how do catamarans float

How Does A Catamaran Float?

A catamaran, or cat, is a watercraft with two parallel hulls of equal size, providing resistance to rolling and overturning. They typically have less hull volume, smaller displacement, and shallower draft than monohulls of comparable length. Catamarans float due to positive buoyancy in their two hulls, achieved through sandwiching, which makes every piece of the hull float individually. The buoyancy force generated by Archimedes principle is when boats displace as much water as they weight, allowing them to float.

A hydrofoil catamaran works by using hydrofoils, which are wing-like structures mounted underneath the hulls of the boat. The air inside the boat is much less dense than water, allowing it to float. When boats displace enough water to counteract their weight, this is known as the buoyancy force. This force is crucial for understanding how ships float, move, and steer, including oar-powered boats, sailing ships, and modern engine-powered vessels.

Foils work in a similar way to aircraft wings, deflecting the flow and exerting a force on the foil. If the force is upward, the faster the boat, the catamaran floats according to the Archimedes law. Catamarans have two hulls in the water, both expanding a volume of water that weighs more than the weight of the catamaran, so the catamaran floats according to the Archimedes law.

The catamaran’s design includes an upside-down T-shaped foil, a horizontal piece under the water acting like a plane wing, and two floats that taper off the bow. The air-deck, another inflatable tube, provides better buoyancy, making the UFO a unique and efficient watercraft.

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I am a school teacher who was bitten by the travel bug many decades ago. My husband Billy has come along for the ride and now shares my dream to travel the world with our three children.The kids Pollyanna, 13, Cooper, 12 and Tommy 9 are in love with plane trips (thank goodness) and discovering new places, experiences and of course Disneyland.

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How Do Boats Float? A Look at How Boats Made of Steel Float

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Boats, from a small toy boat to a cruise ship to a massive cargo ship, manage to stay afloat on water. How do boats float ? Archimedes first recorded the standard definition of floating.

The Archimedes Principle states that 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 pounds (or kilograms), it will sink into the water until it has displaced 1,000 pounds (or kilograms) of water. Provided that the boat displaces 1,000 pounds (or kilograms) of water before being submerged, the boat floats. If not, the ship sinks.

It is not very hard to shape a boat in such a way that the weight of the boat has been displaced before the boat is completely underwater. The reason it is so easy is that a good portion of the interior of any boat is air (unlike a cube of steel , which is solid steel throughout). The average density of a boat — the combination of the steel and the air — is very light compared to the average density of water. So very little of the boat actually has to submerge into the water before it has displaced the weight of the boat.

How Floating Works

Why the titanic sank.

To better understand why cruise ships and small boats do not sink, we must understand how floating works. How do the water molecules know when 1,000 pounds of them have gotten out of the way? It turns out that the actual act of floating has to do with pressure rather than weight.

If you take a column of water 1 square inch (6.5 square cm) and 1 foot (0.3 m) tall, it weighs about 0.44 pounds (0.2 kg) depending on the temperature of the water (if you take a column of water 1 cm square by 1 meter tall, it weighs about 100 grams). That means that a 1-foot-high column of water exerts 0.44 pounds per square inch (psi). Similarly, a 1-meter-high column of water exerts 9,800 pascals (Pa).

If you were to submerge a box with a pressure gauge attached (as shown in this picture) into water, then the pressure gauge would measure the pressure of the water at the submerged depth:

If you were to submerge the box 1 foot into the water, the gauge would read 0.44 psi (if you submerged it 1 meter, it would read 9,800 Pa). What this means is that the bottom of the box has an upward force being applied to it by that pressure.

So, if the box is 1 foot square and it is submerged 1 foot, the bottom of the box is being pushed up by a water pressure of 63 pounds (12 inches x 12 inches x 0.44 psi). (If the box is 1 meter square and submerged 1 meter deep, the upward force is 9,800 newtons.) This just happens to exactly equal the weight of the cubic foot or cubic meter of water that is displaced!

Gravity exerts a downward force on the boat, attempting to pull it down into the water. To remain afloat, the boat must create an upward force that is equal to or greater than its weight. It is this upward water pressure pushing on the bottom of the boat that is causing the boat to float.

Each square inch (or square centimeter) of the boat that is underwater has water pressure pushing it upward, and this combined pressure lets ships float.

At more than 800 feet (243 meters), the Titanic, often referred to as "unsinkable," was not as big as a modern container ship. But despite the highly advanced technology used on the ship, it tragically sank on its maiden voyage in April 1912.

Several factors contributed to the ship's sinking , notably the damage caused to the ship after it struck an iceberg. The ship's designers included 16 watertight compartments so the boat could float even if damaged. However, as the compartments filled with water, the ship's overall density increased.

When the density of an object becomes greater than the density of the fluid it displaces (in this case, seawater), it loses buoyancy. The flooding of multiple compartments caused the Titanic to become denser than the surrounding seawater.

Can a Ship Be Too Big to Float?

Mostly, a boat sinks because its buoyancy cannot support its weight and cargo. But size alone doesn't determine a boat's ability to float. We have to also factor in the boat's design, construction , distribution, and operation. Regardless of its size and what moves the boat forward, a poorly designed ship is more susceptible to sinking.

It's important to note that large commercial vessels, such as cargo ships and cruise liners, are subject to stringent safety regulations and inspections to ensure their seaworthiness and stability. However, no floating boats are entirely immune to the risk of sinking if it encounters extreme conditions, is poorly maintained, or exceeds its operational limits. Proper design, construction, maintenance, and operation are essential for the safety of large boats at sea.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

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How Do Boats Float on Water: The Science Behind Buoyancy

The mesmerizing sight of boats gliding gracefully over the water has captivated human curiosity for centuries. The ability of these vessels to stay afloat, even while carrying considerable weight, has sparked wonder and intrigue. It is through the marvels of buoyancy that boats achieve this seemingly magical feat, and understanding this scientific principle unravels the mystery behind their effortless floatation.

Buoyancy, the force that enables boats to remain buoyant and defy gravity’s pull, lies at the heart of this enigma. In this article, we embark on an informative journey to explore the intricacies of buoyancy and how it works to keep boats afloat. As we delve into the principles of physics and fluid dynamics, we will unlock the secrets that underpin the captivating spectacle of boats gracefully navigating the water’s surface. So, let us set sail on this exploration to uncover the science behind how boats float with ease and grace.

Archimedes’ Principle as the Foundational Concept Behind Buoyancy.

At the heart of the science behind boats floating on water lies Archimedes’ Principle, a fundamental concept in fluid dynamics. This ancient principle, formulated by the Greek mathematician Archimedes, serves as the foundational explanation for buoyancy.

Archimedes’ Principle states that when an object is submerged in a fluid (like water), it experiences an upward buoyant force that is equal in magnitude to the weight of the fluid displaced by the object. In simpler terms, when an object enters the water, it pushes aside an amount of water equal to its own volume. The displaced water exerts an upward force on the object, which we call the buoyant force.

This buoyant force has a crucial role in enabling boats to float on water. When a boat is placed in the water, its hull displaces an amount of water that matches its own volume. This displaced water exerts an upward force on the boat, countering the force of gravity pulling the boat downwards. As long as the buoyant force is greater than or equal to the boat’s weight, the boat will remain afloat, seemingly defying gravity.

It is this exquisite interplay of forces—gravity pulling the boat downwards and the buoyant force pushing it upwards—that allows boats to maintain their equilibrium on the water’s surface. As we uncover more about the principles of buoyancy, we will gain a deeper appreciation for the seamless harmony between science and nature, witnessed every time a boat glides effortlessly over the water.

In-Depth Explanation of Displacement in the Context of Boat Floating

In the context of boat floating, displacement is a key concept that plays a pivotal role in the phenomenon of buoyancy. Displacement refers to the act of pushing aside water as a boat enters the water. It is this displacement of water that creates the upward buoyant force, allowing the boat to float.

When a boat is placed on the water’s surface, its hull comes into contact with the water. The shape and volume of the boat’s hull are purposefully designed to displace a specific amount of water, equal to the boat’s own weight. This design is crucial in ensuring that the upward buoyant force generated by the displaced water is sufficient to counteract the boat’s weight and keep it afloat.

To achieve effective displacement, boat designers carefully consider the hull’s shape and volume. A hull with a greater volume will displace more water, resulting in a larger buoyant force and greater capacity to carry weight. Conversely, a smaller hull volume will displace less water and have a lower buoyant force, limiting the boat’s carrying capacity.

The shape of the hull is also critical for efficient displacement. Boats with hulls that have a wide and rounded bottom displace a larger volume of water, providing excellent stability and buoyancy. On the other hand, boats with narrower and flatter hulls may displace less water, making them less buoyant and potentially less stable.

Moreover, the distribution of weight within the boat can influence its displacement and stability. A well-balanced boat with an even weight distribution will displace water evenly, ensuring optimal buoyancy. Uneven weight distribution, on the other hand, can lead to inefficient displacement and compromised stability.

In essence, displacement is the cornerstone of buoyancy in boats. By cleverly designing hulls to displace water effectively, boat builders harness the principles of Archimedes’ Principle, ensuring that boats stay afloat with grace and stability. Understanding the art of displacement illuminates the science behind boats’ effortless floatation and how their design and shape contribute to their buoyant prowess on the water.

Different Types of Hulls and Their Impact on Buoyancy

The design of a boat’s hull significantly impacts its buoyancy and overall performance on the water. Boats can be classified into two primary types based on their hull design: displacement hulls and planing hulls. Each type offers distinct characteristics that influence buoyancy and stability under different conditions.

Displacement Hulls: 

Displacement hulls are designed to push water aside as they move forward, displacing an amount of water equal to their own weight. These hulls are typically characterized by their V-shaped or rounded bottoms and are commonly found in sailboats, trawlers, and slower-moving vessels. The primary focus of displacement hulls is to ensure efficient buoyancy and stability at lower speeds.

Due to their displacement-focused design, these hulls create minimal wake and resistance, making them ideal for navigating at lower speeds. The buoyant force generated by the displaced water allows these boats to stay afloat while maintaining a steady and stable ride, even in rough waters. Displacement hulls are preferred for long-distance cruising and fuel efficiency, as they require less power to move through the water.

Planing Hulls: 

Planing hulls, on the other hand, are designed to rise on the water’s surface as speed increases, utilizing the hydrodynamic lift to enhance buoyancy during faster navigation. These hulls typically have flatter bottoms and are commonly found in powerboats, speedboats , and watercraft meant for high-speed operation.

As planing hulls gain speed, they create enough lift to rise above the water’s surface, reducing the amount of hull in contact with the water. This reduces friction and resistance, allowing the boat to reach higher speeds with less effort. The increased buoyant force generated by the hydrodynamic lift provides excellent stability and control while planing.

While planing hulls offer impressive speed capabilities, they are less fuel-efficient at lower speeds due to increased resistance when not planing. Additionally, their design may result in a rougher ride in choppy waters compared to displacement hulls.

In conclusion, the choice between displacement hulls and planing hulls depends on the intended use and performance requirements of the boat. Displacement hulls excel in stability and fuel efficiency at lower speeds, while planing hulls are designed for high-speed navigation and dynamic handling. Understanding the characteristics and impact of each hull type on buoyancy allows boaters to select the most suitable design for their specific boating needs and preferences.

Role of Ballast in Maintaining Boat Stability and Buoyancy

Ballast plays a crucial role in maintaining boat stability and buoyancy, especially in sailboats and larger vessels. Ballast refers to the weight, often in the form of lead or other heavy materials, strategically placed in the keel of sailboats or the hull of larger vessels. The primary function of ballast is to lower the boat’s center of gravity, enhancing its stability and preventing excessive rolling or capsizing.

In sailboats, ballast is typically found in the keel, which is the heavy fin-like structure located at the bottom of the hull. The ballast in the keel serves two essential purposes. Firstly, it counterbalances the force exerted by the wind on the sails, preventing the boat from heeling excessively or being pushed over by the wind. Secondly, the weight of the ballast in the keel lowers the boat’s center of gravity, improving overall stability.

For larger vessels like yachts or ships, ballast is commonly placed inside dedicated compartments in the hull. These compartments, known as ballast tanks, can be filled with water to increase the vessel’s weight and lower its center of gravity. When the vessel needs to improve stability, water is pumped into these tanks to provide additional ballast.

Proper ballasting is critical for optimizing buoyancy and safety during sailing. Insufficient ballast can lead to reduced stability, making the boat more susceptible to capsizing or rolling excessively in rough seas. On the other hand, excessive ballast can hinder the boat’s performance and create unnecessary drag, compromising fuel efficiency.

The distribution and management of ballast are carefully considered during the design and operation of a boat. Sailors and boat operators must understand the importance of ballast and ensure its proper maintenance to maintain optimal stability and buoyancy during their sailing adventures. With a well-balanced and properly ballasted vessel, boaters can navigate with confidence and enjoy a smoother, safer experience on the water.

Factors Affecting Buoyancy

A boat’s buoyancy can be influenced by several factors that impact its trim, draught, and overall stability. Understanding these factors is essential for maintaining safe and efficient sailing conditions.

• Load Distribution: The distribution of weight and cargo on board significantly affects a boat’s buoyancy and stability. Proper load distribution is crucial to maintain an even balance and prevent excessive leaning or heeling. Unevenly distributed weight can cause one side of the boat to be lower in the water than the other, leading to compromised stability and potential safety risks.
• Weight: The overall weight of the boat, including its structure, equipment, fuel, passengers, and cargo, plays a vital role in buoyancy. A boat’s design is intended to displace water equal to its weight, as per Archimedes’ Principle. If a boat is overloaded, it may not displace enough water to generate sufficient buoyant force, resulting in reduced stability and potential problems with flotation.
• Water Density: Water density can vary due to factors such as salinity and temperature. Seawater, which typically has higher salinity than freshwater, provides greater buoyancy due to its increased density. Conversely, colder water is denser than warmer water, affecting buoyancy calculations. These variations in water density can influence the boat’s draft and trim, necessitating adjustments in ballast or load distribution for optimal stability.
• Changes in Weight or Water Level: Changes in the weight on board, such as passengers moving around, or variations in the water level due to tides or loading/unloading cargo, can alter a boat’s buoyancy. Operators need to adapt to these changes by adjusting ballast or redistributing weight to maintain proper buoyancy and stability.
• Hull Shape and Design: The shape and design of a boat’s hull significantly impact its buoyancy and stability. Different hull types, as discussed earlier, have distinct characteristics that influence buoyancy. The hull’s shape can also affect how the boat reacts to waves and choppy waters, influencing its stability.
• Wind and Weather Conditions: Strong winds can exert lateral forces on a boat’s hull, causing it to heel or lean. Boats with proper buoyancy and stability can handle these forces better, ensuring a safer and more controlled ride.

In conclusion, numerous factors affect a boat’s buoyancy, trim, draught, and overall stability. Sailors must be mindful of load distribution, weight, water density, and changes in conditions to maintain a well-balanced and safely operating vessel. Adapting to these factors ensures that the boat remains buoyant and stable, providing a comfortable and secure sailing experience in various conditions.

Safety and Buoyancy Considerations

Maintaining proper buoyancy is paramount for ensuring safe and enjoyable boating experiences. Boaters should take specific safety considerations and precautions to optimize their vessel’s buoyancy and overall stability.

• Regular Maintenance and Inspections: Regular maintenance and inspections are essential to ensure the boat’s buoyant capabilities are not compromised. Check the hull for any signs of damage, such as cracks or leaks, which can impact buoyancy. Additionally, inspect all fittings, seals, and ballast systems to prevent potential water ingress or loss of buoyancy.
• Adherence to Weight Limits: Adhering to the manufacturer’s weight limits and load capacity specifications is crucial for preserving the boat’s proper buoyancy. Overloading the boat with excessive weight can lead to reduced buoyancy, diminished stability, and potentially hazardous conditions. Distribute weight evenly and avoid exceeding the recommended load capacity to maintain optimal buoyancy.
• Understanding Emergency Procedures: Understanding the principles of buoyancy is vital for proper emergency procedures and handling in case of water ingress or flooding. Should water enter the boat due to unforeseen circumstances, such as rough weather or hull damage, it’s essential to act quickly and decisively. Keep in mind that water entering the boat will displace air and reduce buoyancy, potentially leading to instability or capsizing. Knowing how to respond in these situations, such as activating bilge pumps or securing the boat’s position, is critical for maintaining buoyancy and ensuring safety.
• Buoyancy Aids and Safety Equipment: Carry buoyancy aids and essential safety equipment, such as life jackets and flotation devices, for all occupants on board. In the event of an emergency, these items can significantly enhance personal buoyancy and increase the chances of survival if the boat’s buoyancy is compromised.
• Boating Education and Training: Proper boating education and training are invaluable for understanding buoyancy principles, handling emergencies, and ensuring safe navigation. Boaters should undergo certified boating courses that cover buoyancy and safety protocols to enhance their knowledge and preparedness on the water.
• Weather and Water Conditions: Always be aware of the current weather and water conditions before embarking on a boating trip. Rough seas, strong currents, or adverse weather can impact buoyancy and stability. Avoid navigating in hazardous conditions and ensure the boat is appropriately equipped to handle potential challenges.

In conclusion, prioritizing buoyancy and safety is essential for all boaters. By performing regular maintenance, adhering to weight limits, understanding buoyancy principles, and being prepared for emergencies, boaters can ensure their vessels remain buoyant and stable, providing a safe and enjoyable experience on the water. A well-maintained and buoyant boat sets the stage for worry-free exploration and enjoyment of the waterways.

Watch How do ships float? (3D Animation) | Video

Top 5 FAQs and answers related to how do boats float on water

How do boats float on water boats float on water due to the principle of buoyancy. .

When a boat is placed in water, it displaces an amount of water equal to its weight. The displaced water exerts an upward force on the boat known as the buoyant force. As long as the buoyant force is greater than the boat’s weight, it will float on the water’s surface.

What is buoyancy? 

Buoyancy is the force that allows objects to float in a fluid, such as water. It is a result of Archimedes’ Principle, which states that any object submerged in a fluid experiences an upward force equal to the weight of the fluid it displaces.

What factors affect a boat’s buoyancy? 

Several factors impact a boat’s buoyancy, including its shape, volume, weight distribution, water density, and the presence of ballast. The boat’s hull design and material, as well as the amount of weight it carries, influence how effectively it displaces water and stays afloat.

Do all boats float the same way? 

Different types of boats float in various ways, depending on their hull design and intended purpose. Some boats, like displacement hulls, are designed to push water aside as they move, providing stability for slower-moving vessels. Planing hulls, on the other hand, rise on the water’s surface at higher speeds, using hydrodynamic lift to maintain buoyancy.

Can a boat sink? 

While boats are designed to float, certain circumstances can compromise their buoyancy and lead to sinking. Damage to the hull, excess water intake, or overloading beyond the weight capacity are some of the factors that can reduce buoyancy and cause a boat to sink. Proper maintenance, adherence to weight limits, and prompt response to emergencies are crucial to prevent sinking and ensure safe boating.

In conclusion, the phenomenon of boats floating on water is attributed to the principle of buoyancy, governed by Archimedes’ Principle. By displacing an amount of water equal to their weight, boats experience an upward buoyant force that keeps them afloat. Understanding the principles of buoyancy is paramount for safe and enjoyable boating experiences.

Throughout the article, we explored various factors affecting buoyancy, such as hull design, weight distribution, and ballast. Different types of boats, like displacement and planing hulls, float in distinct ways, catering to their intended purposes.

Appreciating the marvel of buoyancy in boat design enables boaters to prioritize safety and responsible practices. By adhering to weight limits, conducting regular maintenance, and being prepared for emergencies, readers can ensure their vessels remain buoyant and stable during their boating adventures.

With the knowledge gained from this explanation, readers can now better comprehend the science behind boats floating on water. As they set sail, they can navigate the waters with confidence, appreciating the principles that allow boats to gracefully and effortlessly stay afloat, enriching their boating experiences and ensuring safe voyages.

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April 12, 2012

Buoyant Science: How Metal "Boats" Float

A watery wager from Science Buddies

By Science Buddies

Key concepts Hydrodynamics Fluid dynamics Physics Water

Introduction Have you ever wondered why when you drop a steel nail into water it sinks like a stone, but when a well-built steel ship is in the ocean it floats, even though it weighs much more than a tiny nail?

The answer has to do with the fact that when an object is placed in water, water is pushed out of the way. You may have noticed this when you take a bath in a bathtub. Known as Archimedes' principle, as water is pushed away by an object, the water exerts a force back on the object that is equal to the object's weight. This is what helps make an object float.

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Background More than 2,200 years ago, a scientist named Archimedes sat down in his bath and figured out that when an object is placed in water, water moves out of the way—it gets displaced. If the object is floating, the amount of water that gets displaced weighs the same as the object. There is a force, called a buoyant force, which pushes on an object when it displaces water. The strength of this upward acting force exerted by water is equal to the weight of the water that is displaced. So, if an object displaces just a little bit of water, the weight of that small amount of water is small, and so the buoyant force is small, too. If, on the other hand, the object displaces a lot of water, then there will be a large buoyant force pushing upward. Whether an object sinks or floats depends on its density and the amount of water it displaces to create a strong enough buoyant force. How dense can an aluminum "boat" be before it sinks? Materials •    Cloth towel (or paper towels) •    Large, clear bowl or container •    Water •    Aluminum foil •    Metric ruler •    Pen •    Scissors •    Permanent marker •    Hammer or mallet Preparation •    Spread out the towel or paper towels on a hard work surface. Fill the bowl or container about two thirds full of tap water and set it on the towel(s). •    Measure out a square of aluminum foil that is about 25 centimeters (cm) square. Cut out the square. This will become the metal "boat" you test. •    Mark the four corners of the aluminum foil square with permanent marker. •    Be careful: later in the experiment you will use the hammer. Be sure to pound on a surface that can safely withstand the force and is resistant to damage. Procedure •    Pull the corners of the aluminum foil square together and crumple the square into a loose ball that is approximately six cm in diameter. Rumple the aluminum such that the marked corners stay together and are visible in one spot. •    Set the ball gently in the bowl of water, placing it so that the marked corners are at the top of the ball, as this will help prevent the ball from filling up with water. Immediately observe the ball. Does it sink or float? •    Get down low so that you are at eye level with the aluminum foil ball and quickly observe how much of the ball is below the surface of the water. Is about 10 percent, 25 percent, 33 percent, 50 percent, 67 percent, 75 percent, 90 percent or 100 percent of the ball underwater? •    Remove the ball from the bowl, shake out any water and dry it on the towel. •    Now crumple the ball a little more tightly, into one that is approximately five cm in diameter. If you crumple it too much, just carefully pull apart some of the aluminum foil to get the desired size. •    Again, set the ball gently in the water, placing the marked corners at the top. Does it sink or float? What percentage of the ball is below the top of the water? Remove it, shake out any water and dry it. •    Continue to crumple the ball to be smaller and tighter, and test whether it floats or not (as you have been doing) as it gets smaller. Keep testing smaller diameters until the ball completely sinks. Try testing these diameters (or ones roughly similar): 4.0 cm, 3.0 cm, 2.5 cm, 2.2 cm, 2.0 cm, 1.8 cm, 1.6 cm. If it is too hard to squeeze the ball smaller by hand strength alone, then carefully use the hammer or mallet to gently pound the foil into a smaller ball (or as close to a ball-shape as you can make it). For each diameter you test, what percentage of the ball is submerged? •     At which diameter did the ball sink to the bottom? Do you think that the ball that sank had the lowest or highest density? At which diameter did the ball have a density that was approximately equal to that of water? (When was the ball almost completely submerged or fully submerged but not quite sinking to the bottom?) •     Extra : Cut out at least two additional aluminum foil sheets that are 25 cm square and repeat this activity. Do you get the same results with all the aluminum squares you test, or is there a lot of variation? •     Extra : You can do this activity again, but this time weigh the aluminum sheet on a scale and calculate its mass in grams. Calculate the volume of the spheres for each diameter, using the fact that the volume of a sphere is equal to four thirds times pi (3.14) times the radius cubed. Using the mass and the volumes, compute the average density of the aluminum sheet for each diameter by dividing mass by volume. At what density did the aluminum ball sink? At what density was the aluminum ball approximately equal to that of water? For each diameter of the sphere, what is the mass of the water that was displaced? For more accurate results, continue testing additional 25-cm aluminum squares.

Observations and results Did more and more of the ball end up below the top of the water as the ball's diameter decreased? Was about half of the ball below the water when the ball had a diameter of about 2.5 cm, and did the entire ball sink when its diameter was about 1.6 cm or smaller?

If an object is floating in water, the amount of water that gets displaced weighs the same as the object. Consequently, while it was floating, the ball should have displaced the same amount of water as it decreased in diameter, and so the buoyant force should have remained the same. However, the density of the ball was changing—it increased as the ball's diameter decreased.

Density is the mass per unit volume—it describes how much "stuff" is packed into a volume of space. When the aluminum ball had a diameter of 6.0 cm, the ball should have floated well because it had a density lower than that of water due to the air inside of the ball, just like steel ships that can float because their density has been lowered by encasing air inside the hull. And as long as the ship displaces enough water to create a strong buoyant force, it can stay afloat—even if it is loaded with cargo. As the diameter decreased and density increased, the ball should have sank more and more. When its diameter was about 1.8 cm or 1.6 cm, you may have seen it become 90 percent (just barely) submerged. This is when the ball had a density approximately equal to that of water. With a diameter of about 1.6 cm or smaller, the ball should have completely sank, indicating that its density was greater than that of water, thereby overcoming the buoyant force.

Cleanup Pour the water down a drain and recycle the aluminum foil.

More to explore Archimedes' Principle from Hila Science Videos How Stuff Works Archimedes' Principle by ORACLE ThinkQuest : Education Foundation Science Buddies This activity brought to you in partnership with Science Buddies

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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 own power or using power from the elements (wind, waves, or Sun). Most boats move partly through and partly above water but some (notably hovercraft and hydrofoils) lift up and speed over it while others ( submarines and submersibles, which are small submarines) go entirely under it. These sound like quite pedantic distinctions, but they turn out to be very important—as we'll see in a moment. Why do boats float? All boats can float, but floating is more complex and confusing than it sounds and it's best discussed through a scientific concept called buoyancy , which is the force that causes floating. Any object will either float or sink in water depending on its density (how much a certain volume of 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 small the object is: a gold ring will sink in water, while a piece of plastic as big as a football field will float. The basic rule is that an 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. Sponsored links (adsbygoogle = window.adsbygoogle || []).push({}); Positive, negative, and neutral buoyancy Buoyancy is easiest to understand thinking about a submarine . 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 the tanks are partly filled with air, it's possible to make the submarine float at some middle depth of the water without either rising up or sinking down. That's called neutral buoyancy . The other option is to fill the tanks completely with water. In that case, the submarine is negatively buoyant , which means it sinks to the seabed. 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 of US Navy . Buoyancy on the surface Now most boats don't operate in quite the same way as submarines. They don't sink, but they don't exactly float either. A boat partly floats and partly sinks according to its own weight and how much weight it carries; the greater the total of these two weights, the lower it sits in the water. There's only so much weight a 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 we can put in or on them without getting anywhere near this point. So how can we figure that out? Archimedes' Principle The person who first worked out the answer was Greek 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 crown made for a king was either solid gold or a cheap fake partly made from a mixture of gold and silver. One version of the story says that he was taking a bath and noticed how the water level rose as he immersed his body. He realized that if he dropped 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 measure the volume of a very complex object. By weighing the crown, he could then easily work out its density (its mass divided by its volume) and compare it with that of gold. 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 the whole tale is probably made up anyway! Later, he came up with the famous law of physics now known as Archimedes' Principle: when something is resting in or on water, it feels an upward (buoyant) force equal to the weight of the water that it pushes aside (or displaces). If an object is completely submerged, this buoyant force, pushing upwards, effectively reduces its weight: it seems to weigh less when it's underwater than it does if it were on dry land. That's why something like a rubber diving brick (one of those bricks you train with in a swimming pool) feels lighter when you pick it up from underwater than when you bring it to the surface and lift it through the air: underwater, you're getting a helping hand from the buoyant force. All this explains why the weight of a ship (and its contents) is usually called its displacement : if the ocean were a bowl of water filled right to the brim, a ship's displacement is the weight of water that would spill over the edge when the ship were launched. The USS Enterprise in our top photo has a displacement of about 75,000 tons unloaded or 95,000 tons with a full load, when it sits somewhat lower in the water. Because freshwater is less dense than saltwater, the same ship will sit lower in a river (or an estuary—which has a mixture of freshwater and saltwater) than in the sea. Photo: This relatively small container ship can carry 17,375 tonnes (metric tons) of cargo. The biggest container ships carry over ten times more (around 200,000 tonnes). Photo by Laura A. Moore courtesy of US Navy and archived on Wikimedia Commons . 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 of ocean. Water is a fairly dense liquid that is virtually impossible to compress. Its high density (and therefore heavy weight) means it can exert a lot of pressure: it pushes outward in every 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 except upward; 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 weight and 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

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

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 .

Middle Ages

The great age of shipping, modern ships.

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