How do ships float?
By Nisha Lad
Have you ever wondered why a
150,000 tonne steel boat floats,
whereas a small steel bar sinks
when it is dropped into water? The
bar sinks because the density of
the steel is greater than the
density of the water.
This is similar to when oil is mixed
with water – the oil droplets form
on the surface as they are less dense
compared with water.
But then how does that answer the
question as to how cargo ships and
ocean liners float, when they are
made of steel and thousands of
times larger than a steel bar.
Well, consider Buoyancy, average
density and Archimedes principle.
Firstly, consider buoyant force (or up
thrust), which is the opposing force to
gravity and acts upwards.
To understand this, think of an ice cube in a
glass of water. As the ice cube displaces
some of the water, it causes the level in the
glass to rise and the ice cube to float
partially in and out of the water.
This is because gravity is pulling the ice
cube down and buoyant force is pushing it
up. How far in or out of the water your ice
cube rests depends upon on its density.
The amount of water the ice cube displaces
is equal to the weight of the ice cube
immersed in the fluid.
This is Archimedes principle. An
object in a fluid experiences an
upward force equal to the weight of
the fluid displaced by the object.
So if a boat weighs 1,000 kilograms,
it will sink into the water until it has
displaced 1,000 kilograms of water.
Provided that the boat displaces
1,000 kilos of water before the
whole thing is submerged, then the
boat floats.
So back to our question: The ability of a boat to float depends on its average density.
Average density takes into account not just the weight of the steel hull but also the air
trapped in it. A ship with a large volume of trapped air has a lower density than that of the
water – so it floats.
This principle is vital in transporting goods overseas
today, as cargo ships that transport heavy loads use
especially long and wide hull construction to increase
the maximum amount of buoyant force. As the larger
the hull, the more trapped air the ship has, and
therefore the more buoyancy and it less dense than
water, so it floats.
Archimedes principle has many
applications. Hot air balloons use
this principle in the way that when
air is heated, hot air is less dense
than cold air due to thermal
expansion, and so the balloon will
rise.
Also with submarines; in order for it to sink the crew must pump water into the
ballast tanks, resulting in the submarine becoming denser than the surrounding
water. Archimedes Principle is also used to determine how much cargo you can put
on a ship without it being fully immersed in water.
So in conclusion:
By Archimedes principal every object when immersed in water will displace an
amount equal to its own weight. Also, in the case of the ship, due to the large
amount of air pockets on the ship and wide hull construction, there is a large
buoyant force created, and provided the object has displaced its own weight of
water before becoming fully immersed, it will float.