The shape and name of the propeller
The propeller is commonly known as the vehicle blade, and its appearance is shown in Figure 2-1. The propeller is usually installed at the stern of the ship (but there are also some special ships equipped with propellers at the fore and aft, such as port work boats and ferries, etc.). Ships with only one propeller at the center line of the stern are called single propeller ships, each on the left and right sides. One is called a double-propeller boat, and there are also three-oar, four-oar or even five-oar.
A propeller is usually composed of blades and a hub. As shown in Figure 2-2, the connecting part of the propeller and the tail shaft is called the hub, and the hub is a truncated cone. In order to reduce the water resistance, a fairing is added at the rear end of the diesel hub to form a smooth streamlined body with the propeller hub, which is called a hub cap.
The blade is fixed on the hub. Ordinary propellers are usually three-blade or four-blade or two-blade propellers only used on sailboats or small boats. Some ships (such as large tonnage tankers) use five-blade or more than five-blade propellers to avoid vibration.
When looking forward from behind the stern, one side of the propeller blade seen is called the blade surface, and the other side is called the blade back. The connection between the blade and the hub is called the root, and the outer end of the blade is called the tip. When the screw is rotating, the edge of the blade in the front is called the leading edge, and the other side is called the trailing edge.
When the propeller rotates (suppose there is no forward and backward movement) the circular trajectory of the blade tip is called the tip circle, and the diameter of the tip circle is called the propeller diameter, denoted by D. The area of the tip circle is called the area of the propeller’s disk, denoted by A. Express:
When the spiral diesel is rotating in the right direction, looking forward from the back of the ship, the direction of rotation seen is clockwise, which is called right-handed diesel. On the contrary, it is a left-handed propeller. The propellers installed on both sides of the stern, whose upper part rotates toward the center line of the ship when the main car rotates, are called internal propellers. On the contrary, it is an external propeller.
The blade surface of the blade is usually a part of the spiral surface. In order to clearly understand the geometric characteristics of the propeller, it is necessary to discuss the formation and characteristics of the spiral surface.
Assuming that the line segment ab is at a fixed angle to the axis OO1, and the ab rotates around the axis OO1 at a constant angular velocity while moving upwards along the OO1 at a constant linear velocity, the curved surface drawn by the ab line in space is a constant pitch helix, as shown in Figure 2. -3 shown. The line segment ab is called the bus, and the distance the bus travels around in the axial direction is called the pitch, denoted by P.
According to the shape of the bus bar and the change of the included angle with the axis, different forms of helicoids can be obtained. If the generatrix is a straight line and perpendicular to the axis, the spiral surface formed is a positive spiral surface as shown in Figure 2-4a. If the generatrix is a straight line but not perpendicular to the axis, an oblique spiral surface is formed, as shown in Figure 2-4b. When the bus bar is a curve, a twisted spiral surface is formed as shown in Figure 2-4c and Figure 2-4d.
The trajectory formed by any fixed point on the generatrix during the movement process is a spiral. The intersection line of any coaxial cylindrical surface and the spiral surface is also a spiral. Figure 2-5a shows the spiral line BB1B2 obtained by the intersection of a cylindrical surface with a radius of R and the spiral surface. If the cylindrical surface is developed into a plane, the cylindrical surface becomes a rectangle with a base length of 2ΠR and a height P, and the spiral line becomes a diagonal line (the diagonal of the rectangle), which is called a pitch line. The triangle B’B”B”2 is called the pitch triangle, and the angle θ between the pitch line and the bottom line is called the pitch angle, as shown in Figure 2-5b. It can be seen from the figure that the pitch angle can be determined by the following formula: