ObjectivesAs unmanned aerial vehicles are equipped with total field magnetic sensors to detect the total field anomaly above ships, it is necessary to establish a corresponding total field anomaly inversion model in order to evaluate the underwater magnetic signature and airborne far-field magnetic signature of ships. Methods A ship magnetic field inversion model based on a magnetic monopole array is established on the basis of the total field anomaly data measured on the plane above the ship. Based on this, the intensity of the ship's magnetic monopole array can be obtained, thereby realizing the evaluation and analysis of the ship's three-component magnetic field signatures above and below the ship. A laboratory validation test of a steel ship model is carried out in which the magnetic fields of the ship model are measured from above and below, and inversion modeling is conducted on the basis of the total field anomaly in the measurement plane above the ship model. Results The total field anomaly inversion of the three-dimensional magnetic monopole array distributed on a ship's body can be used to achieve high-precision three-component magnetic field measurements on the basis of the magnetic field evaluation lines above and below the ship.ConclusionsThe proposed total field anomaly inversion modeling technique provides a certain theoretical and technical basis for the engineering application of unmanned aerial vehicles for measuring ship magnetic fields, and provides a viable alternative for ship magnetic field detection and analysis.

ObjectiveAir-independent propulsion (AIP) submarines carry huge quantities of liquid oxygen. However, in the liquid oxygen combustion process, the cold energy is not usually recovered, and there are criticisms concerning CO2 removal in the confined space of a submarine. Aiming at the two major pain points that have long plagued AIP submarines, this paper proposes an integrated solution that organically combines the recovery and utilization of cold energy from liquid oxygen with the removal of CO2 . Method Liquid oxygen can be used as a cold energy source to process the cabin air. According to the different phase transition temperature characteristics of each air component, the CO2 in the air is frozen (condensed) and separated under normal pressure. ResultsThis study uses quantitative data analysis to show that the energy stored in liquid oxygen can basically meet the cold energy demand for freezing the CO2 produced by the crew's respiratory metabolism within the self-sustaining capacity of the submarine, and proposes a three-level workflow and implementation plan. ConclusionThe technical solution proposed in this paper can recycle 5% of cold energy from liquid oxygen, which conforms to the idea of the comprehensive integration of energy saving and noise reduction, and the full integration of overall resources. As such, it can be used for air purification and CO2 removal in confined spaces such as submarines, submersibles and deep-sea space stations powered by AIP systems.

ObjectiveA method for calculating and selecting the minimum specific energy consumption working point is developed to solve the problem of high energy consumption in the short-distance slurry transport of dredgers.MethodsBased on the relevant models of pump pressure, pipeline resistance, critical flow velocity and pump cavitation established in previous research, the minimum energy consumption per cubic meter of particles is taken as the goal, and the pump speed which is most familiar and easily controlled by the workers is taken as the optimization variable. A double optimization method combining the minimum energy consumption search of the transport system and the optimization of multiple pump speeds is adopted to calculate and select the minimum energy consumption working point of the dredger with multiple pumps in a series, and the algorithm is verified by taking the construction of the "Xin Hai Lang" dredger with three different types of centrifugal pumps under specific working conditions as a case. ResultsThe calculation results show that the specific-energy consumption of the working point optimized by this algorithm model is 11.8% lower than that of the maximum production working point. Conclusions The application of these research results can effectively make up for existing optimization methods that aim at production capacity (max. production), providing feasible references for energy saving and emission reduction in dredging projects.

ObjectivesThis study analyzes the influencing factors of the aeroacoustic noise of cruise ship chimney exhaust pipelines, and predicts the noise level in the open area of a domestic cruise ship. MethodsA hybrid approach combining Large Eddy Simulation and Lighthill Analogy is used to conduct a numerical simulation of the pipeline’s aeroacoustic noise under the actual working conditions of a cruise ship. The impacts of the inlet flow rate, outlet pressure, cross-section shape and bending angle on the aerodynamic noise are studied. A real-size cruise ship chimney exhaust pipeline model is then established to analyze the main noise sources and predict the noise level in the open areaa. ResultsThe results show that the larger the inlet velocity and bending angle, the greater the aeroacoustic noise. Furthermore, the aeroacoustic noise of a pipeline with a zero pressure-outlet is greater than that with a non-zero pressure-outlet. It is also found that the aeroacoustic noise of a rectangular cross-sectional pipeline is marginally smaller than that of a circular one. In addition, the narrowband peak frequency decreases as the bending angle increases. Except for the chimney outlet, the noise level in the open area is low. ConclusionsThis study has significant reference value for controlling the aeroacoustic noise of cruise ship chimney exhaust pipelines and improving passenger comfort in open areas.

ObjectiveIn order to monitor the health status of a centrifugal pump in real time, this study proposes a model for the real-time identification of the degradation state of centrifugal pumps. MethodsFirst, based on the operating parameters and degradation mechanism of the centrifugal pump, a combined weighting model using a combination of subjective and objective weights is used to calculate the combined weights, then a health index during the degradation process of the centrifugal pump is constructed. Second, based on the existing pump degradation data, a degradation identification model based on the genetic algorithm-group method of data handling (GA-GMDH) algorithm is proposed.ResultsThe reliability of the GA-GMDH monitoring model is relatively high, with a root mean square error of 0.029216 between the output values of the health index and the actual values. Based on the model's output results, the accuracy of degradation state identification is 93.333%. ConclusionThe results of this study can provide valuable references for the health monitoring and maintenance operation management of centrifugal pumps.

Underwater vehicles (UVs) not only play an important role in civilian fields, but have also been widely used in military fields and have broad application prospects. However, they are susceptible to the influence of the ocean current environment during underwater navigation, which makes their hydrodynamic parameters uncertain and in turn leads to difficulties in motion attitude control, navigation deviation and increased navigation cost, and even affects the completion of underwater combat tasks and the occurrence of collision accidents, thereby endangering navigation safety. Building on a summary of the current status of hydrodynamic research regarding different UVs in ocean currents and UV control methods for dealing with ocean current interference, we propose several perspectives on the development direction of UVs under ocean currents intended to provide references for the study of the influence mechanisms of complex ocean currents on the hydrodynamic parameters of UVs and the design of controllers.

The extra-large unmanned underwater vehicle (XLUUV), as an underwater unmanned combat platform capable of carrying a large number of mission payloads, holds significant research value. This paper aims to provide a comprehensive understanding of the current research status of XLUUVs abroad and to offer insights for their development and application in China. It systematically analyzes the development status and future trends of the five main types of XLUUVs: ordinary, submersible, submarine-like, packable, and container types. The paper explores key technologies such as modularization, multi-domain communication, underwater detection, positioning and navigation, autonomous control, and energy power. It also studies their combat applications in reconnaissance and surveillance, underwater mine laying, underwater attack and defense, and underwater delivery. Research indicates that XLUUVs are trending towards ethnic, remote, fully deepened, and intelligent development. They are capable of multi-dimensional detection, sealing off important maritime routes, conducting asymmetric strikes, and covert delivery, which will be crucial in future underwater confrontations.

Objectives To simulate the complex nonlinear interactions between the free surface and instantaneous wetted hull surface during ship navigation, an automatic mesh generation method is proposed.MethodsThe method takes a whole-hull triangular mesh file as the input and automatically generates meshes for the instantaneously transforming free surface and wetted hull surface. It is able to handle arbitrary hullforms with complex bow and stern geometries while exhibiting strong robustness. Simulations of waves generated by a KCS hull at various speeds are then conducted using the proposed method. ResultsThe predicted ship-side wave profiles, wave patterns around the hull, and wave-making resistance show good agreement with the experimental results. Moreover, at higher speeds, the simulated wave surface at the KCS stern consistently stays below the transom, exhibiting a dry transom characteristic. ConclusionsThe results demonstrate that the proposed method can accurately model nonlinear ship waves and has the potential to simulate the flow fields around transom-stern ships.

Objective In order to improve the actual efficiency of microbubble drag reduction technology, this study develops intelligent airflow control technology for microbubble drag reduction based on an improved ant colony optimization (ACO) algorithm.MethodsBased on the mechanism of microbubble drag reduction, the ideal optimal airflow rate at different speeds is obtained by carrying out microbubble drag reduction tests on a self-developed ship model prototype. The software system of intelligent airflow control technology is developed by employing the improved ACO algorithm. A self-propelled test on a ship model installed with an intelligent control hardware system is carried out to verify the actual drag reduction effect of this technology. ResultsThe technology proposed in this study can effectively control the airflow to reach the optimal microbubble drag reduction condition, and can also monitor the speed change and adaptively control the airflow to achieve the best drag reduction conditions at various speeds.Conclusion This technique improves the automation and intelligence level of microbubble drag reduction technology while enhancing its actual efficacy.

ObjectivesThis paper explores the effects of air flow rate and sailing angle on the air layer retention and energy efficiency of the bottom groove of a ship, focusing on a large scale model of a bulk carrier. MethodsAn air layer drag reduction self-propelled model system and hull cavity scheme are designed, and drag reduction experiments are conducted under open water conditions. The jet drag reduction effect on the model in a positive floating attitude of ship is examined, as well as the impact of a certain trim angle on the speed and shaft power of the model. Results The results indicate that, when the main engine speed is constant, air injection can significantly improve the speed of model; after stopping the jet, the air layer within the air cavity of the bottom groove can be maintained for a long time, with better drag reduction efficiency when the ship is in an upright state and the trimming is within 0.25 degrees. When the trimming angle is too large, the gas will overflow from both sides of the model head and the air layer will not effectively cover the bottom of the ship, decreasing the efficiency of drag reduction. ConclusionsSeveral meaningful conclusions are obtained from the above experiments, providing useful references for the engineering application of air layer drag reduction technology on full-formed ships.

ObjectiveIn order to improve the efficiency of the finite element geometric modeling of hulls, to address the problem in which hull geometry models exported by ship design software cannot be directly used in finite element modeling, this study propose a geometric model processing method based on the XML file data.MethodsAn Extensible Markup Language (XML) file of a hull exported by AVEVA Marine (AM) software is taken as the research object. First, the data expression rules of the geometric model are clarified through the analysis of the data file structure. Then, on the basis of the hull finite element modeling specifications and model data modification, the proposed algorithm can automatically delete and simplify components such as notches and holes, convert bulb flat and brackets, and move profiles and plate seams.ResultsThe processed hull geometry model can be directly imported into finite element software HyperMesh, which complies with the finite element modeling specifications and retains the original model material, plate frame thickness and other related attributes. ConclusionsCompared with manual modeling, the efficiency of testing segmented solid geometric modeling is significantly improved. The proposed method can enable the reuse of ship design models and improve the efficiency of finite element geometric modeling.

ObjectiveThis paper seeks to solve the problem of the partial failure of the actuator of an underwater salvage robot caused by a decrease in the shipborne power supply voltage or corrosion of the control system circuit. Methods Considering the influence of complex ocean conditions on deep-sea torpedoes, cargo shipwrecks, and other engineering salvage operations, the terminal sliding mode observer is used to observe the uncertain disturbance of the system, while the fault-tolerant control method and finite-time control method are used to estimate the fault coefficient of the actuator on-line, and a finite-time trajectory tracking fault-tolerant control scheme with a terminal sliding mode observer is designed. ResultThe system output of this scheme is smooth, stable, and can reach the desired trajectory quickly. At the same time, compared with the traditional fault-tolerant control scheme, the steady-state time of the underwater salvage robot's control system is reduced (the lateral displacement is reduced by 10 s and the longitudinal displacement by 15 s).ConclusionThis study has practical engineering significance and can provide theoretical references for the trajectory tracking of underwater construction machinery.

Objectives To overcome the challenges of tracking small targets in unmanned surface vehicle vision under the conditions of low feature resolution and similar environmental information, a multi-feature fusion-based continuous convolution operator tracking （MCCOT） algorithm is proposed. Methods The resolution of multi-feature maps is enhanced using bicubic interpolation techniques to enable sub-pixel-level localization. Efficiencies in target tracking are achieved through feature projection and sample space generation to mitigate filter overfitting. Furthermore, interference arising from similar environmental features on the filter is addressed by developing an update strategy for high-confidence models. Results As the experimental results show, compared to traditional continuous convolution operator tracking algorithms, the proposed algorithm achieves an average success rate increase of 17.4%, average distance precision increase of 17.8%, and expected average overlap rate increase of 5.1%. Conclusions The proposed algorithm can deal with the problem of small target tracking confusion in marine environments, providing key technical support for improving the intelligent sensing capability of unmanned boats and marine robots.

ObjectivesThis paper seeks to solve the problem of excessive energy consumption caused by the excessive amplitude of the rudder angle and high frequency of rudder angle change during ship course-keeping.MethodsA closed-loop gain shaping algorithm (CGSA) is used to design the controller; a double nonlinear feedback algorithm is introduced; the steering amplitude and frequency are reduced by a combination of the bipolar S-function and arc-tangent function; and the whole control system is controlled by the positive feedback method. ResultsThe simulation results show that the proposed double nonlinear feedback algorithm improves the comprehensive energy-saving evaluation index of steering gear energy consumption by 31.53% and 18.63% under Beaufort No. 6 and Beaufort No. 8 respectively. ConclusionsIt is verified that the double nonlinear feedback method has a positive effect on saving the power consumption of the steering gear and reducing carbon emissions in the course of ship navigation, and an equivalent control effect of positive feedback and negative feedback, providing valuable references for the study of ship course-keeping control.

ObjectiveThis paper investigates the synchronization control problem for supply and supplied ships during underway replenishment in a constantly time-varying ocean environment while considering the ocean environmental disturbances induced by wind, waves and currents, and the interaction effects between ships and their soft connections. It then proposes an event-triggered synchronization anti-disturbance control method based on disturbance estimation and compensation.MethodThe first-order Markov process is used to describe the ocean environmental disturbances induced by waves, wind and currents that affect the supply ship, as well as the unmodeled dynamics. The interaction effects between the ships and their soft connections are included in the unmodeled dynamics. The online disturbance observer estimates time-varying unknown bounded disturbances which are compensated for in the control design using disturbance estimates. The auxiliary dynamic system helps to reduce the control saturation effects on anti-disturbance control. The event-triggered mechanism reduces the execution frequencies of control signals to avoid the excessive wear of the thrusters.ResultsA simulation of a supply ship using the event-triggered control method shows the existence of a minimum sampling interval greater than 0 and the inability to trigger infinitely in a finite time. The controller can achieve synchronous navigation between supply and supplied ships while ensuring the global uniform ultimate boundedness of all closed-loop control signals.ConclusionThe simulation results validate the effectiveness of the proposed event-triggered disturbance estimation and compensation control method.

ObjectiveAs traditional ship crack detection methods based on artificial visual inspection and ultrasonic methods in ship repair and inspection processes have the characteristics of low efficiency, high cost and high danger, a ship crack detection method based on deep learning is proposed. Methods First, a lightweight convolutional structure (GSConv) is used to replace the standard convolution and introduce an attention mechanism in the backbone of YOLOv5s to achieve the reduction of network parameters and computational complexity while enhancing the ability to extract crack features. Second, C3~~Faster constructed by a fast convolutional structure is used instead of the original C3 module in the neck of the network to improve the processing speed of the model and enhance its rapidity. Finally, an improved bidirectional weighted feature fusion network (BiFFN) is used to enrich the semantic and positional information of cracks in the feature map and improve the model's crack identification accuracy and location precision. ResultsBy training on both original and augmented ship crack datasets, the proposed method achieves a detection accuracy of over 94.11% and a recall rate of over 93.50%, while reducing the computational complexity by 17.93% and parameter count by 15.81%. ConclusionThis study demonstrates that the proposed ship crack detection method based on GSConv and BiFFN achieves lightweight model architecture and high detection accuracy and recall rates, providing useful references for the development of UAV/ship autonomous inspection systems.

ObjectivesIn the application of sandwich composite materials in ship structures, to ensure the structural strength characteristics, bolt connections are usually used between the sandwich composite structure and steel main hull. However, due to local stress concentration around the holes and the boundary effects at the connection locations, the bolt connection position is often becomes the most critical area of the structure, necessitating strength and fatigue assessments. MethodsFor a full-scale steel-sandwich composite panel structure, the strength characteristics of the plate frame under a 0.1 MPa uniform load are studied using experimental and finite element methods. The accuracy of the finite element method is then verified by comparing its results with those from the experimental method. Subsequently, based on the finite element method, fatigue tests for two equivalent bolted joints are designed through structural stress equivalence, and the fatigue life of the steel-sandwich composite structure is evaluated and verified. ResultsIt is determined that the fatigue life of the panel structure under the design load meets the design requirement of 5 million cycles.ConclusionsRegarding the fatigue problem of large bolted connections, equivalent joints can be designed for fatigue testing, and load equivalence can be achieved through structural stress equivalence.

ObjectivesIn order to accurately assess the ultimate load-bearing capacity of a hull structure under cyclic loading, it is necessary to consider the effects of the cyclic properties of the load on the ultimate strength of hull stiffened plates. MethodsFinite element software ANSYS is used to carry out the numerical simulation of ultimate strength based on the elastic shakedown limit state under in-plane cyclic loading for 200 T-bar stiffened plates and flat-bar stiffened plates. The material Bauschinger effect, welding initial deformation and residual stresses of the hull stiffened plates under in-plane cyclic loading are considered in detail. Based on the stiffened plate nonlinear finite element numerical simulation results of the elastic shakedown limit state, the dimensionless ultimate strength of the stiffened plates is taken as the objective function, the flexibility coefficients of the plates and stiffeners are taken as the independent variables, and a formula for predicting the ultimate strength of hull stiffened plates under in-plane cyclic loading based on the elastic shakedown limit state is proposed. ResultsThe accuracy of the formula is verified through comparison with the nonlinear finite element simulation results. ConclusionsThis study has positive theoretical and engineering significance for the ultimate strength assessment of hull stiffened plates under cyclic loading.

ObjectivesThis study proposes a structural strain test method for investigating the buckling pressure of a composite shell in a high hydrostatic pressure water environment and the effects of different initial defects. MethodsA high hydrostatic pressure test is conducted on a composite shell, and the strain of the structure is measured under 15 MPa hydrostatic pressure using the deep-water 3D-DIC (digital image correlation) method based on a high pressure shield. The results are compared to numerical simulations to confirm the feasibility of the 3D-DIC test system in a high hydrostatic pressure environment and verify the accuracy of the numerical method. Based on the numerical simulation method, research is then conducted on the buckling behavior of the composite shell. ResultsThe average error between the experimental results and numerical simulation results is found to be 7.2%. In addition, the ultimate load capacity of the composite shell is found to be in an almost quadratic function relationship to the size of the added geometric defects in different modes, and the average gradient of change in the ultimate load capacity is 1.48-2.12 MPa/mm in the first order mode and 1-4.32 MPa/mm in the third order mode. Conclusions3D-DIC test technology allows for the accurate strain measurement of composite shells in high hydrostatic pressure environments. Moreover, in such an environment, geometric defects largely affect the ultimate load capacity of the composite shell, while third-order modes are affected to a greater extent than first-order modes.

ObjectivesTo solve the problem of applying thick carbon fiber laminates in naval architecture and marine engineering, this paper proposes a thick plate structure composed of thin carbon fiber laminates with reinforcing ribs and core material in the middle.MethodsFirst, based on the theory of continuous damage mechanics, two-dimensional Hashin failure criterion and cohesive element technology, a finite element analysis model of a combined thick plate is established. Second, the bending performance of the combined thick plate in different directions is compared and analyzed. Next, the progressive damage process of the adhesive layer and carbon fibre-reinforced polymer (CFRP) is analyzed to investigate the failure behavior and mechanisms of the combined thick plate during the bending process in the direction perpendicular to the stiffeners. Finally, the influence of adhesive strength on the damage behavior of the combined thick plate is discussed. ResultsThe results indicate that the bending stiffness of the combined thick plate is higher in the direction perpendicular to the stiffeners compared to the parallel direction. During the bending process perpendicular to the reinforcing rib direction, the adhesive layer at the interface between the rib plate and panel is the first to suffer damage; it rapidly expands under shear force, ultimately forming a mixed failure mode where interface failure and panel 0° ply fiber tearing promote each other. When the adhesive strength is low, the adhesive layer breaks first; and when the adhesive strength is high, the laminates break first. The adhesive layer mainly bears shear action under Types II and III tearing modes, and the shear stress in the direction of the reinforcing rib is the primary cause of initial damage to the adhesive layer. ConclusionsThis study reveals the damage behavior of a new-type composite thick plates during bending perpendicular to the stiffener direction, thereby providing technical references for the promotion and application of thick composite plates with reinforcing ribs and core material in the middle in major load-bearing structures in the fields of naval architecture and marine engineering.

ObjectiveIn order to meet the development trend of low weight and high strength in the design of compression shell structures for deep-sea submersibles, this paper proposes a structural enhancement method which involves winding a circumferential internal corrugated compression shell with a carbon fiber reinforced polymer (CFRP), then analyzes and optimizes the CFRP layering scheme. MethodFinite element simulation software ABAQUS is used to change the layering parameters of the CFRP according to the stress distribution of the composite layers, and the optimal winding angle under 14 layers of CFRP is explored to obtain the best layering scheme. The relevant strength failure criteria are then combined to check the intensity. ResultsThe results indicate that 40° is the best CFRP winding angle to enhance the circumferential internal corrugated compression shell. When the layering mode is (±40°2/±55°2/±70°2/90°2), the stress distribution between the layers is relatively uniform, meaning that the structure has a lighter weight while its strength fulfills the actual use requirements. ConclusionThe proposed structure can reduce the overall weight of deep-sea compression shells and push the development of the field toward lightweight design.

ObjectiveIn order to design and optimize the typical orthogonally stiffened plates commonly used in naval architecture and ocean engineering, it is necessary to investigate the influence of different stiffener configurations on the intrinsic dynamic characteristics of orthogonally stiffened plates.MethodsThe natural frequencies and mode shapes of orthogonally, obliquely and curvilinearly stiffened plates are calculated using the finite element method under clamped boundary conditions. Comparative and analogy analyses of the intrinsic dynamic characteristics of the three types of stiffened plates reveals the influence of stiffener configurations on their dynamic coupling behavior and intrinsic dynamic characteristics.ResultsThe numerical results show that the natural frequency of obliquely stiffened plates can be increased by 9% compared to that of orthogonally stiffened plates, while the stiffness distributions of obliquely stiffened plates and curvilinearly stiffened plates are more homogeneous than that of orthogonally stiffened plates, and the dynamic stiffness homogenization effect of curvilinearly stiffened plates is more significant. In the frequency range below 900 Hz, the stronger the reinforcement, the greater the influence of the stiffener configuration on the dynamic characteristics of stiffened plates.ConclusionThe results of this study can provide valuable references for the design of structures composed of orthogonally stiffened plates.

ObjectiveIn order to study the stress distribution of circular holed infinite corrugated core sandwich panels under uniaxial tension and seek the most unfavorable opening position, Methodboth submodel analysis method and subproblem optimization method were employed just for calculation stress distribution of different holing positions and seeking the holing position where the hole edge stress concentration factor reached the maximum, respectively.ResultThe stress distribution near the opening is closely related to the mesh division, and the maximum stress always occurs on the core layer board. The stress concentration factor of an infinitely corrugated sandwich panel with a circular opening under uniaxial tension is generally greater than 3, with a maximum of 4.104. Conclusion Undergoing the stress extreme value optimization analysis process of the overall model before the submodel can save computation time, achieve high computational accuracy, and improve optimization efficiency.

ObjectivesAs wheel loading on the vehicle decks of ro-ro ships may introduce difficulties for structural analysis due to the uncertainty of their quantities, ranges, and positions, it is necessary to study the equivalence and simplification of such loads. MethodsUsing a multi-span beam model as an idealized vehicle deck secondary member, wheel loading is simulated by distributed load and concentrated force respectively, and several structural responses are compared and analyzed. Furthermore, an analysis is made of the change in structural response after multiple wheel loadings have been simultaneously transformed into concentrated force. ResultsThe calculation results suggest that for vehicle decks with an ordinary arrangement, such equivalence and simplification tends to be safer and will not bring significant changes to the structural response. ConclusionsBased on the above conclusions, a reasonable equivalent multi-wheel loading method is provided for engineering application, thus contributing to the design simplification and strength verification of vehicle decks.

ObjectiveAs specific and demanding requirements are imposed by measuring the impedance of the anechoic coatings on the hull surface of an underwater vehicle using the traditional water-filled tube experiment, a method of sound absorption coefficient measurement based on a simplified impedance experiment is proposed, and the correction of non-planar wave reflection in the measurement is achieved. MethodsThe velocity at different radii on the surface of the coating is deduced by simplifying the coating as the impedance matrix boundary and combining the fluid boundary conditions. The relationship between the pressure of the non-planar reflected wave and axial velocity is established via the Hankel transformation, and the formula of the sound absorption coefficient of the modified non-planar reflection wave is obtained. Numerical calculation and the impedance experiment are compared with theoretical calculation and the traditional acoustic tube experiment to validate the validity of the proposed method. ResultsThe numerical calculation shows that the results of solid coating are consistent with the theoretical results, but the results of coating with cylindrical cavities are obviously different from the theoretical results. The impedance experiment results have a large error compared with the acoustic tube results after planar wave processing, and are consistent with the acoustic tube results after correction. ConclusionsCavities are among the causes of the reflection of the non-planar wave from the coating, and this needs to be corrected in impedance measurement. The sound absorption coefficient obtained by the modified impedance measurement method is consistent with that obtained by the acoustic tube experiment in the frequency range of interest, and the experiment is simplified, giving the proposed method a certain practical value.

ObjectiveA lightweight and efficient ship detection method based on the YOLO-FNC model is proposed for complex environments such as ports with dense traffic. MethodFirst, a FasterNeXt neural network module is designed on the basis of the FasterNet method and replaces the C3 module in the YOLO model to ensure faster operation without affecting accuracy. Second, a normalization-based attention module (NAM) is integrated into the network structure and the sparse weight penalty is used to suppress the feature weights and ensure more efficient weight calculation. Finally, a new bounding box regression loss is proposed to speed up the prediction frame adjustment and increase the regression rate, thereby improving the convergence rate of the network mode. ResultsThe experimental results show that when performing detection experiments on ship datasets in a self-built complex environment, the proposed method improves the mAP@0.5 by 6.35%, reduces the parameter count by 9.74% and reduces the computational complexity by 11.39%. ConclusionThe proposed method effectively achieves lightweight and high-precision ship detection compared with the YOLOv5s algorithm.

ObjectiveIt is vital to detect and track ships during coastal monitoring and ship navigation over long distances in complex circumstances. However, due to their small size and unclear features, they can be readily confused with shorelines, noise, and rocks, making them sometimes difficult to spot immediately. To address this issue, a novel ship detection method called ShipDet is proposed which significantly improves performance through the design of a dedicated backbone network, improved feature extraction process, and constrained microscopic detection heads. MethodFirst, this method constructs a feature fusion and extraction network that is highly sensitive to small objects by integrating the Swin Transformer module (STR) with the classic CSPDarknet53 network. This enhances the correlation between small target features and the environment, establishing associations between ships and waterways, ships and other ships, and ships and coastlines, while suppressing irrelevant information. Subsequently, considering the uneven distribution and minor scale variations of ship targets in the dataset, two detection layers are retained to reduce model parameters and further enhance model performance. Moreover, the method employs the SCYLLA-IoU (SIoU) loss function to constrain the detection heads, thereby reducing regression freedom and improving detection accuracy and robustness. ResultsTo validate the proposed method, a dataset called 2023ships is established which consists of up to 9 000 samples covering various scenarios such as inland rivers, coastal areas, daytime, nighttime, and foggy weather. During testing, the proposed method demonstrates superior overall ship detection performance compared to other algorithms, with a mAP of 92.9%, a precision rate of 92.1%, and a parameter size of 35 366 310. ConclusionThe proposed method can greatly benefit the fields of maritime monitoring and intelligent navigation.

ObjectiveThis paper proposes a lightweight remote sensing ship target detection algorithm LR-YOLO based on improved YOLOv5s to meet the lightweight and fast inference requirements of ship target detection tasks involving remote sensing images. MethodsFirst, the backbone network adopts the ShuffleNet v2 block stacking method, effectively reducing the number of network model parameters and improving the computational speed; second, a region selection module filter is designed to select regions of interest and extract effective features more fully; finally, a circular smooth label is introduced to calculate angle loss and perform rotation detection on remote sensing ship targets, while deformable convolution is used to adapt to geometric deformation and improve detection performance. ResultsThe experimental results on the HRSC2016 ship dataset show that the detection accuracy of the algorithm reaches 92.90%, an improvement of 1.3%, with the number of network model parameters only 39.33% that of the baseline model. ConclusionThe proposed algorithm achieves a balance between lightweight and detection accuracy, providing references for remote sensing ship target detection.

ObjectiveTo address the problem of ultra-short baseline (USBL) relative navigation and positioning during the dynamic docking of an autonomous underwater vehicle (AUV) in shallow water, a compensated unscented particle filter (CUPF) algorithm based on a disturbance observer is proposed. MethodsThe CUPF algorithm compensates for missing values in the USBL data based on the dead reckoning model, estimates the unknown disturbances in the dynamic docking of the AUV using observer technology and filters outliers in the USBL data combined with the unscented particle filter to realize the estimation of the AUV's state. Results The lake trial data shows that the proposed CUPF algorithm can effectively remove outliers and fill in missing values for USBL positioning with a speed estimation error of less than 15% and computation time reduced by 57% compared with traditional methods. ConclusionsThe CUPF algorithm can improve the positioning accuracy of USBL while smoothing the AUV motion trajectory and state estimation by fully utilizing relative measurement information for AUV docking.

ObjectivesThis paper proposes a multi-source observation-based recovery bucket guidance docking strategy for the reliable recovery observation and motion goal tracking of unmanned surface vessels (USVs). MethodsDuring the entire docking process, the interference zone of the mother ship's wake is first avoided in order to complete the rough alignment of the USV route; the heading tracking guidance line is then maintained to prepare for terminal docking recovery adjustments; finally, the data obtained by the visual sensor and inertial navigation sensor is filtered and fused to calculate the recovery guidance line, which is then transmitted to the USV. The USV completes the tracking of the terminal guidance line and docking recovery task through its own guidance and control systems. A USV docking recovery system is simultaneously designed on the basis of visual and integrated navigation fusion, the hardware and software of the real boat is independently designed, and lake field tests are conducted to verify the feasibility of the system design and docking strategy.ResultsThe experimental results show that the success rate of the USV in performing autonomous docking tasks reaches 91.6%. The proposed docking strategy can meet the high-precision docking and recovery requirements of USVs. ConclusionsThe findings of this study can provide critical technical support for USV recovery operations.