INTRODUCTION

7.1 In preceding chapters we have touched in a rather piecemeal way on a number of considerations which bear on the choice of geometric form of a submarine and on the way in which its contents, both inside and outside the pressure hull, could be arranged to best effect. Our purpose in this chapter is to collect those considerations together and also to introduce several other issues associated with arranging the contents of the submarine.

In dealing with the matter of arrangements we will be addressing an aspect of the activities of the submarine designer which are what one might call architectural by nature. As we are both naval architects by profession we can take that description for granted our regret is that so much of a naval architect's time is taken up with engineering matters that he can be limited in the attention he is able to give to architecture. Yet it is an area of activity in which there is still scope as well as need for some art amongst so much science.

It might appear at first sight that the form and arrangements of a submarine present much simpler problems than those of a surface ship. Essentially a submarine is a long tube in which the disposition of most of its contents is arranged longitudinally with little scope for vertical variations. In a surface ship, on the other hand, with its multi-deck configuration, there is freedom for spatial interaction longitudinally, vertically and athwartships to be taken into account.

INTRODUCTION

10.1 Attention in previous chapters has been focused on the technical considerations in submarine design which bear on achieving the required performance. In the first chapter, however, we did point out that the designer has at all times to keep in mind that the submarine should be capable of being produced at an acceptable cost to the customer and be considered value for money, and also that the resources are available for the detailed design and build. Although it is conceivable that performance might be regarded as paramount, and consequently that any costs incurred either directly or to create the resources necessary for building have to be accepted, in most designs it is necessary to keep a strict balance between performance, cost and resources.

It is pertinent to consider the comparison between the cost of building a submarine and that of a corresponding commercial ship. On the basis of displacement tonnage there is a considerable difference in cost per tonne for the two types of vessel. The difference between the two calls for explanation.

One factor is the difference in the way in which the size of a submarine and a commercial ship is expressed. The gross tonnage of a commercial ship is in fact determined by its cargo or hold space and does not involve the weight of the ship itself. It is better to compare costs on the basis of the actual weight of the ship as constructed, termed its lightweight. Making that change, the cost per tonne for a commercial ship is higher but still significantly less than that of a submarine.

INTRODUCTION

11.1 It can be recognised that throughout the book we have placed emphasis on conceptual formulation in submarine design and on the high level of interaction between the sub-systems. It must nevertheless be appreciated that the successful outcome of a submarine design is ultimately achieved by extensive detailed study of every aspect of the design. It is only by detailed studies that the multitude of interaction effects can be identified and resolved. However, such studies have to be conducted within a framework which adequately defines the total design and that is the purpose of the Concept Design phase. The objective is to determine a size and weight plus geometric configuration within which the detailed studies (often conducted by specialist designers) can take place. For example, there is little point in conducting a detailed design of pressure hull structure if the envisaged configuration of the hull precludes achievement of a weight/buoyancy and longitudinal balance. Similarly, there is little point in a detailed investigation of the weapon compartment layout and tankage if its location within the hull is inconsistent with the fore-end configuration.

11.2 Consequently, the designer has to be able to determine the broad design characteristics having an awareness of the details that must follow. He is unlikely to have much information on those details, particularly in an innovative design, but he must endeavour to ensure that the subsequent stages are capable of achievement.

In design, when the trim and compensation arrangements are being addressed, a procedure is required for sizing the tanks and assessing the amount of water in them, i.e. the weight of variable ballast. The method we now describe, which is widely used for the purpose, is to be read in conjunction with Figure A3.1.

1. (a) A standard condition of the submarine is defined, i.e. one with full crew, stores and fuel on board, with the boat in water of standard density 1.0275. A statement is drawn up of all the weights and locations of items which might vary during the course of a patrol, including buoyancy changes, especially those associated with extremes, e.g. maximum changes of seawater density between say 1.00 and 1.03 (or whatever is appropriate for the submarine under design).

2. (b) A graph is prepared with a horizontal axis for longitudinal moments and a vertical axis for forces, the origin representing the standard condition of the boat. The weight and buoyancy changes in the statement from (a) are plotted as a series of discrete points on the graph to the convention that reductions in weight forward are placed in the first quadrant. The aim is to cater for demanding combinations of weight and buoyancy changes which are sensible but nevertheless still serve to establish the boundary of extreme conditions in which the boat might find itself. Thus towards the end of a patrol a submarine, having consumed nearly all of its stores and fuel, might have to enter an area in which sea-water density is high; alternatively a submarine shortly after leaving base to go on patrol might have to enter water which is almost fresh. […]

INTRODUCTION

In this book we attempt to convey an understanding of those aspects of design which are specific to submarines. However, submarine design is just one of many engineering design activities and there are general features which are common to all. Before proceeding to the specific aspects of submarine design, we consider it worthwhile in this first chapter to address the more general aspects and show how they relate to the submarine design task.

DESIGN OBJECTIVES

1.1 Though there are many variations on what may be considered design objectives we suggest that the following three are primary in all designs and should be sustained throughout the whole design process:

1. (a) that the product should perform the functional purpose of the customer or operator;

2. (b) that the design should be suitable for construction within the capability of the technology and resources available;

3. (c) that the cost should be acceptable to the customer.

Though expressed as separate objectives they are interactive and may on occasion be incompatible. The circumstances within which the design takes place may lead to one or another becoming the prime objective with the others subsidiary.

In some situations the performance of the design is paramount: only a design that is capable of fully performing the required function is of interest. In such circumstances the customer would have to be prepared to pay the cost of such a design and even the capital investment to create the technology and resources necessary to realise the design. Some major nations have accepted this situation for defence equipment in the past but it is less prevalent in current political circumstances.

The two authors of this book have been involved in the design of submarines for the Royal Navy for upwards of thirty years, and have also been involved on and off in the teaching of submarine design for much of that time. They both have connections with Vickers Shipbuilding and Engineering Limited (VSEL) the only builders of submarines in the U.K., Louis Rydill as a design consultant and Roy Burcher as the VSEL Professor of Subsea Design and Engineering at University College London (UCL). Roy Burcher runs a postgraduate design course at UCL, which is attended by students from many countries.

With this background, we are only too well aware of the dearth of textbooks on submarine design and engineering. There are also relatively few technical papers on the subject. There was a seminal paper in 1960, published by the Society of Naval Architects and Marine Engineers of the U.S.A., entitled Naval Architectural Aspects of Submarine Design by Arentzen and Mandel, which we regard as an outstanding contribution to the subject but, no doubt because of security issues involved in military submarine design and operation, that splendid opening up of the vistas made possible by the advent of nuclear propulsion for submarines has subsequently become largely closed off to view.

Yet there is still much about submarine design and engineering which can be said without risk of offending against security obligations. The course at UCL is, in fact, completely unclassified and is open to all prospective students with the appropriate qualifications.

INTRODUCTION

9.1 Whilst all vehicles and ships have operating systems, the submarine requires special systems to enable it to operate in its environment between the surface and fully submerged below the sea. The systems are required both for operation in underwater space and for the crew to work efficiently totally divorced from the atmosphere. In concept formulation and initial sizing the systems do not present issues that need to be considered in detail, though they do require provision of space and weight within the hull. Nevertheless, since they are so important to the general operation of the vessel, we devote this chapter to describing particular aspects of submarine systems, how they are operated and the usual form taken by their design. A particularly important aspect of the systems is their integrity and reliability.

The primary systems in a submarine can be categorised under the following headings, though in some cases they overlap:

1. (a) Hydraulic systems – these are provided for power actuation of many valves, systems and controls.

2. (b) High pressure air system – this is required mainly to initially discharge water from the main ballast tanks in changing from submerged to surface condition, but it also has many other uses on board.

3. (c) Water distribution systems – these are required to control the trim of the boat and to eject unwanted water taken on board during various operations.

4. (d) Ventilation and air-conditioning systems – these are distinctive in a submarine because of the special needs of the enclosed atmosphere once submerged.

INTRODUCTION

6.1 The powering of a submarine vehicle is one of the most important factors in the determination of its size. As shown in Chapter 4, power plants use a high proportion of the weight and space available in a submarine, some 35% of weight and 50% of the total volume being devoted to power generation and storage. As we go on to show, the power requirements are determined in conjunction with speed by the size of the vessel, and hence the designer encounters a loop in the design process whereby the output of the power assessment in terms of a volume requirement for propulsion plant is itself a significant input to that assessment. In control engineering terms this is a positive feed-back loop, which can all too easily cause a growth in the total size of the design.

The powering of a submerged conventional submarine is an exercise in efficient energy storage and its conversion to usable power. It is the solution of this problem which dictates the underwater endurance of the submarine. As described earlier, it was only when a nuclear power plant became available that a true submarine could be produced, because it was the development of an energy source totally independent of atmosphere that enabled the true submarine to become a reality. The successful production of a practical nuclear power source not only enabled the design of a true submarine but also shifted the endurance limits to other factors such as the crew and expendable stores, because the nuclear power source endurance is measured in years rather than days or weeks.

FIRST PRINCIPLES OF FLOTATION

3.1 To naval architects the hydrostatic properties of vessels floating on the water surface represent a fundamental part of their stock in trade, and that familiarity readily reads across to submarines on the surface; the hydrostatic properties of submerged submarines are less familiar to naval architects in general, but they can identify the parallels without difficulty. For most other engineers the subject of hydrostatics may not be so familiar. For that reason Appendix 1 is provided which gives the broad principles on which the following discussion is based.

The submarine, like any other marine vehicle, has to be designed to float in the water where its weight is supported by the buoyancy forces due to the displacement of water by its hull. For a surface ship the objective is not simply that it should float at the surface but also that it should remain afloat even if some damage resulted in part of the hull being flooded. To cater for this means the provision of a substantial watertight volume of hull above the normal waterline, (Figure 3.1). This enables the surface ship to accommodate changes in weight by adjusting its level of flotation, i.e. by slightly sinking or rising. The amount of sinkage is governed by the area of the hull at the waterline (the Waterplane Area).

As well as just floating, the ship must remain upright in calm water. This involves the consideration of transverse stability and the concept of GM (Metacentric Height) and GZ which, as shown in the Appendix, is also governed by the characteristics of the waterplane area. (Figure 3.2(a)).

For a vessel floating on the surface, the hydrostatic properties of relevance are the conditions of flotation and the stability of the vessel in relation to disturbances from the static flotation condition.

It is assumed that the water surface is calm and any movement of the craft is sufficiently slow for any dynamic effects to be discounted.

Accepting Archimedes principle that a vessel will displace its own weight of water, the initial requirement for flotation is that the intact hull is of sufficient volume to displace its own weight whilst having a reasonable freeboard (height of weather deck above the waterline) and that it floats upright. The volume of watertight hull above the waterline constitutes a Reserve of Buoyancy (ROB). This ROB becomes important when considering the safety of the vessel in damaged conditions when water floods part of the hitherto intact displacement volume below the waterline.

The stability of the vessel concerns the outcome of perturbations from the static flotation condition. Sideways motion (sway), change of heading (yaw) and fore or aft motion (surge) do not change the static conditions of the hull and can be considered as neutral. Whereas roll motion (heel), pitching and vertical motion (heave) result in changes in the distribution of buoyant volume and hence variation in the static equilibrium condition. The question to be answered is whether after a disturbance the vessel returns to its initial equilibrium state.

If a vessel heaves upwards then the displacement volume reduces and the excess of unchanged weight over reduced buoyancy provides a force in a direction restoring the vessel to its original position.

INTRODUCTION

8.1 Historically, it was not until the advent of the nuclear submarine with its capability for sustained high speed that the focus of attention of the submarine designer moved from the provision of adequate means for control of motion in the vertical plane – particularly at periscope depth – to achievement also of an appropriate balance between manoeuvrability and dynamic stability, with the emphasis again on motion in the vertical plane. Concentration of attention on depth changing and keeping rather than on course changing and keeping was natural because of the inherent risks in the high speed submarine – even with better diving depth capability through increased pressure hull strength – of accidentally exceeding the allowed maximum depth, a hazard regarded as potentially more dangerous than inadvertent surfacing.

As can be appreciated, there are similarities between the submarine manoeuvring submerged and the airship – though there are also significant differences – and in fact early theoretical and experimental investigations into submarine dynamic stability and control, in the 1940s and 1950s, initially drew on corresponding investigations for airships in the 1920s and 1930s. Subsequent research, specific to submarines, has provided a body of knowledge of which it behoves the submarine designer to have a basic understanding, even though dynamics and control are not primary considerations in determining the size and shape of the submarine at the concept stage.

It might seem from the latter observation that, once size and form have been determined by the dominant considerations, provision of the means for achieving the desired control characteristics could be left to specialist hydrodynamicists and control engineers for development subsequent to the concept stage.

INTRODUCTION

5.1 The main attribute of a submarine is its ability to dive beneath the surface and to go to reasonable operating depths. For a manned submersible there is a requirement for the enclosed volume to be maintained at atmospheric pressure. This need applies not only for the personnel but also for much of the equipment which has been designed to operate in atmospheric conditions. It is desirable to keep the enclosed volume as small as possible so as to limit the weight of structure that is required to withstand the differential pressure between sea pressure at depth and atmosphere. In small unmanned submersibles and ROVs it will usually be possible to minimise the amount of volume that needs to be contained by the pressure carrying structure, but for most large seagoing manned vessels there are inescapable requirements for a considerable amount of volume to be contained within the structural envelope. It may be that the design of a submarine as a whole leads to a decision to include other volumes within the pressure hull although they are not necessarily required to be at atmospheric pressure; for example, some of the main ballast tankage may be included within the pressure hull. It can also be convenient to locate some fuel tanks within the pressure hull so that they can be operated in atmospheric conditions. Variable ballast tanks will usually be located within the pressure hull, even though in some instances they are subject to sea pressure. It is important that where such volumes are inside the pressure hull, steps are taken to enable them to be isolated from sea pressure.

Introduction

Bubbles form in a flowing liquid in areas where the local pressure is close to the vapour pressure level. They form and collapse in a short time, measured in microseconds, and their life history gives rise to local transiently high pressures with flow instability. In pumps this results in noise, vibration and surface damage which can give rise to very considerable material loss.

The inception and collapse mechanisms are discussed briefly in this chapter, as are the conventional empirical rules used to ensure satisfactory pump behaviour. The chapter concludes with a discussion of the design rules to be followed in producing a good pump, and with a treatment of the techniques used to predict cavitation performance.

Bubble inception and collapse

In theory, cavities will form when the local liquid pressure level is equal to the vapour pressure under the local conditions. In practice bubbles form at higher pressure levels, due in part to the presence of very small bubbles or particles of detritus which act as triggers. A very exhaustive treatment of the process will be found in the monograph by Knapp et al. (1970), so a very brief summary will be given here.

Figure 2.1 is based on work done by Worster (1956) who used theoretical equations first published by Rayleigh (1917) to predict the life cycle of an existing small bubble as it grew and then collapsed.