1,720,962 research outputs found
Aspects of twin screw ships semi-empirical maneuvering models
No full textSimplified mathematical models based upon semi-empirical regression formulae describing forces and moments acting on the hull during manoeuvres are commonly used for the preliminary evaluation of ship steering capabilities. Many semi-empirical regressions have been developed, and are broadly adopted, for the manoeuvrability prediction of single screw slow/medium speed ships. Their application for the study of twin screw vessels (cruise ships, RoRo ferries, megayachts, naval vessels, for which dedicated regression formulae are scarce) manoeuvring capabilities could lead to misleading results. These ships are usually characterised by different hull forms and more complex stern configuration due to the presence of appendages like skegs, shaft lines and brackets, which can strongly affect manoeuvrability behaviour. In this work a novel procedure to properly account for the particular geometric and stern appendage characteristics of these kind of vessels is described. A thorough analysis has been performed on a ship model equipped with 13 different stern appendage configurations and new formulae have been developed in order to describe accurately their influence on ship manoeuvring behaviour
Off-Design Propulsion Power Plant Investigations by Means of Free Running Manoeuvring Ship Model Test and Simulation Techniques
No full textTwin screw vessels’ propulsion system experiences strong off design conditions during tight manoeuvres due to the propellers inflow asymmetry arising from the coupled yaw-drift motion. Unfortunately, simplified mathematical models based upon statistical data or ad hoc executed captive model test (PMM or CMT) do not provide such a detailed information. Indeed, free running model tests are the best mean in order to get ship’s trajectory and kinematics parameters data and propulsion behaviour by recording the loads (thrust and torque) on the shafts. More insight into this complex aspect is desired in order to improve and generalize the application of existing manoeuvring mathematical models for the preliminary design of unconventional propulsive configuration control system
Application Of Semiempirical Regression Formulae For The Assessment Of Fast Twin Screw Ships Manoeuvrability
No full textIn present work, the application of previously developed semiempirical regression formulae for the prediction of fast twin screw ships manoeuvrability is shown, considering the test case of the 5415 naval ship. Results in terms of standard manoeuvres macroscopic parameters and also of forces in PMM-like tests are reported, showing the capability of the method and allowing to remark shortcomings and possible improvements
Marine propulsion system dynamics during ship manoeuvres
No full textMarine propulsion plants can experience large power fluctuations during tight manoeuvres. During these critical situations, dramatic increases of shaft torque are possible, up to and over 100% of the steady values in straight course. In the case of a twin-screw ship turning circle, the two shaft lines dynamics can be completely different in terms of required power and torque. This phenomenon, if not correctly considered, is potentially dangerous, especially for propulsion plants with two shaft lines powered via a unique reduction gear, which can be subject to significant unbalances.
The paper presents a simulation approach able to represent the dynamics of a twin-screw ship propulsion plant in these critical working conditions. The numerical model includes the ship manoeuvrability and the dynamic behaviour of prime movers, shaft lines, propellers and propulsion control system. Numerical results obtained have been compared to full-scale measurements in order to validate the proposed simulation approach
Hydrodynamic coefficients regressions analysis and application to twin screw vessels
No full textThe problem of ship manoeuvrability has reached nowadays a significant consideration, both for merchant ships, with the adoption of IMO standards, and naval ships, with the production of various documents by NATO Specialist Teams. In literature many works regarding manoeuvrability of single screw slow/medium speed ships can be found, while a lack of information exists for what regards twin screw ships (cruise ships, RoRo ferries, megayachts, naval vessels), characterised by different form coefficients and more complex stern configuration due to appendages like skegs, shaft lines and brackets that can strongly affect manoeuvrability behaviour. In this hydrodynamic coefficients of a series of twin screw naval vessels have been evaluated considering different semi-empirical formulations proposed by various authors and standard manoeuvres (turning circle and zig-zag) have been simulated with the aid of a simulator tool. Discrepancies of predicted manoeuvres with respect to experimental full scale ones have been analysed considering parameters involving stern appendages and hull main dimensions in order to select most suitable regressions to study twin screw vessel manoeuvring performance in the earlier design stages. Moreover, linear hydrodynamic coefficients corrections related to stern appendages contribution have been developed on the basis of results of a system identification technique and of an analysis of a set of experimental data at PMM for a ship model equipped with different stern appendages configurations
Experimental Investigation of Asymmetrical Propeller Behaviour of Twin Screw Ships During Manoeuvres
No full textTwin screw ships may experience considerably asymmetrical propeller functioning during manoeuvres. This phenomenon may result in large power fluctuations during tight manoeuvres, with increases of shaft torque up to and over 100% of the steady values in straight course and considerable unbalances. A joint research project supported by Italian Navy has been set up in order to deeply investigate the phenomenon, potentially dangerous for ships propulsion systems with coupled shaft lines, by means of large scale model testing and related numerical simulations.
In the present work, the extensive experimental campaign results on a free running model of a twin-screw ship are presented, allowing to obtain a deeper insight of the problem. In particular, tests have been carried out simulating different simplified control schemes, starting from the most common constant rate of revolution tests and including different control strategies (constant torque and power). Usual standard manoeuvres (turning circle, zigzag and spiral) have been carried out, providing results for asymmetrical shaft and ship manoeuvrability behaviour. Results from the present analysis, together with the previously presented results, allow to obtain the complete model for the time domain simulation of asymmetrical shaft functioning
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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