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Technical and research (T&R) | Bulletins and reports
It is the purpose of this bulletin to present in usable form methods for estimating the cavitation speed of two-dimensional and certain three-dimensional flows about shapes of technical interest to naval architects.
In this sense, it is not a research paper (although some new methods are presented), but rather a distillation of research done over the past several decades into a form which can be applied. No attempt will be made to explain the development of the various recommended formulae since those who are interested in such details can consult the references which are cited. On the other hand, a very elementary description is given at the outset of the physical characteristics which have an important bearing on the determination of the minimum pressure attained on the surface of a body or section and hence on the speed at which the phenomenon of cavitation may be expected to ensue. It is pointed out that the onset of cavitation depends upon many things other than the ideal steady-state. pressure in the liquid as given by the estimates presented herein and hence the predictions made must be considered simply as estimates which are nonetheless of use in direct as well as in a comparative sense.
Treatment of the material is divided into three main categories, i.e., flows in two dimensions, axially symmetric bodies in axial- and cross-flows and certain three-dimensional flows. In the first two of these, results for existing forms are condensed and presented to show the importance of such shape parameters as thickness ratio, leading edge radius, slenderness ratio and the integrated effect of overall shape. This is followed by elucidation of a method for computing the pressure distribution about an arbitrary form (one which has not been previously computed) in terms of operations on the offsets of the body. The existence of computer programs for such calculations is announced. The scope of the last section on three-dimensional flows is limited because of the paucity of existing material and the general difficulty in carrying out calculations in this case. Simple superposition techniques are advanced for engineering analysis of juncture flows.
Let us consider the problem of small harmonic oscillations of a body under the free surface of a heavy, incompressible fluid of finite depth.
This sheet is intended to serve as a convenient standard form for presenting the lines, coefficients of form, and smooth-water model-test results for both hard-chine and roundbottom small craft. Now that the format has been developed, the Society intends to publish a number of these sheets, each one dealing with a particular hull form. This undertaking bears a close resemblance, of course, to the previous development and issuance by the Society of datasheets for large ships.
The last decade has seen a marked increase in the study of seakeeping characteristics of ships. There are several reasons for this. From a military standpoint, extending the speed range is an essential naval requirement, so much so that radical new forms such as hydrofoil boats, ground effect machines, etc., are receiving considerable attention. Passenger traffic has always had the requirement of high speed and minimum discomfort but competing air travel has accentuated these needs. Commercial cargo delivery is not so sensitive to increased speed as it is to maintaining speed, safety and structural integrity through avoidance of serious motions, impacts, and deck wetting.
T&R Bulletin 1-25 (1976) has superseded the T&R Bulletin 1-2 (1962).
A Committee on friction formulation was appointed by the American Towing Tank Conference (A. T. T. C.) in 1946 to submit recommendations for the adoption of a uniform practice in the computation of frictional resistance and the expansion of model test results to full-sized ships. This new Bulletin is the result of revisions made in 1964. Much of the text has been deleted and minor changes have been made in the tables of kinematic viscosity for fresh and salt water. These latter are now in agreement with those adopted by the International Towing Tank Conference (I. T. T. C.) in 1963.
The Society of Naval Architects and Marine Engineers gratefully acknowledges the work performed by one of its members, Dr. Bennett Silverstein, and his associates in contacting various agencies participating in Seakeeping Research and processing the replies received from many sources. The cooperation of all involved is sincerely appreciated. Since 1950, Panel H -7 has endeavored to sponsor and encourage research in Seakeeping. It is hoped that the information in this survey will be useful and will assist in the coordination of research activities in this area.
Measurements of hydrodynamic drag and side force plus observations of flow have been accomplished for the 5.5 meter yacht ANTIOPE at the David Taylor Model Basin. These tests are for the purpose of providing a definitive set of data for a typical sailing yacht, principally for correlation with equivalent scale model experiments to check and promote improvements in methods for the model tests. In addition, the detailed understanding of the forces and flow patterns for this one hull will enhance the general understanding about all sailing yachts.
The Society of Naval Architects and Marine Engineers through the work of its Technical and Research (T&R) Program endeavors as part of its assigned task to provide information that could assist the naval architect in designing the most economical and efficient ship hull form.
A relatively new field of transportation endeavor is the use of large ocean-going barges. A paper entitled "Notes On the Hydrodynamics of Seagoing Barges" by Messrs. R.B. Couch and J. Moss, presented at April 3, 1964, Gulf Section SNAME Meeting, proved of such interest that Panel H-2 (Resistance and Propulsion), under the direction of the Society's T&R Hydrodynamics Committee, funded a modest project at the University of Michigan to analyze the data obtained from towing tank testing of many types of ocean-going barges for the purpose of preparing guidelines for the barge designer. Additional Industry support was volunteered and the project expanded in scope.
The results of the investigation are presented in this Bulletin. It is hoped that it will serve as a guide In the design of large ocean-going barges and encourage more research on this subject.
For a long time Panel H-7 of the Hydrodynamics Committee, SNAME, has been concerned with evaluating the seakeeping performance of ships. Both experimental and theoretical techniques for determining the motions of alternative ship designs in regular and irregular waves have advanced rapidly in recent years. But questions have remained as to what is good behavior and what is bad, what is acceptable and what is unacceptable performance. In short, how can standards of "seakeepability" be established? The Panel hopes that this survey will clarify a difficult and important problem and that others will use it as a basis for further development of techniques for evaluating ship environmental operability.
In the past bow bulbs were supposed to be unsuitable for slow, fullform ships. However, the present study reveals that they can cause a considerable reduction of resistance in the ballast condition. This property has been confirmed by model tests and full-scale operation in practice. Parameters for effective bulb design are indicated. The sensitivity to weather conditions and the economic significance of bulbous bow ships are investigated. The second part of the paper describes a series of special experiments conducted with a tanker model for clarifying the physical mechanism by which the bow bulb reduces the resistance of slow, full-form ships. The tests comprised measurements of resistance, viscous wake and wave pattern as well as flow observations by means of film and underwater television. It was found that contrary to previous hypotheses the bulb produced its effect not so much by wave cancellation or improved flow around the bilge as by elimination of the breaking bow wave.
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