The working principle of marine propeller

What is the principle of marine propeller actuation? We can explain from two different perspectives, one is the change of momentum, and the other is the change of pressure. From the point of view of momentum change, simply speaking, the propeller accelerates the passing water, causing the water momentum to increase and generating a reaction force to propel the ship. Since momentum is the product of mass and velocity, you can imagine that different masses with different speed changes can cause varying degrees of momentum.

On the other hand, the principle of propeller actuation can be more clearly explained from the viewpoint of pressure change. The propeller is constructed from a group of airfoils, so its principle of operation is similar to that of a wing. The wing is a geometrical change of the airfoil and an angle of attack of the inflow, so that the fluid flowing through the airfoil has different speeds. According to Bernoulli's law, the difference in speed causes the difference in the pressure on the lower surface of the airfoil. Generate lift.

The airfoil constituting the propeller blade is formed by the advancement and rotation of the propeller. Without considering the influence of the friction between the fluid and the surface, the component of the lift of the airfoil in the forward direction is the thrust of the propeller, and the component in the direction of rotation is the torque force that the ship's main engine must overcome. According to Bernoulli's law, the magnitude of the pressure difference is proportional to the difference between the squares of the synthetic speeds. Therefore, the magnitude of the lift is also proportional to the difference between the squares of the combined speeds. Therefore, the faster the rotation speed of the propeller, the greater the lift generated, and the thrust is also With the increase, the speed of the ship is thus increased. For most ships, the speed of the ship is roughly proportional to the speed of rotation, so the inflow speed synthesized by forward and reverse is also proportional to the speed.

The marine propeller can be seen as a "wing" that rotates on one side and advances on one side. The flow of water through the various sections of the blade creates a forward force. Take a very small section at both propeller radii r1 and r2 (r1 < r2) to discuss the airflow over the blade. V—axial speed; n—propeller speed; φ—air flow angle, that is, the angle between the airflow and the propeller rotation plane; α—blade angle of attack; β—blade angle, that is, the angle between the chord of the blade section and the plane of rotation . It is obvious that β = α + φ. When the air flows through the small sections of the blade, the aerodynamic force, the resistance ΔD and the lift force ΔL are generated, and the total aerodynamic force after the synthesis is ΔR. The component force of ΔR in the flight direction is the pulling force ΔT, and the force ΔP opposite to the direction of rotation of the rotary propeller prevents the propeller from rotating. In this way, a larger pulling force is obtained, a smaller resistance is obtained, thereby increasing the efficiency, and the airflow angle actually reflects the ratio of the forward speed to the tangential speed.