Tides

A Primer for Deck Officers and Officer of the Watch Exams

Philip M. Smith, Philip Smith

• Publisher: Routledge
• Year: 2018
• Print ISBN: 9781138366305
• eBook ISBN: 9781317200345
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

Tides: A Primer for Deck Officers and Officer of the Watch Exams prepares the reader for the Officer of the Watch and Master/Mate certificates required by all officers on commercial seagoing vessels. From the formation of tides and tidal stream data, right through to practice questions with answers, and even mock exam papers, this book will provide you with all the reference material you need in order to pass your exams.

1

TERMINOLOGY USED IN TIDAL CALCULATIONS

1.1 Planetary influences on tides

In order that simple but costly mistakes made by extracting incorrect figures from the Tide Tables can be avoided, it is important that all the relevant terminology to be encountered is clarified. Many people have a broad idea of what the tides are caused by but as a mariner, a slightly deeper understanding is required other than that they are caused by the Moon.

They are indeed caused by the Moon but it is a combination of the effects of the gravitational forces exerted upon the liquid surface of the earth by both the Sun and the Moon which influence the tidal pattern of movement. Between the Sun, the Earth and the Moon there is an attractive force of gravity holding the Earth and all the other planets in orbit around the Sun. Tides result as a consequence of the combined gravitational forces of the Moon, and to a lesser degree, the Sun, acting upon the liquid surface of the Earth. As the Moon is closer to the Earth it exerts much more of an influence on the tides than the Sun which is 93 million miles away. It is possible using Newton’s Laws of Gravity and Motion to quantify how much less the Sun’s gravity affects the Earth’s tides.

Newton’s Law of Motion states that:

$$\mathrm{Acceleration}\left(\mathrm{a}\right)=\frac{\mathrm{Force}\left(\mathrm{F}\right)}{\mathrm{Mass}\left(\mathrm{M}\right)}$$

Newton’s Law of Gravity states that:

A body of mass M exerts a gravitational attraction G on a unit of mass at a distance (r)

$$\mathrm{Fg}=\frac{\mathrm{G}\times \mathrm{M}}{{\mathrm{r}}^2}$$

(Where G is universal gravitational Konstant)

Comparing gravitational attraction of the Sun on the Earth with that of the Moon on the Earth:

Mass of the Sun = 27 million times that of the mass of the Moon

Distance from Sun to the Earth = 390 times the distance from the Moon to the Earth

Thus: Fg (Sun) = 27 × 10⁶

Fg (Moon) = 390²

= 178 times that of the Moon

From this, it appears that the gravitational effect of the Sun on the Earth is much greater than that of the Moon. It is a commonly assumed fact which most people know however, that it is the Moon which has more effective influence on the tides than the Sun. This is perfectly correct and the reason for this is because it is just the proportion of the gravitational force (GF) NOT balanced by centripetal acceleration (Ac) in the Earth’s orbital motion, that produces the tides. This unbalanced portion is proportional to the inverse cube of the distances rather than the inverse square of the distance from the Earth.

Because gravity is still proportional to the mass, we can see that forces causing the tides are approximately:

$$\mathrm{Fg}=\frac{\mathrm{G}\times \mathrm{M}}{{\mathrm{r}}^2}$$

$$=\frac{178}{390}$$

This shows that the gravitational effect of the Sun on tides is equal to 0.46 of the effects caused by the Moon.

1.2 Tidal terminology explained

1.2.1 Chart legend

The basic unit of measurement for tidal calculations is metres and the information extracted from British Admiralty charts will also be in metres. It is important however, that the navigator always consults the Legend of every chart which confirms the units of measurements used for navigational and tidal data on that particular edition of the chart.

As already highlighted, it is extremely important that the terminology used throughout this guide is well understood right from the outset. It is equally important that the relationship between all the various terms and levels used in the tidal calculations are clearly distinguished in order that solutions to tide questions can be solved with a high degree of accuracy.

There are in fact only two questions to be answered in tidal examinations:

A) What is the Height of Tide at a given time? e.g. 21:30 hours tonight.

B) What time will the tide be a given height? e.g. 5.5 metres.

FIGURE 1.1 Admiralty chart legend notes on maritime charts

In order to answer these two questions, we need simply to look up the correct pair of tides relevant to the time of day required, apply some relatively simple maths and with the aid of the tidal curves provided in the Tide Tables, both of these questions can be solved.

1.2.2 Tide

A tide may be described as the regular and predictable movement of water governed by the relative positions of the astronomical bodies, primarily, the Sun and the Moon. It is regarded as the vertical movement of water which rises and then falls.

1.2.3 Flood tide

The term refers to the time during which the tide comes in, normally over about a six hour period after Low Water (LW) until it reaches its maximum height above Chart Datum at High Water (HW).

1.2.4 Ebb tide

The term refers to the time during which the tide recedes, normally over about a six hour period after HW until it reaches its lowest height above Chart Datum at LW.

1.2.5 Slack Water

This term refers to the short period immediately after HW or the period after LW when there is no vertical change in depth of water and the tide effectively stands still. It is this time when it is ideal to swing vessels around in turning basins within ports, as there will be no loss of Under-Keel Clearance (UKC) during the swing as there is no vertical tidal movement of water level. This is sometimes referred to as the ‘Stand’ for similar reasons.

1.2.6 Current

The horizontal movement of water brought about by a number of different conditions but primarily as a result of wind’s influence on the direction, or topography of the surrounding features of the land. Currents may vary in magnitude or even direction depending on the season, differences in sea water temperature or salinity.

1.2.7 Height of Tide (HOT)

The predicted Height of Tide is found in tidal publications for High and Low Water and the HOT found at these times, indicates the height in metres measured above the reference point Chart Datum, to the water level.

Height of Tide is required to be calculated at any point in time in order to determine the more important value which is the Under-Keel Clearance between the keel and the seabed.

1.2.8 The causes of tides

The planet Earth revolves around the Sun in an elliptical orbit and it maintains its position along this path as a result of the gravitational pull exerted on it by the Sun. The Moon revolves around the Earth and once again a constant orbit is maintained as a result of the gravitational force between the Earth and the Moon. As the Moon is closer to the Earth, the gravitational force between them effects the liquid surface of the Earth, the oceans, more than that of the Sun. The Moon revolves around the Earth approximately once every 29 days. This varies slightly due to the changing proximity of the Sun as the Earth continues along its orbital path. The Earth takes 365 days to complete one orbit of the Sun and regularly, all three bodies are aligned resulting in a combination of gravitational forces all acting in the same direction. It is this maximum combined effect of gravitational forces which leads to extra high tides known as Spring Tides and produces greater ranges of tides over the daily tidal cycle.

The point at which the cycle begins, when the Earth, Moon and Sun are in the same ...

Table of contents

• Cover Page
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• List of Figures
• About the Author
• Preface
• Introduction
• Acknowledgements
• Disclaimer
• 1 Terminology Used in Tidal Calculations
• 2 The Admiralty Tide Tables
• 3 The Use of Tidal Curves
• 4 Preparatory Standard Port Exercises
• 5 European Secondary Ports Tidal Calculations
• 6 Pacific Standard Port Tidal Calculations
• 7 Pacific Secondary Ports Tidal Calculations
• 8 Vertical Sextant Angles, Tidal Streams and Co-Tidal Data
• 9 Revision Papers with Worked Solutions
• Appendix 1: European Waters
• Appendix 2: Pacific Tides
• Bibliography
• Index