Inshore Navigation E-Book

Tom Cunliffe

• Herausgeber: Fernhurst Books Limited
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 185

Veröffentlichungsjahr: 2008

Beschreibung

Navigation is a subject best learned outside the classroom so this book is written to get you started as soon as possible. Once you've mastered a few simple principles, it helps you to get afloat and stay out of trouble as you learn the necessities and build confidence. Using straightforward explanations and clear illustrations, Inshore Navigation goes straight to the heart of each topic. This is the definitive guide for everyone who wants to learn navigation easily, quickly and safely.

Leseprobe

Published by Fernhurst Books Limited

62 Brandon Parade, Holly Walk, Leamington Spa, Warwickshire, CV32 4JE, UK

[phone] | www.ferhurstbooks.com

Copyright © 2008 Tom Cunliffe

First published in 1987 by Fernhurst Books Previously published by John WIley & Sons Ltd

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a license issued by The Copyright Licensing Agency Ltd, Saffron House, 6-10 Kirby Street, London EC1N 8TS, UK, without the permission in writing of the Publisher.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book.

This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher accepts no responsibility for any errors or omissions, or for any accidents or mishaps which may arise from the use of this publication.

NOTICE: The UK Hydrographic Office (UKHO) and its licensors make no warranties or representations, express or implied, with respect to this product. The UKHO and its licensors have not verified the information within this product or quality assured it.

THE CHARTS REPRODUCED SHOULD NOT BE USED FOR NAVIGATIONAL PURPOSES.

We would like to thank A&C Black for their kind permission to reproduce material from Neville Featherstone’sReeds Nautical Almanac 2008, Adlard Coles Nautical, an imprint of A&C Black Publishers.

We are also grateful to Northshore Yachts for use of their yacht and for the 3D model of the Southerly 38 swing keel cruising yacht in the illustrations.

British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library

ISBN 978-0-470-75389-7 (paperback) ISBN 978-1-912177-37-0 (eBook) ISBN 978-1-912177-38-7 (Mobi)

Typeset, design and illustration by PPL

Contents

Introduction

1 The chart and the almanac

2 Direction, course and the magnetic compass

3 Position, distance and dead reckoning

4 Man-made aids to navigation

5 Pilotage

6 Tidal streams

7 GPS and the electronic fix

8 Estimated position and the traditional fix

9 Course shaping

10 Use of GPS in onward navigation

11 The electronic chart plotter

12 Tidal heights and depth of water

13 Passage planning

14 Fog

Conclusions

Introduction

The object of this book is to give sailors and motorboaters enough knowledge to navigate safely on short inshore passages. The logical way to begin cruising is to make day sails in reasonable weather and steer clear of night passage-making until you have gained a little confidence. There is enough information here to enable you to do just that.

The book is structured so as to help you get on the water as soon as possible and stay out of trouble while you learn the necessities. All the essential navigational concepts are covered, so that when you feel your sea-going experience is sufficient you will be building on a solid foundation as you move into the broader detail required for offshore and night-time work.

Navigation is not a textbook subject that flourishes in dusty classrooms. It is a living science that draws breath from open water and narrow tidal creeks. Once you have mastered a few basic principles you can only learn the art of its application in the great schoolhouse of the sea. The best teachers are going to be yourself and the ever-changing elements. Here are the rudiments. The rest is up to you.

Tom Cunliffe

1 The chart and the almanac

Given that there is enough water to float your boat, navigation divides into two main sections. The first is working out where you are. The second is deciding how to move from there to your destination.

In the simplest case of all, you can see your goal, there is no current and no obstacles stand in the way. All you have to do is look at where you want to go and steer straight to it. This will hold good in still water and clear visibility over any moderate distance, but if the haven of your choice is too far away to be discerned with any certainty, you’re going to need a chart to find it successfully. An almanac will also be essential in order to refine raw information delivered by the chart.

Charts

The chart is the core unit of navigation. It’s really only a very detailed map produced to the highest standards of accuracy which shows the coast, the foreshore, the water and the nature of the seabed. It also indicates rocks, shoals and a variety of objects that will help you work your way from one location to the next. Charts come in two essential variants: paper and electronic. Both have a number of sub-groups. All use similar sets of symbols, scales and general conventions.

Part of a British Admiralty Chart produced by the UKHO (United Kingdom Hydrographic Office).

Types of chart

Step into a good chandlery anywhere in the world and you’ll find three sorts of chart on sale.

■ National charting authorities publish paper charts for their own coastline, and often many others too. One of the most prolific is the British Admiralty. If you’re operating in home waters, these are the yardstick by which others are measured. They come in full-sized, commercial grade charts also called ‘Standard Charts’ from an official chart agency, but ‘Leisure Charts’ are also issued. These are full-sized charts sold folded down to around A4 size. Unlike Standard Charts, they may not be completely up to date when sold. Smaller charts are issued in ‘Leisure Folios’ of popular sea areas. These sit more comfortably on a small vessel’s chart table and some carry additional data on the reverse side. Occasionally British Admiralty charts carry blow-up plans of specific harbours.

■ Commercial companies also publish paper charts. For example, Imray cover the whole of the British Isles and much of Europe and the West Indies. Commercially produced charts rely on survey data that is owned by the official authority but they may deliver it in a form that carries certain advantages. While an Admiralty chart is the ‘Bible’, a commercial unit can offer extras such as numerous plans which may save you having to buy additional Admiralty charts to achieve the same coverage. Further useful data may also be found on the back of such charts.

An Imray chart. Note that the colour scheme differs from the Admiralty and that soundings are in vertical typeface. Most things, including chart symbols, look the same, however. The box around Fowey Harbour appears elsewhere on the same chart as a helpful ‘blow-up’ plan.

■ Electronic charts for use in GPS chart plotters are really computer-screen representations of the paper equivalent, but two essentially different types are on offer.

Raster scan charts are literally high-definition scans of a given paper chart. With such a chart, you may be able to magnify it on the screen, but by doing so you will not reveal any additional information. If you need more detail, you must call up a chart with a different scale. Raster charts are most often found on PC chart programs. Their similarity to familiar paper charts makes them popular with many navigators.

Vector charts are typically seen in a stand-alone chart plotter unit, although many PC plotter software plotter programs use them as well. They are built up electronically so they look quite different from the traditional paper equivalent. However, complete information for a given area can be accessed via a single chart chip or CD. This sounds like good value, and so it is, but there is no free lunch at sea, so while everything may be in there somewhere, it is often far from obvious at first sight. To use vector charts effectively, you need to familiarise yourself with their little ways. You can read more about this in Chapter 11.

An electronic vector chart showing part of South Cornwall.

Chart symbols

Many of the symbols on charts are self-explanatory and some of them will be covered in this manual, but anyone venturing out to sea should buy and study the modestly priced booklet called ‘Admiralty Chart number 5011’ (AP5011). This colourful publication lists the whole lot. None but the most conscientious student could ever learn them all – nor do most of us need to – but the best way to become familiar with the common ones is to get out on the water and to use a chart. Those that really matter will soon become second nature.

Soundings

The unattached, italicised figures you see all over the chart are known as ‘soundings’. They give the depth of water at that point, measured in metres and tenths of metres, and they give it when the tide is as low as it will ever be. If the figure appears on a green part of the chart and has a line underneath it (thus 13), it means that at the lowest of low tides that area will be 1.3 metres above sea level. It is called a drying height. A ‘sounding’ in normal typeface whose figures are set vertically generally refers to the height of an adjacent rock or object. Heights are given relative to mean high water springs (see Chapters 6 and 12). Normal typeface soundings that are set at oblique angles on a depth contour line indicate the depth of the contour.

Vertical Chart Datum

Whilst the cartographers have sensibly and safely decided to chart their depths at a very low tide, a need exists for something less vague. In fact, all soundings are measured below (or, in the case of drying heights, above) a level called Chart Datum or, to be fully correct in the GPS era, Vertical Chart Datum. This is not to be confused with the ‘datum to which the chart is drawn’ – its horizontal datum (see Chapter 7). If this sounds perplexing, don’t worry. The two subjects have little or nothing to do with one another and you are in no danger of mixing them up. It’s just an unfortunate coincidence that they carry the same name.

Chart Datum is taken as the level of tide at the very bottom of the lowest tide that can be predicted. Tidal predictions are calculated by reference to the gravitational pull of sun and moon and so the lowest prediction is called the Lowest Astronomical Tide, or LAT. In reality, tidal predictions may be modified by local weather and other factors, but such considerations are best left to more advanced navigators. Choosing LAT as Chart Datum was very wise indeed of the head cartographer. It means that except in certain places far from the UK you’ll generally be unlucky to find less water anywhere than the charted soundings. The tide will only add to these and increase your safety margins as it does so.

Chart scale

All charts are drawn to some scale or other. For any but the shortest trips, a chart with sufficient detail to guide you in and out of harbour will perforce be too small in its area of coverage to include your destination. Typically, you might therefore need one large-scale harbour chart for each end and a smaller-scale passage chart to see you safely from one to the other. Be aware of this and don’t try to navigate on the wrong scale of chart.

When it comes to electronic charts, a vector chart can be zoomed from one scale to another, which solves the issue neatly. A raster scan cannot, so if you’re using these it’s important to ensure you have the scales you need.

This is an Admiralty Leisure Chart which is sold folded to make storage and use easier in small craft. The fold is clear here, and note that it doesn’t affect the clarity of the information.

Chart corrections

Admiralty charts

If you buy Admiralty Standard Charts from an official chart agency, they will be corrected up to the date when you walk out of the shop. Leisure Charts may not be. The Admiralty does all it can, but these charts sit on shelves in folios or folded up, and cannot be attended to in the same way as Standard Charts. This doesn’t mean they aren’t any good. So long as you either make every effort yourself to update them, or navigate appropriately if you do not, you should be fine. The Admiralty regularly publishes new corrections for worldwide charts online. Because there is far more information in these notices than will ever be required by yachtsmen and fishermen in home waters, the Leisure Charts are covered by a dedicated website with free access. This gives all the information needed to update charts currently in production. Correcting charts is satisfying work and to do it once in a while will not only give you up-to-date information and some useful plotting practice, it will also keep you out of the pub for an evening.

A section of a passage chart. Note the lack of detail in the inshore sections, then compare it with the detailed chart of the boxed area labelled ‘Harbour Chart’ shown in the illustration on the previous page.

Commercially published charts

Late corrections are often issued with the chart itself, with ‘stop press’ data available on the publisher’s website. Imray do both.

How up to date are you?

By international convention, all chart corrections are given a number and a year. When you correct your charts, your job is to enter this in the margin at the bottom left-hand corner. If someone else has already done some, you’ll find his or her corrections noted down, and by checking them against the Notices to Mariners on the corrections website you can see which ones are missing. It’s beautiful in its simplicity, so long as you don’t forget to note down those numbers.

Electronic charts

A number of ways exist for dealing with electronic chart corrections. In some cases you buy a new chip with financial incentives from the manufacturer. Some can be sent in for updating. Charts issued on CD may well have CD updates, and so on. Find out about this before investing in a chart system, because disappointment can be your lot if you don’t like the deal later on.

The Notices to Mariners web page. Navigate from here to all you need for up-to-date charts – free!

Reference numbers after the words Notices to Mariners are corrections that have been applied to this chart. They are always found in the bottom left-hand corner.

Learning to live with charts

The best way to learn about charts is to get out on the water and bring a paper one up on deck. Orientate it so its North edge is facing North, and look around you. Compare what you are seeing with what it looks like on the chart. You can have a lot of fun referencing everything in sight against AP5011.

It’s simple orienteering really. For much of the time, inshore navigation in practice is little more than this.

The Almanac

If a chart delivers the bread and butter of navigation, the meat in the sandwich is the almanac. A comprehensive almanac such as Reeds is virtually essential for all navigation, if for no other reason than that it tells you what the tide is doing. Without this information, your safety is compromised and any enjoyment you might have been hoping for very often flies out of the porthole. We’ll be dealing with the almanac in more detail further on in the book. For now, let’s just to say that it goes far beyond its basic function of publishing annual data for tides, sunset and sunrise. If ever you find yourself stuck for an answer to any maritime question, from a rule-of-the-road issue to how to deliver a baby personally at sea, your first port of call is the almanac.

If your sailing is likely to be within a single wide area, you may well be able to save money by buying the local almanac, published either by Reeds or some other firm. English Channel sailors don’t necessarily need to know what time high water is at the North Cape of Norway. They won’t find the data in ‘The Channel Almanac’ either, but within its pages they will discover all they need to know to sail their own waters safely and without fuss.

Orienteering.

2 Direction, course and the magnetic compass

In order to find out which way to steer towards an unseen destination, lay out your chart, take a straight edge and a sharp pencil, then draw a neat line from where you are to where you want to go. Sketch one arrow on the line as shown. This is the accepted convention to denote a ‘course to steer’. Try always to use a soft pencil because, not only will the line be clearer, it will also be easier to rub out. The best grade is ‘2B’.

Obviously you cannot draw an equivalent line on the sea to steer along, so you need some form of directional reference to link the line on the chart to the direction you are actually steering. This link is the compass. It’s denoted on the chart by means of a compass rose.

Now look at the compass rose illustration. All charts carry these, often in several places. They relate direction on the chart with degrees of a circle, with 000 – 180° lining up to the geographic, or ‘True’ North and South Poles. 270° is West’, 090° East’.

A line plotted on the chart from the present position towards your destination represents the ‘course to steer’. It is identified by a single arrowhead halfway along its length.

It’s an inconvenient fact that the Earth’s magnetic field doesn’t coincide exactly with the axis of the planet. Instead, the magnetic North Pole wanders around the Arctic some distance from the geographic pole, dragging the world’s compass needles with it. Notice that the illustrated rose has a secondary ‘compass’ inside the outer one. The outer rose lines up on True North along the axis of the globe. The North arrowhead of the inner one points slightly ‘left of centre’ showing that it is oriented with the magnetic pole. Sadly, the practice of displaying the complete magnetic rose on a chart is becoming rare. All too often nowadays, it is replaced by a ‘True’ rose that carries a simple ‘Magnetic North’ arrow to indicate the magnetic pole.

A full ‘magnetic’ compass rose.

The numbers on the magnetic North line show how far off North the magnetic pole is at the time the chart was published (4° and 20 minutes West in this case). The number in brackets (7’E) indicates the annual rate and direction of change, but for practical purposes inshore, you can forget about that unless your chart is as old as this one. In any case, once you have this year’s figure, you can safely round up or down to a whole number of degrees, because it is impossible to read a yacht’s compass to a finer measurement than a degree. In a rough sea even that can be pushing your luck. The important concept to grasp is that your chart is drawn to True North and your compass is pointing towards the Magnetic North. The difference between them is called variation.

Variation changes not only with the years, but also from one location to another. Each chart carries the local values, although moving slowly along the coast the change is often imperceptible from one chart to the next.

Three-figure notation

Courses are always expressed as degrees of a circle in a three-figure notation. This avoids the confusion that can otherwise arise on a windy day. When someone calls out ‘nineteen’, for example, does he mean, ‘19’, or perhaps ‘90’. ‘Oh-one-nine’ (‘zero-one-nine’ is fine too) removes all possibility of error. You may feel somewhat pedantic saying this to start with, but it very soon becomes second nature. To order up a course of ‘thirty-five’ would feel horrible to an experienced sailor. When writing a course in a logbook or on a chart, an equally important discipline is to note whether it’s True (T) or Magnetic (M) – like this: 055°M

Bearings

The direction of any object relative to yourself is defined by a bearing. This might be a relative bearing such as 45° on the port bow, but it’s more likely to be a compass bearing. If you observe the object over the top of a compass, you can read off its bearing just as you would if you were steering a course. On the chart, a line plotted from the object along its compass bearing can only pass through your vessel.

A bearing is always noted in degrees True as seen from seaward. It is given an arrow at its seaward end.

Plotting instruments

In order to determine the direction of the course line you have drawn on the chart, you must use an instrument to relate it to a convenient compass rose. The traditional method, still favoured in North America, is parallel rulers. These have practical drawbacks on small yacht chart tables which are solved in Europe by instruments that used to be call ‘plotters’, or ‘chart plotters’. It’s still correct to refer to them by this name, but because electronic chart plotters are now in common use and are a different thing altogether, I prefer to call them ‘chart protractors’.

Parallel rules

Very often there will not be a convenient compass rose beside the course line so if you are using parallel rules you must ‘walk’ them across the chart. On the chart table of a small yacht, well heeled in a seaway, this can be an infuriating process, which is why most of us living east of the Atlantic opt for one of the commercial protractors instead.