An effective method to eliminate the sound of propeller

Trim the blade side

To change the distribution frequency, just change the thickness of the trailing edge of the blade. If the trailing edge is quite thin and the rear stagnation point is fixed, the alternating vortexes released are almost non-existent.

In addition, the shape of the trailing edge of the profile also has a great influence on the strength and frequency of the vortex distribution, because it determines the ratio of the horizontal and vertical spacing of the vortex street formed behind the trailing edge. If a flat plate with a wedge-shaped trailing edge with different acute angles is used to observe in the flow field, it can be found that the sharp wedge-shaped trailing edge produces a narrower tail vortex, a higher vortex emission frequency, and a weaker vortex strength.

For the trailing edge of the propeller blade, because of the certain strength, the trailing edge cannot be trimmed to be very thin or very sharp. Therefore, to eliminate the sound of the propeller, it can be achieved by controlling the sharpness of the trailing edge and the thickness of the trailing edge. According to our experimental research, after the trimmed tail edge, its release frequency must be far away from the blade sound frequency. Generally, the distribution frequency of the profile tail vortex at 0.7 blade radius should be more than 2.5 times the sound frequency. The beep sound is completely eliminated in the entire speed range. Therefore, we believe that the trimming range of the propeller blades along the edges should be from 0.95 to 0.40 until the trailing edge of the profile at the blade radius. The trailing edge of each section is trimmed into a wedge shape. The wedge angle of the outer radius of the blade is set to 17 degrees (propellers with a diameter less than 2m) or 21 degrees (propellers with a diameter greater than 2m) to gradually transition to 24 degrees or 27 degrees.

The thickness of the trailing edge is set at 0.05% (small paddle) to 0.07% (large paddle) of the section chord at 0.7 blade radius, and the thickness of the trailing edge of other sections is based on this standard. Using this method to trim the edge, can effectively eliminate the sound of the propeller.

Based on the thickness of the trailing edge after trimming, the frequency of tail vortex release in each section of the blade was re-estimated. It can be seen from the estimation results that the emission frequency of the tail vortex of each section is more than 3 times higher than the sound frequency of the propeller, far away from the sound area.

Increased blade damping

In order to change the response frequency of the blade to reduce its vibration, the method of increasing the blade damping can also be used to suppress the occurrence of chirping. The use of high damping alloy materials to manufacture the propeller is an effective measure to eliminate the sound of the propeller. A paddle model made from this material was compared with a copper alloy model that simulates the sound of a propeller in a water cylinder, and it was found that the former did not produce a sound of sound within the range of speeds used. Due to the relatively complicated casting and processing technology of high-damping alloy, it is currently only used in military ships.

In order to eliminate the sound of civil ship propellers, we adopt a simple method, that is, the surface of the blade is coated with diluted high viscosity neoprene to increase blade damping and suppress vibration. The coating thickness (single side) is about 4-6% of the section thickness at 0.7 blade radius. Too thick will affect the hydrodynamic performance of the blade. We conducted a comparison test on the vibration of the blades before and after the glue application. Even if the emission frequency of the tail vortex is coupled with a certain response frequency mentioned above, there is not enough energy to increase the vibration of the blade, so it is impossible to produce a sound.