Introduction

The appeal of a numerical method for estimating ship hull resistance is in the ability to seek the ‘best’ solution from many variations in shape. Such a hull design optimisation process has the potential to find better solutions more rapidly than a conventional design cycle using scale models and associated towing tank tests.

Historically, the capability of the numerical methods has expanded as computers have become more powerful and faster. At present, there still appears to be no diminution in the rate of increase in computational power and, as a result, numerical methods will play an ever increasing role. It is worth noting that the correct application of such techniques has many similarities to that of high-quality experimentation. Great care has to be taken to ensure that the correct values are determined and that there is a clear understanding of the level of uncertainty associated with the results.

Introduction

General

The methods of presenting propeller data are described in Section 12.1.3. A summary of the principal propulsor types is given in Chapter 11. It is important to note that different propulsors are employed for different overall design and operational requirements. For example, a comparison of different propulsors based solely on efficiency is shown in Figure 16.1, [16.1]. This does not, however, take account of other properties such as the excellent manoeuvring capabilities of the vertical axis propeller, the mechanical complexities of the highly efficient contra-rotating propeller or the restriction of the higher efficiency of the ducted propeller to higher thrust loadings.

As described in Chapter 2, the propeller quasi-propulsive coefficient ηD can be written as follows:where ??O is the propeller open water efficiency, and ??H is the hull efficiency, defined as follows:where t is the thrust deduction factor, and wT is the wake fraction. ??R is the relative rotative efficiency. Data for the components of ??H and ??R are included in Section 16.3.

Introduction

An interaction occurs between the hull and the propulsion device which affects the propulsive efficiency and influences the design of the propulsion device. The components of this interaction are wake, thrust deduction and relative rotative efficiency.

Direct detailed measurements of wake velocity at the position of the propeller plane can be carried out in the absence of the propeller. These provide a detailed knowledge of the wake field for detailed aspects of propeller design such as radial pitch variation to suit a particular wake, termed wake adaption, or prediction of the variation in load for propeller strength and/or vibration purposes.

Average wake values can be obtained indirectly by means of model open water and self-propulsion tests. In this case, an integrated average value over the propeller disc is obtained, known as the effective wake. It is normally this average effective wake, derived from self-propulsion tests or data from earlier tests, which is used for basic propeller design purposes.

Introduction

Resistance data suitable for power estimates may be obtained from a number of sources. If model tests are not carried out, the most useful sources are standard series data, whilst regression analysis of model resistance test results provides a good basis for preliminary power estimates. Numerical methods can provide useful inputs for specific investigations of hull form changes and this is discussed in Chapter 9. Methods of presenting resistance data are described in Section 3.1.3. This chapter reviews sources of resistance data. Design charts or tabulations of data for a number of the standard series, together with coefficients of regression analyses, are included in Appendix A3.

Standard series data result from systematic resistance tests that have been carried out on particular series of hull forms. Such tests entail the systematic variation of the main hull form parameters such as CB, L/∇1/3, B/T and LCB. Standard series tests provide an invaluable source of resistance data for use in the power estimate, in particular, for use at the early design stage and/or when model tank tests have not been carried out. The data may typically be used for the following:

1. Deriving power requirements for a given hull form,

2. Selecting suitable hull forms for a particular task, including the investigation of the influence of changes in hull parameters such as CB and B/T, and as

3. A standard for judging the quality of a particular (non-series) hull form.

Components of Propulsive Power

During the course of designing a ship it is necessary to estimate the power required to propel the ship at a particular speed. This allows estimates to be made of:

1. Machinery masses, which are a function of the installed power, and

2. The expected fuel consumption and tank capacities.

The power estimate for a new design is obtained by comparison with an existing similar vessel or from model tests. In either case it is necessary to derive a power estimate for one size of craft from the power requirement of a different size of craft. That is, it is necessary to be able to scale powering estimates.

The different components of the powering problem scale in different ways and it is therefore necessary to estimate each component separately and apply the correct scaling laws to each.

Background

This appendix provides a background to basic fluid flow patterns, terminology and definitions, together with the basic laws governing fluid flow. The depth of description is intended to provide the background necessary to understand the basic fluid flows relating to ship resistance and propulsion. Some topics have been taken, with permission, from Molland and Turnock [A1.1]. Other topics, such as skin friction drag, effects of surface roughness, pressure drag and cavitation are included within the main body of the text. Descriptions of fluid mechanics to a greater depth can be found in standard texts such as Massey and Ward-Smith [A1.2] and Duncan et al. [A1.3].

From an engineering perspective, it is sufficient to consider a fluid to be a continuous medium which will deform continuously to take up the shape of its container, being incapable of remaining in a fixed shape of its own accord.

Propeller Geometry, Coefficients, Characteristics

Propeller Geometry

A marine propeller consists of a number of blades (2–7) mounted on a boss, Figure 12.1. Normal practice is to cast the propeller in one piece. For special applications, built-up propellers with detachable blades may be employed, such as for controllable pitch propellers or when the blades are made from composite materials.

The propeller is defined in relation to a generator line, sometimes referred to as the directrix, Figure 12.1. This line may be drawn at right angles to the shaft line, but more normally it is raked. For normal applications, blades are raked aft to provide the best clearance in the propeller aperture. For high-speed craft, the blades may be raked forward to balance bending moments due to centrifugal forces against those due to thrust loading.

Physical Components of Main Hull Resistance

An understanding of the components of ship resistance and their behaviour is important as they are used in scaling the resistance of one ship to that of another size or, more commonly, scaling resistance from tests at model size to full size. Such resistance estimates are subsequently used in estimating the required propulsive power.

Observation of a ship moving through water indicates two features of the flow, Figure 3.1, namely that there is a wave pattern moving with the hull and there is a region of turbulent flow building up along the length of the hull and extending as a wake behind the hull.

Both of these features of the flow absorb energy from the hull and, hence, constitute a resistance force on the hull. This resistance force is transmitted to the hull as a distribution of pressure and shear forces over the hull; the shear stress arises because of the viscous property of the water.

Introduction

Ship powering relies on a reliable estimate of the relationship between the shaft torque applied and the net thrust generated by a propulsor acting in the presence of a hull. The propeller provides the main means for ship propulsion. This chapter considers numerical methods for propeller analysis and the hierarchy of the possible methods from the elementary through to those that apply the most recent computational fluid dynamics techniques. It concentrates on the blade element momentum approach as the method best suited to gaining an understanding of the physical performance of propeller action. Further sections examine the influence of oblique flow and tangential wake, the design of wake-adapted propellers and finally the assessment of cavitation risk and effects.

Although other propulsors can be used, Chapter 11, the methods of determining their performance have many similarities to those applied to the conventional ship propeller and so will not be explicitly covered. The main details of the computational fluid dynamic (CFD) based approaches are covered in Chapter 9 as are the methods whereby coupled self-propulsion calculations can be applied, Section 9.6.

New ship types and applications continue to be developed in response to economic, societal and technical factors, including changes in operational speeds and fluctuations in fuel costs. These changes in ship design all depend on reliable estimates of ship propulsive power. There is a growing need to minimise power, fuel consumption and operating costs driven by environmental concerns and from an economic perspective. The International Maritime Organisation (IMO) is leading the shipping sector in efforts to reduce emissions such as NOx, SOx and CO2 through the development of legislation and operational guidelines.

The estimation of ship propulsive power is fundamental to the process of designing and operating a ship. Knowledge of the propulsive power enables the size and mass of the propulsion engines to be established and estimates made of the fuel consumption and likely operating costs. The methods whereby ship resistance and propulsion are evaluated will never be an exact science, but require a combination of analysis, experiments, computations and empiricism. This book provides an up-to-date detailed appraisal of the data sources, methods and techniques for establishing propulsive power.

The estimation of ship propulsive power is fundamental to the process of designing and operating a ship. A knowledge of the propulsive power enables the size and mass of the propulsion engines to be established and estimates made of the fuel consumption and operating costs. The estimation of power entails the use of experimental techniques, numerical methods and theoretical analysis for the various aspects of the powering problem. The requirement for this stems from the need to determine the correct match between the installed power and the ship hull form during the design process. An understanding of ship resistance and propulsion derives from the fundamental behaviour of fluid flow. The complexity inherent in ship hydrodynamic design arises from the challenges of scaling from practical model sizes and the unsteady flow interactions between the viscous ship boundary layer, the generated free-surface wave system and a propulsor operating in a spatially varying inflow.

Up to the early 1860s, little was really understood about ship resistance and many of the ideas on powering at that time were erroneous. Propeller design was very much a question of trial and error. The power installed in ships was often wrong and it was clear that there was a need for a method of estimating the power to be installed in order to attain a certain speed.

General

The hydrodynamic behaviour of the hull over the total speed range may be separated into three broad categories as displacement, semi-displacement and planing. The approximate speed range of each of these categories is shown in Figure 14.1. Considering the hydrodynamic behaviour of each, the displacement craft is supported entirely by buoyant forces, the semi-displacement craft is supported by a mixture of buoyant and dynamic lift forces whilst, when planing, the hull is supported entirely by dynamic lift. The basic development of the hull form will be different for each of these categories.

This chapter concentrates on a discussion of displacement craft, with some comments on semi-displacement craft. Further comments and discussion of semi-displacement and planing craft are given in Chapters 3 and 10.