The manoeuvring characteristics of a ship at very large drift angles will be significantly influenced by non-linear hydrodynamic forces, initiated by the cross flow underneath a ship from the inflow to the leeside. It is considered that the magnitude of cross flow drag is very sensitive to the forms of the local cross-sections of a ship, such as thickness of sections and smoothness of the edge.
This paper dealt with the cross sectional aspect ratio and radius of round edge effects on cross flow drag acting on rectangular cross-sections, which are theoretically investigated by using of vortex model developed by the authors. From the results of numerical calculations, cross flow drag acting on rectangular cross-sections decrease abruptly as increasing corner radius, especially for the sections with a breadth to height ratio becoming larger. Thus, the cross sectional aspect ratio and radius of round edge effects will be important factors for cross flow drag acting on rectangular cross-sections, and therefore these effects should be also considered in evaluating the local cross flow drag of a ship moving at very large drift angles.

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Concerning the study of capsizing of a ship and the evaluation of nonlinear wave loads acting on a ship, the numerical simulation method is necessary since the nonharmonic forces are concerned in the problem. To deal with the practical problems, it is recognized that the Strip Theory is useful and in this paper, based on the Strip Theory, the equation of motion described in the time domain with a convolution integral is developed. Numerical results obtained by using of it are compared with those of the second order linear differential equation with constant coefficients.

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In this paper, the authors carried out model tests to estimate directional wave spectra from only a set of simultaneous records of ship motions. Based on the assumption of linearity between the waves and the ship motions, many studies which estimate the wave power spectra by using only data of ship motions have been done. However, there are few results for estimation of directional wave spectra based on only information of ship motions. The estimating calculation in this paper consists of three steps. The first step is a calculation of cross spectra of ship motions by a stochastic approach using a multidimensional auto regressive model. The second one is a theoretical calculation of frequency response functions of the ship from the waves by New Strip Method. And, the last one is an estimation of directional wave spectra by using Bayesian Model. In order to discuss the quantitative accuracy of this method, some results were compared with the ones estimated from the observing data in an array of wave height sensors. Successful agreement between measured and estimated directional wave spectra was obtained. And the power spectra which were obtained by integration of the estimated directional wave spectra also agreed quantitatively with the measured ones.

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A numerical procedure is described for predicting wave drift forces and low frequency motions in short crested waves. In calculations of drift forces, wave directional spreading is treated exactly but difference of frequencies is approximated by Newman's assumption. Quadratic transfer functions of drift forces due to arbitrary combination of two waves of the same frequency from different directions are derived based on Newman's far field theory, and numerical calculations are carried out by using three dimensional source technique.
Two kinds of moored structures (storage oil tanker and semi-submersible platform) are chosen as the numerical calculating models for investigating the influence of wave directional nonlinear interference on slowly varying drift forces. It is shown that nonlinear interference of wave direction on drift forces is dominant for tanker model, and it is relatively weak for semi-submersible platform so that Newman's frequency assumption can be extended to directional spreading. Furthermore, drift forces in mean wave direction are increased with narrow wave directional distribution (approaching long crested waves), and then drift force in other direction and yaw moment are decreased correspondingly. The mean drift forces for a set of parameters of wave directional distribution and the different mean wave periods are compared with results of the model tests.
Finally, effect of wave directional distribution and directional nonlinear interference on low frequency motions of moored semi-submersible platform are discussed by time domain simulations. It is revealed that the neglect of wave directional nonlinear interference can be accepted. The numerical simulations are in good agreement with the model tests by Takezawa etal.

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The techniques of generating directional spectrum waves have already developed in the towing tank of Yokohama National University. So, many kinds of experiments could be conducted in such waves. Of course the experimental data could also be compared with theoretical results and checked each other. We can find new phenomena in such short crested waves.
As one step of these researches, the added resistance experiments were conducted using three models : SR108 (a container ship), SR208 (a tanker), SHSS (a super high speed ship). The mean added resistances were compared with the theoretical calculations which use the conventional Maruo's method.
Three models are different shapes and examined in different velocities. Considering their laternal motions (roll motion) and ship bow reflections, we compared the theoretical calculations with the experimental data. Here, all of them will be reported.
For the analysis of directional spectrum waves, we adopted MLM (Maximum Likelyhood Method) using the Laser wave surface recorder which was developed recently by ourselves. And the motion transfer functions based on the NSM (New Strip Method) calculations which are checked or modified according to experiments are used.
As the results, we got : (1) In the range of short wave length, when the wave directional distribution become wide, the added resistance coefficient seems to be increased both in experiment and theoretical calculation. (2) In the range of short wave length, the wave reflection on the bow can not be reglected. And it can be expressed by Fujii-Takahasi's formula. (3) In directional spectrum waves, the mean added resistances also can be estimated by integrating the accurate added resistance coefficients multiplied by the wave power spectrum. (4) When the ship speed is not so fast, using the response function calculated by NSM method, the added resistances which are calculated by Maruo's theory agree relatively well with the experimental data.

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For evaluating speed loss, self-propulsion test and free running test are carried out in directional spectrum waves with a full ship model in the towing tank of Yokohama National University. Such experiments will be the first time in the world. In the free running test, the model has six degrees of freedom as in real condition, and it is controlled to run straightly by using rudder controlling system, therefore the speed loss can be derived directly by considering the relative speed against carriage by using optical position sensor. It is found that the speed loss in short-crested waves is bigger than the speed loss in long-crested waves in short wave length. The experimental results of speed loss in directional spectrum waves are compared with the results of estimation methods which are originated from the added resistance obtained by experiment and theoretical calculation.

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Many plans to construct very large offshore floating structures are proposed, such as floating cities, airports, and so on. In the first report, the authors studied the response of the flexible structures in directional waves. The main results show that the flexibility of the structures affect their responses due to structural deformation. However, these responses are mainly in wave frequency regions. The low frequency or steady responses sometimes become sgnificant and cause fatal accidents. This paper is about the experiments conducted to investigate the responses of large structures in wide frequency region.
In the first report, the structures are moored by a single buoy of a TLP type. Therefore, the large and slow oscillations are observed which cause rotation around yaw axis and displacement in sway direction. These oscillations cause difficulty in using such offshore structures for any purpose. In these experiments, described in this paper, another buoy is added to the mooring system in order to reduce such motions. Both ends of the structure therefore are moored by two buoys.
The main purpose of this paper is to measure the responses of the structures in an oblique condition relative to the wave direction. In the experiments, two setting angles, 0 and 22.5 degrees are used to obtain comparative results. The results show that in the oblique condition the level of some slow oscillations decreased while the interactions between motions increased especially in low frequency responses.

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We have recently carried out model tests of multi-unit floating structures in waves. Up to 5-unit semi-submersible-type floating units were modeled in scale of 1/80. In the test of wave exciting forces, 1-unit, 3-unit, and 5-unit systems were tested in regular and irregular waves in order to obtain the effects of hydrodynamic interaction among floating bodies on their wave exciting forces.
In the test of dynamic responses, 3 and 5-unit floating structures were connected to one or two foundation units settled in the model basin. Structural wave loadings and motion responses were successfully measured in head, beam and oblique wave conditions.
Results of the tests were compared with theoretical predictions and following conclusions were obtained.
(1) A theoretical method, previously proposed by authors, can estimate wave exciting forces accurately including the interaction effect.
(2) The elastic structural responses of multi-unit systems can be estimated well using the above mentioned theoretical wave exciting forces.

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This paper is concerned with simulations and statistical prediction of total second order responses, including slow drift motions caused by waves and wind, of a full scale offshore structure “POSEIDON”. The at-sea experiment was carried out from June, 1986 till July, 1990 at the Japan Sea. In order to simulate the total second order motion, hydrodynamic and restoring characteristics were examined using data from full scale free oscillation tests. Second order force characteristics have been investigated through comparison between experimental results from cross bispectral analysis of slow drift motion and waves and numerical ones from potential theory. And from multi input analysis of motion, waves, instantaneous wave power and wind fluctuations the contribution rate of wind fluctuation to slow drift motion has also been studied.
Comparisons between measured time series of slow drift motion and simulations on the basis of above investigations have been carried out.
Both a Monte-Carlo simulation model and nonlinear statistical etimate method are newly devloped. The former is based on the assumption that the second order force process can be approximated by diffrence of two gamma processes. The latter is a statistical estimates method taking into account not only second order wave forces but also wind fluctuations and it is an extended method of nonlinear statistical theory suggested previously by authors.
Both measured sample data and statistical values estimated by two new methods are compared.
Main results are as follows :
1) In order to simulate total second order motions, in-line and transverse wind fluctuations should be taken into account even though mean wind direction is head. As a wind spectrum representing wind fluctuations, a spectrum form with significant low frequency power compared with Davenport and Hino spectra, e. g. spectrum forms suggested by Ochi-Shin and one of authors, should be used.
2) It is confirmed that the linear prediction method based on Longuet-Higgins' significantly underestimates the measured results while the present method estimates them very well.
3) Extreme statistics by Monte-Carlo simulation model significantly scatter. Especially, estimates from a simulation data with the number of peaks of about 300, i.e. a five hour simulation, scatter from 2.5 to 5.0 as a value of extreme value/standard deviation. Attention is needed.

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Theoretical estimation methods of diffraction and radiation forces acting upon floating structures partly composed of permeable walls are developed. Combining the technique proposed by B. Molin for the treatment of permeable walls with the conventional singularity distribution method for the hydrodynamical representation of body geometries, the present method can be applied to 3-dimensional bodies of arbitrary geometry with permeable walls. Example structures were tested in a water tank and the calculated diffraction/radiation forces are compared to the experimental data, which verify the validity of the present method. A parametric study is also conducted while systematically varying the permeability as the parameter. From this study, the effects of permeability on the hydrodynamic characteristics of permeable floating offshore structures are discussed.

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Low frequency added mass of a semi-submersible is increased by incident waves, similarly to low frequency damping. It can be 1.6 times value for still water and affects the resonant frequency of the slow drift motion as well as the amplitude of it. This is a very interesting phenomenon and there is no physical explanation to explain this yet. In order to investigate this phenomenon forced oscillation tests in regular waves were carried out not only for a total semi-submersible model but also several constituent models, those being the lowerhull, brace, column, column lowerhull, columm brace, column brace of 2/3 narrowed breadth, column brace of 2/3 shortened length. It is important to elucidate this phenomenon on the design of mooring of a floating body.
The experiments show that the brace plays the most important role of the change of added mass of the semi-submersible model, and that the added mass increases proportionally to square of wave amplitude of incident waves rather than the wave amplitude itself.

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The hydrodynamic forces acting on an oscillating circular cylinder in an uniform and harmonic flow are investigated through the use of direct solution of the incompressible Navier-Stokes equations in order to gain a deeper understanding of the nature of the slow-drift damping force of an ocean structure. In the calculations the time-dependent generalized coordinate system has been employed to obtain an accurate numerical representation of boundary conditions for a moving boundary, which is important for determining the hydrodynamic forces by integrating the pressure distribution on the cylinder surface.
The computed results are compared with other experimental works and show good agreement. Several important conclusion regarding slow-drift damping and the characteristics of a two harmonic flow are obtained : in the range of small reduced velocity the high frequency oscillation of flow field obstructs the formation of an alternate vortex caused by steady flow or slow-drift oscillation, but in the range of large reduced velocity this oscillation excites the alternate vortex shedding. The values of the reduced velocity of the turning point can be determined from the Strouhal number of the cylinder in steady flow.
An attempt to explain the mechanism of the viscous slow-drift damping force is also offered.

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An increasing demand for exploitation of ocean space creates a need for artificial low cost protection of marinas and nurseries. So far floating breakwaters are frequently chosen to provide this protection, particularly in location where large tide is considered, and water depth is large. Recently a breakwater which consists of submerged horizontal plate has attracted much of attention, because this breakwater has several advantages such as a low construction cost, an easy exchange of seawater behind the breakwater, no obstruction of sea scenery etc.
It is, however, of importance to keep the submerged plate with a constant depth from a free surface irrespective of tide, in order to get its good performance fully. Thereby we can not apply the existing mooring system such as a catenary type mooring and tension leg type mooring to it, and it has been a great problem of long standing to moor this type of floating breakwater at seas. A new mooring system has been developed lately. This mooring system consists of sliding double-piles which can follow tide and can prevent the floating body from oscillating in waves.
We have carried out the extensive forced oscillation tests to make clear the hydrodynamic forces in the sliding double-piles. In this paper, we derive the practical formula from the experimental results which is useful for estimation of hydrodynamic forces in various conditions of double-piles.

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This paper is concerned with stability curves and capsizing of a ship in severe quartering waves. The effects of wave height, wave length and heading angles of ship to waves on stability curve are analytically investigated for a container ship. A time domain numerical simulation program for motions and capsizing has been used to investigate motions in a variety of wave configulations. Numerical simulations based on a mathematical model are carried out to predict critical situation leading up to capsizing of a container ship in severe quartering waves. Various phisical mechanism that could be responsible for capsize are pointed out by numerical experiments.

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A globally conservative NS solver for flow past a ship hull has been developed. It uses a 3rd-order accurate upwind differencing of the preprocessing (MUSCL) type for inviscid terms, in which the nonuniformity of grid spacing is taken into account. Using the solver, the drag of a flat plate at zero incidence was computed in the Reynolds number range R_e=4.0 × 10⁵ to R_e=4.0 × 10⁷The dependence of the drag on the degree of clustering of grid points was checked. The computed drag agreed with the Schoenherr value within 4%. An appropriate criterion for the minimum grid spacing Δ_min adjacent to solid wall with this particular scheme seems to be Δ_min= 0.005/ √R_e.
Then the drag of the Series 60 (C_B=0.6) ship hull with the double model assumption was computed in the same Reynolds number range using grids with various Δ_min and various degree of clustering toward bow and stern. Although the computed drag values showed some scattering among different grids, the results with the smallest Δ_min agreed well with the measured values throughout the Reynolds number range.

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This paper presents simulated viscous flow fields with free-surface around two-dimensional floating bodies. As the governing equation, two-dimensional Navier-Stokes equations are adopted and calculated by a finite difference method on a body- and boundary-fitted coordinate system. The characteristics of such flows are the periodic oscillation of the free-surface in front of the body, the wave breaking phenomena F_n≥1.0 and so on. The calculated results simulate well the oscillation and the interactions between the free-surface and the viscous flow behind the body. A critical condition for the appearance of sub-breaking waves is applied to the simulated results to show its validity, although the critical condition is derived from the inviscid instability analysis.

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Unsteady flow field around a rigid 2D NACA 0012 foil perfoming pitching oscillations is studied numerically, changing the reduced frequency and pivot point location. Special attention is payed to the ability of thrust production.
Viscous flow simulation is based on solving the full 2D laminar incompressible Navier-Stokes equations in vorticity-stream function formulation, defined in moving noninertial frame of refrence. An implicit factored finite-difference numerical algorithm is used. Performed numerical tests for simulation indicate that, the simulated flow is of sufficient accuracy to merit a physical analysis. The computed flow pattern is compared with the flow visualized experimentally and both results seem to agree satisfactorily.
It was proved that, for large products of reduced frequency and distance to the pivot point location, the pitching hydrofoil is able to produce thrust but of low efficieny. However, the comparison with available results based on nonviscous flow investigations shows that the thrust and efficiency in these cases is highly overestimated. It is connected with the strong vortical structures near the leading edge. The same combination of parameters but in the case of pivot point located in front of the foil, is found more efficient. The flow pattern analysis shows that in this case the dominant leading edge vortex is weaker and it is washed downstream easier.

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A numerical computation method for the simulation of the flow around a submerged body of complex geometry is developed in the framework of the TUMMAC (Tokyo Univ. Modified Marker And Cell) method. For the three-dimentional problem, the idea of porosity is introduced in order to simplify the treatment of no-slip body boundary conditions. A flow past a submerged mountain ridge is simulated by the present method in shallow water condition. It is experimentally studied in ship model basin and the results are compared with the computed flow patterns. The complicated interaction of eddies behind a submerged mountain ridge is well simulated.

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The velocity distribution downstream of a ducted propeller was measured in a cavitation tunnel by the use of a 3-component Laser Doppler Velocimeter. A considerable amount of change of the hydrodynamic pitch of the ducted propeller was observed in comparison with that of the impeller without duct. Based on these velocity measurements around the ducted propeller, a new wake model considering the effect of impeller loading was proposed to improve the prediction of the hydrodynamic performance for ducted propellers by the surface panel method. The flow fields around the ducted propeller calculated by the use of this method were shown in good agreement with the measured data as expected. The open-water characteristics of ducted propellers calculated by this method also showed good agreement with the experimental data.

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The applications of surface vortex lattice method to marine propellers are considered. The surface vortex lattice method based on general vortex lattice method is possible to simulate a lifting body including thickness and volume effects by distributing horse-shoe vortices on the surface of the body.
The advantages of this method compared to other lifting body theory are the facts that it is easy to simulate a trailing vortex wake geometry behind propeller and that the Kutta-condition is satisfied automatically by convecting the trailing vortices.
The geometry of the wake can be calculated by iterative procedure starting from the linear wake model and all vortices are convected to new positions step by step with a small time interval. In this paper, the propeller open characteristics, pressure distribution on the blade and the geometry of the wake concerning to three type propellers have been calculated by the present method. The results of these calculations are good agreements with experiment or other theoretical calculation results.

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This paper describes an application of CFD (Computational Fluid Dynamics) to the computation of incompressible viscous flow around a marine propeller. High Reynolds-number flow around a marine propeller operating in a uniform flow is simulated using a newly developed CFD scheme, which is based on cell-centered, finite-volume method with global conservation property for mass and momentum flux.
Governing equations are full Navier-Stokes equations and continuity equation with pseudo-compressibility, which are written in boundary-fitted curvilinear coordinate system fixed on a propeller blade. For discretization in space, 3rd-order upwind scheme derived from flux-difference splitting method is adopted for convection terms and 2nd-order central difference scheme for viscous terms, respectively. 1st-order Euler implicit scheme is adopted for the time integration and the resulted discretized equations are solved by Implicit Approximate Factorization (IAF) scheme with high efficiency.
A series of high Reynolds-number flow computations around a SEIUNMARU conventional propeller are made to show validity and availability of the present scheme as a practical design tool. Through quantitative comparison of the computed results with experimental data for surface pressure distributions, thrust and torque coefficients, it is clearly shown that present scheme can predict a viscous flow around a marine propeller qualitatively well and that several problems, such as open boundary conditions and excessive grid skewness, should be solved in order to improve accuracy of the present scheme.
By flow visualization technique using the computed results, some important characteristics of a rotating propeller flow such as the development of boundary layers and separation on a propeller blade, tip vortex generation and a structure of trailing vortices, are found to be well simulated qualitatively by the present scheme, which may be useful tool for the development of the inviscid-flow calculation method.

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Experimental study is made of the hydrodynamical forces and moments on two bodies, a revolutional one and a rectangular one, moving beneath the free-surface at the Froude number 0.11 to 0.44 with yaw and trim angles. Due to the free-surface effects and the nonlinear separating flows the measured results show a lot of interesting features. The structure of the flow-field is investigated by the TUMMAC-VII method in the framework of the rectangular coordinate system. Some aspects of the three-dimensional separating flow are elucidated.

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This paper presents the non-linear theory of a supercavitating foil section, where the transient flow model is applied. The calculated model is based on the Hess & Smith method. The shape of cavity was obtained by the iteration.
The calculated result of a cavitating flat plate agreed well with the exact solution by Wu. The theory was applied to the several cavitating foil sections and compared with experiments. The results agreed well with experiments for the supercavitating as well as partially cavitating conditions except for the following cases;
(1) For the foil whose trailing edge is open, the effect of cavity from the trailing edge should be taken into consideration. Therefore, it is necessary to make the pressure of the trailing edge be the same as the pressure in cavity.
(2) Appropriate modification is indispensable for the foil with cup at the trailing edge. In this case, the separation at the trailing edge of back side of the foil might be important factor for this revision.
The theory has been extended to the foil between two parallel walls, and the cascade.

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This paper describes the experimental result of ventilated cavitation on a two dimensional foil. The ventilated cavitation could be compared directly to the natural cavitation when the cavitation number was evaluated by the pressure in cavity. The flow rate of ventilation was measured at 4 and 8 degrees of angles of attack. The velocity distribution in cavity was also analysed numerically and compared with experiment.
The conclusions are follows ;
(1) Both ventilated and natural cavitations were comparable on cavity length, shape, and Cp distribution at the same cavitation number.
(2) Partial cavity began to pulsate when it grew more than 60 % of the chord. In this region, the flow rate increased rapidly to the cavity length. Whereas, it changed little at the super cavitating condition. The empirical equations for the flow rate were obtained.
(3) The flow in the cavity was reverse near the foil surface, so it made a full rotational flow within the cavity. The reverse flow velocity was about one fourth of the main flow velocity.

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This paper describes the experimental results of a finite span hydrofoil of NACA0012 section with elliptic planform (aspect ratio of 3).
Lift and drag coefficients were measured at two angles of attack (α=10 and 20 deg.), where the cavitation number was changed from non-cavitating condition to super-cavitating condition.
The pressure distribution of the hydrofoil and the wake distribution were measured at some typical cases. With this experiment the following results are obtained :
(1) As the cavity grows, the ratio of cavity length to chord length becomes larger near the tip.
(2) Under the non-cavitating condition or the condition of tip vortex cavitation there is a velocity acceleration near the tip-vortex. This acceleration changes to deceleration after inception of sheet type cavitation.

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A computational study of the flow around the strut-plate juncture formed by the inclined strut and curved plate is presented. The characteristics of the flow are discussed by making use of the solution of the Navier-Stokes equations. To investigate the effects of the inclination angle of the strut and the curvature of the plate on the juncture flows, the computation has been made for three different inclination angles of strut and two different curvature of plate. The flow structure is changed greatly due to the inclination of strut and the forces acting on strut are affected greatly by the inclination angle. However the effects of the curvature of plate on the flow and the forces acting on strut are less than thoes of inclined cases. Although the plate is curved, the local angle is still 90° and it can be concluded that the local angle between strut and plate will be an important parameter for the flow structure around a juncture.

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In recent years, many ideas are proposed in order to pursue the high speed vessel. Many of those ideas are based on unconventional concepts such as the application of hydrodynamic lift generated by wings. In those cases, however, we have many problems for the purpose of sizing up and long range operation. As well known, conventional ship, that is a displacement type, has more merits than unconventional ones at least from point of view of buoyancy. Therefore it can be considered as one of the effective ways to pursue the high speed vessel based on the concept of displacement type.
In the present study, minimization of wave resistance of a special type ship in high speed range is discussed according to the concept of wave making interaction between a main hull and an air-ship form bulb. This bulb is formed by the combination of a hydrodynamic point source and a line sink, semi-splitted from the main hull of displacement type and supported by the strut. Wave resistance of this special type ship is minimized under some design conditions by means of nonlinear programming. In this paper, experimental investigations are carried out for 2 selected models which have the air-ship form bulb and the mathematical water lines and frame lines based on the present theory. After showing the model test results of fixed trim conditions and free trim conditions, some discussions about the direction of model improvements are given. In order to adopt the air-ship form bulb as the practical bulbous bow, the effect of an entrance angle of the main hull, the control of the trim and so on are investigated experimentally.

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In order to investigate the propulsive performances of “Ro” that is oriental sweep used to propel a Japanese traditional wooden ship, the authors analyze geometrically the motion of “Ro” when rowing it first of all and make clear the principle of propulsion by “Ro” in the present paper. According to the results of such analysis of the motion of “Ro”, manufacturing of the rowing machine of “Ro” is discussed.
The open tests have been carried out by rowing the model “Ro” of 1/3 scale installed on the realized rowing machine. Furthermore the self-propulsion tests of a 4 metres long model ship propelled by “Ro” operated by the rowing machine set-up in the model ship have been performed in the towing tank. As the results of such experiments the hydrodynamic characteristics of “Ro” have been made clear experimentally for the first time. For example, the efficiency of “Ro” is possible to be more than 0.70 in a good case but less than 0.40 in a bad case. In the worst case any thrust cannot be gained. Accordingly the propulsive performances of “Ro” strongly depends on the skill of a rower.

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Free-running experiments are carried out to make clear the stable running of a semi-submersible high speed ship with wings. The ship is an unconventional new type of high speed ship consisting of a main hull, main and tail wings with flaps, and a strut to support the super-structure. The ship floats as a conventional ship but the downward lifting force makes the ship semi-submerged during the operation condition.
The model used is a self-propelled free-running model. The carriage follows the model at a constant speed on which all the controllers are placed. Experiments are carried out not only under the calm condition but also among waves of following and head seas, although all the experiments are limited to the forward running tests.
Through the experiments it is made clear that the ship can run stably by the control of flaps except under the following sea condition. The induced motions by waves are notably reduced by the submergence. The thrust estimated from the number of revolution of propellers is almost same as the measured resistance and no appreciable increments in the resistance are experienced even among waves. It may be necessary to study more on the flow phenomena and the forces acting on the hull under the following sea condition.
Although all the knowledge is limited to the model level, it can be concluded that such an unconventional new type of high speed ship is promising from the hydrodynamic aspects.

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The present paper studies the characteristics of the foil load and the vertical motion of the high-speed ships with hydrofoils quantitatively. First, the hydrofoil load was measured directly using dynamometers. Time-domain non-linear simulation was carried out using TSLAM-F_HF^1.2.3.4) and was compared with the experimental data. It was shown that the simulation can predict not only the vertical ship motion but also the foil load quantitatively in case no hydrofoil emergence occurs. The experimental result, however, shows the necessity of further investigation on the characteristics of histeresis of the hydrofoil lift to simulate ship motion in case foil emergence occurs. The eigenvalue analysis for the vertical ship motion of the high-speed ships with hydrofoils was performed based on the linearized equation of motion. The effect of the ship speed, the area and locations of the foils on the ship motion in wave was clarified.

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The motions in waves of high-speed catamarans with hydrofoils were studied through both nonlinear computer simulation and towing tank tests. The category of the ships of our interests is characterized by a set of fully submerged hydrofoils whose dynamic lift support the majority of the ship weight and a set of catamaran hulls whose displacement gives the support for the rest of the ship weight as well as the longitudinal and lateral stability. We describe the longitudinal motions in head and following waves in this first report. The time-domain non-linear computer simulation was based on the strip method for the forces and moments on the hulls. For the dynamic lift of the hydrofoils, the unsteady components stemming from the wave orbital velocity and the ship motions were considered in the quasi-steady way. The simulation results showed good agreements with the towing tank tests in general. Through the simulation runs and the tank tests for the ship Froude number of 1.0 and the wave length to ship length ratios from 1.0 to 4.0, the motions in following waves were found to be significantly larger than those in head waves. In the high following waves (H_w/L=0.06), the fore hydrofoil went out of the water in the downward slopes of the waves and the ship showed large non-linear motions. This kind of wild motions in following waves are described to be a natural tendency of the ships of the subject category. In order to suppress the large motions, we tried controlling the angle of attack of the hydrofoils by feeding the pitch angle and the pitch rate back. Both the simulation and the tank tests showed significant effects of the control. The amplitudes of the pitching motions were reduced to about 30% of those under no control both in head waves and in following waves. Finally, we studied the effects of the hydrofoil configuration with the simulation. Although the studied cases dis not show good improvements of the wild motions in following waves, the developed simulation program in a good tool for designing this kind of ships considering the motions in waves. In the second report, we intend to survey the motions of six degrees of freedom in the waves from various directions.

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The H_∞ roll control system for the Hydrofoil Catamaran is presented in this paper. The roll dynamics of the ship changes as the speed and other state variables change, and the ship is always under the disturbance from the waves and winds. Taking these two aspects of the control into account, the roll conrol system is defined as the mixed sensitivity problem of the H_∞ robust control theory. The system was designed and validated by the actual sea testing using the prototype ship “Exceller.” The robustness and stability to the disturbance from the wave were clearly observed in the actual sea testing.

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It is well known that aerodynamic forces acting on an elastic lifting surface are affected by structural deformations caused by aerodynamic forces on the surface or changes of attack angle due to these deformations. This type of problems are referred to as aeroelastic problems. Aeroelastic effects are much more significant in aerodynamic performance of yacht sails than ordinary wings of aircrafts, because the sail of yacht is formed by an elastic membrane. The pressure distribution on a membrane in a flow can be calculated if the shape of the membrane is determined. However, the shape is a result from the equilibrium of aerodynamic pressure and inner stress of the membrane, which depends on aerodynamic pressure. In order to treat this kind of mutual interaction between aerodynamic forces and elastic forces, some analytical or numerical approaches have been proposed to give reasonable solutions.
In the present paper, Finite Element Method and a modified Vortex Lattice Method are adopted to solve aeroelastic problems of 3-dimensional elastic membrane sails. Made is use of large deformation incremental method to calculate stress and strain in deformed state of membranes, and to determine the sail shape in equilibrium state under loadings. One of the main advantages of the incremental method is that this method can be applied to dynamic problems more easily than the iterative method.
The usefulness of this approach is confirmed by comparing numerical results obtained by the present method with existing numerical solutions of elastic membranes and experimental results of yacht sails as well. As examples of application of the solution proposed in this paper, a series of numerical calculation is carried out to clarify the effect of leech tension and sail material on the aerodynamic performance of sail.

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The sailing performance of a 10.6 m-LOA ocean cruising yacht was measured by sea test and the results were compared with calculated values. The sea test was carried out to examine the steady sailing performance as well as the dynamic performance in waves.
First, the steady sailing performance was estimated by calculating the velocity prediction program (VPP). The VPP calculates the equilibrium sailing state of the boat considering the balance of X, Y forces and K, N moments, respectively. The hydrodynamic force coefficients of hull were derived by the towing tank test using 1/4.275 scale model, and the aerodynamic force coefficients of sail were calculated using the results of a wind tunnel test.
Then, the sailing state parameters of the full-scale were measured under various wind conditions. The measured items were wind velocity, wind direction, boat velocity, leeway angle, heel angle and rudder angle. These values were compared with the results of VPP. The both results were in good agreement and this indicates that the calculating method and fluid dynamic coefficients for VPP were adequate.
Next, The motion of the boat was measured in conjunction with wave measurement using a throw-in type wave meter. A vertical gyro and accelerometers were used for measurement of rolling, pitching and heaving motion of the boat. These data were analyzed by means of spectral analysis, and compared with both model test and numerical calculations. The model test was performed in regular wave conditions at the towing tank. The analyzed results indicated the sail damping effect on the rolling motion quantitatively and the distinctive feature of motion for the sailing cruiser in waves.

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In this paper, the optimal control problem for ship maneuvering motion is discussed. Using nonlinear mathematical models for the prediction of large maneuvering motions, the nonlinear optimal control problems with nondifferential constraints are solved by a sequential conjugate gradient-restoration algorithm. By applying these nonlinear optimization techniques, the operations of course changing maneuver for a small training ship are optimized. Compared the nonlinear optimal solutions obtained here with some linear optimal solutions such as bang-bang control or linear optimal regulator, the importance of nonlinear mathematical models for the optimization of ship maneuvering is discussed. And by using these nonlinear optimal solutions, an automatic feedback controller is proposed. The performance of the controller is checked by numerical simulation with respect to a deviation maneuver and a berthing one. The good performance of the proposed controller is confirmed by the results of these simulations.

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A study on the database system aimed at the prediction of ship's maneuverability has been carried out. The database system covering full-scale ship's performance, hydrodynamic ship model data and ship geometry is presented. The full scale ship maneuvering performance data and model hydrodynamic data are analyzed here. The results are explained by comparison with series ship model test data. It is pointed out that the use of the database system has limited accuracy if only ship's main particulars are used for the database access. To remedy this situation, an idea of using a type ship concept for improvement of the maneuverability prediction is proposed.

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The first commercial ACV service started with Vickers VA-3 in Britain in 1962. Since 1964 Mitsui (MES) has been making R and D efforts in ACV production and constructed 29 hovercraft of MV-PP series.
Recent MV-PP10, a diesel powered 105 passenger hovercraft has achieved high performance and remarkable running cost efficiency. The weight penalty due to the adoption of diesel engines and welded aluminium structures resulted in water drag increase particularly in low speed region. In order to overcome the water scoop, model tests and FEM numerical analyses together with full scale sea trials have been conducted. Full scale measurements with conventional rear skirt systems revealed that the rear skirts would scoop the sea water at about 8 knots.
Through the research and development works new rear skirts were conceived and designed for actual use considering geometric instability of the inflated membrane. There were great improvements in the reduction of the drag compared with the conventional systems.
Two MV-PP10 hovercraft equipped with the improved rear skirts are now in commercial service for access to Oita airport by Oita Hoverferry Co, Ltd.

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The biggest difficulty in the process of finite element (FE) analysis is the modeling of industorial products that have complex configulation. To overcome this, prototype of integrated finite element modeler, MODIFY (MODeling tool for Integrated Finite element analYsis) is developed by the authors [1]. MODIFY has three features : (1) FE modeling by the part object concept, (2) fully object-oriented data structure, (3) fully automated part by part mesh subdivision. In MODIFY, part objects consist of the geometry objects, the analytical condition objects and the relation objects, and the user only need to define and assemble them. The prototype system is applicable to FE modeling for 3-D plate structures.
In this paper, MODIFY is extended for 3-D shell structures. Three enhncement was made. First, the shell object that is represented by B-rep model and rational B-spline surface is defined as a geometry object. Second, the mesh generator for 3-D curved surface is constructed. Finally, MODIFY is linked with external CAD syntem using IGES file, which is most widely used as a conversion in commercial CAD system.
Several examples of finite element model generated by MODIFY are shown.

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Actual geometries of welded structures are slightly deviated from the designed shapes due to the gas-cut surfaces along plate edges and due to the irregular shapes of weld beads with possible undercuts at weld toes. Although fatigue failures of welded structures may be initiated at these highly stressed regions, conventional finite element structural analyses are usually carried out disregarding these imperfection effects at the design stage. A second order perturbation method is proposed to the two-dimensional elastic problem of a semi-infinite plane, whose surface is slightly deviated from a straight line. The deviation, η, of the actual surface from the straight line is assumed to be small compared with the characteristic length of the problem, and it is taken as the perturbation parameter of the present formulation.
The accuracy of the present method is first investigated by solving the problem of semi-elliptical notch in a semi-infinite plane under a combined uniform tension and in-plane bending at infinity. In comparing the present perturbation solution with other results, it is found that the peak stress can be estimated by the first order approximation with satisfactory accuracy, while the second order solution is necessary for the proper determination of the stress gradient at the peak point. The method is then applied to the problem of a slightly wavy surface, which is sometimes observed in actual weld beads. The peak stress and the stress gradient are calculated in closed forms up to the second order in terms of the height and wave length of the wavy surface. It can be seen that the present method is effective for the analyses of geometrical imperfections of arbitrary shapes, which may be difficult to analyze through the conventional structural design procedures of welded structures.

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At present, the finite element method has become a standard tool directly and widely applied to ship structural design. One important application is the overall analysis of ship hull or a large portion of it to evaluate stresses acting on different members. Although accurate results may be obtained by the finite element method, the accuracy of these results depends on the way they are used. For example, when plate buckling strength under combined loads is evaluated approximate buckling interaction equations, the inaccuracy of these equations may diminished the value of the accurate stress analysis by the finite element method.
In this paper, an accurate and efficient calculation method of the buckling strength of a rectangular flat plate is presented. The boundary condition is any combination of being simply supported and fixed, and the plate is subjected to combined two directional axial stresses (σ_x^aaf and σ_y^a), inplane shear (τ_xy) and two directional inplane bending (σ_x^b and σ_y^b). Biharmonic deflection function which satisfy the boundary conditions is assumed and made to satisfy the boundary conditions. The principal of virtual work is applied to formulate a general eigenvalue problem in a matrix form. Solution of this problem provides the buckling stress and the corresponding buckling deformation pattern. The accuracy of this method is estimated. The same method is used with any boundary and loading conditions, and is implemented in one compact and efficient computer program. The buckling values may be more accurate than those by the finite element method with less computer time.

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In the previous paper, a new buckling design formula for simply supported plate panels subjected to combined in-plane and lateral loads was suggested. The effect of welding residual stress was also included. For the plasticity correction, the well-known Johnson-Ostenfeld formula was used.
In the present study, more advanced formula of the plasticity correction is proposed taking account of edge condition effects. The applicability of the proposed formula is demonstrated by comparing with the present and the conventional result.

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Stiffened plate is a fundamental component of ship hull. The main purpose of the study is to present an analysis method which may be easy but effective to get a good estimation of stress-deflection behavior of stiffened plates. A rectangular plate subjected to uni-axial compression is the main target in the first report.
The proposed method is called “AEPM (advanced elastic plastic method)” as it is based both on the elastic large deformation analysis and on the plastic analysis. AEPM has been found to be effective in the region not only pre-ultimate but also post-ultimate by taking advantage of a refined equation derived from FEM data base as a regression curve. It has been verified by comparing its results with FEM that AEPM keeps as good precision as FEM and it is available for the wide range of structural characteristics including plate slenderness ratio, residual stress, initial deflection and plate aspect ratio.

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This paper describes the further improvement of the simplified method of progressive collapse analysis of a ship's hull under longitudinal bending proposed by the authors. Here, the method of analysis of flexural behaviour of stiffener elements was extended to deal with coupled flexural-torsional behaviour of angle bar stiffeners welded to continuous plating.
A series of elastoplastic large deflection analysis was performed and the rationality of the proposed method was examined.
With the improved computer code 'HULLST', two alternative analyses were performed on existing bulk carrier including and neglecting tripping of stiffener elements. It was found that the ultimate strength of stiffener elements decreases when stiffener tripping takes place, which results in reduction of ultimate bending moment of the cross-section. The load carrying capacity of stiffener elements in the post-ultimate strength range was more decreased and so that of the cross-section. The allowable bending moment according to the Lloyd's Register of Shipping was calculated and the reserve strength above it until the initial local collapse was 22.4% of the allowable bending moment. Interaction diagram for longitudinal hull strength under bi-axial bending is also presented.
Progressive collapse analysis was performed also on a large scale frigate model tested by Dow, and the applicability of the present method was demonstrated.

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Vibration of unidirectionally stiffened plate is investigated by means of the calculation based on finite element method and developed by the authors. In order to examine the relative stiffness of stiffeners to that of panels between stifferers, the quantity 'stiffness parametre' is defined. The effects of added mass, inplane loading, initial deflection and irregular disorder in structure are investigated for various stiffness parametres. Some of the calculations are compared with experiments.

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In order to simulate a trapped-air impact due to slamming, as a first step of the study, a flow of the air layer beneath the bottom of a rigid body falling on a deformable surface is numerically analyzed. The air domain examined is two-dimensional and is extended far away from the bottom edge. The physical model of the air is that of compressible and inviscid gas. The Euler equations are written in generalized non-orthogonal curvilinear coordinates. This permits the bottom of the rigid body to have any deadrise angle or to be arbitrarily shaped ; the same holds for the surface beneath. The flow is computed by finite differences ; a strong conservation law form of the equations was used, which is capable of capturing the shock if present. To simulate the flow of the air layer whose height largely changes as the body falls down, a new mesh generation technique is proposed. The proposed algorithm may be also applied to other air-cushion related problems, such as a simulation of pressure control in the air-cushion chamber of SES.

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The bottom plates of cylindrical flat-bottom tanks such as on ground LNG storage tanks generally have detached parts from the foundation due to the welding deformation. And the allowable limit of it from the view point of the strength under the liquid pressure becomes an important problem at the actual construction of tanks. However, there have been few reports which mentioned this problm.
In this paper, the results of theoretical study on this problem, assuming the welding deformation is two dimensinonal, are presented. The behaviors of the detached bottom plate under liquid pressure are found as follows ;
(1) When the liquid pressure begins to exert on the bottom plate, the major part of it touches to the foundation and the considerable amount of bending of the plate occurs in the detached part.
(2) When the liquid pressure reaches to some level, the whole bottom plate touches to the foundation completely in an unstable manner.
The proposal for the method of determining the allowable welding deformation is also presented.

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The paper describes a 3-dimensional numerical simulation technique based on the Marker-And-Cell (MAC) method. In order to simulate sloshing motion of liquid in a tank with such 3-dimensional shapes as shamfers, the governing equations are derived in a tank wall fitted coordinates system which moves with the tank. Comparing with the case where cartesian coordinates system is used, the boundary condition on the tank walls can be expressed simply in a numerical procedure, and the effects of inclined walls of the tank on slosihng motion can be taken into account with reasonable numbers of computational cells.
The 3-dimensional configuration of a free surface caused by chamfers are observed experimentally, though the tank model is forced to oscillate in only longitudinal direction. Numerical results succeed to show the above tendency. And also, the calculated time histories of sloshing pressure on the tank wall agree well with the experimental ones.

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This paper presents methodology for installation of flexible underwater line structure. In the deepwater drilling, subsea oil development, deepsea mining and so on, pipes and structure are so long that the structure become so flexible. Dynamic response and deformation must be controlled during installation or operation to protect the structures from unexpected disturvances. Control method formulated in this paper was tested by basin experiment. Two actuators were used in the experiment. One is servo controlled wing-like appendage which generate variable drag force. The other one is variable buoyancy. Buoyancy is varied by chaging volume of air chamber. Ultrasound ranging sensor was used for measurement of displacement. Accuarcy of this sensor is high, less than 1 mm, and this sensor is also used as deformation sensor. Experiments were conducted for upending of TLP tendon and free fall of horizontal pipe. Tendon is suported at one end by a pin joint and dropped freely. In the free fall experiment the pipe is kept straight and horizontal.

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This paper presents active control of TLP. Dynamic responses, motion and stress, were controlled. Natural frequency of the deepwater TLP tendon decreases with increasing water depth. There is possibility that the dynamic response, which is induced by hull motion, become significant for deepwater. Strain of the tendon was included in the objective function of optimal control formulation. High frequency response induced by wave was cut off and slowly varying wave drifting force was controlled.
The experiment showed that motion by drifting force can be much larger than wave induced motion in the case of TLP model used in the experiment. The magnitude of motion was reduced to comparable to wave induced motion and at the same time the strain of the tendon was limited or reduced by the control.
Displacement of hull was measured by ultrasound ranging sensor and control force is generated by two thrusters. The experiment showed that the maximum control force is less than 1/80 of hull weight which is conventional value found among existing semisubmersibles.

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The uranium adsorption tests using model bed units reduced to a scale of 1/4 from the adsorbent system proposed in the 1st report and packed with the adsorbent of fiber bundle type were carried out towing in the velocity of about 2 knots for 30 hours and mooring for 37 days in Imari Bay. The pressure distribution around the unit was measured and flow distribution in the unit was calculated for the towing test. And the uraniun adsorption of the adsorbent enclosed in the unit was simulated numerically. A good agreement can be seen between the measured and simulated uranium adsorptions. For mooring test, it was proved that the ability of adsorption was not decreased even if the adsorbent was keeping in underwater bed unit covered with the net of shield light during 37 days.
Furthermore, numerical simulation of this system was carried out for practical operation in the ocean and the production cost of uranium extraction was calculated to be about 55, 000 yen/kg-uranium for extraction period of 60 days.

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