Ships and boats | How do they float? | History of ships (2024)

by Chris Woodford. Last updated: March 21, 2023.

You can't walk on water: you're too heavy andyou'll sink like a stone. But this aircraft carrier can float, even though it'sover 300m (1000ft) long, at least a million times heavier than you are, and carries about 70 airplanes and 4000sailors. Ships (large oceangoing vessels) and boats (smaller ones)are a brilliant example of how science can be put to work to solve asimple problem. Over two thirds of Earth's surface is covered inwater so it's just as well that science helps us take to the waves.How exactly do ships do their stuff? Let's take a closer look!

Photo: Running 342m (1123ft) from bow to stern, the aircraft carrierUSS Enterprisewas the longest warship in the world until its retirement in 2013. Despite the huge size of this ship, notice how its bow (front) is quite sharply pointed so it pushes the water aside, creating less resistance and allowing the ship to move faster and more efficiently. Photo by Brooks B. Patton courtesy of US Navy.

What are boats?

Not such a silly question! A ship or a boat (we'll call them all boats from now on) is a vehicle that can float and move on the ocean, a river, or some other watery place, either through its ownpower or using power from the elements (wind, waves, or Sun). Mostboats move partly through and partly above water but some(notably hovercraft and hydrofoils) lift up and speed over itwhile others (submarines and submersibles, which are smallsubmarines) go entirely under it. These sound like quitepedantic distinctions, but they turn out to be very important—aswe'll see in a moment.

Why do boats float?

All boats can float, but floating is more complexand confusing than it sounds and it's best discussed through ascientific concept called buoyancy, which is theforce that causes floating. Any object will either floator sink in water depending on its density (how much a certain volumeof it weighs). If it's more dense than water, it will usually sink;if it's less dense, it will float. It doesn't matter how big or smallthe object is: a gold ring will sink in water, while a piece ofplastic as big as a football field will float. The basic rule is thatan object will sink if it weighs more than exactly the same volume of water.But that doesn't really explain why an aircraft carrier (made from dense metal) can float,so let's explore a bit further.

Positive, negative, and neutral buoyancy

Buoyancy is easiest to understand thinking about asubmarine. It has diving planes (fins mounted on the side) and ballast tanks that it can fill with water or air to make it rise or fall as it needs to. If its tanks are completely filled with air, it's said to be positively buoyant:the tanks weigh less than an equal volume of water and make the sub float on the surface. If thetanks are partly filled with air, it's possible to make the submarinefloat at some middle depth of the water without either rising up orsinking down. That's called neutral buoyancy. The other option isto fill the tanks completely with water. In that case, thesubmarine is negatively buoyant, which means it sinks to theseabed. Find out more about how submarines rise and fall.

Photo: Submarines can rise to the surface or sink to any chosen depth by controlling their buoyancy. They do so by letting precise amounts of water or air into their ballast tanks. Photo courtesy ofUS Navy.

Buoyancy on the surface

Now most boats don't operate in quite the same wayas submarines. They don't sink, but they don't exactly float either.A boat partly floats and partly sinks according to its own weight andhow much weight it carries; the greater the total of these twoweights, the lower it sits in the water. There's only so much weighta boat can carry without sinking into the water so much that it...does actually sink completely! For ships to sail safely, we need to know how much weight wecan put in or on them without getting anywhere near this point. Sohow can we figure that out?

Archimedes' Principle

The person who first worked out the answer wasGreek mathematician Archimedes, some time in the third century BCE.According to the popular legend, he'd been given the job of finding out whether a crownmade for a king was either solid gold or a cheap fake partly made from a mixture ofgold and silver. One version of the story saysthat he was taking a bath and noticed how thewater level rose as he immersed his body. He realized that if hedropped a gold crown into a bath, it would push out or "displace"its own volume of water over the side, effectively giving him an easy way to measurethe volume of a very complex object. By weighing the crown, he could then easily work out itsdensity (its mass divided by its volume) and compare it with that ofgold. If the density was lower than that of gold, the crown was clearly a fake.Other versions of the story tell it a slightly different way—and many people think thewhole tale is probably made up anyway!

Upthrust

Artwork: The weight of a ship pulling down is balanced by upthrust—the pressure of the water underneath, pushing up.

Unfortunately, none of this really explains why an aircraft carrier floats! So why does it?Where does that "magic" buoyant force actually come from?An aircraft carrier occupies a huge volume so its weight is spread across a wide area ofocean. Water is a fairly dense liquid that is virtuallyimpossible to compress. Its high density (and therefore heavy weight) means it can exert a lot of pressure: it pushes outward inevery direction (something you can easily feel swimming underwater, especially scuba diving).When an aircraft carrier sits on water, partly submerged, the water pressure is balanced in every direction exceptupward; in other words, there is a net force (called upthrust)supporting the boat from underneath. The boat sinks into the water, pulled down by its weightand pushed up by the upthrust. How low does it sink? The more it weighs (including the weight it carries), the lower it sinks:

If the boat weighs less than the maximum volume of water it could ever push aside (displace), it floats. But it sinks into the water until its weight and the upthrust exactly balance.
The more load you add to a boat, the more it weighs, and the further it will have to sink for the upthrust to balance its weight. Why? Because the pressure of water increases with depth: the further into the water the boat sinks, without actually submerging, the more upthrust is created.
If the boat keeps on sinking until it disappears, it means it cannot produce enough upthrust. In other words,if the boat weighs more than the total volume of water it can push aside (displaces), it sinks.

Upthrust—made simple

To get the idea of upthrust clear in your mind, think about what happens as you load a ship.

With no load onboard, the ship sinks into the water by a certain amount. The amount of water it displaces (shaded area) weighs as much as the ship. The weight of the ship pulling down (red arrow) and the upthrust pushing up (blue arrow) are equal and opposite forces, so the ship floats.
Now what if we start loading the ship? It sinks down further, displacing more water (bigger shaded area). The weight of the ship and its load pulling down (red arrow) and the upthrust pushing up (blue arrow) are still equal, but now both are bigger.
Suppose we load the ship a bit more so that it just sinks beneath the surface but continues to float. Again, the weight pulling down and the upthrust pushing up are equal, even though both are bigger. But at this critical point, the ship is displacing as much water as it possibly can, so the upthrust cannot get any bigger.
We didn't like the ship much anyway, so let's add a lead weight on top (a weight that's dense enough to sink all by itself).No matter how much weight we add, the ship cannot produce any more upthrust: once it's completely submerged, whatever depth it sinks to, it can only ever displace a certain amount of water and create a certain amount of upthrust. Now the weight of the ship is more than the maximum possible upthrust so it sinks to the bottom. Suppose we attached a giant weighing machine to the top of the ship at this point. The apparent weight of the ship plus its cargo would be much less than expected, by an amount equal to the weight of the displaced water (the size of the upthrust). In other words, if we wanted to raise the ship to the surface from the seabed, we'd need to use a lifting force equal to the difference between the weight and the upthrust (the red arrow minus the blue arrow).

How do we know that the upthrust on something is equal to the weight of fluid it displaces?

If you can bear a little bit of math, it's very easy to prove! We need to know two general bitsof physics to do it. First, that pressure is defined as forceper unit area (force divided by area), so the force on a given surface of the box is the pressure times the area of thatsurface. Second, that the pressure (P) at a given depth (h) in a fluid is equal to the depth times the density (ρ) times g (the acceleration due to gravity). Or P = h × ρ × g.

Now take a look at this submerged box. How big is the upthrust?

The water pressure at the top of the box is h1 × ρ × g while the pressure at the bottom ish2 × ρ × g. The difference in pressure is (h2−h1) × ρ × g = h × ρ × g.

Because the area of the box is the same throughout, the difference in force is simply the difference in pressure times the area of the box: h × ρ × g × (w × l)

But because (h × w × l) is the volume of the box, and ρ is its density (or its mass per unit volume), that's the same as saying the difference in force is equal to m × g, where m is the mass of the fluid the box pushes aside. m × g is another way of writing the weight of this fluid.

So we've very quickly proved that the upthrust is equal to the weight of the fluid the box displaces. In other words, the more fluid the box displaces (the bigger the box), the bigger the upthrust. And that's why bigger boats—ones spread out wider, to occupy more volume—can carry more stuff.

Average density

If you still find the idea of upthrust a bit confusing, just go back to thinking about density. Imagine if the aircraft carrier were a giant metal box of the same volume (like a big shoe box), partly filled with engines, airplanes, sailors, and everything else—so it weighs just as much. That box would float if it weighed less than a box the same size filled with water; otherwise it would sink. So, in short, an aircraft carrier floats because it weighs less than the same volume of water—because its average density is less than that of the water that surrounds it.

Photo: The simplest way of understanding why things float is to forget about Archimedes and think instead about density. A ship floats because its average density is relatively small. This empty military transport ship is effectively a giant empty metal box. Divide its total mass (its own mass plus that of its contents) by its volume and you get its average density. That's less than the density of a solid metal box or a metal box filled with water, and that's why the ship floats. Photo by Gary Keen courtesy of US Navy.

Why don't boats capsize?

If you've ever climbed onto a rowboat tied to ajetty, you'll have a sense of how very unstable small boats can bewhen you start to load them. When you stand in a boat, you effectivelybecome a part of it, so you alter its center of gravity,instantly making it much higher up. Since youenter a boat from the side, the minute you step onboard, you shiftthe center of gravity over toward the place where you're standing.Now the center of gravity is no longer above the center of the boat, and that makes the whole thing rotate toward you. A boatcan swing from side to side relatively freely; because it's positively buoyant, it takes only a smallforce to make it move in the water. All these things combined make asmall boat relatively easy to capsize (rotate onto its side andoften sink). By sitting down in a small boat, you lower the center of gravity and makeit more stable.

Photo: Small boats can be quite unstable, even with a keel. The strong wind couldcapsize this boat, which is why the crew are all sitting along the edge to balance the turningforce with their own weight. Photo by Chad Runge courtesy of US Navy.

Bigger boats don't suffer from this problem.First, they tend to have heavy metal engines very low down beneath thewater level, which gives them a low center of gravity and makes them more stable. Second, they have abigger keel (a vertical board running underwater, from back to front, along thecentral "spine" beneath the boat. The keel helps to stopa boat rolling (moving from side to side) or capsizing,because it means more force is required to push the boat sidewaysthrough the water or rotate it. The keel also plays a part insteering and propelling the boat, as we'll see shortly.

How do boats move?

Gravity is the force we have to work against when we walk on land. Butif you're a keen swimmer,you'll know it's not really an issue when you move on water because your body is relatively buoyant:even though your body is mostly water, it's not all water (it weighs less than a bagof water exactly the same size). Water resistance (drag) is by far the biggest force swimmers have to work against—andthat's also true of boats. The more weight a boat carries, the lowerit sits in the water and the more water resistance it creates. That's why boats have sharpnarrow bows (to push water cleanly out of the way) and curvedfront edges that plane (lift them up out ofthe water as they move along). Hydrofoils push this idea to the limit by usingunderwater wings to lift their hulls up and clear of the water as they move along.

Photo (above and below): A hydrofoil is a type of boat that uses underwater wings to generate lift as it speeds forward, raising the hull above the waves to reduce water resistance. Photo by Mark S. Kettenhofen courtesy of US Navy and US National Archives. Original photos are hereandhere.

Like most other objects, boats propel themselves byharnessing Newton's three laws of motion: 1) They don't go anywhereunless a force of some kind pushes or pulls them; 2) When there is asuitable force, it makes them accelerate (move faster or in a newdirection) and a bigger force will accelerate them more; 3) If a boatwants to go forward, it has to apply a backward-pushingforce—just as you have to kick back with a skateboard to shoot offdown the sidewalk (or push back with your legs to walk forward).

Most boats use one of three different kinds of power: oars or poles, sails, and engines.

Oars and poles

Photo: Human power remains a dependable way to power a boat, but unless you have an equal number of oars on both sides (or keep swapping sides), you'll move to one side more than the other. Photo by Clay Weis courtesy of US Navy.

The oldest form of boat propulsion is simple humanpower. You can row a boat by pulling the water backward with largepaddles or you can punt something like a raft forward by pushing offa*gainst a river or seabed. Oar-power reached its pinnacle with theamazing galleys developed in Greek and Roman times. Biremes (perhapsdating from the second millennium BCE) had two platforms packed withrowers, while triremes (invented a few hundred years BCE) had three,quadriremes had four, and quinquiremes five (although it's uncertainwhether boats with so many people packed in, at such height, wouldhave been either stable or efficient). In any case, they soon gaveway to sail power.

Sails

If you hang bed sheets out to dry in a strongwind, you'll know exactly how sailing boats work! But boats don'talways want to sail away with the wind blowing straight behind them.In practice, that means the sails have to be positioned at an angle,but the wind will then try to blow the boat at that angle instead ofthe direction you want to go in. The force from the wind pushes youone way, so you need other forces as well, in other directions, tocorrect it and produce a resultant (combined) force in thedirection you actually want to go. Two other forces help. One is aforce from the keel. If the wind blows the boat partly sideways, thekeel pushes against the water and helps to power the boat in a forward direction. You can also angle the rudder at the back of theboat so, as the water hits it, it shoots off at an angle, steeringthe boat to one side or the other. (Rudders used to be positioned onthe right of the boat and called steerboards, which is why the rightside of a boat, facing forward, is still referred to as starboardto this day; the left side is called port.)

Photo: Yachts have multiple sails so they can catch the wind from any direction. Photo by Eric Brown courtesy of US Navy.

If you want to sail into a wind, you need to use atriangular or lateen sail hung at the front of your boat at anangle to the oncoming wind. As the wind hits the sail, it blowsaround it and accelerates; the sail acts in a similar way to anairfoil (the curved surface of an airplane wing), throwing air aroundand behind it and pushing the boat forward (just as the downward airfrom a wing propels a plane upward).

Engines

Old-time sailors were lost without wind, butthat's not a problem we have now, thanks to engines, which can beused to power propellers. The first engine-powered boats usedhigh-pressure steam engines fueled by coal; modern engines are invariablydiesel-powered. The only real problem with using internal-combustionengines to power boats is that they need a constant supply of oxygento burn the fuel; that means you can't use them to power submarinesunderwater. There is a workaround, however: you can use a dieselengine near the surface to drive a generator and chargebatteries,which are then used to drive an electric motor and the propeller whenthe sub is underwater. (Nuclear power is another option and means asubmarine can be underwater for weeks or months at a time.)

Not all engine-powered boats use propellers.Jet Skis® and jetfoils (large, hydrofoil versions of jetskis) useengines to power impellers (water pumps) that create a powerfulbackwater-pointing jet of water. The force of the water shooting backinto the water powers the boat forward, much like the hot exhaust gasfrom a jet engine.

What materials are boats made from?

Just about every material you can think of hasbeen used to make boats at one time or another. The first boats weremade from animal skin, bark, and wood; later came dugouts made byscooping the wood from a carefully chosen tree trunk. In ancienttimes, boatbuilders perfected the art of building boats up fromseparate planks, either by fixing the edges of one plank to the edgesof those around it like bricks in a wall (which is knownas carvel building) or, better still, byoverlapping the planks from the bottom upward (a technique known asclinker building), whichmakes for a stronger, lighter, and faster boat. The Industrial Revolution brought another great innovation:the age of mighty iron and steel ships. Most modern ships are stillbuilt from steel today, although it's relatively heavy. That's whysome larger boats are now made from strong, lightweight metals suchas aluminum, while smaller ones are often made from lightcomposites such as fiberglass or super-strong plastics like Kevlar®.

Artwork: Clinker-built boats (left) have overlapping planks, which gives a much stronger hull; carvel-built boats (right) have the planks joined end to end to form a smooth outer surface.

A brief history of ships and boats

The links in this timeline will generally take you to further information in Wikipedia and other sites.

Prehistory and ancient times

~10,000BCE: First boats include rafts, skin, hide and bark boats, kayaks, and dugouts.
~5000BCE–3000BCE: Mesopotamian sailors invent sails.
~3000BCE: Ancient Egyptians make the first boats from wooden planks.
~2500BCE: Minoans andMycenaeans become the first great ocean navigators, exploring the Mediterranean sea.
~1500BCE–27BCE: Greeks build giant warships, including biremes and triremes.
27BCE–400CE: Romans build galleys, the ancient ancestors of modern warships, with innovations including a raisedbridge.
~300BCE: Archimedes (287–212BCE) figures out the science of floatation.
200CE: Lateen sails are being widely used in the Mediterranean (though they are believed to have been inventedsomewhat earlier in the Arab world).

Middle Ages

300: Vikings invent clinker building.
1200: The central rudder starts to replace the "steerboard."
1400–1600: Most large ships are wooden-built andsail powered, and include cogs,carracks,galleys, and galleons.
1620: Cornelis Drebble builds the first submarine from wood.
15th century: Great voyages by Christopher Columbus (1451–1506),Vasco da Gama (1460–1524),Ferdinand Magellan (1480–1521),and others pioneer ocean exploration and circumnavigation.

The great age of shipping

Photo: The USS Constitution, nicknamed "Old Ironsides," is a classic, three-masted frigate (warship) dating from 1797.Long since retired from military duty, it's now a fascinating museum in Boston. Photo by Kathryn E. Macdonald courtesy of US Navy.

1777: First iron-hulled boat is built (in England).
1783: Marquis d'Abbans (1751–1832) builds the first steamboat.
19th century: Regular "packet shipping" services are established, sailing to timetables. Fast clipper shipsset sailing records.
1807: Robert Fulton (1765–1815) sets a record for traveling from New York City to Albany with a steamboat calledClermont.
1819: Steamboat SS Savannah crosses the Atlantic in a record 29 days.
1836: Modern propellers are invented(independently) by Francis Pettit Smith (1808–1874) and John Ericsson(1803–1889).
1837: Great Western, a giant steam-poweredship called built by Isambard Kingdom Brunel (1806–1859) becomes the first iron boat to cross the Atlantic.
1850s: English shipbuilder John Jordan makes probably the first composite ship by adding wooden-plank claddingover an iron framework.
1870-1898: John Holland designs and builds the first practical, engine-powered submarine, but struggles to convince the USNavy of its potential.
1877: English inventor John Thornycroft patents an early form of the hovercraft based on a floating lily pad.
1884: Sir Charles Parsons invents a highly efficient steam engine called the steam turbine. In 1897, he developsa steam-turbine-powered motor launch called Turbinia.
1886: German auto pioneer Gottlieb Daimler uses a gasoline engine to power a boat.
1886: Gluckauf, one of the first ocean-going tankers, is built and launched in Britain.
1880: Retired Swedish naval officer Charles G. Lundborg invents SWATH (smallwaterplane area twin hull) boats that ride high above the waves ontwo submerged hulls.

Modern ships

~1900s: Wealthy people enjoy the romantic,luxurious age of ocean travel onboard liners such as theMauretania,Lusitania, andAquitania.
1906: Enrico Forlanini invents the hydrofoil.Telephone pioneer Alexander Graham Bell also plays a part in its development.
1912: A "practically unsinkable" luxury linercalled Titanic secures a place in history as the most famous ocean catastrophe of all time when it hits an iceberg and sinks,killing over 1500 people.
1930s: Germans develop the snorkel, a breathing tube that allows diesel engines in submarinesto operate underwater, reducing the risk of detection.
1943: Marine gas turbine engines are used for the first time.
1955: US Navy launches first nuclear-poweredsubmarine, USS Nautilus.
1956: Ideal X, the first container ship, islaunched from Newark, NJ.
1959: Christopher co*ckerell's hovercraft makes its maiden voyage.
1960: Jacques Piccard and Lt. Don Walsh of theUS Navy dive to around 11km (6 miles) in the deepest part of theoceans, the Mariana Trench, in Trieste, a super-reinforced diving vessel (bathyscaphe).
1962: Scripps Institution's FLIP ship is first used tostudy the movement of ocean waves.
1964: A deep-diving scientific submersible calledAlvin is developed by Woods Hole Oceanographic Institution inMassachusetts and begins exploring the ocean to depths of almost 5km(3 miles).
1978: Scientific exploration vessel JOIDESResolution is launched, allowing scientists to explore the seabedin more detail than ever before.
1992: US Navy scraps the last of its giantbattleships, USS Missouri.
1993: US Navy launches an experimental, radar-invisible stealthboat called the Sea Shadow, based on a SWATH design. In 2011, the Navy announces it intends to scrap the vessel.
2012: The German-built MS Tûranor PlanetSolar becomes the first vehicle powered byphotovoltaic solar cells to circumnavigate the world.
2014: The Norled MF Ampere,the world's first fully electric, battery-powered car and passenger ferry, begins operating in Norway,and saving one million liters of diesel per year.

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On this website

Hovercraft
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Propellers
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Swimming

Books

General

Ocean Fleets by Allan Ryszka-Onions. Ian Allen, 2023. A detailed catalog of ocean-going ships currently working our seas. The focus is mostly on cargo and tanker ships, with a handful of cruise liners thrown in for good measure.
Ship by Brian Lavery. DK, 2017. A detailed, well-illustrated guide to the last 5000 years of maritime history.
The Great Passenger Ships of the World by Raoul Fiebig et al. Koehlers, 2016. A comprehensive guide to the world's cruise liners and other passenger ships.
The History of the Ship by Richard Woodman. Conway Maritime Press, 2012. A very readable, accessible history that takes us from ancient times all the way to modern container shipping.

Practical books

Know Your Boat: The Guide to Everything That Makes Your Boat Work by David Kroenke. McGraw-Hill Professional, 2002. A practical guide for boat owners.
How Boat Things Work: An Illustrated Guide by Charlie Wing. International Marine, 2007. A practical DIY guide to the nitty gritty of how small powerboats and sailing boats work, covering such things as engines, propellers, compasses, sails, and rigging.

For younger readers

Ships & Boats: Sail, Navigation, Radar, Anchor, Keel (Exploring Science) by Chris Oxlade. Armadillo, 2015. A hands-on experimental guide to the science of shipping. Ages 8–12.
First Book of Ships and Boats by Isabel Thomas. A &C Black, 2014. A 48-page introduction that covers each different type of boat in turn, with simply labeled illustrations (so you can compare their different functions and features). Ages 6–9.
Why do Ships Float? by Susan Markowitz Meredith. Infobase/Chelsea House, 2010. A good little (32-page) introduction to the most basic question about ships, but also covering submarines, military ships, and related topics. Ages 8–10.
Ships and Submarines by Chris Woodford. Facts on File, 2004. My own guide to the history of ship technology runs through chronologically from ancient times to the latest technologies. Although designed for ages 9–12, it's actually written at the same, general sort of level as this article and will be of interest to older readers too. Lots of photos and illustrations.
Ships by Chris Oxlade. Lorenz, 1999. A very simple 64-page introduction with 16 projects for children to try. Ages 9–12.
The Usborne Book of Cutaway Boats by Christopher Maynard. Usborne, 1996. An old but nevertheless wonderful introduction for ages 9–12 (relatively easy to find secondhand, if not new).

Articles

How container ships are built: The New York Times Interactive, 17 June 2020. A fascinating photographic sequence shows how the world's biggest ships are bolted together.