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What Floats a Boat?

The mystery of how ships and boats float is one that baffles many people. Why is it that human beings can't walk on water because we're too heavy - yet a ship that's more than one million times heavier than a person can float with apparent ease?

An item as small as a gold ring will sink to the bottom of the ocean - but a ship with an area the size of a football pitch won't! So, how can a boat carry a mass amount of weight without the worry of sinking?

A boat can float and move on the water using its own power source, or driven by the wind. Most boats move partly above and through the water, apart from hovercrafts, which lift and move over the surface, or submarines, which move wholly under the water.

Cruise Ship

© Suphanat / Adobe Stock

 

Buoyancy

All boats float as a result of the scientific concept of buoyancy. The basic scientific rule is that any object will sink if it's heavier than the same volume of water, but thanks to buoyancy, even an object as heavy as a dense metal aircraft carrier can float.

To understand buoyancy, think of a submarine, which has fins known as diving planes on the side. It also has ballast tanks that can be filled with air or water to make it rise or sink as necessary. When the tanks are full of air, it will float - positively buoyant. This is because the tanks are lighter than the equal volume of water.

When the tanks are partially filled with air, then the submarine will float at a medium depth of water without going up or down - neutral buoyancy. If the tanks are totally filled with water, the submarine is said to be negatively buoyant and it will sink to the ocean bed.

Boats operate in a slightly different way, but with the same principle of buoyancy. A boat floats according to its own weight combined with the weight of anything it's carrying. For a ship to sail safely, we must estimate how much weight it can safely carry.

 

Mathematical formula

The formula used to work out the answer was devised by the Greek mathematician Archimedes in the third century BC. It is known as Archimedes' Principle after its inventor. He developed the theory after noticing how the water level of his bath rose as he climbed in, with his body displacing exactly the same volume of water.

He came up with the famous law of physics which states that when something is resting on water, it feels a buoyant, with an upward force equal to the weight of the water it displaces. When the object is submerged, the buoyant force pushes upwards and the object seems to weigh less when underwater than when on dry land.

The weight of a ship and its contents is also known as its "displacement" - meaning how much water it displaces when it's launched. For example, a ship with a displacement of 75,000 tons when empty and 95,000 tons with a full load sits lower in the water in the latter case.

The apparently magic buoyant force exists because water is a dense liquid that has a heavy weight and is almost impossible to compress. It exerts plenty of pressure - this is what you feel when you're swimming underwater or scuba diving.

When a boat floats partly submerged in the water, the pressure causes a force called up thrust to support it from underneath. When launched, the boat will be pulled down by its weight, sinking into the water, but pushed up at the same time by the up thrust, so it floats. It will sink into the water until its weight and the up thrust balance exactly.

 

History of boats

Although the theory of buoyancy was realised in the third century BC, of course, people have been sailing for thousands of years before this. They knew that boats would float, even though they couldn't understand why. The first boats on record were used in 10,000 BC. People made rafts, kayaks, dugouts from wood and simple boats from animal skins and bark.

Sails were invented between 5000 BC and 3000 BC by Mesopotamian sailors, while the ancient Greeks were the first race to build giant warships in around 1500 BC. The Romans built galleys with new innovations, such as a raised bridge, between 27 BC and 400 AD.

Innovations in the Middle Ages included the central rudder invented in 1200 and large sail-powered ships in 1400. Voyages of discovery were undertaken by Christopher Columbus between 1451 and 1506, Vasco da Gama between 1460 and 1524, Ferdinand Magellan between 1480 and 1521, and other pioneers of ocean exploration.

Cornelius Drebble built the first submarine from wood as long ago as 1620. In England, the first iron-hulled boat was built in 1777 and the first steamboat was built by Marquis d'Abbans in 1783. By the 19th century, regular shipping services were established, with timetables for passengers and cargo.

Modern propellers were invented in 1836 and one year later, the giant steam ship, Great Western, built by Isambard Kingdom Brunel, was the first iron boat to sail across the Atlantic. The first engine-powered submarine was designed by John Holland in 1870.

The first hovercraft was patented by English inventor John Thornycroft in 1877. Retired Swedish navy officer Charles Lundborg invented SWATH boats that floated above the waves on two submerged hulls in 1880.

 

Modern times

In the early 20th century, the upper classes enjoyed the romantic age of ocean travel on cruise liners. They were only for the wealthy, however. In 1943, marine gas turbine engines were used on boats for the first time. The US Navy launched the first nuclear-powered submarine, the USS Nautilus, in 1955.

In 1992, the US Navy scrapped its final giant battleship, USS Missouri. One year on, it launched a radar-invisible stealth boat, the Sea Shadow, which was in operation until 2011.

In recent years, the German-built MS Tûranor Planet Solar became the first vehicle powered by photovoltaic solar cells when it sailed around the world. The first electric battery-powered passenger ferry, the Norled MF Ampere, began operating in Norway in 2014, saving one million litres of diesel fuel annually.

As the marine industry continues to develop at a rapid rate, thanks to advances in technology, there are endless possibilities for the future of ocean travel and transport - with the ability to float still based on the physics principle calculated by Archimedes in the third century BC.

Here at Coruba, we're always on trend with our range of rubber boat matting to help prevent slipping accidents, and rubber fenders to cushion boats when approaching the dock. Contact us for further details of our marine and wet environment products.



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