Showing papers in "International shipbuilding progress in 1995"

Performance of chordwise flexible oscillating propulsors using a time-domain panel method

Abstract: A two-dimensional time-domain constant potential panel method was used for analysis of chordwise flexible oscillating hydrofoils as oscillating propulsors. The oscillatory motions as well as the chordwise deflections were of large amplitude. Results are presented of propulsive thrust coefficient and efficiency over ranges of motion parameters including varied reduced frequency ; pitch amplitude and feathering parameter ; heave amplitude/chord ratio ; pitching axis position ; phase between pitching and heaving motions ; and deflected shape. The ranges were taken about a favourable operating point for a flexible oscillating NACA 0012 foil. A peak in propulsive efficiency was found for each motion parameter variation studied. Propulsive efficiency varied strongly with changes in heave amplitude/chord ratio, pitch amplitude and phase between heaving and pitching motions, but less strongly for variations in pitching axis position and flexibility.

Ship structures: improvement by rational design optimisation

Abstract: The application of rule-based and rational design methodologies for ship structures had been demonstrated in [1] and [2] respectively. The hierarchical superiority of rule-based optimum design over rule-based arbitrary design and rational design over rule-based optimum design was discussed in [2], on the basis of design objectives. However, rational designs presented in [2] used a very approximate conservative approach to prevent overall grillage failures, and logically a more rational approach was advocated for that purpose. This paper presents the results of the more rational approach together with results presented in [2] for the sake of clarity, completeness and consistency of this presentation. A brief comparison of all designs with respect to their merit functions is provided as a guidance for the selection of a design approach for improved design. The use of different price-structures (labour rate to material price ratio) demonstrates the difference of capabilities of different shipyards to produce efficient structures. Post-design safety factors of structures and activeness of design constraints against so called common-mode failures are assessed to suggest further rationalisation of different design approaches

Tip loaded propellers (clt) justification of their advantages over conventional propellers using the momentum theory

Abstract: Nearly 20 years have elapsed since the publication of theoretical arguments justifying the higher efficiency of propellers with non null loads at the blade tips and a peculiar radial load distribution. It is now the proper time to divulge the improvements reached with the application of this type of propeller and to offer to the community of Naval Architects and Marine Engineers an original and very efficient procedure to design any kind of propeller. More than 120 CLT propellers (Contracted and Loaded Tip) developed by the authors have been designed using this procedure and are operating in many type vessels. The reference lists of applications for merchant ships and also for controllable pitch systems are herewith presented. To design the CLT propellers it was necessary first to generalize Lerbs lifting line theory, Refs. (1), (5), (6), and afterwards to develop a new theory (Cascades Theory) in order to conduct the three-dimensional corrections required by the lifting line theory. This Cascades Theory is published in this paper for the very first time. Later on the authors adopted the New Momentum Theory, which fundamental were presented in Refs. (7), (8), (9), and has shown to be valid to design any kind of propeller. The CLT propeller advantage over conventional are comprehensively explained using the New Momentum Theory

Nonlinearly Coupled Pitch and Roll Motions in the Presence of Internal Resonance; Part I, Theory

Abstract: The dynamic stability and complicated motions of a vessel in following or head regular waves are investigated when the frequency in pitch is nearly twice the frequency in roll. The damping in the pitch mode is modeled by a linear visco ins damping term, whereas that of the roll mode is modeled by the sum of a linear viscous part and a quadratic viscous part. The method of multiple scales is used to determine a system of four nonlinear first-order equations governing the modulation of the amplitudes and phases of the pitch and roll. Force-response andfrequency-response curves are generated. Coexistence of multiple solutions is found. The jump phenomenon continues to exist, whereas the saturation phenomenon ceases in the presence of quadratic damping. Hopf bifurcations are found. Near these bifurcations, the modulation equations possess limit-cycle solutions and hence the steady-state motion is a periodically modulated pitch and roll motion. Numerical simulations are used to investigate the bifurcations of these limit cycles and how they lead to chaos and hence chaotically modulated pitch and roll motions.

On the calculations of ship motions and wave loads of high- speed catamarans

Abstract: Two mathematical models are presented for the calculations of ship motions and wave loads acting on high-speed catamarans. One is three-dimensional oscillating source distribution method and the other is three-dimensional translating-pulsating source distribution technique. Comparison of theoretical results of ship motions and wave loads for two fast catamarans is discussed. It is found that the difference between those results of ship motions obtained by these two modellings is insignificant. However, the sophisticated three-dimensional translating pulsating source distribution model predicts some phenomena of wave loads, which have been found by experiments in literature, whereas the simplified three-dimensional oscillating source distribution model cannot provide such evidence.

On the use of neural network techniques in the analysis of free roll decay curves

Abstract: In this work, neural network techniques are used to analyze free roll decay curves. The nonlinearities in the roll equation are lumped in one nonlinear function in the roll angle velocity. The technique is then used to identify this general function. This has the advantage of not having to specify the shape of the nonlinearity a priori. The technique has been applied to roll decay curves obtained using numerical simulation and experiments. The agreement between curves predicted using neural network techniques and the actual curves is excellent.

Study of propeller-rudder interaction based on a linear method

Abstract: A linear method which calculates the steady forces of the propeller-rudder system working in a uniform flow is presented. A propeller with an infinite number of blades is used and modelled by a bound vortex sheet, a hub vortex filament and a series of free vortex sheets with an assumption that the free vortex sheets are undisturbed by the rudder. A lifting-line analysis method is incorporated to estimate the propeller performance characteristics. The rudder is represented by vortex and source/sink filaments on its mean plane. A vortex lattice method is used to estimate the rudder performance characteristics. Comparisons of the steady forces with experiments show a reasonable agreement. A rudder located in the accelerating flow just behind a propeller experiences a pressure drag. It is shown that this drag is an internal force which is counterbalanced by an increased propeller thrust. Due to the propeller induced tangential velocities in the slipstream a rudder thrust is created and the work done by the rudder thrust is a measure of the energy recovered by the rudder. The rudder in the test cases can recover up to 40% of the rotational energy and can increase the open water efficiency by up to 1.7%.

The nonlinear swath ship motion in large longitudinal waves

Abstract: Recently the linear analyses for the SWATH ship motion have been well done in the frequency domain. In the present paper the technique to simulate the time history of motions for a SWATH ship running in large longitudinal waves is developed. The nonlinear effect treated here mainly arises from the large motion which is due to the large wave amplitude. Besides, the nonlinear viscous force is also included in the study. Because of large motion, the corresponding hydrodynamic coefficients vary with the instant draft, i.e : each time step, and must be carefully treated. The complexity of the speed effect on the hydrodynamic coefficients at each time step is also taken into consideration. To verify the validity of the present technique, the experiment is also carried out to compare with the theoretical results. Generally the results prove that the technique developed in the present study is successful for predicting the time history simulation of SWATH ship motions in large longitudinal waves. With the basis of the present study, the further analysis on the nonlinear phenomena, e.g. fin effect, of the SWATH ship in waves can be more extensively done.

Prediction of sway force and yaw moment on slender ships

Abstract: A method based on the slender body theory was developed to calculate the sway force and yaw moment on a slender ship performing drifted forward motion. The force and moment were considered to be caused by the purely potential flow around the ship and the vortex shedding from the ship bilge corner. Source elements and discrete vortices were used to represent the sections of the ship hull, and a vortex shedding model was adopted to simulate the shedding and convection of vortices. Calculations were conducted for three different ships and the results were compared with PMM tests. The comparison showed reasonable agreement between the calculated and measured sway forces. For the yaw moment, the agreement between the calculation and the measurement varies notably with the shape of ships.

The mechanics of shaft alignment

Abstract: The static load/deflection behaviour of a flexible shaft resting on elastically-supported journal bearings is examined. Each journal is in contact with its bearing but may move in a circular orbit around the bearing centre. Also examined is the change in configuration when shaft rotation causes contact-support at the bearings to be replaced by fluid-film support. A procedure is devised whereby the static alignment required to achieve a desired operating alignment can be determined. Numerical examples are presented.

Analysis of diffraction-radiation problem of a twin-hull barge system

Abstract: Ocean wave is an important marine environmental phenomena which influences the performance of a marine structure. This necessitates a detailed hydrodynamic study of the wave-structure interaction problem. In this paper the finite element method has been demonstrated as an effective tool to solve the complex linearised three-dimensional hydrodynamic problem. The developed software has been validated for regular bodies and applied to a multi-body (twin-barge) system.