Analysis of Fault Causes of Marine Propeller

The propeller of a ship transfers the thrust generated to the ship itself. It is a component that converts the torque generated by the outboard engine into kinetic energy. If the propeller fails, the normal movement of the ship will be affected to varying degrees. When different faults occur, the treatment methods are different. Choosing the appropriate method should be based on the reasonable analysis of the faults, it means the cause analysis is also very important. The accuracy of cause analysis directly affects the result of judging the type of fault.

Fault of Ship Propeller Winding Fishing Net

Propellers are often entangled by fishing nets. This situation will increase the torque of the propeller, resulting in an increase in outboard motor load. When working under overload, it may stop the shafting or propeller, which will damage and deform the shafting and blade of the propeller. And sometimes it produce greater shafting vibration and noise. When the shock signal based on the speed pole appears in the shafting, it proves that the shafting machinery is not working properly. In this case, you should do further inspection of the shafting or propeller to find out the reasons of the shock signal. 

The following are possible failures:

1. Shield fin deformed.

2. Damage of shaft stern flange.

3. Damage or missing of fixed hexagonal bolt of stern bearing.

4. Displacement of coupling flange.

5. Damage of thrust block in reverse of thrust bearing.

6. The vibration noise of the shafting system increase. On the one hand, the fishing net debris blocked the cooling water passage of the bearing. On the other hand, the flange displacement of the shafting coupling made the shafting parts loose.

Friction between Propeller and Accessory Components

Due to the complex working environment of the stern shaft, bearing and stern shaft sometimes produce louder vibration because of poor lubrication effect and unsatisfactory clearance. It may cause by the incorrect installation of the sealing device, the contact between the blade and the guide hood, the stern shaft shield and the hub of the propeller, etc. The contact point between propeller and its accessories may appear at the protective cover of propeller hub and stern shaft. In the docking inspection room, the protective cover near the propeller hub is properly removed to ensure that no contact friction occurs.

Unbalance Caused by Breakage of Propeller Blade

When a ship is traveling on the river, the violent vibration of the ship hull occurs instantaneously. The crew will generally slow down, the vibration will be weakened, the continuous deceleration, and the vibration will become smaller. It is likely that the propeller blade has been broken, which makes the propeller lose its balance and makes the hull vibrate violently. Generally, this may be due to the poor performance of propeller materials, casting defects, and serious corrosion of propeller blades, which results in the unstable center of gravity of each propeller blade losing balance. Then it cause to strike the stern and shaft or hull vibration.

Deformation of Blade

When the outboard engine propeller works, the blade and water exert force, which makes the water react to the hull and then it can move forward. In the long-term use of the blade, the deformation and fracture of the material will occur. In addition to this force, there are still many uncertainties, such as the erosion of water body makes the blade thickness thinner, which makes the blade deformation increase.

Propeller Utters Noise

The sound of a propeller occurs because the frequency of the blade whirlpool is the same as the frequency of the structural response of the maximum amplitude of the blade vibration. These energies cause the self-excited vibration of the propeller blade.

Propeller Cavitation

In the operation of propeller, if the cavitation is near its surface, the blade material will be corroded by pits because of hydraulic impact. Cavitation often produces vibration and noise, and the metal surface will be spotted corrosion, which can cause blade perforation or rupture. It will result in the unstable center of gravity of the whole propeller, resulting in vibration and noise.

The fault of ship propeller is a kind of normalized fault. The propellers of different ship materials have different working conditions. The actual water level should be taken into account when dealing with the fault and analyzing it, so as to avoid judging the cause of the fault by experience. At the same time, we should pay attention to outboard engine maintenance.