In order to elucidate the free surface phenomena around the ship bow, experiments of simple wedge-shaped models are conducted in the towing tank. It is found that the surface tension has a remarkable effect on the free surface shape around the model. It is shown that the surface activator is very effective in order to eliminate the effect of the surface tension. Free surface configurations free from the surface tension observed by photographs after application of the surface activator are examined in detail, and the mechanism of the inception of wave breaking at the bow is discussed under the light of rational hydrodynamic theories.

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Theoretical and experimental studies are reported on a two-dimensional shallow water flow over isolated obstacles. The Froude number based on the depth of water ranges from O.2 to O.8, and the obstacles have bluff configurations. The flow separates on the obstacles and it is under the influence of the presence of the free surface with breaking waves.
In the experimental part of the paper the results of a variety of measurements are described and discussed. The free surface appearance is photographed and the flow field is visualized using tracers. The pressure on the obstacles is measured. The distribution of mean velocities and the depthwise distribution of turbulence are measured by a five-hole pitot tube and a hot-film anemometer, respectively. For the numerical study a finite-difference method based on the Navier-Stokes equations is employed. The higher-order upstream differencing scheme is used for the convective terms so that a high Reynolds number flow can be simulated. An ingenious treatment of the nonlinear free surface condition enables the simulation of the breaking wave motions. No-slip body boundary conditions are approximately implemented in the framework of an inflexible rectangular mesh system. The computed results agree well with the measurement and they seem to elucidate the complicated flow full of nonlinearities with the interaction of a recirculation flow with a breaking wave both involving intense turbulence.

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The author has investigated the application of the wave resistance theory to the hull form design by means of nonlinear programming. In this study two problems are discussed, one of which is the problem of hull form improvement, and the other is the problem of minimum wave resistance. The present paper concerns the concluding result of these series, and the feasibility of this approach has been proved by showing the high performance hull form of SR 138 container ship being improved further as an example.
We can regard the hull form design as a kind of the problem of nonlinear programming, because the objective function is generally specified as the nonlinear functional of design variables and many constraints of equalities or inequalities are needed in the practical design stage. The nonlinear programming technique provides a rational method for the determination of lines of practical hull forms.

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In this report, we deal with statistical analysis for resistance of high speed ship forms in still water.
Ship forms are classified into two types, i. e. naked hull with bow dome and without bow dome.
Multiple regression equations for residual resistance are obtained by regression analysis on results of resistance test. And also, multiple regression equations for wetted surface area are obtained.
Using these formulae, it will be able to estimate easily effective horse power over a wide range of speed.
Estimation formulae are very available when designed, and in the future there is a possibility of finding the direction to improve ship forms.

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The author has tried to introduce reciprocity theorems and represent the flow field of Navier-Stokes flow by Oseen's scheme.
The present report deals with the three-dimensional steady flow.
For practical examples of application, taking up a slender thin ship, he shows her flow fields and resistance approximately but visually.
Then, taking up a slender thin square wing, he extends Flax's reciprocity theorem to non-linear domain and obtains a simple but accurate formula of the non-linear lift.

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The Quasi-Continuous Method (QCM) which was originally applied to the steady planar thin wing problems possesses both advantages of continuous and discrete loading methods. The QCM has been successfully applied to steady propeller problems. In the present paper, the QCM is extended to the unsteady propeller problems. First, explanations are made on unsteady two-dimensional wing problems for the evaluation of the validity of numerical method and then a numerical procedure to solve unsteady propeller problems is described in detail. It is then shown that the fluctuating thrust of one blade and propeller vibratory shaft forces calculated by the present method agree well with experiments. Further, an example of calculation is shown for pressure contours on blade surface varying with angular position of a propeller blade in nonuniform flow.

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New propellers with better cavitation performance in comparison to a MAU type one were designed, matching the equivalent two-dimensional foils obtained from lifting surface calculations with those obtained from foil section design theory by prescribing the pressure distribution. In designing the new propellers only the section shape and pitch were altered, keeping all other parameters same as the MAU type one. Flat pressure distribution except near the leading edge, based on the experimental results of the two-dimensional foils presented in a previous paper, was adopted so as to give less cavitation.
Experiments were performed, using both the new and the MAU type propellers. The following results were obtained and the validity of the above-mentioned design concept and method was verified.
(1) Both cavity extent and thickness of the new propellers were remarkably reduced in comparison to the MAU type one. Cavity volume of the new propellers reduced to less than half of that of the MAU type one.
(2) Fluctuating pressure due to the new propellers was about half of that due to the MAU type one. This pressure level agreed approximately with that in non-cavitating condition.
(3) The new propellers had nearly the same efficiency as the MAU type one.
(4) The load distribution of the new propellers became higher in the aft part of the blade section. If the region near the trailing edge was too highly loaded, the thrust of the new propeller became lower than that of the lifting surface calculation. This fact should be kept in mind while designing propellers in a similar manner.

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For high speed crafts it is often necessary that the propeller be operated under inclined flow conditions. This results in generation of cloud cavitation in the root region leading to erosion. Therefore it is thought necessary that the effect of boss be taken into account in such a case. In the present report as a first step in analysing this problem, the effect of boss in uniform flow was studied. In the theory based on Vortex Lattice Method the boss was represented by a series of source rings the strengths of which were determined from tangency condition on the boss surface. Experiments were performed to measure the pressure distribution on the blade near the root region and also on the boss surface. Theoretical results had been found to agree well with the experimental ones.

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In order to determine the cause of propeller nozzle vibration attaining 8, 000 Gal experienced on two ocean-going tugboats constructed by IHI and ranking among the world's largest, the data acquired on the actual ships were supplemented by model tank tests for hydrodynamic force measurements, flow visualization, reproduction of self-excited vibration, and cavitation tests, using 2-dimensional profiles representing the propeller nozzle, and 3-dimensional model of propeller-nozzle assembly.
The mechanism causing the inordinate vibration was identified as “galloping” -self-excited vibration due to negative damping-of the nozzle, from the evidences observed of :
(1) Negative hydrodynamic damping of nozzle exerted hydrodynamically on the structure, ascribable to a negative slope developing on the curve relating nozzle lift to incident angle, and which is induced by flow separation over entire outside surface of nozzle.
(2) Galloping phenomenon reproduced in model experiment
(3) Kármán's vortex street discarded : The vortex-induced vibration developed in test with 2-dimensional model, but not with 3-dimensional model, the latter due to mutual counteraction between vortexes generated at points situated diametrically opposite each other on the nozzle circle.
(4) The other conceivable factors causing the inordinate vibration such as cavitation, twin propeller wake interaction and propeller blade stall flutter were eliminated by model experiment and analysis of actual ship data.
Measures considered effective for preventing the occurrence of galloping are :
(1) In design, to select and specify nozzle profile and operating parameters such as to prevent flow separation over entire nozzle surface
(2) As remedial measure, to ensure a vibration mode of large structural damping
(3) In service, to reduce ship speed, so as to lower the absolute value of negative hydrodynamic damping of nozzle.

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The Surface Effect Ship (SES) has been under Research & Development for over 20 years and many kind of commercial SES are now in service. Generally, cushion pressure of SES is kept by two side-hulls and two seals (or skirts) each located at fore and aft end of SES cushion chamber.
The restoring forces and moments of these seals are considered to affect on craft's dynamics but we can find very few reports about seal dynamic characteristics up to the present.
In this study, the two dimensional dynamic model test was carried out to inspect fundamental characteristics of 3 types seal configurations which are considered to be proper for high speed SES, namely, Bag finger seal, Planing seal and Lobe seal. The experimental equipment is consist of fan-duct system, plenum-chamber, load-cells, pressure-gages, oscillating plate, power units and controller to simulate SES airflow system. The propriety of the mathematical model which corresponds to the SES seal-airflow system was confirmed by these experimental results.
The conclusion may be summarized as follows :
1) The mathematical model is composed of two equation groups, namely, equations of seal geometrical shapes for each configurations and equations of cushion airflow system. This mathematical model is confirmed to show a good agreement with model experimental results except of some high frequency region.
2) The lobe seal has different characteristics, especially shows good responsibility to wave elevation (in this experiment replaced with flat plate oscillator).
SES seal dynamics was fundamentally clear by this work but a further study including craft's motion in waves is necessary to discuss actual SES seal dynamic restoring forces and moments.

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The purpose of this paper is to present a new composite expansion of slender body theory, which is more suitable for antisymmetric motions than an ordinary composite expansion.
A new composite solution is gained by retaining second order terms in far field solution and in matching of far field and near field solution. This solution has three dimensional correction terms due to antisymmetric flow field component, and an ordinary composite solution has no these three dimensional antisymmetric correction terms.
When applying this method to lifting flow problems, order of lateral motion must be small, because slenderness assumption is also necessary for free vortex sheets.
In the case of small lateral motion, simple approximate formulas are shown for lateral force distribution, sway force and yaw moment. These formulas are composed of usual slender body approximation formulas and new correction terms.
As to non-lifting potential flow problems, even in the case that order of lateral motion is unity, this method can be applied effectively as well as the case that order of lateral motion is small.
The effectiveness of this method is shown for a potential flow about a spheroid, by comparing results of analytical method, usual slender body theory and this method.

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Wind force affects on a ship manoeuvrability, especially when the superstructure is very large or the ship's speed is much lower than the wind velocity. It is indispensable for the safety analysis of the harbor manoeuvrablity to estimate the ship motion in wind.
In this paper, the predicting techiniques are investigated confirming by the successfull free-sailing model tests. Then, the manoeuvrable upper limit of wind velocity is discussed under the unsteady condition when a frequent engine telegraph is used.
Through the present study, the following conclusion are obtained.
(1) The numerucal simulation based on the moduler type mathematical model can give a fairly good agreement with experimental results in wind.
(2) Although a dynamic yaw damping force is included in the wind force of a superstructure, it does not so much affect on the manoeuvrablity in wind.
(3) From the numerical simulations with accelerating and decerelating conditions in wind, it is found that the behaviour of ship almost depends on the main engine power against the wind. This suggests that the manoeuvrable upper limit of wind velocity should be described by the ratio of main engine power to wind velocity, not by the conventional ratio of ship speed to wind velocity.

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Course stability is one important factor while taking into consideration safety of navigation for towed vessels. Recently, the problem of towing operation, towing equipment etc. has been considered at the design stage.
Course stability will depend on various factors and especially in this paper emphasis has been placed upon the effects of wind on the two towed systems. A theoretical approach to the calculations is also elaborated. Calculations for three different types of systems, composed of cargo and tanker vessels, i. e. for the case when the towed vessels are identical, small and large, large and small respectively, are performed and the effects of wind on each system are as follows.
For higher wind velocity, the above mentioned systems have a tendency to become stable, however as the wind direction changes from the against wind to the following wind, there is a tendency for the systems to become unstable. Besides these, the tow line length, the location of the tow and towed points also influence the stability of the systems.

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A rapid progress in the marine transportation brings traffic congestion in sea areas near important ports and harbours. Needs for the development of marine traffic control system and improvement for vessel traffic regulations are increasing year by year.
A fundamental study on the marine traffic control system is carried out. As an extention of an algorism proposed by Ishitani and others (1983), a method of marine traffic control for larger area is developed, introduction of a grounding avoidance term into the evaluation function for optimal route setting and a technique to achieve the 'keep to the right' rule, makes it possible to apply the method to the much more complicated and realistic sea area model than Ishitani's examples.
Computer simulation results for simplified model indicate the characteristics of the method and its effectiveness.
As an example of practical application, a simulation of the marine traffic in the Tokyo Bay is performed. Useful information for the further improvement of the method, and some interesting features of the traffic flows are obtained.

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Maneuverability of the MURS 300 Mark II (a Remotely Operated Vehicle), developed for inspecting underwater parts of hydroelectric dam facilities, is discussed in this paper based on the results of the computer simulation and the water tank test.
A mathematical model of the submersible operation is built on the equation of a six degree freedom motion of a rigid body. Gravity force, buoyancy, hydrodynamic forces and thrusts are considered as external forces in the model. Cable tension is not included because of its negligibly small force in calm water. Hydrodynamic forces, drag, lift, added mass and so on are determined by the wind tunnel test and the water tank test.
Thrusts are also measured in a water tank test. Being equipped with neither tail wing nor stabilizer, ROVs are usually characterized with their relatively poor directional stability. If a step input of thrust is added, ROVs are unlikely to move on a straight track without supplemental manipulation of thrusters.
An automatic heading control system can be a solution to improve directional stability of ROVs.
In case of the MURS 300 Mark II, a step input of its foward thrust causes its ascending because of the center of its horizontal thrust located slightly below its gravity center unless an automatic depth control system is installed. Given a rectangular pulse thrust, the MURS 300 Mark II equipped with automatic heading and depth control systems automatically returns to an original place where an operator sets a joystick in a neutral position.
This operational characteristic of the MURS 300 Mark II is quite useful for underwater structure inspections. The results of a full scale experiment performed in a water tank agree to and prove the computer simulation outputs.

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Through the previous papers, the authors presented a method of predicting nonlinear wave loads acting on a ship in large-amplitude waves, and verified its validity for evaluating nonlinearities of wave loads to a satisfactory extent in both quantitative and qualitative sense. Furthermore, this paper deals with nonlinear wave loads of a ship in most general case, that is to say, wave loads on a free-sailing ship in large amplitude waves, and effects of ship's advance speed on their nonlinearities are investigated in detail.
Vertical wave loads are more sensitive to wave height compared with lateral wave loads except torsional moment. Computed results of vertical wave loads agree well with experimental results, and clarify that a main reason of the nonlinearity of vertical wave loads is time-varying relative vertical displacement of a ship and that the nonlinear coupling between vertical and horizontal hydrodynamic forces come to be important at and near the frequency of roll resonance. For the nonlinearity of torsional moment, the above-mentioned coupling effect plays a dominant role in relatively short waves where roll is small.
It is also found that as the ship's speed increases, in general, nonlinear component of vertical wave loads increases remarkably so that the peak values of nonlinear wave loads may exceed by far the peak values obtained by linear calculation. The almost same trend is found in torsional moment while the effect of ship speed is negligible for horizontal shear force and horizontal bending moment.

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Authors study on the characteristics of the wave load in the reliability analysis of the ship strength from the view point of practically encountered wave condition of ships.
In this report, authors research the statistical character of wave and wind conditions observed by 59 voluntary ships in the Pacific Ocean during the period of 6 years (1967-1981), to obtain the fundamental data of the wave load for an ocean going vessel. To verify the character η_μ that exaress the observed wider scatter band of such meteorological phenomena as wind velocity and wave condition, authors research the wind mesurement recorded at 24 weather stations in Japan for 20 years. It is confirmed that the value of η_μ is equal to the result of the 2 nd report.
In the 2 nd report, it is concluded that the scattering of each wave conditions is caused by the above η_μ and the variation of the ship operationg condition, and the effect of the latter become smaller when the ship service period is such long time as 20 years. But, it is found by the above research that the latter effect can not be disregarded when the area where ship will sail become wider.
In general, an ocean going vessel does not sail in a certain limited sea zone, so it may be required to regard that the latter effct is larger than the result of the above research when we analyse the reliability of strength of such ships.
The maximum value of the wave load of a ship is governed by the maximum wave height and its frequency in the encountered wave condition. Authors study on the distribution of the maximum wave height and clarify that the theoretical distribution well agree with observed maximum distribution of each wave period.
From the above result, it is concluded that the coefficient of variation of the maximum wave load of a ship which operates in the same sea zone as the voluntary ships amounts between 18% and 22% and that the value increase in proportion to ship size.
In the same manner as the variation of wave condition, as the area of operating zone increases the scatter band of distribution of the maximum wave load released increases and the value of the coefficient of variation of the maximum wave load of the general ocean going vessel should be taken more than the amount of 18-22%.

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In order to increase the ocean space exploitation by using the floating structures, it ceems necessary to develop the tension mooring system. This system has many advantages such that it becomes relatively inexpensive with depth, suppresses the motions of the floating structure and needs only minimum mooring spaces.
An at-sea experiment is planned to establish those fundamental technologies. The semi-submersible consists of twelve columns with footing, and its principal dimensions are 30 × 20 × 12.5 m. This is supposed as a proto-type of bigger scale floating platform.
The structure will be moored mainly by the six catenary chain lines with anchors. Further mooring by the four tension legs will be tested. The tension leg is made of rubber and chain. This composite material has the higher breaking load of chain and also anti-impact and corrosion resistance of rubber.
In this study the tension mooring system has been designed to match with the environmental conditions of test site. The mooring tension characteristics have been estimated both by the model tests and the theoretical calculations.

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This paper deals with the probability distribution and the distribution of maxima of the horizontal motions including slow drift motion of a moored floating offshore structure. If the motions can be described in two term Volterra functional series, the probability distribution of the motions can be solved exactly by use of the statistical value of quadratic form. The distribution of maxima can be found by the joint probability density function between the motion displacement and the motion velocity under Powell's assumptions in the field of structural dynamics.
In this paper, the authors propose the new method which can be obtained by matching between finite Gram-Charlier expansion and the asymptotic form derived from the exact solution of the probability distribution. Furthermore the relationship between the statistical values of the probability distribution and the characteristics of the motion responses including the nonlinear response are found. The approximate solution for the distribution of maxima is discussed by introducing the assumptions which the motion displacement and the motion velocity are independent and which the motion velocity is Gaussian process with zero mean in addition to Powell's assumptions.
In order to confirm the applicability of the present method, the measurements of wave elevation and horizontal motions in long duration have been carried out in the long tank, the characteristics of the first and the second order transfer functions of the motion have been found through cross spectrum and cross bispectrum analyses, the experimental results are compared with the results of the present method.
As the results, the following items have been found :
1) Pinkster's approximation is applicable to the second order transfer function of motion. Further the linear transfer function can be estimated by the usual linear motion prediction method.
2) The present method can be applied to the estimation of the probability distribution and the distribution of maxima of the horizontal motions including slow drift motion.

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It has been pointed out that under same KC number condition the lift force acting on a vertical pipe in regular wave can exceed the force acting in the wave progressing direction. In this study a model for the lift force is presented, and using this model dynamical response of pipes are calculated employing a analysis program of under water line structure. And these results are compared with those of experiments. The model is consist of two formulae, the first formula expresses two alternation of lift force acting on a element of the pipe during one cycle of wave elevation and the other one expresses three alternation of lift force.
f_L2=1/2ρDC_LV²n× (V_N-v_N) /|n× (V_N-v_N) |sgn (dH/dt) f (H)
f_L3=1/2ρDC_LV² n× (V_N-v_N) /|n× (V_N-v_N) |sgn (H-H_max√3/2) sgn (H+H_max√3/2) g(H)
Here ρ represents the fluid density, and D is the diameter of the element, V denotes the velocity of fluid particle, and C_L represents lift force coefficient, n is unit vector parallel to the axis of element, V_N and v_N represent components of fluid particle velocity and element velocity normal to the element axis respectively. And sgn is a sign function and take its value +1 and -1 according to positive and negative of its argument value respectively. 1/2ρDC_LV² represents the amplitude of lift force, and the following functions detect the crest, the maximum slope position and the definite height position of free surface elevation, f and g are smooth functions and their maximum values are unity. C_L values used here were determined experimentally as to the solid pipes. As a result next conclusions are obtained.
(1) Using the first formula, the predicted displacements of the bottom end are in good coincidence with experimental result especially in case of resonance. The results, obtained using the second formulae, didn't show good coincidence with the those of experiment, but the resonance similar to the experiment were observed.
(2) C_L calculated from the measured lift force, increased in case of the resonance, but the calculated displacement, using the fixed C_L value obtained through experiments with solid pipes, were in good agreement with the experimental results. Therefore, major part of increase in C_L in case of resonance could be explained as a result of dynamic magnification.

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It can be said that the mechanism and the characteristics of wave energy absorption of attenuator type devices have not yet been clarified experimentally and theoretically. Therefore, the authors deal with an attenuator type device which consists of five air chambers with and without a bottom plate. In the last report, the authors whowed a part of results of series of experiments in transient waves and the values of numerical result of the strip method, and gave following two conclusion
(1) Taking into account the bottom plate on the device, the maximum coefficient of wave energy absorption became large and moved to low frequency range.
(2) Without coupling terms between the device and OWC in strip method, the motion of device could be estimated well but the coefficient of wave energy absorption of each OWC could not be estimated well.
This paper presents the mathematical model using the 3-D source distribution method and the limitation of successful application of the strip method considering the coupling hydrodynamic forces by means of comparison between the experimental results and the calculated results. From these results, this paper shows the characteristics of wave energy absorption of the device in the head and oblique sea condition, and shows the effect of the bottom plate for the wave energy absorption and the reason of its improvement.

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The system simulation for a fixed O. W. C. type wave energy absorber has been studied. It shows both the transient states of the system while the turbine revolution becomes high velocity and linear calculation method for high velocity revolution. However, because of the radius and the number of revolutions of turbine were not optimum controlled in previous studies, both efficiencies of primary energy conversion and turbine were not necessarily optimum.
This paper shows the method of optimum design for turbine on a O. W. C. type wave energy absorber. That is the optimum radius and the number of revolutions of turbine. Furthermore, in order to improve the efficiency the authors consider the system simulation with phase control and discuss the effects of phase control.

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The system simulation for a fixed O. W. C. type wave evergy absorber was proposed and have been studied. In this simulation, the form of air-chamber and turbine were refered to what was used in the real sea trial at SANZE. Since the device was fixed in very shallow water, the nonlinearity of hydrodynamic force on O. W. C. must be discussed.
In this paper, the authors try to know whether equations and performances of turbine used in the system simulation are appropriate or not. Furthermore, they carry out experiments by using a small model, and consider nonlinearity of hydrodynamic forces on O. W. C. on the basis of experimental results.

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Nowadays many researches select topics on CAD/CAM. This population comes from the rapid progress of computer technology and the inovation of computer graphics and geometric modeling technology. This paper aims to reveal the tashs to be solved on the boundary area of navalarchitecture and information technology by considering the feature of the shipbuilding industry. The basic theory of solid modeling is expanded to enable ship's hull structure representation. In order to realize the integreated CAD/CAM, the concept of system architecture have a great importance. This paper examines to design a open architecture using network processing methodology based on ship's product modeling and LAN technology.

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The paper describes a calculation procedure developed at Lloyd's Register of Shipping for design purposes, based on a two-dimensional numerical finite-difference method which predicts the sloshing behaviour of fluids in arbitrary shaped tanks when excited by ship motions in a seaway.
The method overcomes some problems associated with model experiments. Furthermore, tank shape, internal structure and excitation can be readily modelled and the calculation provides realistic free surface behaviour and pressure time histories, including impacts. A standard excitation method is suggested which takes into account the important interactions of fluid and ship natural periods and amplitudes of ship motions.
Other modes of excitation are also examined including one based on records of irregular ships' motions and another based on a numerical model of the coupled phenomenon of sloshing and ship motions.

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Sea trials were conducted featuring measurement of wave impact loads and hull responses for a patrol boat with displacement about 1, 300 ton, while the boat was run in severe waves at various kinds of wave encounters and speeds.
Measured items are ship motions, longitudinal bending stresses on upper deck and impact pressure on the bow flare and at the bottom of fore part of the hull, as well as waves in the test area.
This paper presents some results of the analysis of the data, as follows.
1. Bow flare slammings occured so frequently, but bottom slammings did a few times even in head seas. Bow flare slammings induced very high whipping stresses on the deck, but the latter did not.
2. Impact pressure on the bow flare, which was showed to be the main cause of bow flare slammings, were estimated by means of a method which was derived on the analogy between water impact phenomina of a wedge. and that of a ship. The estimated pressure showed very good agreements with the measured one.
3. Numerical analysis of impact pressure on the bow flare by the method showed that ship speed had to be taken account of for the estimation of the impact pressure.
4. According to spectrum analysis of whipping stresses and acceleration, up to 3 rd mode of hull vibration is enough to simulate the hull response to slams.
5. Analysis of fundamental mode of whipping vibration showed that logarithmic decrements at the inititial stage of whipping were very large, but those of the following vibration were comparatively small, which were still almost twice as large as Kumai's formula.

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The impact pressure due to the water impact was studied experimentally and numerically.
The two dimensional impact tests of a rigid flat plate, a 1-deg wedge and a 2-deg wedge were carried out under the condition that drop velocity was kept constant during the water impact.
Experimentally it is confirmed that in case of the water impact of 2-deg wedge the impact pressure obeys Wagner's theory and the air layer is not formed between the wedge and water surface. In case of 1-deg wedge the air cushioning effects appear when the drop velocity is higher than 0.4 m/s.
In order to investigate the mechanisms of the flat plate impact, the phenomena during the water impact were filmed with a high speed camera at 2000 fr/s.
The pictures show that during the impact the edges of the flat plate doesn't touch closely the water surface, but the air-water mixture is formed between the plate and water surface near the edges. Since the density of the air-water mixture may be higher than that of the air, the high density mixture chokes the air flow under the plate. Thereby, the pressure between the plate and water surface ascends rapidly.
We present a mathematical model considered the mixture leakage. Numerical simulation was carried out of this model.
The air layer thickness, added mass and coefficient of the air leakage, which cause the impact pressure ascending, were estimated by means of the comparison of the numerical results with experimental results.
The estimated values obtained from the simulation almost agree with the observed values.

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Simulation program has been developed to simulate comprehensive ship responses in bidirectional irregular waves. Statistical analysis has been made on the simulated time history to have “theoretical distribution” of response amplitude. Model experiments corresponding to the simulation were set up using an elastic model of a bulk carrier in three kinds of loading condition, i. e. full loaded condition and two ballast conditions of the same displacement with different trim conditions. Comparisons were made between the simulation and the experiment on histograms of longitudinal bending moment and variations of significant values of the moments along the hull. Discussions cover effects of slamming and whipping to the bending moment and changes of responses to the loading conditions and effects of heading angle in the bi-directional waves.

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In 1935 the Forth Fleet of the Imperial Navy encountered a typhoon in the occasion of the naval grand maneuvers in the Pacific Ocean off the northern coast of Honshu Island of Japan, and many naval vessels of the fleet were damaged. Destroyers “HATSUYUKI” and “YUGIRI” lost their fore bodies being splitted in front of bridges with 51 crews on board. They were designed on the basis of the theories and experiments of those days ; they had long forecastles with large flare for high performance in rough seas, and their section moduli were designed to be in proportion to the estimated longitudinal bending moment for the reduction of hull weight.
In this paper the authors attempt to analyze the disaster of the two destroyers theoretically with the aid of the knowledge available today. The wave bending moments acting on the destroyers are calculated for several wave conditions by TSLAM based on the nonlinear theory of ship motions, and they are compared with the ultimate moment. It is shown from the calculated results that the sagging moments are increased by slamming to the flare and buoyancy of the long forecastle dipped into a wave. If the longitudinal strength were designed in such a manner that the section modulus had an almost constant value sufficiently far towards the fore body of the vessel, the disaster of the two destroyers could have been avoided.

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This paper presents a predicting method of failure probability of ultimate collapse under extreme wave load in deck plates (panels) between longitudinals when initial distortions (deflections) of the welded plates are represented by an exponential distribution.
Since failure equation is non-linear and non-normal in this case, the advanced level 2 method is applied by introducing equivalent Gaussian parameters and calculation is performed by numerical successive method.
The probability of ultimate collapse is predicted as a total collapse which occurs due to decrease of axial load carrying capacity introduced by successive collapses of weaker panels which lie in a transverse line across the deck.
The probability of extreme wave load is derived from binomial distribution of parent Weibull distribution. Similarly the probability of collapsing capability of weaker panels is derived from parent distribution of the initial deflection.
The obtained safety index β and failure probability P_f show approximately a linear relation with maximum expected stress during ships' life and still water stress. They indicate also a large effect of initial deflection.
A linear approximate method using equivalent Gaussian parameters is proposed in last Chapter.

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This paper is concerned with reliability assessment of ship structures which are modeled as relatively rigid frame and subjected to quasi-static extreme loads. At first, a linearized plasticity condition of the section is introduced which takes into account of combined effects of bending moment, shearing force and axial force on the yielding. The failure criterion greatly facilitates generation of the safety margins and calculation of the failure probabilities. Structural failure is defined as large reduction of total structure stiffness. Second, the socalled branch-and-bound method is used to select the probabilistically dominant failure modes, which saves the computational efforts performing the reliability analysis of large-scale structures. The failure probabilities of the selected modes are evaluated. Third, the proposed methods are applied to a plane frame structure idealized for a transverse ring of a tanker. Through the numerical example, the following results are obtained.
(1) The linearized plasticity condition is effectively applied to consider the effect of shearing force on probabilistic collapse analysis of such a structure consisting of the girder with deep web, which results in underestimation of the strength of the girder.
(2) In the case of plane frame structure idealized for a transverse ring of a tanker, the probabilities of failure modes with shearing force effect including are very larger than those excluding the effect.

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The purpose of the study is to estimate the strength of frames against impulsive load in relation to the strength of ships subjected to wave impact. Especially authors are interested in the dynamic lateral buckling behavior of frames.
In the previous report, the T-shaped beams were tested in a pendulum type impact testing machine. According to the results of experiments and elastic-analysis based on the finite element method, the dynamic lateral buckling behavior of beams were discussed.
In the present report, the dynamic load is calculated by using simple model of collision between the beam and the swing weight, and elastic-plastic analysis of the beam is carried out.

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Previously, the authors developed the plastic node method for elastic-plastic analyses of homogeneous continuum in any geometrical shape. In this method, using ordinary finite elements, plastification is examined only at the nodes. Regarding the yield conditions at the nodes as plastic potentials and applying the plastic flow theory, the elastic-plastic stiffness matrices can be derived without integration over the element. The successful applicability of this method has so far been verified.
By the way, the accuracy of the elastic-plastic behavior obtained by this method depends on that of the stresses in the elements and the yield conditions. Especially on the former one, it is generally agreed that the stresses evaluated at the Gaussian points or by means of averaging may be more relevant rather than nodal ones.
From this point of view, in this paper, the basic theory of the plastic node method is further extended so that plastification can be examined at any point in the element. In this sense, the authors name this extended theory the “plastic collocation method”. The theoretical background of this new method is also argued especially aiming at the mechanism of the plastic deformation in the element. Applying this plastic collocation method, several examples are analysed, and the effectiveness of this method is demonstrated.

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The plastic node method previously developed by the authors is very effective one for elastic-plastic analyses of the framed structures. In this method, the new mechanism of plastic hinge based on the plastic flow theory is introduced to the fully yielded nodal point of one dimensional finite element. Therfore, the elastic-plastic behavior of the framed structures subjected only to the concentrated loads can be exactly evaluated using the discrete model where nodal points are placed on the loading points and joints.
On the other hand, when the distributed loads act on the framed structures, possible yielded zones cannot be estimated in advance. Inappropriate discrete models lead to overestimation of the collapse load.
In this paper, applying the plastic collocation method developed newly by the authors, the advanced theory for elastic-plastic analyses of the framed structures subjected either to concentrated or to distributed mid-span loads is proposed. In this new theory, the exact collapse load, collapse mechanism, and mid-span deformation of such structures can be obtained by the discrete model where nodal points are placed only on the joints.

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Offshore structures are exposed to higher probability of collision with ships or icebergs because of their limited mobility. In general, the consequence of the collision is disastrous. It is important to study the behavior of offshore structures under collision to ensure their safety.
In this paper, dynamic elastic-plastic response of frame structures carrying heavy mass on the top is investigated to provide better understandings of the dynamic response. For this purpose, the Plastic Node Method, which is superior to ordinary Finite Element Methods for elastic-plastic problems, is employed. The numerical results show a clear transition in plastic deformation modes. It is also found that these deformation modes can be correlated with equivalent static deformation modes. Considering these characteristics of the phenomena, a simple method based on a modified rigid-plastic model is proposed to estimate the plastic deformation due to collision. Further, it is applied to classify the dynamic response with respect to the mass and the initial velocity of the colliding body.

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The behavior of double bottom structures is studied through the model tests for two types of grounding. The one type is associated with the case where a ship runs on a rock which penetrates through the bottom plate. In the other type, a ship strikes a rock and suffers the intrusion of the rock into the double bottom from the forepart of the ship. The test results for the former type of grounding are compared with a nonlinear finite element program. It is demonstrated that the finite element method represents a useful tool for the analysis of load-rock penetration curves of double bottom structures.
In addition, the height of double bottom required for the inner bottom plate intact against rock penetration is examined in a usual tide range, through a simplified analytical procedure.

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Interframe collapse of stiffened plate subjected to combined loading is investigated and empirical formulas for the ultimate strength are proposed based on the nonlinear finite element analysis in the present paper.
The effects of existence of shear stress on the ultimate strength of plates is mainly investigated in the present paper. Reduction of ultimate strength by shear stress is more significant in the case of longitudinal compression than transverse one. Unified expression for the interaction of two direct and shear stresses is derived by modifying von Mises ellipsoid which is corresponding to in-plane crushing by yielding. In the formula the deviation from the ellipsoid is expressed as a function of plate slenderness ratio.

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An inelastic bifurcation analysis method is proposed to investigate collapse behavior of submarine pipelines under the combined loads. Computer program “PCOLAS” is developed by combining this method with simplified analytical method developed in the first report where limit load types of collapse were considered.
Four-point bending tests are carried out on aluminium alloy and stainless steel pipes. Theoretical results are evaluated from the test results and also from available test results for combined bending and external pressure ; the theoretical values agree with test results.
Main results are :
-PCOLAS enable us to investigate the complicated collapse behavior of submarine pipeline subjected to combined loads in a few CPU-seconds (HITAC M-280 H).
-For pure bending load, and combined bending and external pressure, bifurcation occures before limit load type of collapse occures.
-Axial tensile force increases the bifurcation curvature ; however, this axial force decreases the moment carrying capacities.

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In order to know the behavior of deformation and fatigue strength of structural members under compressive and tensile loads, theoretical and experimental investigations were performed on a column with rectangular cross section in the first report.
In this paper, the problem of thin tubular columns which are usually used for bracing in offshore structures subjected to periodic forces by waves and winds is studied. The following items are studied, 1) the gloval buckling strength of column 2) the post-buckling behavior 3) the local buckling behavior occured at the compressive side of wall in the cross section at midlength 4) the initiation mechanism of fatigue crack during repeated loads 5) the fatigue strength at the hot-spot.
The following conclusions are made.
1) It is confirmed that not only the buckling and postbuckling behavior of deformation due to compressive loads but also the cyclic behavior of a column under repeated loads are easily calculated by the analytical method proposed in this paper. And the analytical results agree well with the experimental ones.
2) Definition of the critical strain at which the local buckling occurs in a pipe under combined loads of compresion and bending moment is made clear. The local buckling strain obtained by experiments shows good agreement with that by analysis.
3) The initiation mechanism of fatigue crack can be explained through the change of load P, ovalization of cross section and local strain at the hot spot during repeated load.
4) A design proposal for the fatigue strength is made by using the above-mentioned local buckling strain.

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An Experimental and analytical invetigation of shell buckling of circular cylindrical shells under hydrostatic pressure was carried out in order to clarify the effect of initial deflection.
Fifty-nine models with one bay and two rings, which consisted of 32-models without the initial deflection and 27-models with that, were fabricated with machine tools and were tested. The geometrical characteristics of those models are shown in Table 1.
The analyses of shell buckling of those models were carried out by finite element method.
The following results were obtained from the experiments and the analyses.
1) It seems, from comparison of the analytical results with the experiments, that the shell buckling strength of the cylindrical shells with fixed ends will be predicted by the finite element method.
2) In the lower range of the thinness factor λ (that is, 0. 7<λ<1.0) the buckling strength of the cylindrical shells with the initial deflection being one-half to equel size of the shell thicknness were 30% to 40% lower than the buckling strength of the perfect cylindrical shells having same geometrical characteristics, but, in the higher range of λ (λ>2.0), the initial deflection did not apeares to be very important.
3) The cylindrical shells, in the lower range of λ (0.7<λ<0.9), could resist to higher pressure than that at which the shell buckling was caused, and those pressure almost equal to the shell buckling pressure of the perfect cylindrical shells having the same geometrical characteristics.

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General instability is one of the important collapse modes of cylindrical shells reinforced by ring stiffners under uniform pressure. Few research, however, has appeared in this field because of difficulties and complexity of the problem.
Pressure hulls of deep diving submersibles have been designed on the basis of research works which were developed under idealized conditions for boundaries, stress-strain relations, and geometries. Recently, the development of high strength materials and the increase of operating pressure and diameter of pressure hulls have brought a change in proportion of primary structural members of cylindrical ring-reinforced shells, which leads to the review of the design procedure. In order to satisfy the above-mentioned requirements, pressure hulls should be designed on a reasonable basis.
In the present paper, the elasto-plastic analysis of the general instability of ring reinforced cylindrical shells is performed on the basis of the finite element method, and results obtained are compared with collapse tests using two machined models and one welded model, which shows good coincidence with the theory. On the basis of a series of calculation, the authors propose a design criteria for the pressure hulls of deep diving submersibles. Futheremore, they investigate the influence of boundary conditions and initial imperfections on the general instability theoretically.

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The strength of ship structures in collision is an important matter in terms of saving life, preventing pollution and from an economic point of view, especially in the case of hazardous cargo carriers, oil tankers and so on.
The authors investigated the strength of double hulled structures in collision, which were typical of the side structures of hazardous cargo carriers. In the former paper, the types of collisions were classified into five groups. Two types among those five groups were examined ; one was the type where a stem collided against the ship's side, and the other was where a bulbous bow collided against it.
In this paper, another type in which a bulbous bow collided against the bilge part of a ship was examined. Such a type of collision is likely to occur between fully loaded ships, which means that the two ships have the greatest kinetic energy before collision. These three types are considered as the most critical ones among those five groups. Static destruction tests were carried out using simplified and detailed large scale models of a bilge structure representative of those used in double hulled ships, where the bow model was assumed to be rigid.
A simplified method to analyse the damage of bilge structures in collision was developed on the basis of the results from the experiments. The basic concept of it is similar to the former paper ; a bilge structure is considered to consist of two main structural members, that is an outer shell which acts as a membrane and a trans web which supports such a membrane.
The analysis method was examined by comparing the calculated values with the experimental ones. From these, a good correlation emerged.

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In this paper the numerical method on the structural crashworthiness is presented by using the Rigid Bodies-Spring Models (which is abbreviated as the RBSM).
In the design of engineering structures it is important to estimate the energy absorption characteristics by plastic deformation. But it is very difficult to simulate the crushing behaviour precisely by using the conventional numerical methods such as the finite element analysis because of the complexities of the phenomena. In the crushing behaviour of thin-walled steel structures plastic region is localized and the collapse mechanism is similar to the link mechanism which consists of rigid plates and plastic hinge lines. This behaviour is often idealized as rigid-plastic and the rigid-plastic analysis can be used approximately.
In case the collapse modes are known, the crushing behaviour of structures can be simulated by using the RBSM with much less number of degrees of freedom than the conventional finite element method, furthermore, the internal plastic work by the membrane shearing force, which can not be calculated accurately in the rigid-plastic analysis, can be taken into account generally.
In this paper the updated Lagrangian formulation for a flat rigid plate element with the moving local coordinate systems is presented and the crushing behaviour of some typical structural members is numerically simulated. The obtained results for the mean crushing load are compared with the experimental ones. All of the calculations are carried out as the static analysis where the inertial forces and the strain rate effect are neglected.

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A fundamental study on vibration characteristics of the air cushion craft was performed taking dynamic behavior of the pneumatic seal structure into consideration.
A linearized mathematical model of the air cushion craft dynamics was derived. The effect of each parameter on the dynamic stability was obtained analytically and experimentally from the view point of negative resistance excitation system.
The conclusions are as follows :
1) The dynamic stability of pneumatic seal structural vibration depends on the air transmission line characteristics and the seal configuration. The effects of these parameters on the stability criteria of the system are numerically examined.
2) In the model test, the self excited vibration caused by the instability of the pneumatic seal system was observed in some range of the inlet area of fan and feedhole area in the air chamber.
3) The calculated fundamental natural frequency for air cushion craft model agreed very well with model test results. The stable region varied in accordance with feedhole area in the model test, and a similar tendencies were also observed in the numerical analysis.
A further study is necessary to discuss the difference between model test results and numerical analysis results.

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In the conventional thin walled beam theory, transverse sections of a beam are assumed being stiffened rigidly by frames and bulkheads, and our main concerns are longitudinal stresses caused by bending and torsion. On the contrary, considering the beam structure with weak transverse members, which is the case for the hull girder of a pure car carrier, its transverse section sometimes deforms significantly by the shearing effect so-called racking.
Based upon an equivalent truss idealization of shell plating, the hull girder is analyzed as a three-dimensional framed structure. We examine the effects of the structural patterns of large openings of the transverse bulkheads by numerical calculations and experiments. It is found that the racking deformation is effectively restrained by the bulkhead with a diagonal pattern of plating. Numerical values of racking angle are in fairly good agreement with experiments.
Cracking failures caused by racking deformation of the transverse sections are sometimes found at the intersections of transverse girders and frames, and also at the corner of an opening in a partial bulkhead. Having calculated the stress concentration factors at the round intersections of the transverse girders and frames by the finite element method, the depth of the girder and the radius of the round intersection are properly determined in such a manner that the maximum local stresses are within the allowable levels.
Numerical computations are carried out for an actual car/container carrier, whose bulkheads have large openings for the purpose of the vehicle access on decks. The racking load is derived along the line of Classification Notes by DnV, and we assume the rolling angle of 27 degrees under the fully loaded condition. Strength of the intersections of the transverse girders and the local deformation of the openings at the partial bulkhead sections are found to be very sensitive to the racking deformation of the hull girder.

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Jack-up type oil rigs are composed of legs and platforms. The degree of redundancies of legs are generally fewer than those of platforms. As a structure becomes less redundant, reserved strength after the initial yielding tends to decrease. So, it is important to clarify the ultimate strengths of tubular joints which are leading causes of overall failures of the legs.
In the 1st report, the authors carried out theoretical and experimental investigations into the rigidity and local strength of T-joint of circular tube under a concentrated normal load transmitted from the brace. The T-joint model used in the study is composed of a chord reinforced with a built-in center-rib and a brace, and is supported by a leg guide.
In the present paper, they studied the rigidity and local strength of the same T-joint model under both a concentrated normal load and bending moment transmitted from the brace. They performed series of theoretical analyses and experiments on the models.
Main conclusions obtained in this study are shown as follows.
1) The rigidity of the T-joint can be evaluated by formulae developed in consideration of the effects of various sizes of the chord and brace.
2) The ultimate strength interaction curves of the T-joint are presented.
3) The failure modes of the T-joint are clarified.

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Foundation failure has been the main cause of several jack-up accidents ranging from total loss of lives to a varying degree of damage to the structure and the drilling equipments. In addition, it can be anticipated that sea bottom soil will restrain the rotational movement of spud tanks, which means the overturning moment can be partly shared by the sea floor.
This is very important concept in designing the unit and for their safety assessment but little are known on this matter due to very complicated and uncertain behaviour of the units.
The behaviour of soil under spud tanks, however, is extremely complicated depending on soil condition, loading pattern and shape of spud tanks etc..
In this experimental study, authors performed model tests by 4-legged rigid rig model and half-circular footing model.
Several points are confirmed as the followings;
(1) In case of 4-legged platform, the skew direction of horizontal force gives more critical condition than beam direction.
(2) About 25% of overturning moment is shared by reacting moment of soil in case of 4-legged model.
(3) Failure patterns of soil under footing are observed but for inclined load not in good agreement with Hansen's slip line.
(4) Hansen's formula gives conservative estimation of bearing capacity ratio for eccentric and/or inclined load.

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This paper investigates impact loads on the legs of a jack-up rig during the stand-on/off-bottom procedure. The magnitude of the impact loads is theoretically estimated by introducing a simplified simulation derived from the energy concept, which means the decrement of kinematic energy of the rig during bottom contact is considered to be the absorbed energy by soil. The equivalent linearization method is applied in frequency domain analysis to solve the equations of rig's motion with/without bottom contact. The calculation results are compared with the experimental ones to show the validity of present new approach. It is found that present study will be of suitable help to improve conventional design criteria.

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A method is proposed to analyze the forced vibration of jack-up rig using a model made by replacing its legs and platform with equivalent beams and a rigid body, respectively, and connecting them by equivalent springs. The exciting forces by waves and currents are calculated by replacing legs with hydrodynamically equivalent cylinders. The transfer matrix method is applied to analyze the dynamic response of the above mentioned model.
The accuracy of this method is studied by comparing the results calculated by this method with tank test results by 1/45 scale model, then it has become clear that the method in this paper gives a practically sufficient results.

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The present paper deals with the elastic response of the large scale barge type floating structures in short head waves.
The floating structures were approximated with the free-free beam and the displacement responses of structures were expressed by using the principal coordinates and the principal modes consisted of rigid and elastic modes.
The hydrodynamic and wave exciting forces were computed with the source distribution method utilizing the assumption of zero draft body.
The experiments were carried out by using a transient water waves and the three types of elastic beam model.
The numerical results of displacements and bending moments obtained by the present and strip methods were compared with the experimental results and some discussions were made.

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