The minimization of the wave resistance is discussed with respect to the hull form having a bulb at the bow. The main hull is represented by a continuous distribution of doublets and a point doublet is added at the forward end to illustrate the bow bulb. The main hull distribution and the intensity of the bulb doublet are determined in such a way that the wave resistance becomes minimum for a given displacement volume of the hull. Model tests involving the total resistance measurement and the wave pattern analysis are carried out to confirm the validity of the theory.

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The equivalent source singularities distributed over the hull are analysed based upon the exact linear free surface condition. The role of “Line Integral” on the Neumann-Kelvin problem is primarily studied, which since Brard's suggestion calls intensive discussions.
The present analysis shows two kinds of solutions for Wigley model. One is for the surface piercing body. The other is for the limiting case of the submerged body. Comparing the both results the prominent contribution of the line integral term is clarified.
1) “Line Integral” has a serious contribution on the determination of the source of the main hull just beneath the free surface.
2) It brings the better prediction of the wavemaking resistance.
3) “Line Integral” should be taken into account for mathematical consistency of the boundary value problem.

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In the previous papers, the characteristics of directly measured wave resistance, amplitude functions and wave spectra were discussed in comparison with Michell's theory, and some examinations were made how closely the principle of linear superposition of free waves is realized. In the former part of this paper, using the results described in the previous papers, some practical applications are shown for the estimation of wave resistance of ship of given principal particulars, and a method to determine the optimum principal particulars of high speed ships is established. The latter part of this paper is devoted mainly to the investigations of the effect of bow bulb on wave resistance which are examined by the tests of series models with airship type bulbs. Furthermore, a method for applying the results of the series tests to the hull form improvement is considered, according to the principle of linear superposition of free waves, which is confirmed by the experimental fact.

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The distortion and the outward convection of the bow wave pattern due to the disturbance velocity of the ship hull itself is studied based upon the Ursell's wave trace theory.
The flow line of wave energy for each elementary wave is traced and local changes of both the wave length and the wave crest line are calculated to give eventually the Kelvin's ship wave pattern.
The local variation in wave amplitudes is considered in relation to the conservation law of wave energy.
Results are applied to the analysis of the bow wave pattern of the car ferry “SHIRETOKO” at F_n=0.258.
The analysis indicates that;
(1) Subject to the local disturbance velocity of the existing hull, each elementary wave propagates with convection changing its flow direction and amplitude.
(2) The Kelvin pattern calculated on the double model flow yields the deformed but more realistic wave feature.
(3) The analyzed bow wave pattern of “SHIRETOKO” gives a good agreement with the measured.
(4) The present analysis suggests the necessity of introducing the local dependencies of the wave phase and amplitude. The current wave analysis assuming the linear propagation of ship waves must be reexamined.

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Instability of stern flow in the coexistence of 2-dimensional separated eddies and bilge vortices are observed using the two 2m full model ships both of which have the same vertical framelines, but one with usual round bilge M 30 R, and the another with sharp edged bilge M 30 S.
The three velocity components are measured in the rear of M 30 5 at four sections to study the diffusive process of bilge vortices, and evaluate the induce drag.
Wake survey and resistance tests are performed and viscous resistance components are compared.
(1) Instability or queer stable state seems to occur on stern flow with interaction between 2-dimensional separated eddies and bilge vortices.
(2) Hydrodynamical properties of strong bilge vortices have been studied. Path of vortex center, vorticity distribution, slight decay of circulation, velocity profiles of vortex core, diffusion of vorticity and induced drag were analyzed.
(3) There seems no breakdown in case of strong bilge vortices in a certain range.
(4) Viscous resistance obtained from wake survey at 0. 5 L aft of AP gave a good agreement with that from resistance test.

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To know the velocity distributions in ship wake is one of the most essential problems in ship design. Here a method is proposed to estimate them and some results of numerical calculations are shown compared with those of experiments.
The velocity is assumed to consist of the potential and the viscous components. The former is calculated from the source distributions which are obtained from the wave analysis and the hull surface condition. The latter is calculated as induced velocity of vorticity distributed in the two. regions, one is the boundary layer on ship surface and the other is the ship wake. For the boundary layer, the three dimensional turbulent boundary layer calculations are carried out and the vorticity distribution and the separation position are determined. On the other hand, the vorticity in ship wake is assumed that it is diffused from the boundary layer which can be estimated by solving the diffusion equation.
The estimated vorticity and velocity distributions show fairly well agreements with those of measurements. As an application of this method, the viscous resistance is calculated from the estimated velocity distributions, which shows a little smaller resistance coefficients than those of the wake survey.

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Three-dimensional turbulent boundary layer equation is expanded according to the ordinary perturbation method with the flat plate flow as the zeroth order solution. The momentum thickness, the local skin friction coefficient and the cross flow angle at the wall including the first order term effects are compared with the measured values on a ship model in acceptable agreement. As applications of this first order solution, an approximate formula to the frictional form factor K_F is obtained and the scale effect of the velocity and vorticity distribution in the boundary layer over ship hulls is discussed.

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This paper presents a new device for the measurement of local shear stress, based on floating-element technique, on a three-dimensional body surface submerged in water. Improvements are made for the new meter to eliminate the flow passing through the gap around its element completely, to reduce the force due to the pressure acting on the side of the floating element, and to satisfy approximately the no-slip condition for the flow at the gap. The measurements of local shear stress are carried out on the surfaces of a flat plate, a two-dimensional body and a streamlined body. The experimental results agree very well with the calculated ones, which shows that the device determines the wall shear stress of a submerged three-dimensional body accurately.

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With increase of fullness of ship, the self-propulsion test is becoming more and more difficult because of many causes such as flow instability and decrease of measured quantities, etc. So each model tank is improving its own method of the test.
In this paper, the authors present a theoretical approach to the self-propulsion test of full ship, which utilizes measured hull resistance and wake data. The full ship adopted here consists of the actual ship form and the infinitely bladed propeller. One side of the after body surface of the ship hull is divided into 180 (12×15) platelets. Solving the hull surface boundary condition by Hess-Smith's method, the densities of platelet sources are determined. On the other hand, the propeller plane is divided into 180 (5×36) sectors and the strength of bound vortex is assumed to be constant in each sector. Solving the propeller plane boundary condition using the wake data, the bound vortex are determined. From the source and vortex distributions, the thrust and torque-of the propeller and the increase of the hull resistance are calculated. Then the thrust of the propeller and the resistance of the hull are made to be in agreement by iteration procedure and at the end the self-propulsion factors are obtained by the thrust-identity method using the results of open water tests on the propeller. Here the contraction of flow due to propeller suction is taken into consideration in evaluating the nominal wake distribution for the full load condition. The presence of the rudder and the viscous part of the thrust deduction and the wavemaking phenomena are ignored in the calculation. The obtained results seem to be reasonable.

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For studying the mechanism of the inception of cavitation, especially the inception of cavitation by stream bubble nuclei, it is important to know and/or to control the size of bubbles contained in water.
It is found out that the minute bubbles can be filtered as if they are solids by special filter paper under moderate pressure difference. Using this filtering method for controling the size of bubbles in water, the relation between the size of bubble and the critical pressure of inception of cavitation of water in a glass nozzle is studied and it is found out that this relation is shown by a simple equation introduced by a theory of mechanically critical balance at the interface of the gas and the liquid of a bubble. Then using this theory the distribution of bubble diameter in water is measured. It is found that the degassed water does not have always small number and small size of bubbles, i.e., the air content in water has no interrelationship with the number and/or the size of bubbles.

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In this paper the pressure distributions on partially cavitating three dimensional wings are calculated and measured. In calculating the pressure distribution Hanaoka's linearised lifting-line. theory has been developed and the coupled integral equations have been practically solved for the elliptical and the rectangular wing with the circular ogival profile. In addition, the iterative. method for calculation of lifting-surface using Prandtl's lifting-line theory has been improved and satisfactory agreements with the calculated results by the existing lifting-surface theory are; obtained. Numerical results for the pressure distribution on the sub-cavitating finite wings can explain well the experimental results. And the result that the calculated values of derivative of cavity profiles are nearly zero is found independently of the given shape of the planform of finite wings. The advantage of this method over the other specious method lies in its ability to estimate the pressure distribution, cavity profile and lift coefficient etc. theoretically.

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This report proposes an approximate calculus for the evaluation of the propeller-induced pressure fluctuation on practical hull forms. The basic integral equation of the present method is deduced from hydrodynamic assumptions of the pressure field about a hull and a propeller.
The potential flow method that has been developed in the past to evaluate the surface force is not well suited for practical hull forms, because the evaluation of the propeller-induced velocity is not easy. In contrast, the present method, as applied to the surface force, requires only the evaluation of the propeller-induced pressure, which can be done more simply than that of the propeller-induced velocity, to make it applicable more conveniently for practical hull forms than the potential flow method.
This is demonstrated for a practical hull-propeller combination by means of some numerical results evaluated by the present method, which are in good accord with experimental results.

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This paper is concerned with the development of a method for calculating the hydrodynamic characteristics of marine propellers in a viscous fluid.
Firstly, the characteristics in a non-viscous fluid are calculated by applying the propeller lifting surface theory and at each propeller section, a so-called corresponding two-dimensional wing which has the same hydrodynamic property and geometrical thickness distribution as the corresponding propeller section, is derived from the calculated circulation distribution. Then circulations corrected by the effects of viscosity and drag coefficients of propeller sections are obtained by using calculated boundary layer properties of the corresponding two-dimensional wings. Finally, the propeller characteristics in a viscous fluid can be obtained from these circulations and drag coefficients.
Calculated results of propeller performances are compared with open-water test results and agreement between calculation and experiment is good. Also scale effects of propeller performance are examined by this method.

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Results of model experiments on an ore carrier are presented to investigate hydrodynamic impact upon the hull surface in two-directional irregular waves. Short term analysis of peak values of impulsive pressures indicates that the expected peak value distributions of the pressures are well approximated by Rayleigh distributions.
Distributions of impulsive pressure peaks over the stem and the hull side above water are examined. Effects of ship speed, course angle and mean wave period of irregular waves on impulsive pressures are also discussed.

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The ship maneuverability has been described with the first order linear equation or the second order linear equation.
However, we are under the necessity of the adoption of the second order equation with a nonlinear term as the mathematical model of the ship maneuverability, especially in case of full ships.
This mathematical model is expressed by
T₁T₂ψ+ (T₁+T₂) ψ+ψ+αψ³=K (δ-δ_m) +KT₃δ
This paper relates a new approach to the parameter identification of this mathematical model.
The authors propose the adaptation of the parameter optimization method to the above problem. The outline of the method may be described as follows ; the response of mathematical model to which the values of parameters are provisionally given is compared with the measured response of real ship, then the values of parameters are corrected with the particular criterion, and these procedures are repeated.
In this paper, the result of analysis applying this method to the motion of real ship is described.
Moreover, the authors wish to investigate the applicability of the second order non-linear equation to the ship maneuverability.

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In the previous report, the authors proposed a new procedure of analysing zig-zag test, by which it is possible to identify the parameters of the second-order mathematical model, i. e.
(L/V) ²T₁'T₂'ψ+ (L/V) (T₁'+T₂') ψ+ψ+ (L/V) ²α'ψ³= (V/L) K'δ+K'T₃'δ
In the present report, a new method is proposed, to predict the various manoeuvring test results from the parameters of only one zig-zag test; the results of the spiral test, reverse spiral test, modefied zig-zag test, yaw-rate zig-zag test, course change test and so on can be obtained by numerical calculations.
Next, we make also some considerations about the speed drop effect on the results of the zig-zag test, and propose a method to analyse the zig-zag test, taking account of it.
Lastly, we make mention of the wind effect on manoeuvring tests.

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The circular motion test technique (CMT) is introduced in this paper, which was developed for the captive maneuverability model test at the Maneuvering Tank of the University of Tokyo.
This test makes fully use of the X-Y carriage system at the Maneuvering. Tank. The.X-and Y-carriages are controlled by digital computer along the prescribed circles. The model rotating device which is attached to the Y-carriage controls the direction of a model to keep the drift angle constant. In this way, the CMT technique is completely equivalent to the rotating arm, test. The most important particular of this system is that the very large radius of circle can be get easily in a 30 × 50 m water surface which was practically impossible in the conventional rotating arm test.
A trial run of this system was carried out with a 3 m Series 60 model. The obtained results coincide fairly well with the results of Eda and Crane. It can be convinced that the CMT will be a standard captive model test technique in our laboratory after several improvements in the. carriage control and data acquisition systems.
The harmonic motion test (HMT) and the transient motion test (TMT) are also going to be developed. HMT and TMT tow a model along the path which corresponds to the sinusoidal velocity and a random path respectively. As the HMT technique is equivalent to the very large amplitude PMM (30 m), it will be possible to get data in between of the conventional rotating arm test and PMM test.

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In this paper unusual phenomena in manoeuvring quality of full ships which are characterized by unusual course stability in small rudder angles are treated in relation to the unstable phenomena in propulsion tests.
Manoeuvring tests as well as propulsion tests were conducted on a model which is liable to both phenomena. Effect of fins near the water surface was also investigated which were designed to prevent the downward stream and intermittent air drawing before the propeller.
As a result, followings were found showing interrelation between both phenomena.
(1) Unusual turning moment in manoeuvring motions is considered to be due to lateral force accompanied with unstable phenomena.
(2) Both unusual phenomena and unstable phenomena are induced by action of propeller.
(3) By fitting appropriate stern fins, unusual phenomena and unstable phenomena can be suppressed.

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In the initial design of ships, a convenient method to estimate the maximum velocity of wind in which a ship is manoeuverable and a method to adjust the results of the turning trials are required. In order to establish them, the authors investigated the motions of a ship in wind by solving the nonlinear equations of motion of a ship, which involved the experimental wind resistance data such as the equation (1), by using a digital computer.
In this report, the author investigated the course stability and the method to estimate the freely manoeuverable range of the wind velocity, and came to the following conclusions.
(1) A ship whose course keeping quality is unstable is made stable in the range of the wind velocity showed by (13) and the wind direction between the maximum C_FN and the maximum ∂C_FN/∂α.
(2) In the aft and quarter wind a ship becomes unstable, and in the head and bow wind the.ship whose lateral projected area above the water line is large becomes unstable.
(3) The unstable equilibrium condition becomes stable by the steering in proportion to the heading angle φ, that is, δ_r=δ_r0-K₂ (φ-φ₁), and the degree of the instability can be showed by the minimum of the proportional constant K₂.
(4) The centroid of the lateral projection above the water line is drawing toward the bow, the limited manoeuverable range grows wide, however, the instability grows great in the bow and head wind, but decreases in the quarter and aft wind.

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The purpose of this research is to study the free surface effects on the side force and the yaw moment acting on a ship hull advancing with a steady drift angle.
First, on the assumptions of thin ship, small drift angle and low wave height, the problem was divided into a symmetric and an antisymmetric ones. Since the symmetric term had no effects on this problem, for the antisymmetric problem, approximate formulas to estimate the free surface effects were obtained on the assumption of slenderness.
Next, by means of model experiment, free surface effects were examined, and it was ascertained that the above approximate formulas could explain the experiment results on the whole.

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We have discussed the steady turning motion of the towed ships system in this Journal No. 137. In this paper, we discussed again the turning of the towed ships system by the parameters as used in No. 137 and the additional parameter κ (radius of gyration of ship), the results are as follows :
1. By steered the rudder of the tugboat, the effects of the fore-towing point, towing ropes' length and the towed ships to the values ofA_d0, T_r0, T_ad0 and R_e0 are small and it can be neglected, and those values are given in equation (7).
2. The larger the rudder angle and the rudder area ratio of the tugboat are the smaller the values of A_d, T_r and T_ad are.
3. As the rudder angle and rudder area of the tugboat are constant, a). the values of A_d, T_r and T_ad decrease, when the number of towed ship increases, b). because of the increase of the radius of steady turning, the values of A_d, T_r, T_ad become larger, when the the towing points are placed apart from the centre of gravity of the towed ship, except the last towed ship, c). the effects of the derivatives of towed ships are included in the parameter it as given in equation (4),
d). the effect of radius of gyration of tugboat itself is large, when the value of radius of gyration of tugboat increase, the values of A_d, T_r and T_ad grow large, and the effect of radius of gyration of towed ship is small and negligible.

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In recent years, casualties at sea have occurred frequently due to the overcrowding of sea traffic, especially on major routes or in harbors, and to the increase in ship size. This has become a serious social problem not only as regards disasters at sea but also in view of environmental pollution. Under such circumstances, a ship maneuvering simulator for the research into the safe sea traffic or the training of crew has stood keenly in need.
The authors have constructed a visual maneuvering simulator which can recreate the similar conditions as appearing to a navigator on board a ship. The most sophisticated factor from a technical point of view is to produce a visual display system which can project the prospect looked over from a wheel-house and a target ship in motion as they actually appear. This can be accomplished only by integrating the following three factors; (1) a simulation program which can compute the precise motions of ships of any type, together with the influence of waves, winds, currents, etc., (2) an optical instrument system which can display a variety of changing images on a screen and (3) the servo control system which connects the former two in a. high degree of precision and controls with dead slow action in concert with the ship motion.
A whole aspect of the simulator is outlined herein with some examples of its application.

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This paper deals with the most basic and fundamental problems, the motion analysis of an unmanned submersible for underwater exploration. Some shape parameters derived from the analytical results for designing the vessel to be automatically controlled are also considered.
The analysis is based upon the slender body theory for obtaining the linearized characteristics of the motion. The shape parameters related to the motion and control of the vessel are extracted, discussing optimum relationship in order to design the control system to be stable and controllable. Finally, some results of experiment are shown to verify the theoretical characteristics of the parameters to be valid.

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Observations on a ship at sea carried out to verify the usefulness of a method of analysis developed for the safety assessment of LNG carriers.
The ship is 60 m in length between perpendiculars and has two LNG tanks (spherical and prismatic).
In the observations, various kinds of dynamic responses such as ship motions, pressures on tanks, stresses on tanks and their supports etc. totaling about 60 channels are measured simultaneously. Besides them, wave is measured using a wave buoy with a pressure transducer.
For the first report of the observations, this paper deals with measurments and analyses of wave data, and evaluation of wave spectra dfrom measured ship responses.
The wave spectra evaluated from measured ship responses show better agreement with measured ones in head seas than in following seas. The errors of the evaluated wave heights and wave periods are within 20% mostly.

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In these years off-shore structures are built with various purposes. It is important problem to study about their mooring system. And this study is also important for ships. In this paper it is aimed to study the tension of mooring lines.
Mooring tension was formerly calculated by what is called “catenary theory”, but it is calculated here by “corrected theory”, which includes the effect of elasticity of the mooring line. Using this theory, the tension is calculated not only in ordinary state but in severe surroundings of wave, wind and current. And in this paper, there are mentioned some practical applications and results of experiments as follows ;
I. Calculating formulas are composed for various situations. The result shows good agreement with experimental one. Calculating formulas are also made in case that a sinker or a buoy is installed in the middle of the mooring line.
II. The tension calculated by these formulas is applied to analysis of the motion of a moored body. A simple one of two dimensional model is used, that is semi-submerged circular cylinder. The motion of the moored body and the tension of mooring lines are calculated by numerical integral methods. Experiment is carried out under same conditions. Comparison of these results is fairly satisfactory.
III. The dynamical effect and the similarity law of the mooring line are considered and a few conclusions are shown.

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In the previous paper, the authors investigated the relationship between the ship motion and the critical frequency of the side wall effect on the longitudinal ship motions by shifting a model from the center line of the tank to a tank side wall.
In this paper, the authors applied this model setting method, called Shift in the previons paper, to the test for the longitudinal ship motions of catamaran.
Two kinds of experiments were carried out in this paper. One was the forced oscillation test, Pure Heave, for the catamaran model whose distance between twin hulls was 2 P. The other was the forced oscillation tests, Pure Heave and Pure Pitch, and the longitudinal ship motion test in waves for one of the twin hulls of catamaran, which was set at a distance P from the tank side wall.
Beside the two kinds of experiments above mentioned, the usual forced oscillation tests, called Center in the previous paper, were also carrid out.
Then the comparison between the results obtained from the two kinds of experiments was made with respect to the transfer functions and the coefficients of the equation of the motion.
The authors could certify that there was hardly difference between the results obtained from the two kinds of experiments, called Catamaran and Shift in this paper.

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To study the longitudinal motion of the submerged body, the fundamental equations of motion and kinematic relationships of the motion coordinates were established, and then by use of proper assumptions, the equations of motion were linealized to obtain equations amenable to analytic solution by perturbation methods.
The characteristics of the eigen values of the equations of motion of the typical model due to the changes in Froude number, and proper stability derivatives were discussed by the use of Root Locus Method and the results of the solution by the digital computer.

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The critical speed of a high speed container ship in irregular waves is investigated theoretically by strip method and statistical calculations. In this paper, “deliberate loss of speed” is mainly discussed in details.
The purpose of this study is the followings :
(1) To find the best way of shiphandling in rough sea.
(2) To inquire into the ship forms which have better seakeeping qualities.
Seven factors of seaworthiness are discussed :
(1) Vertical Acceleration at F. P.
(2) Deck Wetness at F. P.
(3) Slamming
(4) Vertical Wave Bending Moment
(5) Propeller Racing
(6) Lateral Acceleration at S. S. 8¹/2
(7) Rolling
The critical values and probabilities of these factors are checked, and the maximum ship speed, . below which a ship safely navigates across the rough sea, is determined.
These analyses are possible for all kinds of ships, and they will be usefull not only for ship-handlers but also for ship designers who are concerned about seakeeping qualities of ships.

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This paper studies the statistical prediction of the expectations of the extreme values in some dynamical systems which have non-linear and non-memory elements, as seen in the problem of the structural strength of the ship among the irregular waves.
In case of a single input, when number of observations is large enough, the expectation of the extreme value of the output can be obtained by substituting the extreme value of the input into the response function.
In case of multiple inputs, it is difficult to obtain the generality. In this paper, two cases are studied for the impulsive pressures which act upon the fore part of the ship.

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In the previous paper, an analytical method for periodic response of floating framed structures subjected to regular waves was presented, where flexibilities of structural members were included in the motion equations so that the effects of flexibilities pn the motions could be calculated and also that member stresses could be evaluated with good accuracy.
This paper is a successive issue to the previous one. By comparing calculations with experiments of several kinds of foundamental models, the relation between motions and member stresses was made clear.
Moreover, the method was extended to make it possible to analyze floating framed structures with pins and rollers.

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The statistical predictions of vertical wave bending moment were made for a gigantic oil tanker of 310 meters length and the similar ships of different lengths, in order to find out the long-term trends of vertical wave bending moment and to examine the design values of those.
Firstly, the response functions of vertical wave bending moment were calculated by the aid of the modified strip theory. Then, the standard deviations of vertical wave bending moment for different short-term sea conditions were evaluated based upon the linear superposition theory, by using the calculated response functions and the formulated wave spectra for given sea conditions. Finally, the long-term predictions of vertical wave bending moment were determined by using the obtained standard deviations of vertical wave bending moment for the individual short-term sea conditions and the wave statistics of the North Atlantic Ocean.
According to the systematic prediction works, the longitudinal distributions of vertical wave bending moment predicted with the exceeding probability of 10^-8 are investigated in relation to the ship speed and heading angle, and the predicted results were compared with the design values of vertical wave bending moment given by the different classification societies.

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Recently, several papers have been presented with respect to the behavior of iron are pellets in cargo holds of are carriers. However, the property of the behavior of are pellets is so complicated that studies from various points of view are still thought to be necessary and effective to establish a practical procedure to compute the are pressure in cargo holds.
In this paper, the behavior of the are pellets in a hold is experimentally studied by means of two large scaled models of hulls and holds of two different are carriers. As for the essential property of the stress-strain relations of are pellets, a series of triaxial compression tests are carried out. A rather simple equation to calculate the are pressure distribution in a hold after loading is derived using the earth pressure coefficient at rest which is determined according to the results of the triaxial compression tests. A set of nonlinear elastic springs whose characteristics are determined by the triaxial compression tests are introduced as a substitute for the are pellets to calculate the interaction between the hull and the pellets. They are applied to calculate the increase of the are pressure induced by wave loads on the side shells, where the hull is replaced by a frame structure.

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This is the first report of our research about the estimation of the ultimate strength of wing tank structures of tankers and ore carriers under wave pressures.
A transverse ring of the wing tank structure with a single strut is approximated as a simple frame structure. The collapsing phenomena of this frame structure are analysed and its collapse loads are calculated under the various values of the factor which have influence upon them, such as the dimensions of frames, the local buckling strength and the full plastic moment of the section of frames under bending force, the buckling strength of strut under axial force, the support condition of full structure, ore pressure and so forth.
Fairly good agreement is found between the calculated values of collapse loads and the measured ones obtained by the experiments carried out recently by Shipbuilding Research Association of Japan.

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In a previous paper, two of the authors have developed an effective method for the ultimate strength analysis of large size structures. The method is named as “Idealized Structural Unit Method”.
In the method, a large size element with idealized nonlinear character was necessary and an example element, “Girder Element”, was developed. In this paper, this method is applied to double bottom structures. The state of two-dimensional stress in the tank top and the bottom shell plating is considered. Conditions for buckling of these plates are established and its post buckling stiffness is determined. The condition for their ultimate strength and stiffness at the ultimate strength are determined in connection with the girder element.
An example structure is analysed and the results of the analysis are presented.

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This paper deals with the developments of FEM buckling analysis computer code for hull structures.
Basic theorem is firstly described for stress analysis of three-dimensional plate structures. The method which covers bifurcation buckling analysis in elasto-plastic range is also described.
Some examples of analysis including transverse ring gunwale corner of 240 KDWT tanker show the capability and usefulness of the present computer code.

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Seven curved corner models of transverse frame were tested and the process to the buckling collapse was experimentally traced to know the effect of plate thickness, dimension of face plate and stiffener on the compression strength.
The condition to take the critical strength at which the increment of deformation of the structure arises abruptly and the function as structural member can not be maintained, is introduced and the experimental values to obtain the critical strength and load-carrying capacity are shown according to the classification of the collapse mode.

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Generally speaking, the bigger hull blocks become, the larger grows the possibility of local break or buckling during turnover and transfer by means of cranes.
Prior to constructing our first ULCC of 400, 000 D. W. T., authors calculated the stress distribution in 430-ton deck block which was assumed to have the probability of local damages.
The deck block was substituted by a plain frame structure for the convenience of calculation which was carried out by our computer program STAN II.
In accordance with the above results and our long experience regarding VLCC, authors reinforced some local structural members of the deck block and measured it's actual stress distribution during turnover.
As the result, we found little difference between calculation and measurement.

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In addition to ordinary vibration study in conventional ships, study on the following vibrations is necessary in Moss type LNG carrier in order to avoid damages in structures and equipments caused by critical vibration as a result of resonance of structures with blade frequency.
1) Vibration of tank system including spherical tank, skirt and foundation deck.
2) Vibration of spherical tank, i. e., spherical shell.
3) Vibration of tank cover, dome at upper part of spherical tank, pipe tower in spherical tank, etc.
The natural frequency of the vibration in item 3) can be easily estimated, and it can be changed by small modification of design, and that of the spherical tank vibration in item 2) was already obtained by our detailed study so far.
As for the vibration of the tank system in item 1), on the other hand, it seems that there is not any established estimating method of the natural frequency and the vibration response.
The present papers deal with the study on the vibration of the tank system for a 128, 600 m³ Moss type LNG carrier which we will builds.
First, we have made a theoretical calculation on an ideal mathematical model considering coupled vibration of tank system and hull. In order to verify the calculation method, we have carried out vibration experiment using a 1/20 scale model ship, and the experimental results were compared with the theoretical calculation on the 1/20 scale model, and reasonable agreement has been confirmed. Then, using the calculation method, we have estimated actual ship's natural frequency and vibration response, and studied a possibility of resonance of the tank system vibration with blade frequency and a relation between the vibration acceleration of tank and blade frequency.By these studies, it has become possible to determine the number of propeller blade.

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The paper reports on HICASS-H, a computer aided system for hull structure design, which is developed as a subsystem of previously presented HICASS.
The system deals with consistently the hull structural information ranging from design to production line.
The operation starts with lines fairing and is followed by definition of seams and longitudinals on the shell then successively by disposition of internal hull structures. Most of data, input or generated during these operations, are kept in/taken out of the data base which is one of the key elements of the system. So as to support this, a number of commands, in the form of usual terms, are provided for the convenience of communication between designers and the data base.
With a view to the fact that the design process is subjected to fairly frequent trial and error and that the data increase and become full gradually as the design proceeds, the data base is so schemed as to face up to such circumstances.
Various drawings are taken out of the data base at designer's option. It is also utilized for the development calculation of various curved surfaces as well as the manipulation of NC data including those of the secondary members such as brackets, stiffeners etc.

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In the preliminary design stage of ships, the ship's characteristics are estimated by the statistic correlation between particulars. This situation is the same in the case where the computer aided design techniques are adopted. Therefore it is very important to have the reliable correlation data of particulars with the rational background. This can only be made by the reasonable classification of ships.
The classification of ships has usually been made according to the mission of ships, cargo boats, bulk carriers and so on. However, there would be another method to classify ships which provides us with more reliable statistic correlation depending on the specified purposes.
The cluster analysis and the principal component analysis were adopted in this paper for the data set of 628 existing ships.
The grouping of ships was made automatically by the cluster analysis connecting a group to the nearest neighbour. The obtained results were displayed on the two dimmensional plane which was spanned by the first and second components.
The classification of ships obtained by this method were as follows :
1. Car ferry boats
2. Container ships and cargo liners
3. Cargo boats and bulk carriers
4. Medium size bulk carriers and are carriers
5. Very large tankers and combination carriers
These results seem to be a conventional classification. However, the characteristics of these group can be explained very well and the boundary of two groups were made fairly clear by this method.

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HICASS-P covers a broad range of piping from design to production line, enabling the designer to execute his work faster and more accurately. The system can be applied to any pipe line on, any type of ship.
HICASS-P provides a lot of advantages as summarized below :
(1) Easiness to Use
(2) Broad Applicability
(3) Unequalled Data Management Capability
(4) Automatic Joint Positioning
(5) Interactive Input System
(6) Effective Drafting Capability for Piping Arrangement
(7) Furnishing the Piece Drawings with comprehensive Information
(8) Combination with Production Control Systems
The present paper reports the system features, especially (3) (6) of the above.

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Two kinds of tests were carried out using aluminum alloy 5083-O plate and butt weldments at several temperatures : One is the tensile test and the other is the fatigue test.
Tensile tests were conducted on solid cylinder specimens of 10 mm in diameter at several temperatures ranging from 10°C to-196°C. The tests revealed that there was the following relationship between ultimate strength (S_u) and absolute temperature (T);
log (S_u) =A+B/T
where A is a constant dependent on specimen type and B is a material constant.
On the other hand, the fatigue tests were carried out at 10°C and-170°C on the 35 mm thick test specimens : base metal, butt welded joint without reinforcement and butt welded joint with reinforcement. The results showed that the fatigue strength in terms of nominal stress range (?S) could be expressed by the following formula ;
log (?S) =log (C)-klog (N_f)
where k is a constant, C is a constant dependent on specimen type and temperature, and N_f s the number of cycles to failure.
Through these experiments, it has been found that there is a linear relationship between C and ultimate strength (S_u). And the following empirical formula for normarized stress range (?S/S_u) was obtained ;
log (?S/S_u) =0.612-0.16 log (N_f)
where ?S is the actual stress range in consideration of strain concentration due to the weld toe.
This means that fatigue strength at any temperature can easily be evaluated by conducting tensile test for specimens at a temperature under consideration together with relatively easy fatigue test at room temperature.

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Deflection controlled reversed bending fatigue tests in low cycle fatigue range were carried out at ambient temperature and at-196°C on five 9% Ni steels of ASTM A553 grade A and transverse butt joint specimens made by MIG arc welding and manual arc welding. In addition, notch effects were investigated by introducing a surface notch of the elastic stress concentration factor of approximately 3 into the test section of both base and weld metals. Cyclic amplitude of surface strain at the test section, which was continuously measured by a plastic-range type strain 'gauge at ambient temperature and-196°C, showed a stationary behaviour after a number of imposed cycles corresponding to a few percent of crack initiation life. This stationary strain amplitude ea is used for the evaluation of low cycle fatigue strength which is plotted against number of cycles to crack initiation N_c. In case where measurement of surface strain amplitude failed, this was estimated from a dynamic calibration curve of ε_a versus bending deflection.
The relation between ε_a and N_c was expressed by a modified Stromeyer's formula, providing any given percent probability curve for crack initiation basis. Notch effects are observed at room and low temperatures by showing approximately 1. 7 of fatigue strength reduction factor, while weld metals are insensitive to the effects. Fatigue strength is generally higher at-196 °C than at ambient temperature. Weld metal shows lower fatigue strength than base metal in the whole life range.

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In our previous paper, a cummulative damage law for the prediction of fatigue life was proposed and in this report, to study a effect of mean stress and a fatigue strength reduction factor, the cummulative damage law was applied to plane, notched and butt welded specimens under axial loaded condition.
σ_a-σ_m diagram and σ_-1/σ₀-N diagram were obtained by the cummulative damage law and experimental results, where σ_a, σ_m are stress amplitude and mean stress respectively, σ_-1, σ₀ are fatigue strength with stress ratio-1 and 0 respectively, and N is number of cycles.
σ_a-σ_m diagram is expressed by two straight line segments σ_-1-σ₀ and σ₀-σ_B. Relation between σ_-1/σ₀and N is expressed the form of σ_-1/σ₀=α·N^β, and constants α, β are as follows.
for plane specimen ; α=2, β=-0.0353, 10⁰≤N≤2×10⁶
for notched and butt welded specimens ; α=2, β=-0.0753, 10⁰≤N≤10⁴
α=1, β=0, 10⁴<N
Using the above σ_a-σ_m diagram, σ_-1/σ₀-N diagram and one S-N diagram, S-N diagrams with arbitary mean stress or stress ratio can be obtained, and from these S-N diagrams, fatigue strength reduction factor may be estimated.

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For welded ship hull structural members, fatigue strength determined through fatigue test with their scale models and that determined through fatigue test with test specimens of welded joints often are at variance. Accordingly, to find out a clue to more accurate fatigue design of ship hull structural members, the author made a comparative study of fatigue data obtained by the two different specimens or models. During the course of their study, the author (1) established a definition of fatigue crack initiation in scale models, (2) examined how cracks would initiate in certain parts of scale models and affect the behavior of those cracks which later initiate in other parts, (3) determined characteristics of S-N diagram drawn for scale models, (4) compared standard fatigue strength specified for welded joints in steelworks other than ship hull structural members and fatigue strength determined by scale models, (5) examined the manners in which fatigue cracks propagate in scale models and simple shaped test specimens, respectively, and (6) determined the size effect of the scale model on the propagation of fatigue crack.

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An application of Bayesian reliability theory to fatigue design of ship structural members was discussed. The proposed method can make use of the past data on the failed members to chose an appropriate design stress through the posterior probability of the critical cumulative (Miner) damage. The numerical examples showed that this method is satisfactory if the actual cumulative damage at failure distributes in a small range and the shift of S-N relation is equivalent to decrease or increase of the critical damage giving nearly constant probability of failure. This means the proper value of the critical damage should be chosen relatively to S-N relation used.

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The welding material for 9 % Ni steel is usually face centered cubic Ni-bace alloy, the yield strength and Young's modulus being considerably lower than those of steels. Welded joints of 9 Ni steel are usually soft welded joints ; the stress for the brittle fracture initiation of the joints, whose HAZ has a crack, is influenced distinctly by the mechanical properties, aside from the metallurgical properties. The linear fracture mechanics, therefore, can not be applied to btittle fracture initiation of the soft welded joint.
A modified BCS model was introduced in this paper ; the COD of a crack in the vicinity of the soft weld metal was expressed quantitatively by using this model.
Deep notch tests of the HAZ in 9 % Ni steel welded joints were carried out by using specimens which have several notch lengths. The application of the modified BCS model to the experimental results showed that COD-concept could be applied to the brittle fracture initiation of the soft welded joint. Therefore, size effects on the brittle fracture initiation of the soft welded joint was expressed quantitatively. The experimental results also showed that the fracture stress of base metal which has a crack in the vicinity of the soft welded joint is lower than that of base metal free from the effect of the welding. These facts were also explained quantitatively by the modified BCS model.

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In the welded joint having the pre-existing defect, the crack tip is subjected to thermal-strain cycle by post-welding. This strain cycle has effect upon the mechanical properties, especially upon fracture toughness. Consequently brittle fracture behaviors of the welded joint are influenced.considerably by this strain cycle.
Wells-Kihara Test is carried out comparing so-called “Before Weld Notch” with “After Weld Notch”. Brittle fracture initiation temperature of BWN specimens is much higher than that of AWN ones. It is clarified by FEM analysis that the concentration of plastic strain during welding at the crack tip is occurred in BWN specimen. This hot-straining causes considerable deterioration of fracture toughness. BWN specimen shows a considerable susceptibility to effects of hot-straining embrittlement. The influence of this hot-straining on fracture toughness can be estimated quantitatively by COD Bending Test.

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The variation of stress intensity factors along the crack front of a crack penetrating through the thickness of a plate is analysed by the finite-element method on the basis of concept of the superposition of analytic and finite-element solutions, which has been successfully employed for two-dimensional and axi-symmetric three-dimensional crack problems.
In three-dimensional problems, difficulties exist in finding analytic solutions of the governing differential equations, which have the singularity for cracks. In the present paper the authors propose a method of constructing appropriate series of analytic solution which does not satisfy the governing equation but can easily approximate the variation of stress intensity factors.
As a numerical example the stress intensity factor of a compact tension specimen is calculated. It is shown that the stress intensity factor takes the maximum on the middle-surface, which is 7-15% higher than the two-dimensional value.

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A series of research so far carried out, has clarified that among various kinds of multiple notches existing in steel structures, the collinear notch has the largest influence upon brittle fracture initiations. In this research, therefore, the collinear notch is taken up as the subject of study, and examination is made on deformation behaviors ahead of notch tip and on fracture characteristics by elastic-plastic analysis based on the finite element method (FEM) and by experiments.
The results are summarized as follows :
(1) A crack opening displacement (COD) obtained by FEM is approximately equal to the theoretical value by E. Smith and a measured value by Moiré method, which enables the coefficient of interaction effects of notches to be estimated by FEM.
(2) When notches are contiguous very closely to each other, fracture stress is decreased and at the same time the critical COD is decreased. Therefore the concept of COD is not simply applicable to that case. The reason for the occurrence of such phenomenon is clarified through examination of the relation between the plastic strain in notch tip zone and the COD which are respectively calculated by FEM.

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