Archimedes' Principle explains why steel ships float

Some of the things around where you are were probably imported from the overseas or brought in through the shipping route. The ship could handle heavy loads and is still able to float without dropping like a rock in the seas.

In fact, even the world's largest cruise ship, Symphony of the Seas, can accommodate up to 6400 guests. It weighs a stunning 230,000 gross tons. That is more than 79,000 average car weight. This monster of the seas that would be available in April 2018 would float on water.

^{[Image from Wikipedia]Source: A Cruise Ship}


The reason the ship could float has something to do with buoyancy. So when next someone tells you whatever floats your boat, be quick to reply that it is buoyancy that does that.

This buoyancy is the upward force that counteracts the downward pull due to gravity to keeps the ship semi-submerged.

Buoyancy or buoyant force is defined by the Archimedes' principle.

Archimedes was a Greek inventor and mathematician born some 2304 years ago or in 287 BCE in Syracuse, Italy and killed in 212 BCE or 211 BCE when Romans overran the city.

Archimedes' principle states that when an object is wholly or partially immersed in a fluid (gas or liquid) that an upward (buoyant) force acts on the object. The volume of displaced water is equal to the volume of the immersed portion of the object.

This upward force or buoyancy is equal to the weight of the displaced fluid.

For example, an object in water sinks into the water till a point where the displaced water is same as the object's weight.

In the case of a ship, the same principle applies. It sinks further down in the water if there is an increase in the load it's carrying, but this is simultaneously matched by the more buoyant force acting in opposite direction that ensures that the ship floats.

So for an object to float or flotation to occur in a fluid the following holds true.

i. Buoyant force (a net force that the fluid applies to the solid during immersion in fluid) must be greater than and in the opposite direction to the weight of an object.

Buoyant Force > Weight

ii. The Volume displaced( V_d should be less than the total volume (V) of the solid

V_d < V

If V_d = V (object would sink)

It is impossible to have V_d greater than V.

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Upthrust

In other to explain how a ship floats in water, there is need to understand a certain property of water. The buoyant force keeps heavy ships all nice and afloat while carrying thousands of tons. The water is a moderately dense material and difficult to fairly impossible to compress.

That implies that it could exert a fairly large amount of pressure; this it does by having an upward omnidirectional push. A heavy ship, sitting on the water, meets a balanced water pressure acting in every direction except in the upward direction. These net forces are known as the upthrust, and it is what keeps the ship from sinking. But that is not all; the ship sinks but only to a certain degree which is determined by its weight, with the upthrust counteracting it.

The following is true of a ship on top of water

i. A ship always weighs less than water displaced; the submerge in water stops when weight and upthrust are in equilibrium. Note: Upthrust produced is dependent on the volume of water displaced. A solid block of steel of same mass with a ship sinks, but a ship of same mass floats.

^Source


The ship floats as it is not solid steel but rather hollow steel. At every particular moment, its average density is always less than that of water. It displaces water equal to its weight thereby producing an upthrust that keeps it afloat. A solid block of steel is denser than water and cannot displace enough water to create enough upthrust, so it sinks.

ii. If a ship sinks, one thing is evident: the upthrust produced is not enough. In other words, the ship weighs more than the sum of all volume of displaced water.

iii. The pressure of water increases with increase in depth. For every 33 feet dive into the ocean, the pressure increases by 14.5psi. That is roughly one third the recommended pressure of most light car's tire. That invariably implies that the more the water a ship displaces without sinking, the more the upthrust produced to keep it afloat.

The good news is now we all know what it takes to float our boats.

Thank you.

Reference

• ^{Biography of Achimedes}
• ^{Wikipedia: Archimedes Principle}
• Floating and Sinking Principle

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