Ayrton Oneill
Boats are vehicles that float and move on the ocean, a river, or any other body of water. The main feature of a successful boat design is its ability to stay afloat, and this is made possible by a scientific concept called buoyancy. Buoyancy is the force that supports things in a liquid or gas, allowing boats to float even though they are made of materials denser than water. The principle of flotation states that a floating object displaces a weight of liquid equal to its own weight. In other words, a boat will float if it weighs less than the volume of water it pushes out of the way when floating.
| Characteristics | Values |
|---|---|
| Definition | A vehicle that can float and move on the ocean, a river, or some other body of water, either through its own power or using power from the elements (wind, waves, or sun) |
| Propulsion | Oars or poles, sails, and engines |
| Materials | Animal skin, bark, wood, iron, steel, aluminium, fiberglass, high-quality plastics, concrete |
| Design | Must be able to stay afloat; must be strong enough to withstand the upward force of buoyancy, the downward force of gravity, and the force of ocean waves; must be streamlined for speed |
| Buoyancy | The force that supports things in a liquid or gas; the upward force that keeps boats afloat |
Buoyancy and flotation
Buoyancy is the force that supports things in a liquid or gas. When a boat is floating in still water, the pressure of the water on the boat below the waterline pushes upward, creating a buoyant force. The net buoyant force on an object is the difference between the ability of the liquid to support that object and the gravitational force working to sink it.
When the net buoyant force on the object is zero, the object floats and is stationary. When the net buoyant force is positive, the object rises. When it is negative, the object sinks. The buoyant force is equal to the mass of the water displaced by a boat.
The principle of flotation explains how ships float. It states that a floating object displaces a weight of liquid equal to its own weight. The principle of flotation was first discovered in 250 BC by Archimedes.
The density of an object determines whether it will float or sink in water. If an object is denser than water, it will sink; if it is less dense, it will float. The size of the object does not matter. For example, a gold ring will sink in water, while a piece of plastic the size of a football field will float.
The basic rule is that an object will sink if it weighs more than the same volume of water. However, this does not explain why large ships made of steel can float. The answer lies in understanding the concept of displacement.
A boat hull is lighter than the total amount of water that the boat's hull pushes away or displaces. As long as the boat weighs less than the weight of the water it would take to fill the hole it creates, it can float. So, a boat floats when it has displaced just enough water to equal its own original weight.
The more weight you add to a boat, the more it weighs, and the further it will sink into the water. There is only so much weight a boat can carry without sinking completely.
The weight of a ship (and its contents) is usually called its displacement. The USS Enterprise, for example, has a displacement of about 75,000 tons unloaded or 95,000 tons with a full load, when it sits lower in the water.
The more load you add to a ship, the further it will sink for the upthrust to balance its weight. The pressure of water increases with depth, and the greater the volume of water displaced, the greater the upthrust.
If the boat weighs less than the maximum volume of water it could ever push aside, it floats. If it weighs more, it sinks.
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Boat materials
Boats are made from a wide range of materials, including animal skin, bark, wood, iron, steel, aluminium, fiberglass, and super-strong plastics like Kevlar.
Wood was used by ancient boat builders who perfected the art of constructing boats from separate planks. They would fix the edges of one plank to the edges of those around it, like bricks in a wall, in a technique known as carvel building. A stronger, lighter, and faster boat could be built using the clinker-building method, which involves overlapping the planks from the bottom up.
The Industrial Revolution brought the age of mighty iron and steel ships. Most modern ships are still built from steel, although it is relatively heavy. That's why some larger boats are now made from strong, lightweight metals such as aluminium, while smaller ones are often made from light composites such as fiberglass.
The buoyancy of a boat is critical to its ability to float and carry weight. The boat's hull must be lighter than the total amount of water that the boat's hull pushes away or displaces. As long as the boat weighs less than the weight of the water it would take to fill the hole it creates, it can float. This is achieved through a principle called buoyancy.
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Boat shapes
Boats come in many different shapes and sizes. The main feature of a successful boat design is its ability to stay afloat in water. This is achieved through a scientific concept called buoyancy, which is the force that causes floating. The more water a boat displaces, the more buoyant force is created.
The shape of a boat is important for its stability and manoeuvrability. For example, a boat with a sharp, narrow bow will push water aside more efficiently, creating less resistance and allowing the boat to move faster and more efficiently. Conversely, a boat with a flat bottom, such as a barge, will have more stability due to its wide stance at the base, which provides leverage stability.
Another factor to consider is the weight distribution of the boat. A boat with a weighted hull will have better weight stability, as the centre of gravity will be lower and closer to the densest portion of the boat. This is especially important for cargo ships, which may have ballast systems to compensate for changes in weight during a journey.
Additionally, the shape of the hull can affect the speed and efficiency of a boat. A curved front edge, for example, can act as a plane, lifting the hull out of the water as the boat moves forward. Hydrofoils take this idea further by using underwater wings to lift the hull completely out of the water, reducing water resistance.
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Propulsion methods
The three main methods of boat propulsion are:
- Oars or poles: The oldest form of boat propulsion, this involves using human power to row a boat by pulling the water backward with large paddles, or punt something like a raft forward by pushing off against a river or seabed.
- Sails: If you hang bed sheets out to dry in a strong wind, you'll know how sailing boats work! However, in practice, boats rarely sail with the wind blowing straight behind them. The sails have to be positioned at an angle, and the force from the wind will then try to blow the boat in that direction. To counter this, the force from the keel and the rudder can be used to correct the direction and produce a resultant force in the direction desired.
- Engines: Modern boats are often propelled by diesel-powered engines. These engines power propellers, which push the boat forward. Alternatively, engines may power impellers (water pumps) that create a powerful backward-pointing jet of water, which in turn powers the boat forward.
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Boat stability
The centre of gravity (CG) is the force that pulls the boat into the water, while the centre of buoyancy (CB) is the force that pushes the boat back up, counteracting the downward pull of gravity. These two forces interact to determine the stability of the boat. If the boat tips to one side, the centre of buoyancy also shifts to that side, with the water pushing up on the boat's tilted side instead of its centreline.
The point at which the centre of gravity and centre of buoyancy intersect is known as the metacenter. The stability of the boat is closely tied to the position of the metacenter. If the metacenter is located below the centre of gravity, the boat will roll over. Therefore, naval architects aim to keep the metacenter above the centre of gravity to maintain stability.
The delicate balance between the centre of gravity and centre of buoyancy can be disrupted by factors such as excessive weight distribution or overloading the hull. For example, placing too much weight on the flybridge can upset the equilibrium. It is crucial to keep the weight balanced and not exceed the hull's capacity to ensure a stable voyage.
Additionally, the shape and design of the hull play a significant role in boat stability. A well-designed hull improves stability and efficiency. Transverse and longitudinal bulkheads, for instance, enhance the likelihood of ship survival in the event of hull damage by limiting flooding to specific compartments.
Furthermore, add-on stability systems, such as bilge keels and outriggers, are employed to mitigate the impact of waves and wind gusts. These systems do not increase stability in calm seas but help reduce the vessel's response to external forces.
Stability calculations in naval architecture consider centres of gravity, centres of buoyancy, and metacentres, along with damage stability and intact stability, to ensure the vessel's safe behaviour at sea.
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Frequently asked questions
Boats float due to a scientific concept called buoyancy, which is the force that causes floating. Buoyancy is the force that supports things in a liquid or gas. The more water a boat pushes away or displaces, the more upward force is created, pushing the boat upwards.
The principle of flotation states that a floating object displaces a weight of liquid equal to its own weight.
Most boats move partly through and partly above the water. They use one of three different kinds of power: oars or poles, sails, and engines.
Boats have been made from a variety of materials, including animal skin, bark, wood, iron, steel, fiberglass, high-quality plastics, aluminium, and super-strong plastics like Kevlar.
