Why boats float and elephants sink (buoyancy)
How does a boat or ship carrying hundreds of pounds worth of stuff float while that same stuff would sink to the bottom of the ocean if dumped overboard? How come when you're in a pool and you stretch your body out flat you float. But, if you wrap your arms around your legs and curl up into a ball you sink? Well, it all has to do with how much water is pushing against you and a little scientific principle called buoyancy or floatation. When you stretch out flat more water pushes against you since your body is laid out flatter. When you curl up into a ball, less water is pushing against you. Want to test this for yourself? Try this experiment:
1. Take a piece of clay and split it into 2 identically sized pieces. Take one of the pieces and roll it into a ball. Take the other piece and fashion it into a flat boat shaped object (if needed, get mom or dad to help - that's what they're there for).
2. Now place both pieces into a sink full of water. Which one floats and which one sinks? Both? Neither?
So you see, if the total area of the object that makes contact with the water is large enough, the object floats. The object must make room for its own volume by pushing aside, or displacing, an equivalent (or equal) volume of liquid. The object is exerting a downward force on the water and the water is therefore exerting a upward force on the object. Of course the floating object's weight comes into play also. The solid body floats when it has displaced just enough water to equal its own original weight.
This principle is called buoyancy. Buoyancy is the loss in weight an object seems to undergo when placed in a liquid, as compared to its weight in air. Archimedes' principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. From this principle, he concluded that a floating object displaces an amount of liquid equal to its own weight.
How does a heavy boat float?
A boat, or any other object designed to float, is based on a theory by a very old guy, even older than Capt. Matt. Though he is old and, by the way, dead, he was really a cool guy and his name was Archimedes (Ark-i'-meed-eez). His principle, cleverly named the Archimedes' Principle, explains how things float.
If you fill your bathtub with water, what happens when you get in? The water rises, right? (And sometimes goes over the side.) That is because you "displaced" some of the water with your body and it had to go somewhere. The key to floating is that the object must displace an amount of water which is equal to its own weight.
For example, suppose you had a block of wood that was 1 foot square. Let's say that this block of wood weighs about 50 pounds. Now say we lower that wood into the water. The wood will move down into the water until it has displaced 50 pounds of water. That means that fifty pounds of water are pushing back up on the block and making it float.
The principle of floating is pretty easy, however, if you want to remain inside the boat and actually get where you want to go, your boat must have "stability" as well as being able to float. Stability means that it is designed not to tip over easily. That doesn't mean it won't ever tip over.
On a large ship like an ocean liner or tanker, the movement of one person doesn't affect the stability of the ship because it was designed to safely carry lots of weight. But on a small boat, like a fishing boat, your weight and the weight of your gear (and where you put it) has an effect on the stability of the boat.
A boat is said to "heel" (no not the one on your foot) when it leans over to one side. This is why you never want to sit or step onto the side of a boat. Your weight could make it "heel" too much and it may tip over. You should also balance the weight of all the stuff you bring with you. In a small boat, you and your gear should always stay low and to the center of the boat. When getting into a small boat, always try to step into the center and keep "one hand for yourself and one for the boat."
Of course, because you have on your PFD and are displacing enough water to float, you would be okay, just a little wet and cold. If this should ever happen to you and you can't right the boat (turn it back over), stay with the boat, blow your whistle or yell for help.
So . . . the next time someone says "Whatever floats your boat" tell them about Archimedes and stability and why it's a very good idea to always wear your life jacket!
Find out why some boats ride through the water and some ride on top.
Why Does a Boat Plane?
To answer this question, let's first look at what kinds of boats you may see.
Displacement Boat Planing Boat Hovercraft Hydrofoil
Hovercraft and hydrofoil boats are not as common so we won't cover them here. (A later article!) Most recreational boats are either planing or displacement. To get a better understanding, you should first read the article How Boats Float.
In the olden days, before there were engines, all boats were displacement boats. A displacement boat is designed to glide through the water smoothly with a minimum of power (like a canoe with oars or a sailboat under sail). Generally, these boats are very stable and ride smoothly. Larger displacement vessels were also designed to efficiently and safely carry lots of cargo or people.
Planing boats, on the other hand, are designed to rise up on top of the water. They can go very fast, but need more power to get up on top of the water. The heavier the boat, the more power required to get it "on plane."
To understand how these boats work, try putting your hand in a sink or bucket full of water. Start slowly and smoothly, then speed up. The faster your hand hits the water, the more resistance the water presents against it. (Ever do a belly flop? The water felt like concrete, didn't it?)
The displacement boat has smooth lines and curves that gently move water out of its way, both sideways and down. Because the water moves gradually, there is less resistance against the boat. The planing boat moves quickly and the water can't get out of its way, it resists the boat so much that the boat stays on top of the water. If the planing boat slows down, as it sometimes must do, it sinks into the water just as a displacement boat does. Since a planing boat is not designed to glide through the water, the ride can sometimes be bumpy and unstable. Also, if you overload a planing boat, it might not have enough power to get up on top of the water and it will also be much more likely to tip over. A good reason to wear your PFD at all times!
So, even though the boat does not weigh less when planing, the water resistance at high speeds keeps it on top of the water. When the speed of the boat is slow enough to let the water move away, the planing boat no longer planes.
The design of the boat's hull also has an effect on how it works. A flat bottom boat will get up on top of the water more easily than a boat with a round or V shaped bottom. To see the most common types of boat hulls. Both designs have their purpose and this is why you choose a boat suited to the kind of water activities you will be doing. If you want to water ski, for example, you wouldn't use a canoe. Nor would you attempt to cross the ocean on a jet-ski.
The more you know about boats and boating, the more fun and safe your boating experiences will be.
Types of Boat Hulls
Types Of Hulls
Flat bottom boat - These boats are generally less expensive to build and have a shallow draft (the part of the boat that's under the water). They can get up on plane easily but unless the water is very calm they tend to give a rough ride because of the flat bottom pounding on each wave. They also tend to be less stable and require careful balancing of cargo and crew. Examples of flat bottom boats might be Jon boats, small utility boats, and some high speed runabouts.
Vee bottom boat - The vee bottom tends to have a sharper entry into the water which provides for a smoother ride in rough water. They do, however, require more power to achieve the same speed. Many runabouts use the vee-bottom design.
Round bottom boat - These move easily through the water, especially at slow speeds. They do, however, tend to roll unless they are outfitted with a deep keel or stabilizers. Many trawlers, canoes and sailboats have round bottoms.
Multi-hull boat - Catamarans, trimarans, pontoon boats and some house boats use a multi-hull design. The wide stance provides greater stability. Each of the hulls may carry any of the above bottom designs.
Why can boats made of steel float on water when a bar of steel sinks?
The standard definition of floating was first recorded by Archimedes and goes something like this: 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 of water before the whole thing is submerged, the boat floats.
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.
The next question to ask involves floating itself. 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 inch square and 1 foot tall, it weighs about 0.44 pounds depending on the temperature of the water (if you take a column of water 1 cm square by 1 meter tall, it weights 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 (12 inches * 12 inches * 0.44 psi) 62 pounds (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!
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 floats the boat.
HOW DOES SUBMARINE WORKS?
Diving and Surfacing
Photo courtesy U.S. Navy
A submarine or a ship can float because the weight of water that it displaces is equal to the weight of the ship. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will.
To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air (see animation below). When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy). A supply of compressed air is maintained aboard the submarine in air flasks for life support and for use with the ballast tanks. In addition, the submarine has movable sets of short "wings" called hydroplanes on the stern (back) that help to control the angle of the dive. The hydroplanes are angled so that water moves over the stern, which forces the stern upward; therefore, the submarine is angled downward.
To keep the submarine level at any set depth, the submarine maintains a balance of air and water in the trim tanks so that its overall density is equal to the surrounding water (neutral buoyancy). When the submarine reaches its cruising depth, the hydroplanes are leveled so that the submarine travels level through the water. Water is also forced between the bow and stern trim tanks to keep the sub level. The submarine can steer in the water by using the tail rudder to turn starboard (right) or port (left) and the hydroplanes to control the fore-aft angle of the submarine. In addition, some submarines are equipped with a retractable secondary propulsion motor that can swivel 360 degrees.
When the submarine surfaces, compressed air flows from the air flasks into the ballast tanks and the water is forced out of the submarine until its overall density is less than the surrounding water (positive buoyancy) and the submarine rises. The hydroplanes are angled so that water moves up over the stern, which forces the stern downward; therefore, the submarine is angled upward. In an emergency, the ballast tanks can be filled quickly with high-pressure air to take the submarine to the surface very rapidly.
Submarines are incredible pieces of technology. Not so long ago, a naval force worked entirely above the water; with the addition of the submarine to the standard naval arsenal, the world below the surface became a battleground as well. The adaptations and inventions that allow sailors to not only fight a battle, but also live for months or even years underwater are some of the most brilliant developments in military history.
The USS Bremerton performing an emergency rapid surfacing
Photo courtesy U.S. Navy
In this article, you will see how a submarine dives and surfaces in the water, how life support is maintained, how the submarine gets its power, how a submarine finds its way in the deep ocean and how submarines might be rescued.
Question - Which items float in water, which do not and why?
Did you ever notice that when something floats in water, part of it is
actually under water? As it sinks (even a little bit) it pushes away the
water until that amount of water weighs the same as the thing that is
floating. If the thing you try to float is too heavy, it cannot push
away enough water to be the same as how much it weighs. If that happens,
the thing will sink.
Ask an adult to help you with an experiment (a test) that can show you
how this works: Float a small plastic boat in water and notice how deep
the boat sinks when it is empty. Then add pennies to the boat and watch
how the boat sinks deeper and deeper the more pennies you add. The
pennies make the boat weigh more and more. If you add enough pennies,
the boat will sink deep enough so that water reaches the top and then
the whole thing sinks.
Blow up a balloon and float it on water. It will not sink very far because
it is not very heavy. If you look really close, where the balloon touches
the water, you can see a little dent in the water under the balloon.
That's the place where the water is pushed out of the way. If you try
this test with a ball that is exactly the same size as the balloon, the
ball will sink deeper before it floats. Because the ball is heavier than
the balloon, it has to push more water out of the way before it can
It is not just how heavy something is that makes it float or sink. Look
how heavy real boats are -- and they still float. Floating or sinking
has to do with the amount of water pushed out of the way. Any boat will
sink if you put enough stuff inside it -- just like your experiment
showed. Small, heavy things like a marble or a rock cannot float because
they cannot push enough water out of the way to be the same as how much
So remember, anything that floats weighs the same as the water pushed
out of the way.
The object is buoyed up (pushed up) by a force that is equal to the weight
of the water that the object occupies that was previously occupied by the
water. If you shape the object is in such a way that it occupies a volume of
water whose weight equals that of the object, the object will float. If it
occupies a volume of water whose weight is less than the weight of the
object, the object will sink.
You can show this to yourself by taking a piece of aluminum foil and making
a water-tight boat out of it. If you carefully put the boat in a dish or pan
of water, you will see it float.
Now take the aluminum foil boat and crumple it up into a ball and put it
back on the water. It sinks! There is the same amount of aluminum foil in
both cases, but in the case of the boat, you shaped it so that it displaced
a lot of water compared to the amount of water that is displaced when you
crumpled the aluminum foil into a ball.
Objects that are heavier than the same amount of water will sink. Things
that are lighter than the same amount of water will float. If you push an
empty bottle under water, you push a lot of water out of the way. That
water would weigh much more than the bottle. The bottle will float. A rock
put under water also pushes a lot of water out of the way. That water would
weigh less than the rock, so the rock can sink.
Dr. Ken Mellendorf
Illinois Central College
Whether an item will float in water has to do with a property called
density. Density relates to the weight of items that are of a specific
Things that have a lower density than water will float in water. This
is because the item weighs less than the water that it displaces.
Because the water is pulled (by gravity) towards the earth with more
force than the item it flows around the item and pushes it out of the
way so that it is closer to the earth. The result is that the item is
further from the earth and thus floating on top of the water.
You can "model" this result using a shallow can full of small beads or
BBs and a polystyrene foam packing peanut. Bury the peanut in the beads so you
can't see it from the top. Now gently 'bounce' the can on the floor
(drop it a small distance, like 1/2 inch) several times so that the
beads move around, just like water molecules do. The peanut should come
to the surface.