force on the carrier and a is acceleration of the ship due to the net force.

So we can see that, when W > B, that gives us W - B = ma > 0. This means a > 0 because m is always positive value. And a > 0 means the ship is accelerating at some rate into the ocean...it is sinking.

But B = Vrhog = Ah rho g; where rho is sea water density, V is the volume of sea water displaced by the carrier, A is the footprint or area of water displaced, and h is the depth of water displaced. h is how far the ship has sunk into the water. If you check the units of B, you will see that it is simply the weight of the displaced sea water.

A is fixed, it's the horizontal cross sectional area of the carrier. But h is not; as the ship sinks h gets larger. This clearly means B gets larger as the ship sinks. Eventually, it reaches a critical point where W = B; the buoyancy is equal to the ship's weight. That is to say, the weight of the displaced water (B) equals the weight of the ship (W). In which case W - B = f = ma = 0. Thus, a = 0 and the carrier no longer accelerates downward.

But it hasn't stopped sinking quite yet. The ship still has momentum p = mv and velocity downward. So we reach a point where W < B because the ship goes below the critical depth where weight and bouyancy are in balance. When W - B = -f = -ma, we have a < 0 and the ship accelerates back up towards the surface of the water.

This reduces B once again so W > B and the acceleration is again downward. This alternation between minus and positive acceleration because of the changes in B eventually dampen out. In the end, we have W = B at some depth h and there are no more oscillations around that depth. The carrier is floating and ready to receive and launch aircraft.

Answer 2

The bouyant force is equal to the weight of the displaced water. Think about it this way, if you lowered the carrier into the water, the carrier actually sinks until it displaces enough water to equal its weight.

Answer 3

you are right its bouyancy ... how it works is that it traps air in the bulk heads inside the carrier which makes it lighter than the water around it

Answer 4

Because if they didn't float the planes would never get off the deck.