Why is the hull streamlined? What physical knowledge is used?

The invention relates to a streamlined open-water hull of a drainage boat which imitates fish living in water. The invention belongs to the streamlined hull design technology, and the streamlined hull design is carried out in order to adapt to the propeller position that makes the propeller efficiency reach the open water efficiency, that is, the stern of the ship is W-shaped, and the propeller is located in the W-shaped hull at 3_ 1/2 to 4_ 1/2 stations in front of the stern of the hull, so that the power is constant. It greatly improves the propulsion efficiency and speed of all kinds of drainage ships, provides a large swing arm, and improves the stability, seakeeping and maneuverability of navigation, especially suitable for large and medium-sized ships in inland rivers and coastal areas. The navigation performance of the ship mainly includes: 1. It can carry a specified number of loads and float on the water; 2. When the ship is hit by wind and waves and passengers and goods move on board, the ship will tilt to a certain extent and will not capsize; When the external force disappears, the ship has the ability to return to the original positive floating state. 3. When the ship hits the rocks, collides or is attacked and injured by the enemy at sea, it can still keep floating. It should have higher speed and consume less machine power. 5. It has good course stability and sensitivity. 6. When sailing on the rough sea, it won't sway violently to avoid the crew. Passengers get seasick, which hinders the normal operation of the ship's machinery and equipment and the accurate launch of weapons. (1) The floating ship is subjected to water pressure, and the pressures on the left and right sides are balanced with each other, and the bottom pressure is balanced with the weight of the ship itself. The balance condition of the ship is: 1. Gravity p and buoyancy d act on the same vertical line; 2. The displacement is equal to the total weight of the ship, P=D (2). Stability If the ship is tilted by external force, gravity and buoyancy form a moment to push the ship back to its original upright position, and the ship is stable and balanced. How can we make the ship have good stability? 1. The center of gravity of the ship should be as low as possible; 2. Increase the width of the ship; 3. Keep a certain freeboard. However, for a ship with too wide width and too low center of gravity, the distance between gravity line and buoyancy line is very large, so the recovery torque is also very large. This kind of ship swings from side to side on the rough sea with high frequency, which is not conducive to people's work and equipment operation. (3) The water resistance of fast ships is usually divided into the following categories: Friction resistance: Water is a viscous liquid, and when ships sail, they must overcome the resistance caused by the viscosity of water, which is called friction resistance. The friction resistance is related to the wet surface area of the hull, the relative speed between the ship and the water, the roughness of the hull surface and other factors. (1) Wave-making resistance: When a ship is sailing, the bow exerts pressure on the water, which splits the water forward, thus arousing a group of waves that follow the ship. This is the bow wave. There is a low pressure area in the water, which becomes a trough, forming a group of waves caused by the stern, called coda. It also consumes energy to cause waves, which is called wave-making resistance. Because it is caused by water pressure change, it is also called pressure resistance. (2) The wave-making resistance is related to the length and speed of the ship. The higher the ship speed, the greater the wave resistance. In order to reduce this resistance, the ship is streamlined below the waterline. Using the low pressure formed by the spherical part, the height of the head wave is reduced, thus reducing the wave-making resistance. This is an economical and effective method to improve the speed of the ship. (3) Vortex resistance: When a ship sails, the water absorbs the energy of the ship through the vortex formed at the tail and hinders the ship's progress, which is called vortex resistance. Try to design the hull as streamlined as possible, paying special attention to the rationality of the tail and tail modeling. Eddy current resistance can be reduced. Ships will encounter other resistances when sailing at sea, such as air resistance, wind and wave resistance, etc. The total resistance of a ship is the sum of the above three resistances, that is, total resistance = friction resistance+wave resistance+vortex resistance. The product of the total ship resistance and the required ship speed obtained from the model test is the power needed to overcome the water resistance. If we know the efficiency of the ship's power plant and propeller, we can determine how big the main engine should be installed on the ship. (4) The rocking boat will swing from side to side and back and forth under the action of external force, which is called rocking. In order to reduce the sway of the ship, anti-rolling equipment can be used. (5) Some cabins of an anti-sinking ship can still float on the sea surface after being attacked by the enemy or flooded in a general accident, which is called anti-sinking. Almost all modern ships are equipped with double bottoms and watertight transverse bulkheads. The whole hull is divided into several independent watertight compartments, and sufficient freeboard height is reserved above the waterline to maintain a certain reserve buoyancy. In this way, when some parts are damaged or flooded, they can still maintain a certain floating state and stability. (6) The rotation of a rotorcraft includes sensitivity and course stability. Course stability is the ability of a ship to maintain course stability when sailing. The course stability is related to the projected area of the ship's underwater side, the rudder area, the tail wood area and the waves when sailing. The ship can quickly change course at any time according to the intention of the pilot, which is called ship sensitivity. When the rudder surface is kept in the full rudder position, the ship sails along a circular trajectory, which is called a turning circle, and the diameter d of the turning circle indicates the sensitivity of the ship.