Showing papers in "International shipbuilding progress in 1975"

Further computer analyzed data of the Wageningen B-screw series

Abstract: In this paper the open-water characteristics of the Wageningen B-series propellers are given in polynomials for use in preliminary ship design studies by means of a computer. These polynomials were obtained with the aid of a multiple regression analysis of the original open-water test data of the 120 propeller models comprising the B-series. All test data was corrected for Reynolds effects by means of an equivalent profile method developed by Lerbs. For this Reynolds number effect additional polynomials are given. Criteria are included to facilitate the choice of expanded blade area and blade thickness. Finally, a number of new type of diagrams are given with which the optimum diameter and optimum RPM can easily be determined.

A consideration on scaling laws of cavitation erosion

Abstract: This paper presents an attempt to establish scaling laws. The author restricts the problem to find the method of estimation from a model test to its prototype. So, the hydrodynamic similarities such as the cavitation number and the load distribution along a blade are assumed to be similar between the model and its prototype. Also the working fluid is assumed same in both cases. Then there is no scaling effect on the fluid property. The problem is reduced to how to estimate the depth of erosion of the prototype from that of model where the flow condition and the property of the material are known. First an empirical formula which is based on experiments will be proposed, then a more complicated analysis will be presented where a modeling of mechanism of cavitation erosion will be made.

Equilibrium of offshore cables and pipelines during laying

Abstract: An efficient solution technique for determination of the static equilibrium form of a cable or a pipeline suspended between the ocean floor and a laying barge or a stinger is presented. Variations in bending stiffness, weight and buoyancy and forces due to ocean current are taken into account. The governing non-linear boundary value problem is transformed into a non-dimensional form such that the a priori unknown suspended length of the pipeline or cable acts as a scaling parameter. The numerical solution is then based on successive integrations. The application of the method is illustrated by the analysis of equilibrium curves and stresses in pipelines laid with or without the use of stingers and during abandon and recovery operations.

Prediction of full scale wake characteristics based on model wake survey

Abstract: This paper deals with the problem of the prediction of the effective wake distribution in the propeller plane of a ship, using the measured nominal wake of a model as a starting point The chosen prediction method consists of two successive steps viz a scale effect correction to obtain the nominal wake field of the actual ship and the prediction of the difference between nominal and effective wake With regard to the scale effect, a method is introduced based on a threefold contraction of the model nominal wake field in dependency of its harmonic content An empirical approach is used to account for the scale effect on the diffusion of the wake peak in the centerplane which starts as soon as the flow has left the stern Further, the scale effect on bilge vortices, assuming that they are generated near the same point on ship and model, is calculated Application of the method shows promising results It is felt, however, that our knowledge of the scale effect on generation of bilge vortices is unsufficient at present The need for more correlation data is emphasized Concerning the transition from nominal to effective wake, a rough estimate is made of the influence of the propeller-hull interaction on the flow by a) an approximation of the influence of the image of the propeller in the hull, b) a concentric contraction of the wake field to account for changes in the boundary layer profile and c) the calculation of the effect of screw race contraction on the axial velocity distribution in the bilge vortex, if present

Regression analysis of ship data

Abstract: Previous applications of regression analysis to ship data are reviewed and these reveal a lack of appreciation of the consequences of statistical theory. The paper therefore considers this theory and its influence on the choice of data and of independent variables, on the features and the accuracy of a good regression equation, and on the scope of proposed designs to which the equations can be applied. Examples of regression analysis of some hydro-static and resistance data are given which show that neat, consistent and accurate equations can be derived. Recommendations are given on the presentation of regression analysis results and it is concluded that the technique is ideally suited for developing empirical expressions for use as aids to ship design.

A stability analysis of the roll motion of a ship in an irregular seaway

Abstract: The restoring moment coefficient in the differential equation describing the roll motion of a ship is assumed time dependent - being either sinusoidal or random in form. For a sinusoidal coefficient the motion equation reduces to the Mathieu type and the stability of motion discussed. For a ship in an irregular seaway, the coefficient will be a random time dependent function. The concepts of stability from a probabilistic viewpoint are defined and a method summarized how the stability of motion in this situation may be investigated.

Method for the assessment of the cavitation performance of marine propellers

Abstract: In this paper a method is given for the assessment of cavitation inception, for the calculation of the type and extent of cavitation and for the calculation of the effect of cavitation on lift and drag. A boundary layer analysis of experimental results of cavitation studies on hydrofoils indicates that cavitation inception occurs in the laminar- transition region of the boundary layer. This fact is used to derive an approximate relation for the inception location and for the corresponding value of the cavitation number. Results of cavitation studies on hydrofoils are also used to derive a relation for the type and chordwise extent of cavitation and for the assessment of the influence of cavitation on lift and drag. The validity of these relations are demonstrated by comparing theoretical and experimental results for the case of marine propellers.

A new method for the measurement of the acoustical source strength of cavitating ship propellers

Abstract: The paper considers the problem of propeller noise aboard ships. It is proposed to describe a cavitating ship propeller as a source of sound in terms of the source strength of a monopole which is situated near the position of a propeller blade tip. A new method for the measurement of this source strength is presented. The method is based on the application of the reciprocity principle. The source description and the measurement method can also be applied in relation to the problem of propeller induced vibrations. The first experiences with the method are very promising.

Hybrid computer simulation of maneuvering during underway replenishment in calm and regular seas

Abstract: This paper describes the second phase of a hybrid computer simulation of two ships maneuvering during underway replenishment operations. Two identical Mariner class merchant ships are used in this study. However, the simulation program is easily adapted to any class of surface ship. The ships' dynamics are simulated on an EAI 680 analog computer, while the nonlinear interaction forces are generated on a PDP 15 (DEC) digital computer. The first phase of the project considered only the interaction effects on the leading ship due to the close proximity of the tracking ship. This second phase considers the interaction effects on both the leading and tracking ships in calm water and in two different regular seas. Computer runs are made to study the control variables necessary for manual control of the leading and tracking ships. A third phase will be directed to establishing the essential variables needed for ship control during underway replenishment and their required accuracy. Selection and display of these manual control parameters for monitoring by the conning officer and/or helmsmen may extend the operating conditions under which underway replenishment can be carried out.

Wave induced motions and loads on ships in oblique waves.

Abstract: In this report procedures for the calculation of ship motion and sea loads on ships proceeding in oblique regular waves with six degrees of freedom are given. Results of calculations are compared with model experiments for two ships.

Propeller manufacture – propeller materials – propeller strength

Abstract: The paper discusses the casting, molding materials, machinability, and alloying materials in the manufacture of propellers. Included are the fatigue properties, fatigue strength with respect to corrosion, the effect of cavities and grain structure, fracture and the use of various materials for propellers. The final section deals with the strength of propellers, including loading, damage and blade stress variations under different wake conditions.

On the added mass of a rectangular cylinder moving in a rectangular channel

Abstract: This paper deals with the effect of restricted waters on the added mass of rectangular cylinders moving in a free surface of rectangular channels, because the rectangular forms can be regarded a a representative of the main part of an actual ship. The moving directions of the cylinder are either parallel or perpendicular to the free surface. Moreover, the frequency of motion is either infinitesimal or infinite. That is, the former is concerned with the manoeuvring motion and the latter is concerned with the ship's hull vibration. The content of this paper overlaps a part of previous articles, but also includes new results obtained after those articles were completed.

A new cavitation erosion test method and its application to a ducted propeller

Abstract: The report presents a model testing technique to predict cavitation erosion. The method is based on fitting a piece of soft, polished aluminium flush with the eroding surface. After a relatively short time of exposure to eroding cavitation tunnel the aluminium surface becomes rough. This roughness is measured, and it represents a quantitative measure of the erosion intensity. Various results from systematic tests with erosion in a propeller duct are presented.