Introduction

Hypersonic flows are synonymous with high-Mach number flows and therefore are characterized by very strong shock waves. Every hypersonic vehicle has a bow shock wave in front of it, which bounds the flow around the vehicle. On the windward side of a vehicle, the bow shock usually is aligned closely with the vehicle surface, and the distance between the surface and the shock wave is usually small relative to the characteristic dimension of the vehicle. Thus, this shock-layer region is usually quite thin. Hypersonic vehicles tend to fly at high altitudes so that convective heating levels can be managed. Thus, the characteristic Reynolds numbers tend to be low and boundary layers are usually thick. In addition, shear heating in hypersonic boundary layers increases the temperature and viscosity, which also increases the thickness. The low Reynolds number and the relative stability of hypersonic boundary layers mean that many practical hypersonic flows are laminar or transitional. If the flow is turbulent, it is often only marginally turbulent. Therefore, hypersonic flows are particularly susceptible to shock wave–boundary-layer interactions (SBLIs).

Selection of Marine Propulsion Machinery

The selection of propulsion machinery and plant layout will depend on design features such as space, weight and noise levels, together with overall requirements including areas of operation, running costs and maintenance. All of these factors will depend on the ship type, its function and operational patterns.

Propeller Geometry, Coefficients, Characteristics

1. Compactness and weight: Extra deadweight and space. Height may be important in ships such as ferries and offshore supply vessels which require long clear decks.

2. Initial cost.

3. Fuel consumption: Influence on running costs and bunker capacity (deadweight and space).

4. Grade of fuel (lower grade/higher viscosity, cheaper).

5. Level of emission of NOx, SOx and CO2.

6. Noise and vibration levels: Becoming increasingly important.

7. Maintenance requirements/costs, costs of spares.

8. Rotational speed: Lower propeller speed plus larger diameter generally leads to increased efficiency.

Purpose

When making conventional power predictions, no account is usually taken of scale effects on:

1. Hull form effect,

2. Wake and thrust deduction factors,

3. Scale effect on propeller efficiency,

4. Uncertainty of scaling laws for appendage drag.

Experience shows that power predictions can be in error and corrections need to be applied to obtain a realistic trials power estimate. Suitable correction (or correlation) factors have been found using voyage analysis techniques applied to trials data. The errors in predictions are most significant with large, slow-speed, high CB vessels.

Model-ship correlation should not be confused with model-ship extrapolation. The extrapolation process entails extrapolating the model results to full scale to create the ship power prediction. The correlation process compares the full-scale ship power prediction with measured or expected full-scale ship results.

Background

The accurate experimental measurement of ship model resistance components relies on access to high-quality facilities. Typically these include towing tanks, cavitation tunnels, circulating water channels and wind tunnels. Detailed description of appropriate experimental methodology and uncertainty analysis are contained within the procedures and guidance of the International Towing Tank Conference (ITTC) [7.1]. There are two approaches to understanding the resistance of a ship form. The first examines the direct body forces acting on the surface of the hull and the second examines the induced changes to pressure and velocity acting at a distance away from the ship. It is possible to use measurements at model scale to obtain global forces and moments with the use of either approach. This chapter considers experimental methods that can be applied, typically at model scale, to measure pressure, velocity and shear stress. When applied, such measurements should be made in a systematic manner that allows quantification of uncertainty in all stages of the analysis process. Guidance on best practice can be found in the excellent text of Coleman and Steele [7.2], the processes recommended by the International Standards Organisation (ISO) [7.3] or in specific procedures of the ITTC, the main ones of which are identified in Table 7.1.

In general, if the model is made larger (smaller scale factor), the flow will be steadier, and if the experimental facility is made larger, there will be less uncertainty in the experimental measurements. Facilities such as cavitation tunnels, circulating water channels and wind tunnels provide a steady flow regime more suited to measurements at many spatially distributed locations around and on ship hulls. Alternatively, the towing tank provides a straightforward means of obtaining global forces and moments as well as capturing the unsteady interaction of a ship with a head or following sea.

Background

The overall ship powering process is shown in Figure 2.3. A number of worked examples are presented to illustrate typical applications of the resistance and propulsor data and methodologies for estimating ship propulsive power for various ship types and size. The examples are grouped broadly into the estimation of effective power and propeller/propulsor design for large and small displacement ships, semi-displacement ships, planing craft and sailing vessels.

The resistance data are presented in Chapter 10 together with tables of data in Appendix A3. The propeller data are presented in Chapter 16, together with tables of data in Appendix A4.

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.