This paper describes a numerical method to calculate viscous flow for a body of complex geometry. Grid embedding technique is employed in order to enable the flow calculation of complex geometry and the three-dimensional viscous flow calculation method with overset grid is developed.
This method is adopted to the calculation of flow passing through waterjet inlets, and the results are compared with the Kashiwadani's experiments. Pressure distributions on the ramp and the lip surface reveal good agreement with measured results compared with two-dimensional viscous flow calculation. Three-dimensional effects on the total pressure loss is explained through computational results.

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A perturbational study is made of the bow flow field with spray phenomena around a planing plate with a small dead rise angle and a small trim angle in order to get basis on the configuration of the wetted surface area. It is set forth a hypothesis such that the plate planes on a water surface deformed as a parabolic cylinder with transverse curvature near the bow, but the deformation probably seems to be caused by displacement effect. The hypothesis allows to assume a similarity flow changing the scale of transverse section as a square root of longitudinal length. The flow field is divided into three regions for the analysis. By analyzing the bow near field, the spray root line is obtained as forming a parabola on the plate, so that good coincidence with an experimental result of pressure contour is shown with help of the same flow near the spray root line as Wagner's local flow. The spray flow developing on the plate outside the spray root line is analyzed. The flow rate and thickness of the spray and the spray drag show good coincidence with an experimental result. Spray edge line is shown by assuming a lower bound of spray thickness.

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A numerical study on the turbulent flow around the strut-plate juncture is carried out for the inclined strut and for the flow with an angle of attack. Computations are made by solving three-dimensional incompressible Reynolds-averaged Navier-Stokes equation with MAC method. Turbulence is simulated by using Baldwin-Lomax model with a modified length scale for the treatment of corner flow. The flows, at the Reynolds number of 10⁵, show intensive unsteady behavior near the plate while the flows are steady enough off the plate. Inclination of the strut induces a lift which is generated mainly near the plate. On the other hand, the lift by an angle of attack reduces near the plate.

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The craft by Wing In Ground effect (WIG) is considered as one of the future high speed vessel on seaway. As well known, the lift of the wing advancing over the rigid surface becomes larger than the case of the wing in infinite air field by the ground effect. In the present investigation, the free surface effect on aerodynamic characteristics of WIG is computed by means of boundary element techniques. The air flow field around WIG is analyzed by the panel method, and interactions between WIG and the free surface are represented as pressure distributions acting on the free surface by this method. In order to analyze the wave making phenomena caused by the pressure distributions on the water surface, Rankine source method is employed, in which the linearized free surface condition is imposed on the still water surface. The interaction problem between WIG and the free surface can be solved by the interative procedure based on the panel method for the air flow field and Rankine source method for water flow field. From results of these computations, the free surface effect on aerodynamic characteristics of WIG can be clarified.

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A Power-Augmented Ram Wing In Ground (PAR-WIG) is a wing in ground effect, whose lift is enhanced by using the airflow of a thruster, which is placed in the upstream of the wing, so as to create a high ram pressure region under the wing.
In this study, through a NS solver based on a finite-volume method, a flowfield around a two-dimensional wing in ground effect is computed. Relations between a ram pressure and a height from ground to the wing are considered. Next, by soving a flowfield around a two-dimensional PAR-WIG, the PAR effect is mentioned.

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Unsteady viscous flow field around an oscillating 2D hydrofoil is studied numerically. The main objectives are to study the flow phenomena and the abilities of an oscillating foil to produce thrust. In the general case, the foil is performing a combination of symmetric pitching, heaving and surging oscillations. Numerical simulations are performed over a modified NAC0012 foil. Investigations consider the basic flow features and the effect of main governing parameters. Flow is simulated by solving the full 2D unsteady Navier-Stokes equations, in a vorticity-stream function formulation. An implicit numerical algorithm and a compact finite difference scheme are employed.
The validity of the flow simulation is studied through the comparison with the available experimental results. Flow analysis designates that the strong unsteady viscous effects with massive flow separation and intensive vortical structures are valid for the most of the studied motions. Such flow patterns are result of the leading and trailing edge separation and the further flow evolution including a complicated wake interaction. From a propulsive point of view, the combined pitching and heaving motion with a shifted phase of pitching is the most effective one. Surging oscillations introduce additional dynamic effects. Governing parameters have an interrelated influence on the flow penomena and induced forces.

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A surface panel method for the analysis of propellers operating in non-uniform flow is described. The surface panel method which was originally developed for the analysis of steady propeller problems is extended to the unsteady propeller problems. The surface of propeller blade and hub is approximated by a number of small hyperboloidal quadrilateral panels with constant source and doublet distributions whose strengths are varying with time. The surface of trailing vortex sheet is also represented by hyperboloidal quadrilateral panels with constant doublet distributions whose strengths are changing along downstream direction. The strengths of source and doublet are determined by solving the boundary value problem at each time step. First, calculations were conducted for a one bladed propeller rotating impulsively from rest in a uniform flow to confirm the accuracy and the applicability of the present panel method. Next, the problems of the propellers operating in non-uniform flow were solved by the present method. The unsteady pressure distributions on the propeller blades calculated by the present method are in good agreement with the experimental data in full scale propellers. Further, it is shown that the fluctuating thrust of one blade and propeller shaft forces calculated by the present method agree well with experiments.

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Three (3) propellers were newly designed for the container ship on which comparatively large pressure fluctuation of higher order blade frequencies (2nd5th) was observed. 1) The following concepts were applied in the new propeller design in order to decrease the propeller tip-vortex cavitation, consequently to decrease the pressure fluctuation of higher order blade frequencies, without changing the number of propeller blades.
(1) Tip-unload
(2) Smaller diameter
(3) Increase of blade area
(4) Increase of skew
Model experiments were performed in the cavitation tunnel by using these propeller models. These propellers were evaluated after investigating the repeatability of the fluctuating pressure and the effects of the number of propeller revolutions (at the test) and the cavitation number on the fluctuating pressure. The fluctuating pressure of one (1) of these propellers was found to have decreases by about 40 % on average.

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This paper firstly describes a new design method of supercavitating (hereafter, SC) propellers. The circulation distribution of propeller blades was calculated by a lifting line theory as an existing method, while the hydrodynamic characteristics of blade sections at each radial position were calculated by a nonlinear cavity flow theory based on a higher-order singularity panel method, “Linear Vortex Panel Method ; LVPM”.
Three SC propellers were designed for the same design conditions, that is, the ship speed is 50 kts, the required thrust per shaft is 100 tons and the propeller immersion is 4 meters. The SSPA SC propeller was chosen as a target SC propeller, since reliable design charts were published. The optimum propeller revolution rate was determined from the chart under the constrained condition of supercavitation. For the target propeller, a SSPA propeller model was made and the hydrodynamic characteristics were measured in the SRI large cavitation tunnel.
From the calculated circulation distribution and the assumed blade contour, the design lift coefficient at each radial position and SC section were determined by the iteration so that the strength requirement was satisfied. The lifting surface correction based on Ludwieg-Ginzel's method was applied to the blade profile on the face side of the first designed SC propeller (SRIJ-I). The efficiency of the SRIJ-I SC propeller at the design condition (J=1.1) was 0.676 and about 4 % higher than the target propeller. The measured thrust of the SRIJ-I SC propeller was 15 % higher than the predicted one and this propeller emitted relatively higher cavitation noise.
Secondly effort was made to improve the performance of the first designed SC propeller. From the examination by the experiments, inappropriate camber correction was cleared up. Using the load distribution of the SC sections given by the LVPM, the camber distribution at each radial position was calculated by the propeller design method based on “Quasi-Continuous Method”. The second SC propeller was tested at the cavitation tunnel. The measured propeller efficiency at the design condition was 0.720 and the increase of 11 % on the efficiency was obtained against the target propeller. The obtained thrust was 4 % higher than the predicted one, while the propeller was fully cavitating. It is concluded that a new favorable SC propeller could be designed by the present design method.

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This paper describes a computational method to predict the propeller performance under the supercavitating condition. The present calculation is based on a vortex lattice method. Constant pressure condition on the mean camber surface in the cavitating region is satisfied by reforming the mean camber surface iteratively according to the change of load distribution due to cavitation. Calculated results on the propeller force and the efficiency of two supercavitating propellers showed good agreements with experimental ones.

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It is well recognized that the ice resistance in continuous breaking mode consists of several components. However, the quantitative recognition of those components in the ice resistance is poorly understood. To understand the ice resistance quantitatively, the dependency on such factors as a thickness of ice sheet, a bending strength of ice sheet, a velocity of ship and so on has to be clarified for each component.
Among those components in the ice resistance, it is unanimously recognized that the breaking component is of primary importance. It has been considered by many investigators that the breaking component strongly relates to the moment capability of ice sheet, σ_ƒh² and does not depend on the ship velocity. However, the authors have had doubt on this recognition that the breaking component is independent of the velocity of ship. Such doubt comes partly from the experimental study by Kato et al. and partly from the theoretical study by Sorensen and Tawara & Horimizu.
In order to clear this doubt, it is necessary to separate the breaking component from other components. For this purpose, a new component separation test-named the TBI test-was developed and employed to the 1/20 model test on medium size icebreaker. The TBI test can use an almost completely preserved broken ice field which has been made just before by the test of the same model in intact level ice. Thus the TBI test employs a realistic breaking patters of ice field. The model experiments were performed on three different velocities. Six or seven tests were conducted at each velocity under different combinations of ice thickness and bending strength to verify the prediction. It was found from experiments that a relationship between the breaking component and the moment capability of ice sheet is linear when the velocity is specified. It is in agreement with authors' prediction. A relationship between the submersion component and ice thickness is not strictly defined, but a linear relation is again tenable.
Coefficients of both the relationship between the breaking component and the moment capability of ice sheet and the relationship between the submersion component and ice thickness were plotted with respect to velocity. It was found that both coefficients increase with velocities. This indicates that both the breaking component and the submersion component increase with velocities. It is concluded that the velocity dependence of the breaking component in ice resistance has been clearly evidenced. However, in order to understand the ice resistance more quantitatively, the authors should concern a study on the velocity dependency of the breaking component, which was not accomplished in the present study.

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Two optical methods to measure wave profiles are proposed with some accuracy analysis and examples. Although these are independent two methods, both are new techniques to get clear lines of free-surface boundaries. The methods provide enough contrast there for the boundary to be identified accurately by the image processing manner.
The one is the method to measure wave profiles along straight cutlines where the air-side of free-surface is visualized by mist of dry-ice (CO₂ gas). The mist, which is put on the water surface at upstream of the circulating water channel, covers all the surface in contact by the shearing stress of the moving water surface. The laser light sheet clearly visualizes the edge boundary of the air-side.
The second is for the measurement of the hull-side wave profiles by illuminating the wetted surface of the hull through the water. The surface looks brightly white as if some kinds of white paints were painted. It has enough contrast along the free-surface to be identified definitely.
Because the original pictures by the both methods have clear change in contrast across the free-surface boundary, the methods can provide relatively accurate results even after image processing.
Because both methods are enough simple and automatic. they can be applied for daily measurements of the wave height in the circulating water channel.

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In the previous paper, one of the authors and Prof. Fujino, based on a nonlinear strip synthesis, proposed a method to calculate the vertical motions and vertical wave loads of a high-speed craft in head sea. The validity of this method are verified practically through the comparison with model tests data. Furthermore, in this paper, following the similar synthesis scheme, a practical method for predicting the motions of a high-speed craft in oblique waves is presented and applied to an existing craft to clarify its fundamental characteristics of vertical and transverse motions in bow (or beam) sea.
In the method presented, the time-varying submerged hull surface and the coupling effect between transverse and vertical motions are considered. Moreover, using the momentum theory, the nonlinear flare impact and dynamic lift are also taken into account. Besides, the equations of motions are described by the body-fixed coordinte, rather than the vehicle-carried vertical frame which is used conventionally.
To prevent the numerical divergence due to the drift of sway and/or yaw motions in the time domain simulation, artificial springs in sway and yaw modes are introduced. The effects of the spring constants on the predicted ship motions are also discussed.

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An investigation on the effect of the steady disturbances on the unsteady flow of a ship in waves is made by the newly developed combined boundary-integral equation method (CBIEM), where the radiation condition is satisfied accurately. The boundary-value problem is solved in which the double-body flow has been accounted for consistently, and the results are compared to the experiments, as well as the results obtained by 3-D panel method with the free surface condition which only involves the uniform incoming flow. Considerable interaction between the steady disturbance and the unsteady flow is obtained, and the comparisons show that the calculations based on the double-body flow achieve better accuracy not only in the wave exciting forces and hydrodynamic coefficients, but also in the wave patterns and added wave-resistance. Therefore, CBIEM achieves a significant improvement on theoretical prediction.

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In the previous paper¹⁾ the authors reported some capsizing motions of a small craft in concentric transient waves with two successive crests in beam sea conditions. Large drift motions on the wave crests were observed, which seems to be characteristic of small crafts because of their light bodies and small drafts. As mentioned by Tanaka et al.²⁾ sway drag force can be significant in that large drifting motion because of the complicated under-water hull forms of small crafts.
In this paper forced sway motion test was conducted. It is clarified that in a large amplitude sway motion drag component explains the main part of sway damping force, and that the moment coefficient due to the drag force varies widely according to heel angle. It is also clarified by a simulation program that the moment due to sway drag played a significant roll in the roll motion reported in the previous paper.
This paper also deals with drag force and moment in a constant sway drift motion and in wind. The moment coefficient in a constant drift varies according not only to heel angle and but also to drift speed. As a result the heel angle is proportional to U³ in a free drift test in wind.

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This paper deals with development and application of the new regression equation for the bow freeboard which consists of the various parameters indicating the ship's principal particulars and the hull form features. The response characteristics of relative bow motion are first obtained for 67 existing ships in regular waves based on the strip theory. Using these response characteristics of 67 existing ships, a regression equation is introduced to express the magnitude of the relative bow motion in irregular waves. By using the regression equation, the relation between bow freeboard and deck wetness probability can be easily derived under any given sea conditions. This method is used for assessment of the current formula of the 1966 Load Line Convention and the Chinese formula which is recently proposed at SLF/IMO. Finally we discuss the future works necessary for the revision work of the convention.

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In the previous paper, a new theory was developed for computing the radiation forces on a catamaran by extending Newman's unified slender-ship theory. The present paper is a sequel and concerned with the prediction of wave-exciting forces and motions of a catamaran in waves.
The forward-speed version of Haskind-Newman's relation is applied to compute the wave-exciting forces by use of only the radiation-problem solutions. The obtained calculation formula is related to the Kochin function to be computed from the outer solution, which is expressed with the source and doublet distributions along the centerline of each demi-hull.
At zero speed, using Haskind-Newman's relation is theoretically exact and in fact the results are in excellent agreement with independent results by a more rigorous 3-D integral-equation method. In the forward-speed case, measurements of the heave and pitch exciting forces are conducted at Fn=0.15 and 0.3, using the same model as in the forced oscillation tests shown in the previous paper. The agreement between the measured and computed results is not that good particularly in the pitch exciting moment, but the computed hydrodynamic forces are used in studying the motion characteristics in waves of a catamaran.
Although the hydrodynamic interactions between twin hulls affect greatly on the radiation and wave-exciting forces, the resulting motions are similar to those of a single-hull ship, suggesting a cancellation of twin-hull interaction effects between the right- and left-hand sides of ship-motion equations.

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The problem of interaction among multiple floating cylinders which are fixed in waves and current is considered. The current velocity is assumed small. The problem is linearized based on a small parameter τ= ωU/g. Problems of different orders are solved for single cylinder to determine the diffraction property of the cylinder, which gives the relationship between the incident wave amplitude and the diffracted wave amplitude. The resuls are then applied to the problems of multiple cylinders. The interaction among cylinders is represented by some additional incident waves emitted from the other cylinders towards the one under consideration. Different from the previous work in which only the cylinders mounted to the sea bottom are considered, the evanescent wave modes are included in the present work when this method is applied to the floating cylinders. After solving the velocity potential near each cylinder, the hydrodynamic forces can be calculated by integrating the pressure along the wetted surface of each cylinder. It turns out that current and steady potential have a strong effect on the wave-drifting force. The present work also gives a way to evaluate the wave-drifting damping which plays an important role in the slow drift oscillation of moored ocean structures.

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This paper for the first time presents experimental results of the effect of random waves on the slow drift viscous damping forces acting on a vertical circular cylinder. The experiment was carried out using a forced oscillation apparatus, so a wide range of parameter combinations was easy to obtain. The effect of velocity ratio, reduced velocity and period ratio were investigated. Results from the experiment have revealed some different features between the effect of random waves and that of regular waves on the slow drift viscous damping forces.
This problem was also studied by a numerical method, in which the hydrodynamic forces were obtained from simulating the flow field using a finite difference method. Results of slow drift viscous damping force coefficients show generally good agreement between computed and measured values.

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The objectives of present paper are to investigate the properties of second-order sum-frequency wave exciting forces on column-footing type floating bodies and to confirm the existence of wave periods where the second-order wave exciting forces in vertical direction become very small.
The second-order sum-frequency wave exciting forces are obtained from the linear velocity potentials and the radiation potentials of sum-frequency utilizing the extended Haskind formula. The linear and the radiation potentials are computed by the hybrid boundary element method.
The vertical second-order sum-frequency wave exciting forces on column-footing type floating bodies are computed systematically and the numerical resalts are compared with the experimental resalts, so that those properties are made clear. Further, the existence of periods where the vertical exciting forces become very small are discussed.

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This paper describes the new type of anti-rolling system that is capable of reducing the rolling motion of a ship by active control of the moving mass. Similar system has been already realized for the reduction of the vibration of large-bridge towers and high-rise building structures. Present anti-rolling system is an application of the system.
Model experiments in a wave tank were carried out with the ship model installed with the anti-rolling system. Roll amplitudes in regular waves with the anti-rolling system are similar to the amplitudes of the ship model with an anti-rolling tank in which the weight of moving water is about two times of the moving mass of the present system. Then the moving mass system are more effective for roll reduction than anti-rolling tank. Effects of the incident wave height, forward speed, and position of the moving mass on the anti-rolling performance are investigated.
Next, at-sea experiment were performed with a small vessel (L=5.93 m) having the smaller prototype anti-rolling system. The effectiveness of the present system was confirmed from the measured data of roll amplitudes.

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Results of model experiments and numerical simulations concerning the dynamic behaviours of riser system in hung-off (disconected mode) are presented.
To design marine riser system and to decide the most suitable material and size of the riser pipe, estimation of nonlinear behaviours in hung-off mode is essential to check the riser strength especially in case of deep water riser system.
In this paper, behaviours of hung-off riser in 400500 m water depth are studied by model experiments and numerical simulations and the results are discussed to show the dynamic properties of the system.
In model experiments using 4 m long elastic riser model, (1/100 scale), deflections and bending moment are measured at some points along the model and axial force was also measured at the top.
In numerical simulation, the riser system is expressed as assembly of many beam elements. Dynamic bending and twisting behaviours are expressed using Lagrange function. Bending and twisting behaviours are numerically simulated by solving these equations using Newmark-β method.
As the results of the studies, the numerical simulation presented is shown to be sufficient to predict the hung-off riser behaviours mainly consists of bending and twisting oscilations.

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A free-run experiment of a VLCC model ship was carried out in short regular waves. The wave length and the heading angle were varied in range of 0.2≤λ/L≤1.5 and 0°≤χ≤180° respectively. The ship motion, vertical & lateral bending moment at midship, relative water level and wave pressure were measured. Among them, wave pressure on the side wall near the load water line was the particular interest of this experiment.
The first harmonics of the measured data were plotted with calculations of the strip method. We tried two strip methods to compare the accuracy. The first one is so called new strip method “NSM” and the second one is improved NSM we have developed. The solution method for the diffraction problem is improved to satisfy the exact body surface boundary condition. The plots of the measured wave pressure with the calculated value show that the accuracy of NSM is poor when λ/L≤0.5. On the other hand, the accuracy of the improved NSM is still good even when λ/L=0.2.
Another difficulty for the estimation of wave pressure amplitude near the load water line comes from the exposure of wall in the air. This phenomenon is essentially non-liner. However, we found a very simple mehod to correct the pressure given by NSM. This method is based on the experimental fact that quasistatic pressuure, which is identical to the static pressure measured from water surface, can be substituted for the wave pressure near the load water line.

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On analysis of wave exciting forces of large scale floating structure consisted of multiple elements, a part of authors proposed a computation method taking hydrodymamic interaction between elements into account, and proved effectiveness of the method by model testes in previous papers.
In this method, such an assumption is laid down, which gives a restriction in application of the method, that a fictitious, bottom-mounted vertical cylinder circumscribing one element does not contain any part of the other element itself. Although existence of lower-hull connecting column elements violates the above assumption, it can be proved that the effect of violation is small in case of lower-hull. This fact is confirmed by model tests. The computer program for this method is leveled up for the sake of making large scale computation possible, and the upper limit of computation is discussed.

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First and second order finite difference approximations are used in the simulations of this study to generate progressive waves in a three dimensional numerical wave tank. Some segmented numerical wave makers are set at the left end of the tank which is considered as the wave generating end. Waves propagating at different angles with longitudinal axis are generated at this end and sent towards the right end of the tank which is treated as an open boundary. A velocity reduction technique is applied to reduce the vertical velocity within a zone named Velocity Reduction Zone (VRZ) adjacent to the open boundary. In order to control the fluid motion at the open boundary, Sommerfeld Radiation Condition (SRC) is applied on that reduced waves. The generated waves with first and second order computations are compared and the superiority of the second order approximations is discussed. An example of three dimensional wave generation is also presented in this paper.

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As for the recent structural design of a ship, direct strength calculations are approved in classification society's rules, and wave loads for the calculation are given. However, such wave loads may be taken on the premise that the buckling or collapse strength of a ship will be checked by the direct strength calculation. In consequence, it may be questionable to apply such wave loads to the investigation of fatigue strength of a ship. In fatigue analysis of a ship, it is most important to estimate accurate pressure fluctuations in time and space. With the use of the pressure fluctuations, the stress fluctuation can be calculated.
In the present paper, the fluctuation in time and space of pressures acting on a bulk carrier in waves were investigated. Time histories of the pressure along ship's hull were calculated by Boundary Element Method. Surveying the pressures, distribution patterns along the ship's hull of the pressure fluctuation were classified ; 4 typical patterns of pressure distribution were introduced. With the use of these pressure distributions, structural analyses were performed for 1/2 hold spans fore and aft part of a bulkhead in port side of the ship by Finite Element Method. The stress distributions on the inner bottom plate and the bilge hopper tank plate were investigated. Finally, design loads for fatigue analysis of a large ship are discussed.

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Experiments in transient water waves are well known in getting the transfer functions of ship motions. Here, we introduce a new simple method to estimate added resistance coefficients from these experiments. Using this method, we can obtain the added resistance coefficients in single transient water wave experiment instead of doing many experiments in regular waves.
This new estimation method is based on Hsu's assumption, and using the Fourier spectra to separate the added resistance from surge force. Based on NSM (new strip method) and Maruo's theory, we calculated the added resistance coefficients theoretically, and compared them with the estimation results. It is another important point that the data in encounter frequency must be changed into absolute frequency domain.
In experiments, three models were used : SR208 (a tanker), SR108 (a container ship), and SHSS (a super high speed container ship). Three models have different shapes and examined in different advance velocities. Here, all of their results will be reported.
Present method is not only limited in transient water wave experiments. It can also be used in general irregular waves directly. In irregular long crested wave experiments, we obtained relatively good results as in transient water wave experiments.
Using present method, we can also analyse the surge force from experimental data and can simulate the total resistance time history numerically by using the results from experiments in transient water waves.

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In the paper, the simple estimation method of the time domin slow drift motion of a moored semi-submersible platform in directional spectrum waves is presented. This method is obtained expanding the Hsu's method to short crested irregular waves (directional spectrum waves). Based on this method, if we can obtain the time record of the short crested surface wave slope, at the position of the platform, by calculation or experiment, the low frequency drift force can be estimated practically only using the regular wave drift force coefficients.
The time domin estimations of the sway drift motion are compared with the model experiment in directional spectrum waves and they showed good agreement each other.

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Directional irregular waves were generated in a square seakeeping basin by using 80 element wave maker. The basin is 60 metre in long, 25 metre in wide, 3.2 metre in deep, and has a X-Y towing carriage. Each element of wave maker is only 0.31 metre in wide to realize shorter component waves. Side-wall reflections were positively utilized in order to obtain uniform directional spectra in any regions of the basin. Because, uniform directional spectra in position are prerequisite for model experiments of ships running in directional waves. However, the technique generating uniform directional waves with side-wall reflections has not yet been examined at a square basin, although limited results by Takezawa were obtained at a long towing tank. Therefore, the directional spectra were analyzed from wave height records measured by a wave gauge array at various positions of the square basin.
As a result, it was confirmed that spectral parameters, such as mean wave period, band width parameter, mean wave direction, mean spreading angle, are almost uniform in any position of the basin, although variance of water surface is not so uniform in position. Further, these parameters are not significantly varied when wave height increases even up to wave breaking or when directional spreading of wave energy changes. These results prove that model experiments in desired directional irregular waves can be carried out for a ship running with any heading angle.

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This paper deals with guidance and control of a newly developed autonomous underwater vehicle named Aqua Explorer 1000 for inspection of underwater telecommunication cables up to 1000 m in depth. The guidance and control process for altitude keeping control from seabed and depth keeping control in the longitudinal plane is discussed here performing the numerical simualations and the experiments at a towing tank and at sea.
The control strategy for altitude control and depth control was proposed and tested successfully by using fuzzy algorithm.
Optimization of fuzzy controller was successfully performed based on the mathematical dynamic model of the vehicle.
Adaptive fuzzy controller for altitude control consisting of fuzzy controller, system identfication block and output modification block was proposed to keep the effectiveness of fuzzy controller in unknown circumstances such as irregularly wavy seabed.

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In the numerical simulations of ship maneuvering, a set of ordinary differential equations with constant coefficients is traditionally used. However, when the transient external forces due to rudder motions and waves are concerned, this type of equations of motions is not precise in a strict sense. But the research on the relationship between two types equations is sparse. Therefore, in this paper, the study has been made to clear the relations between them.
For this purpose, based on the Strip Theory which is practicaly used to evaluate the seakeeping quality of a ship, the equation of motion described in the time domain with a convolution integral is derived. And by the analytical and numerical considerations, the comparisons of two types equations of motions have been made.

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The berthing is made up of an approach in the restricted water area and final berthing maneuver to full stop. In this paper, the expert system is used to include decision making processes in the berthing.
The expert system was made on the desktop simulator on the Sun-sparc 10 work station combined with the OPS-83 expert system shell. The expert system essentially consists of two independent expert systems which are in charge of planning and control execution respctively. The planning expert system takes not only the ship's maneuverability but also the environmental restrictions to provide the course to go. The control execution system has subcontrol systems such as auto pilot, turning controller, optimal regulator and so forth, and the system chooses the most suitable strategy to follow the planned course or to make the final berthing.
The prototype system for the 150, 000 ton crude oil carrier was built and the knowledges were obtained by authors with simulations under many cases. The simulation results were very satisfactory.

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In the previous reports^{1) 2)} , the method for solving nonlinear optimal control problems of ship maneuvering motions is discussed and the optimal solutions for operating the control devices, such as rudder, CPP and side thruster, are given by them. In this paper, the above method is applied to obtain optimal control references for the feedback control systems. As the closed loop system is driven by the control references, the control reference is regarded as a control input for the closed loop dynamics. Therefore the optimizing technique presented in the provious reports can be applicable. Usually the control references are given by the step function. By using optimal control references instead of the step function, the motion controlled by the feedback system is improved. As the presented method overrides the previous controller, we call it OCS (Override Control System).
Using a small training ship, some numerical simulations are carried out. Then some applications of OCS are shown for the dynamic positioning system controlled by the linear quadratic regulator (LQR). According to the results, the LQR can not be applicable for the nonlinear motion. However, by using the OCS it can be applicable even in the nonlinear region.

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An autopilot system is necessary in order to perform self-propulsion free running test. The autopilot based on the theory like a PID control is generally used in the past. However, a tendency to apply the fuzzy theory to the process control in various industrial fields becomes popular recently. It is because of the fact that the fuzzy control is most similar with the control by an expert, who controls process by his intuition and experience.
In this report, fuzzy sets is applied to design the autopilot system to perform the free running model experiment in towing tank with radio controlled maneuvering system. The experiment is carried out in course-keeping and course-changing mode both in still water and directional spectrum waves with fuzzy and PD control. Here three scale factors are used, which have relations with heading angle, turning rate and rudder angle respectively. Further a computer simulation using the M. M. G. model is compared with the experimental results, and a performance index is investigated to appraise the autopilot system and to determine the optimum scale factors.

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It is of importance to evaluate the ship manoeuvring motion, such as the berthing in a harbour, the drifting behaviour of a disabled ship. A significant part of the lateral hydrodynamic forces acting on a ship will be due to cross flow drag, as the ship's drift velocity becomes relatively larger compared to the ahead speed. For the prediction of ship manoeuvrability at larger drifting angles, it will be required to estimate the cross flow forces accurately.
In this paper, a calculation is given to the longitudinal distribution of the cross flow drag coefficient over the length of ship moving transversely. The theoretical method on basis of vortex model developed by the authors is applied to the Wigley hull and a container ship hull.
By comparing with the measured results of captive model tests, the prediction results agree well with model tests. Therefore, this method will be useful for practical prediction of cross flow forces acting on a ship moving laterally, taking into account the forms of the ship's cross sections

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Focussing on the sea-going maneuverability of a 105-passenger hovercraft, the authors made investigations through series of model tests and computer simulations. The wind forces acting on a scale model, which represents superstructure and cushion form, were measured in a wind tunnel test. Oblique towing tests with self-lifting model were carried out for the hydrodynamic characteristics between cushion skirt and water plane. Regarding the characteristics of control surfaces, the force measurement tests with ducted propeller and rudder in wind tunnel were carried out. Based on these results of model tests, the authors developed a simulation program which employs proper mathematical models for each force components, namely cushion skirt, wind, propellers, rudders and thruster. The output of simulation showed fairly good agreement with the results of actual sea trial. Especially in the case of course keeping in beam wind, the reverse of check-helm direction around Froude number of 1.0 experienced in actual operations were confirmed through computer simulation. Lastly the results of extensive investigations on the sea-going maneuverability of the hovercraft are explained.

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“What is the best tacking procedure ?” has been long-pending problem for yachtsmen. In order to solve this problem, a numerical simulation of tacking motion was conducted, and the results were compared with data from the full-scale sea tests.
The equations of motion were described in the horizontal body axes coordinate system, and the sailing parameters and the trajectory of the boat were calculated using the Runge-Kutta method. The full-scale sea tests were performed using a 10.6 m-LOA sailing cruiser, whose steady sailing performance and dynamic performance in waves were examined in the previous report.
The trajectory of the boat was measured using differential GPS receivers. First, powering run turning test and zigzag maneuver test were conducted to confirm the adequateness of the equations of motion and the hydrodynamic derivatives of the hull. Then, the tacking test was performed under various wind conditions and steering processes. By the full-scale sea tests, the mechanism of tacking motion was revealed, and the results of the numerical simulation coincided well with the sea test data.

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In this paper, an attempt is introduced which applied the fuzzy control principle to the dynamic positioning (DP) system. In order to apply the advantages of fuzzy control such as quick calculation, simple construction, and ease operation of logic conditional statements to the DP system, overcome the weak point of the difficulty to decide the control coefficients in PID control system, a computer simulation system of dynamic positioning with fuzzy controller (FDP) is set up. Instead of conventional MAX-MIN operation, triangular norms is led into calculation of the fuzzy relation in the chosen fuzzy control model so as to avoid neglecting the characteristics of every fuzzy element and make the model work better. In order to examine the feasibility of the FDP system and show its properties, the simulations of a model of tanker floating on the sea in the conditions of wind, irregular waves, and current are carried out. And the results are compared with the ones of model tests with PID control system of J. A. Pinkster et al⁸⁾ . The results of simulations show that the FDP system is easy to establish the control model, quick to operate, and has better control effect than the PID-DP system although former needs lower maximum delivering power of thruster.

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Although many experiments in irregular waves are carried out at towing tanks, these experiments have been generally carried out without the explanation how long the measuring of waves, ship motion and/or added resistance etc. are required. As the necessary record length effects on the size of towing tanks and model ships and also the power estimation of wave maker, it should be enough investigated. Since under the present situation as experimental accuracy has been discussed severely, this investigation is essential. The matters of experimental accuracy include the accuracy of facilities, measuring instruments and also the accuracy of the data obtained in limited time. The former problem has been studied in many ways but the later problem is still insufficient.
We research this problem under some assumptions and apply that procedure to estimate an accuracy of the sea wave-correction for ship trial experimental results, which is performed in only one mile interval.

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A modified ARMA model is proposed for the simulations of slowly varying drift forces in short crested waves from a given force spectrum for the sake of improving calculational time of drift forces. Herein, the property of non-Gaussian distribution of drift forces is considered and the results of the calulations are compared with those obtained by the drift force formula which is derived by Pinkster. It is shown that a lower order ARMA model and suitable time increment can bring about good precision of simulation. Numerical calculations by the modified ARMA model and the drift force formula give almost the same results in spectral analysis. In addition, by statistical analysis of the drift forces, the probability density functions of the forces indicate different in direction of drift forces. The drift forces simulated by two methods show little different probability density function in surge direction, but almost the same in sway and yaw direction.
Furthermore, the computational efficiency and applicability of the modified ARMA model are discussed by conducting simulations of low frequency motions of moored tanker of FPSO. It is revealed that the modified ARMA model can be accepted. The numerical simulations based on modified ARMA model and drift formula are in agreement with each other in spectra of motion responses. In computational time, ARMA model is very short. On the other hand, the simulations by drift force formula take too long time to put practical use.

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A comparative study of designed values and practical results on fatigue damage of 'POSEIDON' a proto-type floating structure was described in part 5. It was shown that the estimation of response transfer function is important to prediction of fatigue life in part 5.
This paper describes effects of hydrodynamic interaction between multiple columns on structural response in waves.
Three-dimensional source distribution method was used to estimation of hydrodynamic interaction. Model test on wave exciting force was carried out.
Theoretical results and test results were compared and it was confirmed that the technique of entimation is valid.
Structural response functions were solved in frequency domain by Finite Element Method. Results of hydrodynamic culculation was used in F. E. M. analysis.
Effect of hydrodinamic interaction against structural responses was confirmed by comparison of predicted results and practical data.
The fatigue damage was predicted by using to power spectrum density of strain response. This study showed that effect of interaction and wave directional function have influence on prediction of fatigue life.

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Second-order long period wave forces acting on a two-dimensional fixed obstacle in bichromatic waves on the water of finite depth are investigated. Although the second-order diffraction problem has been solved by many researchers, only a few paper has been discussed the effect of second-order free waves generated by a wave maker. Therefore, in this paper, the second-order diffraction problem is solved exactly including the effects of second-order free waves generated by the wave maker, and the effect of it on the second-order diffraction potentials and wave forces are investigated. In this paper, for the theoretical simplicity, the second-order diffraction problem of a vertical flat plate in a two-dimensional water channel with paddle type wave maker is considerd, and the second-order long period wave forces calculated by exact method are compared to the results of more convenient method using Green's second identity and experimental results.

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In order to increase the safety of plates structures such as ships, ultimate strength analysis should be performed taking into account of local failures of buckling and yielding of component plate elements. Direct application of the finite element method still needs enormous computation time and is not economical to use at the design stage, even most advanced computers are provided. In order to achieve this purpose, one of the authors proposed the “Idealized Structural Unit Method (ISUM)” and proved its usefulness by many applications. The previous rectangular plate element was developed based on the numerical results on buckling and post-buckling and formulation of its stiffness equation was complicated. In this paper, a new rectangular plate element has been developed. The formulation of its stiffness equation has been performed completely analytically for buckling and post-buckling behaviors by introducing eigen-functions, being paid attention to use only one or two parameters to reduce computation time. For partial yielding of the element, the plastic node method is introduced. This new element can be applied to the complete range of the behavior incluing buckling, post-buckling, ultimate strength and post-ultimate strength. The numerical results indicate that necessary computation time is very short for any aspect ratios and the accuracy is very high in comparison with FEM. There is some improvement is expected to reduce higher shear force at the ultimate strength under a combined action of compression and shear.

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The impact behavior of composite sandwich panels subjected to a cylindrical impactor is investigated numerically and experimentally. Three kinds of facesheets and two different core densities of sandwich panels are considered in the impact analysis. During the experiment, the impact force and strain history are measured, and the failure is inspected by visual observations. The experimental results show that the impact behavior is mainly controlled by core material. The ABAQUS finite element software is used to analyze the impact responses of sandwich panels. Because of line load conditions, the sandwich panels impacted by the cylindrical impactor is modeled as a 2-D plane strain problem. The numerical results of impact force and strain history are compared with those measured in experiment, it is shown that the experimental results are in good agreement with the finite element solutions. Further, the stresses of sandwich panels are examined. The maximum stress failure criterion is used to estimate the fracture initiation of sandwich panels. Then the experimental and numerical results are compared and discussed.

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In International Maritime Organization, they decided that an oil tanker newly constructed after 1992 A. D. should have the devices for prevention of oil pollution, and one choice of these devices was to have the double hull.
In such an oil tanker, we often adopt more space between longitudinal bulkheads. And the more the heigh tensile steel is used, the heigher the stress level in hull structure will become generally. It is, therefore, expected that the effects of warping in bending upon the stress distribution due to longitudinal bending can not be neglected under the heavy ballast condition in which the rate of change of shearing force becomes large.
In this paper, at first, we have carried out some numerical calculations on longitudinal bending stress distributions in the double hull tanker of 140, 300 DWT., applying the method of analysis which has been proposed by the two of us previously. Sea state has been chosen as the regular wave whose wave height equal to 10 metres and wave length equal to the ship's length.
After careful consideration, we have following conclusions :
(1) The values of normal stresses due to warping in bending come to about from 20% to 30% of those due to bending under the heavy ballast condition. Especially at the gunwale part and/or at the bilge part near in the midship section, it reaches its maximum.
(2) Under the hogging condition, the normal stress due to warping in bending is in the same plus/ minus sign of those due to bending at a gunwale/bilge part, within the region where the shearing force is increasing. Under the sagging condition, the situations are reversible.
(3) We can regard the ship's hull girder as thin walled constant cross section beam, so far as the longitudinal stress components in the parallel part of ship's hull girder are concerned.

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In this study, the previously proposed Adaptively Shifted Integration (ASI) technique is applied to the elasto-plastic buckling analysis of framed structures using the cubic beam element based on the Bernoulli-Euler hypothesis. The stiffness matrices with the positions of numerical integration points for the large deformation analysis are described. The minimum number of cubic beam elements needed in the buckling analysis using ASI technique is discussed by studying several cases under different patterns of combined loading. This paper is also concerned with a technique to subdivide each structural member automatically in the process of calculation into the most appropriate number of elements for the frame analysis. By using ASI technique with this automatic resubdivision, sufficiently accurate solutions can be obtained in a minimum calculation time. Elasto-plastic buckling problems of framed structures are analyzed by using this method, and the results are compared with those obtained by using the conventional method.

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This paper gives one method to estimate the structural response of ship structures under water impact pressure.
A series of drop tests of a cylinder with diameter of 312 mm is carried out to measure water impact pressures and strains on the surface of a cylinder.
Numerical analyses are carried out for one of the drop tests to reproduce the structural response under measured time history of pressures. In spite of the existance of large local vibrations shown in the numerically estimated strain values, the numerically estimated strain values and the measured values show the same tendency on the whole. This result shows the numerical method, we employed here, can be used for this purpose.
Then we tried to evaluate the effects of maximum impact pressure on the maximum strain value. Two kinds of pressure histograms are used to estimate the maximum strain values. The strain values obtained from the pressure histogram with maximum impact pressure show less effects than that without maximum impact pressure.
Finally, we modified the maximum impact pressure value of the measured time history of pressures to be used for numerical analysis. One is with the value of twice as much as measured value and the other is with the mean value of before and after the peak value. The results show that the maximum impact pressure value itself has little effects on the maximum strain value, while the time integration of pressure seems to have more effects than that of maximum impact pressure.

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This paper deals with buckling strength of axially loaded cylinders considering interaction between initial imperfection and plasticity.
We conducted 14 axial buckling tests of large-scale fabricated steel cylinders in the 1980s to investigate the effects of yield stress and length of cylinder on buckling strength. Then, we found the discrepancies between the test results and the predictions by the existing buckling design codes like the buckling design guideline of DnV for marine structures and the code case of ASME for nuclear containment vessels and recognized that it was important to consider the interactive effect of initial imperfection and plasticity.
We apply the reduced stiffness theory by Dr. J. G. A. Croll and show a simple prediction method of buckling strength considering interaction between initial imperfection and plasticity.
The predictions by the reduced stiffness theory give a good agreement with results of bucking tests and buckling analysis using nonlinear finite element method. Then, we verify the validity of the reduced stiffness theory in the wide range of dimensions of cylinders.

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In this paper, the hull construction cost of one tank hold length in the midship segment of double hull tanker is taken as the objective function to be minimized by the internal penalty method and the multiplier method based on plastic design. Emphases of this study are placed on the cost minimization compared with the weight minimization and the hull construction cost varied with the number of transverse ring spaces as well as the result comparison between the internal penalty method and the multiplier method. Through the numerical analysis, it has been found that :
(1) The weight minimization is more costly than the cost minimization. It is suggested that the cost minimization is the first choice for structural optimum design of double hull tanker except the case that there is a special limitation on the weight.
(2) Similar to weight minimization reported by Ref. 1), there is also a proper number of transverse ring spaces that makes the total construction cost get the minimum value.
(3) Since the multiplier method has good convergence and numerical stability, it can be taken as a practical method for engineering optimum designs.

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Many studies on system reliability of marine structures have been done using framework models. There are, however, many structures which are difficult to idealize by a framework model. So a new system for plate structures using spatial membrane elements was developed to offer the expansion of the applicability of the structural system reliability by authors in the previous studies.
In this paper, the features of the structural reliability of a large bulk carrier (150, 000 DWT) based on the collapse mode analysis are investigated by successively applying the newly developed system.
From the analytical results for the spatial plate structures idealized for modeling the midship part and fore/aft L/4 parts of the ship hull structure under two loading conditions, main conclusions are summarized as follows :
(1) New formulas of the compressive strength are derived for the longitudinal and transverse frame system panels under in-plane compressive and shearing stresses and lateral pressure, and the effects of the shearing stress and lateral pressure on collapse modes and the system reliabilities are investigated.
(2) Effects of the shearing stress on the dominant collapse modes including damages of side shell elements and their occurrence probabilities in the fore/aft L/4 parts of the ship hull structure are more remarkable than in the midship part.
(3) It is also pointed out that another results for deteriorated structures considering the notional corrosion damage referred to the class NK report are in comparatively good correspondence to the features of structural damages due to corrosion at fore/aft parts of aged bulk carriers.

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