The rapidity of a ship is an important content of its navigation performance and an important performance index of the ship. Refers to the ship's ability to increase the speed of the ship as much as possible under the condition that the output power of the main engine is certain. The speed is slow and the rapidity is not good. The rapidity of a ship contains two meanings: energy saving and rapidity, and the rapidity of a ship depends on two factors: the resistance when the ship moves forward and the efficiency of the ship propulsion device. Therefore, to improve the rapidity of the ship, we should also start from these two aspects, that is, to increase the thrust of the propeller as much as possible and reduce the resistance of the ship's navigation.
In order to improve the rapidity of the ship, it is necessary to do everything possible to reduce the navigation resistance of the ship, which requires the study of the navigation resistance of the ship. When a ship sails, the underwater part of the hull is immersed in water and the rest is in the air. Therefore, the total resistance of a ship in motion includes water resistance and air resistance. Because the density of water is much greater than that of air, the resistance of water is the main resistance. Water resistance can be divided into viscous resistance and wave-making resistance according to the causes.
Viscous resistance resistance caused by the viscosity of water, including frictional resistance and eddy current resistance. Friction resistance comes from the adhesion of water to the hull surface, which accounts for the largest proportion in the total resistance of ships. For low-speed ships, the frictional resistance can account for 80% of the total resistance; For high-speed ships, it also accounts for about 50%. The way to reduce friction resistance is to shorten the captain, reduce the submerged surface area and improve the surface smoothness of the hull. Vortex resistance, also known as shape resistance or viscous pressure resistance, is formed by the head-tail pressure difference caused by viscosity when water flows on the hull surface, and its value is related to the hull shape, especially to the stern of the hull. If the tail line is too full, it is easy to produce eddy current and increase eddy current resistance. The ways to reduce vortex drag are to increase the aspect ratio of the ship and adopt streamlined hull.
Wave-making resistance is the resistance produced by gravity waves when ships are sailing, which is especially important for high-speed ships. Its size depends on the speed and length of the ship. Their relationship can be expressed by Froude number Fr:
Where v is the speed (m/s); G is the acceleration of gravity (m/s); L is the captain (m). If Fr is greater than 0.35, the wave-making resistance is greater than the friction resistance, but the Froude number of a general transport ship is below 0.35. The main ways to reduce the wave-making resistance are to improve the ship form and change the navigation mode. Through a series of ship model tests, we can now get a ship with small wave-making resistance, reasonable main scale ratio and shape coefficient. Generally, there are two wave systems in the waves generated by ships sailing: the bow wave and the coda wave. If the ship type is selected properly, the two wave systems can produce favorable interference and reduce the wave-making resistance. For example, if a bulbous nose is arranged at the bow, an additional wave system can also be generated. Therefore, the interference of waves is beneficial to reduce the wave-making resistance. If the ship can sail in the air or dive out of the water, it can avoid the generation of waves and do not produce wave-making resistance.
The propulsive efficiency of a ship In order to make the ship advance at a certain speed, there must be a thrust equal to the resistance and opposite to it. This thrust is usually produced by the propeller pushing the water backwards. The most common propeller is the propeller installed under the stern water. When the propeller works, it will make part of the water flow move backward and rotate, so it will consume part of the power, which makes it impossible for the efficiency of the propeller to approach 1 in theory. At the same time, because the propeller works in the complex flow field at the stern, it is affected by the uneven water flow, which makes the efficiency lower. When the propeller is running at high speed, the water flow pressure on the blade drops, and when it drops to the vaporization pressure of water, the water turns into steam, forming bubbles, further reducing the efficiency and making the propulsion efficiency very low. Therefore, it is urgent to study the ship propulsion system. It is necessary to carry out theoretical discussion and scientific experiments on the working conditions of the propeller itself, analyze the specific working conditions of the propeller in the stern water flow, and study the influence of the hull on the propeller, so as to design an ideal propeller and make the ship get the highest propulsion efficiency as much as possible.
A fast ship should not only have a good ship form to minimize the resistance during sailing, but also have good propulsion performance to make full use of the power of the main engine. There are three methods to study ship rapidity: theoretical analysis, ship model test and real ship test. Among them, ship model test is still the main means to obtain ship rapidity data at present.