In the classic Chinese book "Yi Jing", there is a hexagram that reads: "Pack the wilderness and use the Feng River. The meaning of this sentence is to carry a hollow gourd to cross the river. Before Archimedes discovered the law of buoyancy, people had already learned to use floating bodies in nature to cross water. Later, floating devices with greater buoyancy, such as hollow gourds used for crossing water, were iterated: many gourds were tied together with ropes, tied on the back or tied around the waist, which was also known as "waist boats". After people started raising livestock, some people in certain areas would use animal skins to inflate and make leather floats to cross water. From a physics perspective, a ten thousand ton steel giant ship floating on the sea is no different in principle from ancient people holding gourds and floating on the water surface. However, steel is still steel, with a much higher density than water. So, how does the steel giant ship achieve stable floating on the sea surface? Under what circumstances will this stable state be broken? The 'waterline' on board the vessel 'China Coast Guard 1001'. If a person approaches a ship and observes it carefully, they will see a scale like mark on the hull, which is the waterline of the ship. In the expected design, when the ship floats freely on still water, the scale corresponding to the intersection line between the hull surface and the water surface is the depth at which the hull is submerged in water at this time. To navigate, a ship must first be able to float stably on the water. Floatability refers to the ability of a ship to float on a certain surface or position in water under certain loading conditions. If the buoyancy is insufficient, the ship will sink, or even sink. Like any object in water, when a boat floats on a still surface, it is subjected to two forces: gravity and buoyancy. Here, the magnitude of gravity is not only the weight of the ship itself, such as a warship, but also includes the weight of mechanical and electrical equipment, weapons and equipment, ammunition, personnel, and various loads. These weights form a vertical downward force, and the position of the center of gravity depends on the distribution of the overall weight of the ship. A steel plate sinks to the bottom of the water, but why can a ship made of steel plate float on the surface of the water? Because it is a hollow and water tight shell (referring to the airtight sealing performance maintained inside the hull under a certain water pressure), it can displace a considerable weight of water and obtain great buoyancy. Buoyancy is vertically upward, acting through the center of the ship's buoyancy. When a ship floats on a still water surface, every part of the hull surface immersed in water is subjected to water pressure, which is perpendicular to the hull surface, and the magnitude of the force is proportional to the depth of the water. No matter what shape the object or hull is, when viewed horizontally, the pressure from water cancels each other out; From a vertical perspective, a vertical upward force is formed, which is the buoyancy exerted on the ship. So, when a ship is stationary and floating at a certain position on the water surface, it is in a state of equilibrium. At this point, gravity and buoyancy are equal in magnitude and opposite in direction. What is the displacement? When people in the industry exchange basic information about a ship, they often ask, "How many tons?" This "how many tons" refers to the displacement of the ship. Because the weight of the drained water is also the buoyancy experienced by the ship, the displacement can also be understood as the weight and tonnage of the ship itself. Even for a giant ship of tens of thousands of tons, the calculation of displacement cannot be separated from this simple physical formula: Δ=ρ V. Among them, V refers to the volume of water discharged by the ship, ρ refers to the density of water, and the density of fresh water is 1.000g/cm?, The density of seawater is 1.025g/cm?, The calculated result Δ is the displacement of the ship, also known as the "tonnage of the ship". However, in practical use, the carrying capacity of a ship often changes, especially during loading and unloading. When the load is reduced, the weight of the ship is less than the buoyancy, causing the ship to float up and the drainage volume to decrease. During this process, the buoyancy decreases, and when the buoyancy decreases to equal the weight, a new equilibrium is reached; vice versa. So, during the process of loading and unloading goods or equipment, cargo ships and warships will also float and sink with changes in the amount of water they consume. The load capacity of a ship is basically divided into two categories: constant weight (A) and variable weight (B). Among them, constant weight refers to the fixed weight and center of gravity of a ship during use, such as the hull, weapons and equipment, various devices and equipment, and fixed ballast. Variable weight refers to the load capacity that changes in weight and center of gravity during use. Among them, fuel, lubricating oil, and spare boiler water belong to B2, while the rest belong to B1, including personnel, food, fresh water, and ammunition. Depending on the variable weight loading situation, the displacement of a ship can be divided into: unloaded displacement, standard displacement, normal displacement, fully loaded displacement, and maximum displacement. Among them, the unloaded displacement refers to the displacement of a ship with complete equipment but no variable weight. This is the lightest loading condition that a ship can achieve after construction, which means only a complete ship and equipment. The standard displacement refers to the unloaded displacement plus the full amount of B1, but does not include B2. This is equivalent to the loading condition when all the fuel, lubricating oil, and spare boiler water on the ship are consumed - at this time, people can live normally on the ship, but the ship cannot leave. The normal displacement is the addition of half of B2 to the standard displacement, which is also commonly referred to as the displacement of a ship. Normal displacement is often used as an indicator in ship design. Full load displacement refers to the displacement of an empty load plus a percentage of variable load (B). This is the loading condition of a ship when it sets sail under normal circumstances, filled with personnel, food, fresh water, ammunition, as well as full fuel and boiler water. The maximum displacement refers to the situation where the ship is fully loaded, plus excess fuel, lubricating oil, boiler water, ammunition, as well as overstaffed personnel, food, fresh water, etc. This is also the maximum loading situation that a ship can achieve. Why do ships need to reserve buoyancy? The waterline of a ship reflects its loading status. The heavier the load on the ship, the deeper the waterline. If it exceeds a certain scale, it may mean that the ship needs to unload some load. In order to ensure navigation safety, there will be a safety mark on the waterline. As long as the water surface is below this safety mark, it means that the displacement is sufficient and the buoyancy "meets the standard". But just meeting the standards is not enough, designers often leave some extra space. So, the space above the waterline will also be designed as a sealed space that is impermeable and does not allow water to enter, serving as the watertight volume of the ship and generating reserve buoyancy when necessary. In 2017, the US warship Fitzgerald collided with a civilian ship, causing severe damage to the hull and water ingress into the hull. However, the ship did not sink because there was still some watertight space above the hull, which could generate reserve buoyancy. Ensuring the prescribed reserve buoyancy is the main measure to ensure the buoyancy of the ship. The reserve buoyancy of a ship is usually represented by "freeboard", which is the vertical distance from the fully loaded waterline in the middle of the ship to the upper edge of the deck. The larger the freeboard, the greater the reserve buoyancy. The magnitude of reserve buoyancy is usually expressed as a percentage of normal displacement. From the perspective of operational safety, warships generally reserve a reserve buoyancy of twice their own displacement, and the specific value varies depending on the type of ship. For example, destroyers typically have a reserve buoyancy of 100% to 150%, cruisers have a reserve buoyancy of 80% to 130%, and submarines are relatively small, typically 16% to 50%. Compared to warships, civilian ships have smaller reserve buoyancy, and their size depends on the type of vessel, navigation area, and type of cargo being carried. The reserve buoyancy of inland barges is 10% to 15%, while that of sea vessels is 20% to 50%. How important is reserve buoyancy? You should know that many shipwrecks at sea are caused by overloading of ships. Overloading not only reduces the maneuverability of the vessel, but also lowers the freeboard and reduces the reserve buoyancy. Therefore, as a waterproof space reserved for ships to resist adverse external conditions, it is very important to reserve buoyancy. To ensure the safety of navigation, reserves must be made in all aspects, whether it is visible "provisions" or invisible "buoyancy". (New Society)