Testing device for measuring underwater vibration characteristics of composite propeller and measuring method thereof

Abstract

The invention belongs to the technical field of ship propeller test analysis equipment, and particularly relates to a composite propeller underwater vibration characteristic measurement test device and a measurement method thereof. Comprising a fixed base fixed underwater, a paddle supporting seat mounted at the top of the fixed base, a longitudinal cutting propeller hub fixed at the front side of the paddle supporting seat, paddles arranged at the front side of the longitudinal cutting propeller hub, an acceleration sensor arranged at a vibration pick-up point of the paddles, and a vibration excitation device arranged right above a vibration excitation point of the paddles; according to the test device and method for measuring the underwater vibration characteristics of the composite propeller of the real ship, the acceleration sensor and the data collecting and processing system are used for collecting and recording excitation signals of excitation points and response signals of vibration pick-up points of the blade, and simulation of vibration modes and inherent frequencies of the blade in all orders of modes underwater is combined; the vibration characteristics such as the underwater modal shape, the inherent frequency and the modal damping of the composite propeller blade of the real ship are obtained, and a test foundation is laid for analyzing the underwater inherent characteristics and low-noise pre-deformation design of the composite propeller blade.

Description

Technical Field

[0001] The invention belongs to the technical field of ship propeller testing and analysis equipment, and in particular relates to a test device and a method for measuring underwater vibration characteristics of a composite material propeller. Background Art

[0002] Propellers are still the most widely used propellers for ships. As the core energy conversion device for ship propulsion, they are also the main source of vibration and noise for ships. Composite propellers are made of reinforced fiber composite materials and are manufactured by layering carbon or glass fiber, composite skin wrapped around high-damping core material, high-damping material skin wrapped around composite core material, and other configurations. The engineering application of composite propellers in China still faces major technical challenges: First, there is a lack of large-scale performance verification. Research is mostly focused on small-scale model propellers, and is mostly based on theoretical research and numerical calculations, lacking experimental testing of real ship composite propellers; second, there is a lack of means to test underwater vibration characteristics. Traditional modal tests are mostly based on air environments and cannot simulate the impact of underwater loads on the vibration characteristics of real ship composite propellers. Summary of the Invention

[0003] The object of the present invention is to provide a test device and method that can be used to measure the underwater vibration characteristics of a composite propeller on a real ship. The device uses an exciter or a hammer to excite the blade of the composite propeller on a real ship. The acceleration sensor and the data acquisition and processing system are used to collect and record the excitation signal of the blade excitation point and the response signal of the vibration pickup point. The frequency response function of the blade is obtained through time-frequency conversion. Combined with the vibration shape and natural frequency simulation of each mode of the blade under water, the modal parameters of the frequency response function of the blade are further identified. Finally, the vibration characteristics of the composite propeller blade under water, such as the underwater modal shape, natural frequency and modal damping, are obtained, laying an experimental foundation for analyzing its underwater natural characteristics and low-noise pre-deformation design.

[0004] To achieve the above objectives, the present invention adopts the following technical solutions.

[0005] The test device for measuring underwater vibration characteristics of composite propellers includes a fixed base 1 fixed underwater, a blade support base 2 installed on the top of the fixed base 1, a longitudinally sectioned propeller hub 6 fixed to the front side of the blade support base 2, a blade 3 arranged in front of the longitudinally sectioned propeller hub 6, an acceleration sensor 4 arranged at the vibration pickup point of the blade 3, and an excitation device 5 arranged directly above the excitation point of the blade 3;

[0006] The depth of the blade 3 entering the water is not less than 1m.

[0007] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristics measurement test device is that the blade support seat 2 is fixed to the base 1 by a bolt group; the root of the blade 3 is fixedly connected to the half-split hub 6; and the acceleration sensor 4 is bonded to the blade 3 by waterproof glue.

[0008] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristics measurement test device, the blade support seat 2 includes a stainless steel plate frame and a square stainless steel plate located on the front and bottom of the stainless steel plate frame, and the square stainless steel plate is provided with threaded holes; a square stainless steel mounting plate is provided on the top of the base 1, and the square stainless steel mounting plate and the square stainless steel plate at the bottom of the blade support seat 2 are connected by a bolt group; the half-cut hub 6 is obtained by longitudinally half-cutting a normal hub, and the cross-section of the half-cut hub 6 is connected to the square stainless steel plate on the front side of the blade support seat 2 by bolts.

[0009] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristics measurement test device is that the base 1 is a rectangular frame structure, which is made by welding a stainless steel plate with a thickness of 15 to 25 mm and a channel steel; the thickness of the square stainless steel plate on the front side of the blade support seat 2 is 20 to 30 mm.

[0010] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristics measurement test device, wherein the excitation device 5 refers to an exciter or a hammer; the acceleration sensor 4 refers to a waterproof acceleration sensor, and the contact portion of the exciter or hammer is provided with a soft hammer cap.

[0011] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristic measurement test device is that a pressure sensor is provided on the hammer.

[0012] A method for measuring underwater vibration characteristics of a composite propeller includes the following steps:

[0013] (1) Using the structural finite element modal analysis method, the underwater vibration characteristics of the blade are calculated to obtain the simulated values ​​of the blade's various modes and natural frequencies underwater;

[0014] (2) Based on the simulated underwater vibration modes of the blades calculated in step (1), select and mark the positions of the excitation point and the vibration pickup point;

[0015] (3) Establish a data acquisition, analysis and processing system to obtain real-time excitation signals and response signals from the pressure sensor and acceleration sensor of the vibrator or hammer.

[0016] (4) Use the test device and data acquisition, analysis and processing system to perform measurements; the steps are as follows:

[0017] (4.1) Calibration equipment: Place the calibration equipment on a stable horizontal surface and apply a stable excitation force to the accelerometer for calibration; calibrate the pressure sensor on the exciter or hammer;

[0018] (4.2) Install and secure the test apparatus. Secure the blade support to the front side of the underwater base, the blade root to the front side of the blade support, the acceleration sensor at the vibration pickup point on the blade surface, and the exciter or hammer at the excitation point above the blade. Arrange and secure the entire apparatus underwater.

[0019] (4.3) Use signal transmission lines to connect the accelerometer and exciter or hammer to the data acquisition and processing system;

[0020] (4.4) Establish a blade modal analysis model based on data acquisition and processing;

[0021] (4.5) Excite the blades using a single-point excitation or multi-point excitation method, using a vibrator or a hammer to excite the blades;

[0022] (4.6) Measure and collect the excitation signal and response signal. Set the excitation signal, data transmission channel, and excitation signal force window to ensure a quiet test environment. Collect the excitation signal and response signal. Repeat the excitation 3-10 times and perform frequency domain averaging on the collected data. During repeated excitation, apply the next excitation only when the previous excitation decays to almost zero. Maintain consistency in the force and duration of each excitation.

[0023] (5) The test data are processed and analyzed, and the excitation signal and response signal collected by the measurement are subjected to time-frequency conversion to obtain the average estimated frequency response function of the blade; the modal parameters of the average estimated frequency response function are identified by combining and comparing the simulated vibration shapes and natural frequencies of each mode of the blade underwater, and the underwater modal vibration shape, natural frequency and modal damping vibration characteristics of the composite propeller blade of the actual ship are obtained.

[0024] A further improvement or preferred embodiment of the aforementioned composite propeller underwater vibration characteristic measurement method, wherein step (2) specifically comprises: dividing the blade into S radial radii and T chord lengths, obtaining S×T marking points on the blade surface; combining the simulated underwater modal vibration modes of the blade calculated in step (1), selecting the excitation point and the vibration pickup point from these S×T marking points.

[0025] In a further improvement or preferred embodiment of the aforementioned method for measuring underwater vibration characteristics of composite propellers, the data acquisition, analysis and processing system of step (3) includes a power amplifier, a signal generator, a signal amplifier, a signal collector and a data acquisition and analysis computer.

[0026] Its beneficial effects are:

[0027] This patent proposes a test device and method for measuring the underwater vibration characteristics of a real ship composite propeller. It can measure the vibration characteristics of a real ship composite propeller underwater, determine its underwater modal vibration shape, natural frequency and modal damping, and lay an experimental foundation for the development of low-noise and high-damping composite propellers. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a schematic diagram of the structure of the test device for measuring the underwater vibration characteristics of a composite propeller on a real ship;

[0029] Figure 2 This is the left view of the test device for measuring underwater vibration characteristics of composite propellers on a real ship;

[0030] Figure 3 It is a schematic diagram of the composition of the data acquisition and processing system;

[0031] Figure 4 It is a schematic diagram of the fixed base structure of the base;

[0032] Figure 5 It is the vibration shape diagram of each mode of the propeller blade under water;

[0033] Figure 6 It is a schematic diagram of the positions of underwater excitation points and vibration pickup points of the propeller blade.

[0034] The reference numerals include:

[0035] 1 underwater fixed base, 2 blade support base, 3 blade, 4 acceleration sensor, 5 exciter or hammer, 6 longitudinally sectioned propeller hub. DETAILED DESCRIPTION

[0036] The present invention is described in detail below with reference to specific embodiments.

[0037] The technical problem to be solved by the present invention is to measure the underwater vibration characteristics of the composite propeller blades of a real ship. At present, domestic research on composite propellers is mostly based on small-scale models, and mainly on theoretical research and numerical calculations. In engineering applications, there are still problems such as the lack of large-scale performance verification and the lack of underwater vibration characteristics testing methods. The present invention can accurately measure and determine the frequency response function of the composite propeller blades of a real ship underwater, and obtain the underwater modal vibration shape, natural frequency and modal damping and other vibration characteristics of the composite propeller blades of a real ship through modal parameter identification, laying an experimental foundation for analyzing its underwater inherent characteristics and vibration reduction and noise reduction design.

[0038] 1. Test equipment for measuring underwater vibration characteristics of composite propellers

[0039] like Figure 1 、 Figure 2 As shown, the test device includes a base 1 arranged underwater, a blade support seat 2, an acceleration sensor waterproof 4, an exciter or a hammer 5, and a semi-split hub 6.

[0040] The blade support 2 is fixed to the front side of the base 1, the root of the blade 3 is fixed to the front side of the blade support 2, the acceleration sensor 4 is arranged at the vibration pickup point on the blade surface, and the exciter or hammer 5 is arranged at the excitation point above the blade 3. The present invention arranges and fixes the blade 3 underwater by designing the base 1 and the blade support 2 and fixing the base 1 to the bottom of the pool or a submersible lifting dock.

[0041] like Figure 3 As shown, base 1 consists of a trapezoidal fixed main frame welded together by a rectangular fixed main plate, trapezoidal fixed side plates, and a rectangular fixed bottom plate, and a fixed base formed by connecting a trapezoidal fixed base frame 101 and a cross-shaped locking frame 102. The trapezoidal fixed main frame is entirely made of cast iron or stainless steel, with a thickness of 15 to 25 mm; the fixed base is entirely made of channel steel, with a thickness of 5 to 10 mm. A lifting hole is machined at the center of mass of the trapezoidal fixed side plates; two vertical rows of approximately 20 bolt holes are machined on the rectangular fixed main plate for bolting and securing the blade support 2 with adjustable height; a U-shaped groove is machined on the rectangular fixed bottom plate for screwing and securing it to the cross-shaped locking frame 102; the cross-shaped locking frame 102 is bolted to the ladder-shaped fixed base frame 101; and bolt holes are machined on the ladder-shaped fixed base frame 101 for fastening to the bottom of a pool or a submersible lifting dock with high-strength bolts.

[0042] Propeller support 2: Consists of a square plate and half a propeller hub connected and secured via threaded holes. The square plate is made of stainless steel and is 20 to 30 mm thick. The half propeller hub is a normal propeller hub cut in half. Threaded holes are machined into the square plate for connection to the rectangular fixed main plate on base 1, allowing for rotational adjustment. The square plate and half propeller hub are bolted together. Propeller blades 3 are fixed to the half propeller hub at their bases. Rotating the square plate allows the propeller blades 3 to be adjusted to a roughly horizontal position.

[0043] Accelerometer waterproofing 4: Must be waterproof. It consists of an adhesive base and a sensor component connected by screws. Use waterproof strong glue to stick the adhesive base to the vibration pickup point on the surface of blade 3.

[0044] Vibrator or hammer 5: It is suspended above the blade 3 through a rigid rod and can be freely adjusted to apply excitation at different excitation points. In case of multi-point excitation, multiple vibrators or hammers can be arranged through multiple rigid rods. According to the simulation calculation of the finite element modal analysis method, the underwater low-order natural frequency of the composite propeller blades of the actual ship is all below 500Hz. The hammer cap of the vibrator or hammer 5 is made of a relatively soft material, which is easier to excite the low-frequency vibration of the blade and avoid the skin on the surface of the blade 3 from being crushed. When using a hammer, a pressure sensor is arranged on the hammer to measure the excitation signal.

[0045] The embodiment of the present invention uses a hammer 5 to apply excitation. The test device is fastened to a sinkable lifting dock and is installed using the following method:

[0046] First, use M30 bolts to fasten the ladder-shaped fixed base frame 101 of the base 1 to the sinkable lifting dock; adjust the position of the crisscross locking frame 102, and use M30 bolts to lock it to the ladder-shaped fixed base frame 101; place the trapezoidal fixed main frame of the base 1 on the crisscross locking frame 102, and use M30 bolts to fasten the entire trapezoidal fixed main frame to the crisscross locking frame 102 through the U-shaped groove on its rectangular fixed bottom plate; install the blade 3 on the blade support seat 2, and use M30 bolts to fasten the blade support seat 2 to the rectangular fixed main board of the trapezoidal fixed main frame, adjust the height of the blade 3, and ensure that the blade 3 is immersed for more than 1 meter, so that its static flow field in the water is similar to that on the actual ship, adjust the blade 3 to be horizontal and lock it.

[0047] Secondly, use waterproof strong glue to stick the adhesive base of the acceleration sensor waterproof 4 to the vibration pickup point on the surface of the blade 3, and then tighten the sensor of the acceleration sensor waterproof 4 to its adhesive base with bolts.

[0048] like Figure 4 As shown, a signal transmission line is used to connect the pressure sensor of the hammer 5, the acceleration sensor waterproof 4 with the power amplifier, the signal generator, the signal amplifier, the signal collector and the data acquisition and analysis system computer to form a data acquisition and processing system for real-time acquisition of excitation signals and response signals, and for data processing and analysis.

[0049] 2. Test Method for Measuring Underwater Vibration Characteristics of Composite Propellers

[0050] The test method includes the following steps:

[0051] (1) The structural finite element modal analysis method is used to calculate the underwater vibration characteristics of the blade, and the simulation values ​​of the underwater modal shapes and natural frequencies of the blade are obtained, such as Figure 5 shown.

[0052] (2) Combined with the simulated underwater modal vibration modes of the blade calculated in step (1), select and mark the positions of the excitation points and the pickup points. Specifically: First, divide the blade into S radial radii and T chord lengths, and obtain S×T marking points on the blade surface. Then, combined with the simulated underwater modal vibration modes of the blade calculated by S1, select the excitation points and the pickup points from these S×T marking points. Among them, the excitation point can be selected as a single point or multiple points to ensure that when it is excited, the vibration of the entire blade can be excited as wide as possible, and the vibration response characteristics of the entire blade can be reflected by the point instead of the surface; the pickup points must be selected at multiple points, and avoid the blade with low stiffness, structural symmetry and vibration stationary points. The selection of the excitation point and the pickup point is very critical. The more reasonable the selected position and number, the more accurate the excitation and measurement of the blade's underwater modal vibration modes and natural frequencies. During the field test, the position and number of the excitation point and the pickup point can be appropriately fine-tuned and encrypted according to the effect, such as Figure 6 shown.

[0053] (3) Establish a data acquisition, analysis, and processing system. Use signal transmission lines to connect the pressure sensor and acceleration sensor 4 of the vibrator 5 or hammer 5 to the power amplifier, signal generator, signal amplifier, signal collector, and data acquisition and analysis system computer to collect the excitation signal and response signal in real time.

[0054] (4) Use the test device and data acquisition, analysis and processing system to perform measurements. The steps are as follows:

[0055] (4.1) Calibration equipment

[0056] Place the dedicated calibration equipment on a stable, horizontal surface and apply a steady excitation force to the accelerometer 4 for calibration. Keep the equipment stable during calibration to avoid any shaking that could cause large errors. Additionally, calibrate the pressure sensor on the exciter or hammer 5 according to the operating instructions.

[0057] (4.2) Install and secure the test device

[0058] The blade support seat 2 is fixed to the front side of the base 1, the root of the blade 3 is fixed to the front side of the blade support seat 2, the acceleration sensor 4 is arranged at the vibration pickup point on the blade surface, and the exciter or hammer 5 is arranged at the excitation point above the blade 3. The entire device is arranged and fixed underwater.

[0059] (4.3) Use signal transmission lines to connect the acceleration sensor 4 and the exciter or hammer 5 to the data acquisition and processing system. Note that the signal transmission line must be routed above the blade 3 and must not directly contact the surface of the blade 3 to minimize the effect of its added mass on the vibration characteristics of the blade 3.

[0060] (4.4) Establish the geometric model of blade 3 modal analysis

[0061] The data acquisition and processing system provides two methods for establishing modal analysis geometric models: one is the manual input method, which directly inputs the characteristic geometric coordinates of blade 3, and then generates a modal analysis geometric model through point-to-line and line-to-surface methods. When using this method, in order to facilitate subsequent data processing and analysis, the serial number of the vibration pickup points on the surface of blade 3, the measurement point number in the system, and the data transmission channel number must correspond one to one. The second is the system generation method, which defines the same number of nodes as the vibration pickup points on the surface of blade 3 on the rectangular plane generated by the system, and adjusts and changes the node coordinates so that the rectangular plane is in the shape of blade 3. The first method is more flexible and can select any number of vibration pickup points, but the actual operation is more complicated; the second method is simple and reliable, but the number of vibration pickup points can only be relatively fixed, and the number of geometric modeling nodes and test vibration pickup points cannot sometimes be guaranteed to be consistent. During on-site measurements, reasonable selection can be made according to needs.

[0062] (4.5) Apply excitation to blade 3

[0063] Using single-point or multi-point excitation, excite blade 3 using a vibrator or hammer 5. Note that the cap of the vibrator or hammer 5 must remain perpendicular to the local surface of blade 3 at the excitation point. When using a vibrator 5, a signal generator drives it through a power amplifier to apply excitation. When using a hammer, excitation is applied by holding a rigid rod connected to the hammer.

[0064] like Figure 6 As shown, the embodiment of the present invention adopts a single-point excitation method, and the excitation point is selected at the midpoint of the chord length at 0.85 times the radius of the blade 3; the pickup points are selected at the chord length guide, middle, and anywhere at 0.3, 0.5, 0.6, 0.8, and 0.95 radii of the blade 3, for a total of 15 pickup points.

[0065] (4.6) Measure and collect excitation signals and response signals

[0066] Before measurement, the excitation signal, data transmission channel, and excitation signal force window must be set, and the test environment must be quiet to meet the frequency band requirements for acquisition and analysis and reduce the impact of electrical noise, random noise, and environmental vibration sources.

[0067] During measurement, to further reduce the impact of random noise, repeat the excitation 3-10 times and perform frequency domain averaging on the acquired data. Note that during repeated excitation, carefully observe the response signal. When it decays to almost zero, apply the next excitation, and maintain consistent excitation intensity and duration each time.

[0068] (5) Process and analyze the test data. Use the data acquisition and processing system to perform time-frequency conversion on the measured excitation signal and response signal to obtain the average estimated frequency response function of blade 3. Combined with the simulated vibration mode and natural frequency of each mode of the blade under water, the modal parameters of the average estimated frequency response function are identified in the data acquisition and processing system. Finally, the vibration characteristics of the composite propeller blade under water, such as the vibration mode mode, natural frequency and modal damping, are obtained.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. Composite propeller underwater vibration characteristics measurement test device, characterized by: It comprises a fixed base (1) fixed underwater, a blade support base (2) mounted on the top of the fixed base (1), a longitudinally sectioned propeller hub (6) fixed on the front side of the blade support base (2), a propeller blade (3) arranged on the front side of the longitudinally sectioned propeller hub (6), an acceleration sensor (4) arranged at a vibration pickup point of the propeller blade (3), and an excitation device (5) arranged directly above the excitation point of the propeller blade (3); The blade (3) is immersed in water to a depth of not less than 1 m.

2. The composite propeller underwater vibration characteristics measurement test device according to claim 1, characterized in that: The blade support seat (2) is fixed to the base (1) by a bolt group; the blade root of the blade (3) is fixedly connected to the half-split propeller hub (6); and the acceleration sensor (4) is bonded to the blade (3) by a waterproof glue.

3. The composite propeller underwater vibration characteristics measurement test device according to claim 1, characterized in that: The blade support seat (2) includes a stainless steel plate frame and a square stainless steel plate located at the front side and bottom of the stainless steel plate frame, and the square stainless steel plate is provided with a threaded hole; a square stainless steel mounting plate is provided on the top of the base (1), and the square stainless steel mounting plate and the square stainless steel plate at the bottom of the blade support seat (2) are connected by a bolt group; the half-section hub (6) is obtained by longitudinally half-sectioning a normal hub, and the cross section of the half-section hub (6) is connected to the square stainless steel plate on the front side of the blade support seat (2) by bolts.

4. The composite propeller underwater vibration characteristics measurement test device according to claim 3, characterized in that: The base (1) is a rectangular frame structure, which is made by welding a stainless steel plate with a thickness of 15 to 25 mm and a channel steel; the thickness of the square stainless steel plate on the front side of the blade support seat (2) is 20 to 30 mm.

5. The composite propeller underwater vibration characteristics measurement test device according to claim 1, characterized in that: The excitation device (5) refers to an exciter or a force hammer; the acceleration sensor (4) refers to a waterproof acceleration sensor, and the contact portion of the exciter or the force hammer is provided with a soft hammer cap.

6. The composite propeller underwater vibration characteristics measurement test device according to claim 5, characterized in that: A pressure sensor is provided on the hammer.

7. A method for measuring underwater vibration characteristics of composite propellers, characterized in that: The steps include: (1) Using the structural finite element modal analysis method, the underwater vibration characteristics of the blade are calculated to obtain the simulated values ​​of the blade's various modes and natural frequencies underwater; (2) Based on the simulated underwater vibration modes of the blades calculated in step (1), select and mark the positions of the excitation point and the vibration pickup point; (3) Establish a data acquisition, analysis and processing system to obtain real-time excitation signals and response signals from the pressure sensor and acceleration sensor of the vibrator or hammer; (4) Use the test device and data acquisition, analysis and processing system to perform measurements; the steps are as follows: (4.1) Calibration equipment: Place the calibration equipment on a stable horizontal surface and apply a stable excitation force to the accelerometer for calibration; calibrate the pressure sensor on the exciter or hammer; (4.2) Install and secure the test apparatus. Secure the blade support to the front side of the underwater base, the blade root to the front side of the blade support, the acceleration sensor at the vibration pickup point on the blade surface, and the exciter or hammer at the excitation point above the blade. Arrange and secure the entire apparatus underwater. (4.3) Use signal transmission lines to connect the acceleration sensor and exciter or hammer to the data acquisition and processing system (4.4) Establish a blade modal analysis model based on data acquisition and processing; (4.5) Excite the blades using a single-point excitation or multi-point excitation method, using a vibrator or a hammer to excite the blades; (4.6) Measure and collect the excitation signal and response signal. Set the excitation signal, data transmission channel, and excitation signal force window to ensure a quiet test environment. Collect the excitation signal and response signal. Repeat the excitation 3-10 times and perform frequency domain averaging on the collected data. During repeated excitation, apply the next excitation only when the previous excitation decays to almost zero. Maintain consistency in the force and duration of each excitation. (5) The test data are processed and analyzed, and the excitation signal and response signal collected by the measurement are subjected to time-frequency conversion to obtain the average estimated frequency response function of the blade; the modal parameters of the average estimated frequency response function are identified by combining and comparing the simulated vibration shapes and natural frequencies of each mode of the blade underwater, and the underwater modal vibration shape, natural frequency and modal damping vibration characteristics of the composite propeller blade of the actual ship are obtained.

8. The method for measuring underwater vibration characteristics of a composite propeller according to claim 7, characterized in that: The step (2) specifically includes: dividing the blade into S radial radii and T chord lengths, obtaining S×T marking points on the blade surface; combining the simulated underwater modal vibration modes of the blade calculated in step (1), selecting the excitation point and the vibration pickup point from these S×T marking points.

9. The method for measuring underwater vibration characteristics of a composite propeller according to claim 7, characterized in that: The data acquisition, analysis and processing system of step (3) includes a power amplifier, a signal generator, a signal amplifier, a signal collector and a data acquisition and analysis computer.

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