Marine propellers, in a less technical way, are equivalent to car tires. Converting the power of the engine into the driving force of the boat is also the most used propulsion device on the boat.
The history of propellers
In 1776, the British wizard James Watt invented the steam engine, which not only greatly promoted the first industrial revolution in human history but also created a precedent for mechanical power as marine power. It also laid the technical foundation for Britain to become a maritime hegemon. Since the propeller at that time had not yet been born, engineers limited to imagination invented the paddle wheel according to the principle of the propeller. It is a boat with oars installed on both sides of the boat. Because part of the wheel is exposed to the water, it is called a paddle steamer. But paddle steamers also have many shortcomings, such as extremely poor operability, low transmission efficiency, and so on when docking at the dock.
In 1836, the British “Archimedes” used a screw propeller, that is, a long piece of wood made like a screw. At the beginning of the test, the boat moved forward at a speed of 4 knots. Later, because the boat encountered an obstacle in the water, the screw was broken, and a small section of the screw remained, but the speed of the boat was increased to 13 knots. This little accident inspired engineers to turn the long screw into a short screw, and then turn the short screw into a blade shape, and the propeller was invented step by step through experiments. Of course, there are many interludes. For example, the shaft seal of the underwater propeller, underwater vibration, and other issues. With the advancement of human technology bit by bit, the propeller is widely used by shipyards around the world due to its great advantages.
Know the main parameters of the propeller
1. Pitch P:
That is, the distance that the propeller rotates in one revolution in the axial direction. It is equivalent to the distance that the screw rotates once in the wood and moves in the wood. But this is theoretical, and in practice, propeller blades slip when in water, resulting in reduced efficiency. The issue of slippage is also involved here.
The larger the pitch of the blade, the larger the pitch angle of the blade relative to the propeller surface. But relative to a certain speed, there is an optimal pitch angle. The blades of a fixed-pitch propeller cannot turn. Therefore, it is most efficient at the design speed. Controlled pitch propellers are different. Its blades can turn, so the pitch can also be adjusted. Therefore, for ships with variable speed requirements, the use of variable pitch propellers can greatly improve efficiency and save fuel.
2. Propeller diameter D:
That is, the diameter of the outermost ring of the blade. In the process of propeller design, it is always desirable to use the largest possible diameter to improve the propeller’s transmission force. However, on yachts, due to the limitation of draft and installation position, the propeller that is too large cannot be protected by the hull and is easy to touch, so it is suitable.
3. Exp. arearatio EAR:
First of all, we need to know that the area of the disk refers to the area of the plane where the diameter of the propeller is, that is, S=π(D/2)². And if the sum of the area occupied by all the blades of the propeller on the plane is S2, then EAR=S2/S. Therefore, the larger the exp. area ratio, the fuller the blade, and the lighter the load per unit area on the blade, which is more conducive to avoiding the generation of cavitation. However, increasing the exp. area ratio will reduce the efficiency of the propeller. Therefore, under the premise of compensating for the occurrence of cavitation, the exp. area ratio should be made as small as possible to improve the efficiency of the propeller. Most of the propellers have a exp. area ratio of less than 1, but some of the high-speed craft propellers have a exp. area ratio of more than 1.
Types of propellers commonly used in yachts
a fixed-pitch propeller. Due to its simple structure, it is most commonly used on small and medium-sized yachts. There are three to four blades. Advantages: simple structure, durable, simple maintenance. Disadvantages: The pitch is not variable, so the efficiency is the highest only at a certain speed. Therefore, the blades of the propeller are generally large in diameter and low in rotation speed to improve efficiency. Of course, the folding propellers used in sailboats also have fixed-pitch propellers.
Controllable pitch propeller:
A propeller with adjustable pitch. It can change according to the speed and maintain a high transmission efficiency. Maximum fuel savings. But its shortcomings are also obvious, the structure is complex, the cost is expensive, and the maintenance is inconvenient. Generally used on large yachts.
two propellers with opposite directions are installed on the same shaft. Such as VOLVO, ZF’s rotating paddle. With the counter-rotating propeller, the rotating water flow generated by the front propeller is just absorbed by the rear propeller, and at the same time, part of the rotating energy is obtained, which further improves the propulsion efficiency. Disadvantages: At the same time, the structure is complex, and due to the limitation of thrust in materials, it is less used in large ships.
Of course, there are other types of propellers, but the application is relatively less common.