In 1914 Captain HP Douglas Superintendent of Charts in the Office of the Hydrographer in the royal navy invented a new navigational protractor that was far simpler and lighter than previous designs. Although it would not see much use on naval vessels, except in very small craft, it was ideal for use by airmen and has continued to be a navigational tool right up to the present day. The RFC did not immediately see the potential of the Douglas Protractor and it did not become RAF issue until the 1930s. The protractor is square in form and made of transparent plastic:

This example dates from the 1970s and was issued in a small vinyl case for protection:

Wartime examples came in a cardboard case with a paper instruction label pasted to the outside. Despite dating from the 1970s, the protractor is still marked up with an Air Ministry property mark:

It is also marked up as having been made by the London Name Plate Manufacturing Company Ltd:

This company had the sole contract to produce these navigational aids right back to the 1930s, as they proudly declared in this pre-WW2 advertisement:

Using the Douglas protractor was apparently fairly simple, but a full set of instructions were printed onto a piece of card and included in the case with the protractor:

The ‘Adlard Coles Book of Navigation’ has this to say on the Douglas Protractor and its use:

An alternative way of measuring directions on the chart is by using one of many different types of protractor. These have their own scale of degrees so they do not rely on compass roses and do not, therefore, have to be moved about the chart. Using them calls for less manual dexterity than parallel rulers, but in most cases more mental dexterity.

The Douglas Protractor has long been popular with air navigators, though it was originally devised for marine use. It consists of a square of transparent plastic- five or ten inches for a genuine Douglas Protractor, though other sizes are available from different manufacturers. Around its edge are two rows of markings from 0° to 360°, with an outer scale reading in a clockwise direction and an inner scale running anticlockwise. Inside this is a grid of equally-spaced lines, parallel to the edges of the protractor; and right in the centre is a small hole.

The most intuitive way of using a Douglas protractor to measure the direction of a line on the chart is simply to place it on the chart, with the central hole on the line and its grid aligned, by eye, with the nearest meridians and parallels. The direction of the line can then be read directly from the clockwise scale.

A similar technique can be used to draw a line in a particular direction; place the central hole of the protractor over the starting point of the line, ensure that the protractor’s grid is lined up with the meridians and parallels, and make a light pencil mark on the chart alongside the required direction on the outer (clockwise) scale. Then move the protractor and use its edge to rule a straight line from the starting point through the pencil mark.

Even today Douglas Protractors are still produced and available for purchase to those navigating in small boats or aircraft and they remain a relatively simple back up to more modern GPS type navigational systems.