An Underwater Visual Navigation Method Based on Multiple ArUco Markers

Underwater navigation presents crucial issues because of the rapid attenuation of electronic magnetic waves. The conventional underwater navigation methods are achieved by acoustic equipment, such as the ultra-short-baseline localisation systems and Doppler velocity logs, etc. However, they suffer from low fresh rate, low bandwidth, environmental disturbance and high cost. In the paper, a novel underwater visual navigation is investigated based on the multiple ArUco markers. Unlike other underwater navigation approaches based on the artificial markers, the noise model of the pose estimation of a single marker and an optimal algorithm of the multiple markers are developed to increase the precision of the method. The experimental tests are conducted in the towing tank. The results show that the proposed method is able to localise the underwater vehicle accurately.

SLAM CHARACTERISTICS OF A HIGH-SPEED WAVE PIERCING CATAMARAN IN IRREGULAR WAVES

Slam characteristics of a 112m INCAT wave piercing catamaran in a range of realistic irregular sea conditions are presented in this paper. Towing tank testing of a 2.5 m hydroelastic segmented catamaran model was used to gather a database of slam events in irregular seas. The model was instrumented to measure motions, centrebow surface pressures and forces, encountered wave elevations and wave elevations within the bow area tunnel arches. From these measurements characteristics of the vessel slamming behaviour are examined: in particular relative vertical velocity, centrebow immersion, archway wave elevations and slam load distributions. A total of 2,098 slam events were identified over 22 different conditions, each containing about 80 to 100 slam events. The data, although inherently scattered, shows that encounter wave frequency and significant wave height are important parameters with regard to centrebow slamming. Relative vertical velocity was found to be a poor indicator of slam magnitude and slams were found to occur before the centrebow arch tunnel was completely filled, supporting the application of a two-dimensional filling height parameter as a slam indicator.

SHAFT LOADS ON AZIMUTH PROPULSORS IN OBLIQUE FLOW AND WAVES

A range of model experiments have been carried out in calm water and waves for an oil spill vessel model with twin tractor azimuth thrusters at different heading angles and advance coefficients in the large towing tank at the Marine Technology Centre in Trondheim, Norway. Propeller shaft bending loads have been measured using a shaft dynamometer capable of measuring all shaft side force and bending moment components as well as propeller torque and thrust. The results include the loads on the propeller shaft with and without the presence of a ship hull model at the same heading angles and advance velocities in order to study the wake influence from the ship hull on the hydrodynamic loads. Results show that the ship hull wake has a much stronger effect on the propeller loads when the propeller is azimuthed outward from the ship hull centreline than inward. Measurements from the experiments in waves are also presented for the same thruster model in a straight-line course for both the head and following sea states under different wave conditions. Larger bending loads are found in head sea conditions compared with the following sea conditions. Generally it is found that the shaft bending loads and lateral forces are quite large, which is important to consider in the mechanical design layout and for dimensioning of components.

THE INFLUENCE OF CENTRE BOW LENGTH ON SLAMMING LOADS AND MOTIONS OF LARGE WAVE-PIERCING CATAMARANS

The effect of various centre bow lengths on the motions and wave-induced slamming loads on wave-piercing catamarans is investigated. A 2.5 m hydroelastic segmented model was tested with three different centre bow lengths and towed in regular waves in a towing tank. Measurements were made of the model motions, slam loads and vertical bending moments in the model demi-hulls. The model experiments were carried out for a test condition equivalent to a wave height of 2.68 m and a speed of 20 knots at full scale. Bow accelerations and vertical bending moments due to slamming showed significant changes with the change in centre bow, the longest centre bow having the highest wave-induced loads and accelerations. The increased volume of displaced water which is constrained beneath the bow archways is identified as the reason for this increase in the slamming load. In contrast it was found that the length of centre bow has a relatively small effect on the heave and pitch motions in slamming conditions.

STUDY ON PROPULSIVE PERFORMANCE OF TWO GEOMETRICALLY SIMILAR PODDED PROPULSORS

This paper presents the outcome of a research to evaluate the effect of size on the propulsive performance of podded propulsors in cavitating and non-cavitating open water conditions. Two cases are examined, namely: propeller-only case and pod-unit case. In the propeller-only case, a commercial propeller dynamometer is used to measure the thrust and torque of two propellers of different size at the four quadrants of propellers with varied shaft and flow speeds. Also, both propellers are tested at different tunnel pressure to study and compare the behaviour under similar cavitation conditions. In the pod-unit case, two geometrically similar but different sized pod-units are tested using two separate custom-made pod dynamometer systems in two towing tank facilities in straight-ahead and static azimuthing conditions. The study showed that the performance characteristics stabilize at lower Reynolds Number for the smaller propeller than the larger propeller. The propulsive performance of the two propellers was comparable in the four-quadrant experiments. Also, the experiments at the cavitating conditions showed that the cavitation characteristics of the two propellers were consistent at corresponding operating conditions. The experiment results of the two pod-units were also comparable for forces and moments in the three coordinate directions in the straight-ahead and static azimuthing conditions. A brief discussion on the uncertainty assessments for each of the measurements is also presented.

STUDY OF FROUDE AND HUGHES METHODS BY NUMERICAL TOWING TANK

The resistance of a cargo ship is calculated by numerical towing tank. RANSE multi-phase parallel solver with K-Z SSTturbulent model and VOF formulation is applied. Computational results from double model (without free surface) areused to obtain 1+k in Hughes’ method and those with free surface are analyzed by both Froude and Hughes’ approachesto investigate model and full scale correlation. ITTC recommended uncertainty study is carried out to evaluate numericalerror due to grid density. The computed wave elevation, wake distribution and resistance components by fine, mediumand coarse meshes are cross-compared and validated against experiment data where applicable. It is found that gridresolution has most effect on wave pattern. The predicted friction and viscous-pressure resistance coefficients arerelatively grid independent from present numerical simulation.

DETERMINATION OF WAVE SLAMMING LOADS ON HIGH-SPEED CATAMARANS BY HYDROELASTIC SEGMENTED MODEL EXPERIMENTS

A 2.5m hydroelastic segmented catamaran model has been developed based on the 112m INCAT wave-piercer catamaran to simulate the vibration response during the measurement of dynamic slam loads in head seas. Towing tank tests were performed in regular seas to measure the dynamic slam loads acting on the centre bow and vertical bending moments acting in the demihulls of the catamaran model as a function of wave frequency and wave height to establish the operational loads acting on the full-scale 112m INCAT catamaran vessel. Peak slam forces measured on the bow of the model are found to approach the weight of the model, this being similar to the findings of full-scale vessel trials. A review of the motions of the hydroelastic segmented catamaran model found that the heave and pitch motions give a good indication of slamming severity in terms of the dimensionless heave and pitch accelerations. The dynamic wave slam forces are closely related to the relative motion between the bow and the incident wave profile.

Experimental model tests to improve a tanker resistance performance

The ship resistance is one of the most important hydrodynamics performances, being related to the contractual ship speed. The experimental model tests can be used to measure and improve the resistance performance. In this paper, the possibility of using the experimental techniques in order to improve a tanker model resistance is demonstrated, based on a bulbous bow modelling solution. In this context, the results obtained in the Towing Tank of the Naval Architecture Faculty of “Dunarea de Jos” University of Galati, related to a tanker model resistance with and without bulbous bow are presented. The bulbous bow form was realised based on the hydrodynamics principles adapted to the bow forms of the tanker. In the case of the bulbous bow solution, a significant reduction of over 8% of the tanker model resistance was obtained, in the design speed domain.

Full-scale self-propulsion simulation with a discretized propeller

A comparison between towing tank testing and full-scale CFD simulations is presented at three different target speeds. For the current self-propulsion simulation, the self-propulsion point was obtained using polynomial interpolation. The studies of boundary layer thickness, a basic grid uncertainty assessment and verification were performed to give some confidence of grid application to current self-propulsion simulation. All simulations are performed using a commercial CFD software STAR-CCM+. It is concluded that with high-fidelity numerical methods, it’s possible to treat hull roughness and directly calculate full-scale flow characteristics, including the effects of the free surface, none-linearity, turbulence and the interaction between propeller, hull and the flow field.

Effect of water resistance on predictions of ship ice resistance using towing tests in ice basin

Object and purpose of research. The object under study is a method to determine ice resistance using towing tests of ship models. The purpose of the work is to develop a method that takes into account the water resistance effect on predictions of full-scale ship ice resistance. Materials and methods. The materials for development are model test data and earlier methods for determination of ice resistance on models, as well as recommendations of the International Towing Tank Conference (ITTC). Main results. The method is suggested to take into account the water resistance in analyzing the towing test data obtained in the ice basin, as well as the method for extrapolating the ice resistance due to hydrodynamic interaction of ice floes with underwater hull, including the scale effect. Conclusions. The methods that take into account the water resistance effect on predictions of ship ice resistance based on towing test data obtained in ice basins are reviewed and analyzed. An improved method to include the water resistance effect in a more correct way is suggested. For better comparison of test results in ice basin it is required to introduce a common method of including the water resistance effect using the method suggested in this work.