Marine Navigation Square Protractor also known as a Navigation Square Protractor or Douglas Navigational Protractor for Air and Sea 5" x 5" for measuring on charts, determining bearings, or to use as a parallel rule. Navigation protractor 360° marked in 1° intervals. Sometimes referred to as a Douglas protractor, the 5" Square Protractor is best suited for rapid plotting of courses and bearings in a confined space. Notations can be easily made directly on matte plastic surface with pencil. A cursory inspection of this protractor will show its many advantages over other forms, especially to the seaman, for in using it dividers and parallel rulers are not needed.
Size 5" x 5" x .030"
- Square protractor with a compass graduated around the edges in 360 degrees
- Hole in the center so that it can be easily placed over markings and landmarks
- Transparent so charts and maps are visible throughout
How to use the Douglas Navigational Protractor
To layoff a course or bearing. Use the left side as a ruling edge and the graduation in italic figures on the meridian or parallel - rule the desired course or bearing. For example try 10°, 170°, 230°, 350°. The left edge can always be distinguished by the writing "Douglas Combined Protractor and Parallel Rule".
To read off a course or bearing. - Align the protractor to the course or bearing and the angle can be read off at once from the nearest meridian or parallel. It is here that the great use of the parallel lines is so noticeable, as any line can be used within the limits of the size of the protractor.
To use a parallel rule - or to take a bearing to or from a compass on the chart. Align the protractor by one of its parallel lines as necessary on the bearing or on the compass and move protractor to the point required by drawing pencil lines either side, of the protractor as necessary. By this means a bearing or course can be moved anywhere over the chart with no chance of unknown displacement.
To find dep. a lat. between two positions - Align the protractor to the nearest meridian and a perpendicular from the course line (at the distance given) will cut off on the 90° - 270° line the numbers of divisions equally the departure. Another perpendicular will cut off on the 0° - 180° line the number of divisions equaling the difference of latitude. The same applies to laying or reading off a position for Lat. or Long. as will readily be seen.
To find Magnetic Course or Bearing from true - Set the protractor to the course or bearing, e.g. 340° and then given the Compass Error as 5° W treat the protractor as a movable compass card and move it 5° to the West. This will give 345° or N.15° W. as the magnetic course. By using the protractor in the same way the True Course or bearing can be found from the magnetic, or given both true and magnetic the Compass Error may be determined.
To fix positions by angles - Lay off the angles observed on the matt side in the opposite direction to that in which they are observed. This can easily be done without calculation by using the italic figures if necessary. Then reverse the protractor so that the pencil lines are in contact with the objects charted and prick through position as usual.
To plot lines of position and other such problems This can easily be done as the upper surface of the protractor readily allows of lines being ruled and rubbed out. This upper surface also allows of notes and calculations being made, positions transferred from one chart to another by tracing and so,on.