Why doesn't the ship capsize or shake when the sea is windy and rough? There is a mystery behind the hull

The weather at sea is like a child's face, unpredictable. So why don't ships capsize when sailing on the sea?

Of course, there are cases where a boat capsizes, but this is considered a low-probability event. In most cases, boats will not capsize due to wind and waves, otherwise no one would dare to take a boat. If we pay attention to some small boats, we will find that they always sway left and right with the fluctuations of the sea water, sometimes with a big sway, sometimes with a small sway, but no matter how big or small the sway is, they can eventually get back to their normal position and will not capsize. Why is this so? The mystery is still to be found on the hull. Any two objects with mass have mutual gravitational effects. For all things that exist on the earth, they are all subject to the gravitational effect from the earth, and the direction points to the center of the earth.

The gravity exerted on an object acts on every point on the object, and the resultant force of gravity exerted on each point is the weight of the object. The point where the resultant force of gravity exerted on each point acts is called the "center of gravity" of the object.

Any object has a center of gravity, regardless of its regular shape, and the center of gravity of an object is not necessarily on the object. Back to the question of the ship, it is obvious that the ship is a relatively regular shape, so the center of gravity of a ship is probably located at the center of the ship. This has nothing to do with the size and height of the ship, because even the giant cruise ship "Oasis of the Seas" with a height of 64 meters, its center of gravity is still at the center of the ship. Because no matter how high the hull is, the heaviest part of a ship is still located at the bottom of the ship.

Gravity pulls the boat downward, but the boat does not sink into the water. Why?

Because there is a force that is fighting against gravity, it is buoyancy, the buoyancy of seawater. Buoyancy is vertically upward, and it is exactly opposite to the direction of gravity, but the difference is that gravity acts on the entire hull, while buoyancy acts on the part of the hull submerged in the seawater, so we often say that the larger the displacement volume of an object, the greater the buoyancy it is subject to. Buoyancy is also similar to gravity in that it also acts on every point of the hull submerged in water, so the resultant force of the buoyancy at each point will converge on one point, which is called the "center of buoyancy." For a ship sailing steadily on the water, the center of buoyancy is located below the center of gravity, and both are located on the same straight line perpendicular to the water surface.

It is because the center of buoyancy is below the center of gravity that the ship can float on the water.

However, it is impossible for a ship to sail smoothly on the water forever. In fact, the ship is shaking all the time. When a ship tilts to the right due to the waves on the sea, the position of the ship's center of gravity will not change, because as long as the volume and shape of the object do not change, the center of gravity will not change. But the center of buoyancy is different. When the hull tilts to the right, the displacement volume on the right side will be greater than that on the left side, and the buoyancy acts on the displacement part of the hull, so the center of buoyancy will also move to the right. At this time, the center of gravity and the center of buoyancy are not on a straight line perpendicular to the sleep direction, so these two opposite forces will generate a torque to twist the hull back from the state of tilting to the right.

The righted hull will begin to tilt to the left due to overcorrection, and the same story will happen again. The combined force of the center of gravity and the center of buoyancy will once again right the hull from the left-leaning state, and the torque generated by the combined force of gravity and buoyancy is called the "righting moment."

So under the action of the righting torque, the ship sways from side to side but does not capsize. Although the ship will not capsize, it is unbearable to sway from side to side all the time. It is okay for a small ship, but if it is a large cruise ship, all the cargo on it will be swayed into the sea, and all the items in the cabin will be smashed to pieces. Therefore, in order to ensure that the ship can always sail steadily without shaking, people installed a component called "bilge keel" on both sides of the bottom of the ship. This thing is basically installed on both large and small ships.

When the hull shakes, turbulence in the opposite direction of the shaking will be generated on both sides of the bilge keel, which can effectively reduce the shaking amplitude of the hull.

But for large cruise ships, bilge keels alone are not enough. They are also equipped with a component that looks like a fish fin, which is called a "stabilizer fin". Stabilizer fins can be passive or active. Active stabilizer fins can be controlled manually to slow down the ship's rolling tendency in a targeted manner. For ships, this is considered a high-end configuration, so small ships definitely don't have it. If the bilge keel and stabilizer fins are installed, but the ship's rolling amplitude is still not satisfactory, there is a way to install anti-roll water tanks inside the hull. The water tanks located on both sides of the hull and connected to each other can generate a force opposite to the rolling direction, thereby stabilizing the hull.

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