|
The course between A and B in this example is 56° and
the contracourse is 236°.
|
Speed, time and distance
Distances at sea are measured in nautical miles (M). One M is equal to 1,852 metres. You measure distances on charts using the (vertical) latitude scales and a pair of dividers. Pick out e.g. 1 M on the latitude scale and “walk” the dividers between the points.
You can also do the opposite: put the tips of the dividers on the two points and then read it against the latitude scale. Speed at sea is measured in knots. One knot is equal to 1 M per hour. The speed of the boat is measured with a log or distance covered. If you have certain input values, you can calculate speed, time and distance with a simple formula as illustrated below.
| What speed? |
M x 60 |
|
time in min. |
|
|
| What distance? |
knots x min. |
|
60 |
|
|
| How much time? |
M x 60 |
|
knots |
Plotting a course
At sea, you steer according to the compass. The compass is divided into 360°, where north is 0° or 360°, east is 90°, south is 180° and west is 270°. You chart the course you want to steer using the chart and a ruler and course plotter. A course (true course, TC) is the angle between the meridian and the boat’s fore-and-aft line.
When you are going to steer according to the compass between two points, you chart the course as follows: lay the ruler between A and B on the chart, lay the course plotter alongside the ruler and slide the course plotter to the nearest meridian. The centre hole/cross on the course plotter should lie exactly over the meridian. Two course headings can now be read on the protractor scale. Since you have a fixed destination, you will know whether you should head northeast or southwest.
So that you will be able to navigate more precisely, you should learn how to account for drift, variation and deviation. It is too complex to get into here - but you will learn as you study for your certificate of competence.
Bearings
The angle between north (0°) and the direction to a sighted object is called the bearing. The easiest way to determine bearings is to steer straight towards the object and read the compass, in which case the bearing is the same as your course.
You can also use a special bearing compass. If you take the bearings of two objects one immediately after the other, you get two bearings. When these are plotted on the chart, you will get a point of intersection, which is your position. If you calculate compass error of approximately 5°, you will at least gain a reasonable position. This method is called taking crossbearings – there are several other methods. |