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Why Does A Boat Float

by Lyndon Langley
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Why Does A Boat Float

Why Does A Boat Float

I have seen many times people asking questions like “why does a boat float?” It seems so simple, but you would think everyone would know this answer already. Well, for those who don’t, here we go.
The air that is inside a ship is much less dense than water. That’s what keeps it floating! The average density of the total volume of the ship and everything inside of it (including the air) must be less than the same volume of water. When you add up all of the components in your boat, they may not be lighter or heavier than water individually, but their combined weight is still less than the same volume of water. This also applies to the air in your lungs and other organs as well. The body has a natural ability to keep its internal pressures low enough to allow the blood and fluids to circulate properly, even if some of them are denser than water.
Think about it. If every component of your boat were actually heavier than water, then the pressure within the hull would build up until it was higher than the surrounding water. Then the boat would sink. So why doesn’t this happen? Because there are things that are less dense than water, which counteract the increase in pressure. There are two main factors. One is buoyancy, which is caused by the displacement of the water around the submerged part of the boat. And the second factor is surface tension, which prevents the formation of bubbles on the outside of the boat. Surface tension is a very strong force between opposite sides of an interface. For example, when you blow into a balloon, you can see how the air molecules at the edge of the balloon hold together despite the fact that they want to escape the surface tension of the liquid. The same thing happens with water. Water molecules always want to move away from each other and get out of the way of anything else that wants to come near. They do this because of the strong attraction between them called surface tension.
Now let’s look at the three different types of buoyancy. All three come from the principle that the amount of upward force exerted by a displaced fluid depends upon the shape and size of the object immersed in the fluid. In order to understand these forces, imagine placing a ball bearing inside a glass jar filled with water. Now place the jar upside down and empty the water out of the jar, leaving only the ball bearing and the air trapped inside. Since the ball bearing is lighter than water, it will stay afloat without any help. On the other hand, if you placed a heavy rock in the jar, it wouldn’t float either. Why? Remember that the reason the ball bearing floated was due to the upward force of the displaced water. But now the heavy stone didn’t displace any water. It just sat there in the middle of the jar. Therefore, no upward force was created to lift the ball bearing.
On the other hand, the air inside the jar provides buoyancy. Without the air, the ball bearing would simply fall through the bottom of the jar. However, since the air is less dense than water, it displaces more water, resulting in a net upward force. The greater the volume of the air inside the jar, the greater the upward buoyant force. You can clearly see that the ball bearing floats at the top of the jar while the heavy rock sinks to the bottom.
Another interesting observation is that the smaller the opening of the jar, the larger the buoyant force. Imagine taking a straw and sucking air through one end of it. What happens? The air pushes the water up against the side of the container. This creates a partial vacuum on the other side of the straw where the water level gets lower. As long as the straw isn’t blocked, the water continues to rise above the level of the normal water column. Of course, once the straw is plugged, the rising water stops. With the plug removed, the water falls back down again.
So far we’ve talked about buoyants provided by objects that are immersed in water. But what about buoyants provided by objects that aren’t immersed in water? An aircraft carrier uses one type of buoyancy known as hydrodynamic lift. Hydrodynamic lift is produced by accelerating a large volume of water relative to another large volume of water. The most common form of hydrodynamic lift occurs when a boat moves forward through the water. At slow speeds, the forward movement causes the bow wave to appear as a small hump ahead of the boat. This kind of hydrodynamic lift works best when the speed of the boat is fairly high, say 10 knots (11 mph). To understand this concept better, take a piece of paper and draw a straight line on it. Now fold the paper along that line. Open up the folded paper and place it flat on a table. The crease forms a 45-degree angle with the horizontal plane of the table. Now start moving your finger across the surface of the paper gently. What happened? Your finger made waves in the water. These waves appeared to bend toward the crease in the paper. As a result, a small bulge formed on the surface of the water next to the paper. This is exactly what happens with hydrodynamic lift. The faster you move your finger, the bigger the waves become.
In addition to hydrodynamic lift, ships use another type of buoyancy known as dynamic stability. Dynamic stability refers to maintaining stability even though the center of gravity of the craft shifts slightly during travel. Let’s say you’re standing on the deck of a ferryboat and suddenly someone opens the door leading below decks. Everyone knows what happens next — the ferry tips over completely. How did the ferry maintain stability? By shifting the center of gravity downward with ballast (weighted metal plates), thus keeping the overall center of gravity relatively constant. Note that even though the center of gravity shifted during transit, the ferry remained stable.
There are several conditions that affect the amount of buoyancy available. First, the specific gravity (density) of the material being used determines how deep it can penetrate underwater. Second, the cross sectional area of the object plays a role. The wider the object is, the deeper it can dive. Finally, the depth of the sea water affects the buoyancy. Buoyancy decreases rapidly with increasing depth.
Surface tension is another important aspect of buoyancy. The term means attractive force between opposing surfaces. In the case of a bubble, the bubble is surrounded by a thin layer of liquid. The thickness of the layer is determined by surface tension. The stronger the surface tension, the thinner the layer of liquid becomes. The surface tension of water is approximately 70 pounds per square inch. Consequently, the diameter of a bubble containing one quart of water would be roughly 1/7th of an inch thick. In comparison, the surface tension of oil is zero. Oil therefore fills the entire space between the bubble and the wall of the container. Thus, instead of buoyancy providing support, oil acts as a solid ballast.
Buoys provide support by creating a resisting force against the motion of liquids such as water. Buoys can also create resistance against the motion of gases. For example, the buoyancy compensator found in submarines helps regulate the body’s thermoregulation system. A person sitting in a submarine chair can adjust the position of the joystick to control the rate of ascent or descent. The buoyancy device functions by changing the volume of gas contained within the vestibule. Gas expands when heated, causing the occupant to feel warmer. Conversely, the buoyancy device contracts when cooled, forcing the occupant to sit closer to the seat.
Airplanes are able to fly in the atmosphere because of buoyancy. Airplane wings generate lift by deflecting the flow of air downward. The deflection of the airstream results in increased velocity and reduced drag. This mechanism accounts for both vertical flight and forward propulsion.
When an airplane flies horizontally, it experiences aerodynamic drag. Aerodynamic drag is the sum of friction and induced or parasite drag. Friction is caused by contact with the air and is proportional to the velocity of the wing. Parasite drag is caused by turbulence generated behind the wing. Unlike friction, parasite drag increases with the cube of wing velocity. The combination of friction and parasite drag produces induced or apparent drag. Induced drag is proportional to the length of the wing. Apparent drag is directly related to the weight of the airplane. Since airplanes typically weigh hundreds of thousands of pounds, their bodies are constructed primarily of lightweight metals. Lightweight metals have extremely low strength compared to their weight. Hence, most of the structural elements of an airplane are designed using materials having a high tensile strength-to-weight ratio.

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