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    The working principle of marine propeller

    The working principle of marine propeller can be explained from two different points of view, one is the change of momentum, the other is the change of pressure. In terms of the change in momentum, in short, it is the propeller through accelerate the water, and cause the water momentum increase, producing reaction to push the boat. Because the momentum is the product of quality and speed, different quality coordinate different speed change, can have varying degrees of momentum change.

    On the other hand, the viewpoint of pressure change can more clear explain the working principle of propeller. The propeller is constructed from a group of airfoil, so the working principle is similar with airfoil. Wing is on the wing surface geometry change and the inflow of Angle of attack, the flowing fluid under the wing surface has different speed, and by the Bernoulli’s law known speed under different will cause the wing surface pressure difference and thus create lift. The movement of the wing, which constitutes a propeller blade, is made by the advance and rotation of the propellers. If we do not consider the influence of friction between the fluid and the surface, on the surface of the wing lift in the direction of the component is the propeller thrust, and component in the direction of rotation is host must overcome the torque force of the ship.

    Take a section of the blade as an example: when the ship is static, the propeller is starting working. If the propeller is not moving, then the flow of the current is going to the blade of the paddle, and its velocity is 2 PI nr (n is rotational speed; r is the radius of the section). According to the hydrofoil principle, the blades are to be driven by lift and drag, pushing the propellers forward, pushing the boat forward. The boat movement produces a top flow and a wake. Continue to see the boat as not moving, then the top flow is equal to the speed of the boat, the opposite flow velocity toward the propeller flow, and the wake flow is the same as the direction of the boat, the velocity is ur to the propeller flow. With velocity synthesis, we can get the combined flow of the propeller into the Angle of attack. The blades of the blades are affected by the combined flow lift and drag, and the lifting force and drag force are decomposed, and the component of the parallel and vertical boat is obtained.

    If the speed V is constant, if the flow velocity increases (the speed decreases), the angle of attack increases, and the thrust and resistance are also large. If the propeller speed increases (the speed increases), the angle of attack increases, and the thrust and resistance are large. When the boat is still, when the propeller is turning, the flow of the water is very large, and the thrust and drag can be very high. Excessive resistance to host work. So the boat is not suitable to use high speed when starting from the rest. In the same way, when the boat is changing in reverse or changing from the back, it should pass through the parking phase, so that the boat will be reduced and then converted, instead of directly changing. It is mainly to prevent the occurrence of large angle of attack, generate huge rotation resistance, causing the host overload.

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