Dedekam Illustrated Navigation

Traditional, Electronic & Celestial Navigation
3. Auflage 2017
ISBN: 978-1-909911-99-4
Verlag: Fernhurst Books Limited
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Book 3, 88 Seiten

Reihe: Illustrated Nautical Manuals

Today, yachts are often equipped with radar, GPS, chart plotters, AIS, etc. This equipment has also become much more reliable, making it possible to make long offshore passages without a great knowledge of navigation. However, such equipment can be set up wrongly, interpreted incorrectly, malfunction or lose power. In these circumstances, knowledge of traditional navigation can become extremely important. This book will teach you how to navigate in the traditional way using compass, log and plotter; and also how to navigate using electronic aids like GPS, radar and chart plotter. In addition, you will learn some basic celestial navigation using the sun and stars to obtain your position using sextant, almanacs, tables and a watch. Each method of navigation is explained alongside detailed illustrations and examples, combining to make a straightforward and easy-to-follow guide. This new version has an improved layout and the eBook's functionality has been improved significantly.

Ivar Dedekam, of Dedekam Design, specialises in uniquely illustrated nautical manuals which are easy to understand and remember. Ivar has been sailing for over 30 years and has logged more than 50,000 nautical miles, including three Atlantic crossings. His book Illustrated Sail & Rig Tuning is an international success and has been published in 24 languages.

Autoren/Hrsg.

Dedekam, Ivar

Leseproben

10

Variation and deviation

The magnetic north pole is situated just outside northern Canada as this is written, but it moves continuously and slowly in these northerly areas.

Variation is the difference in degrees the compass

is showing in reference to true north. If the magnetic

north pole had coincided with the geographic,

the compass needle would always point towards the

north pole along the meridian through your position.

The variation may be really high sometimes in some

places! Always check the chart and calculate the current variation in the area (Fig.18).

Deviation

The compass is also affected by local disturbances

from the boat where it is mounted. Iron and steel on

board create magnetic fields. Electrical circuits and loudspeakers create electromagnetic fields which also cause the compass needle to deviate from the magnetic north pole. This is what we call deviation.

Deviation alters with course (and heeling), but also

over time. Far too many recreational skippers fail to

take deviation into account. This is a practice which

could have serious consequences in situations where

one is totally reliant on the compass, like in thick fog.

NB! Some of the deviation can be eliminated by

adjusting the compass with the built in correction

magnets, something which should normally be done

by experts.

Take every opportunity to check deviation! Write down a deviation table!

Even if you are not able to set up a complete table,

it is necessary to note down the deviation for the

main directions so as to give an indication of the

inaccuracy of the compass. A quick and easy way

to set up a table is given below.

Handbearing compass method

Go out on a day with calm water and position your-self on board in a place with little or no deviation.

This will differ from vessel to vessel, but on a boat with stainless stays (non-magnetic), all the way aft could be a good place. Get the crew to steer the main headings on the main compass while you note the course on the handbearing compass. You could get a table as shown in the illustration. You can also draw up a deviation graph as shown.

Note how the deviation changes in relation to changes in course. The engine often has a large effect. In the figure at the bottom you see how it attracts the compass needle in the simplified situations shown.

In principle, every time you are sure of your true

course (when on a leading line, for example) figure

out your deviation just to check it.

Variation

Magnetic North pole

West (4oW) None (0o) East (10oE)

West (8oW)

Course: 270o Dev.: 3oE Course: 90o Dev.: 2oW

Variation is ever changing and differs...