The rear-admiral Jean Cras (1879-1932) invented the rule of Cras that still bears his name today.
It has been used since 1917, this double protractor ruler makes it possible to draw routes and bearings on sea or air navigation charts as well as to mark them.
Practical and quick to use, this rule is mainly used in France.
It is used aboard pleasure craft, merchant ships, and warships, as well as in aeronautics.
What is the point of the Cras rule?
To ensure the follow-up of its navigation on a nautical or air chart, one must have 2 tools:
A protractor to carry the angles.
A ruler for plotting routes and bearings.
The Cras rule combines these 2 instruments.
Its use, without being difficult, is not very intuitive.
Indeed, the rule associates 2 inverted half-protractors, with double graduation, nested one in the other.
This arrangement, which may put off novices, deserves some explanation.
On the outer edges of the ruler, there are 2 measurement scales:
One expressed in nautical miles, usable on coastal maps at 1 / 100,000th scale, the most used by boaters.
The other was expressed in centimeters.
The ruler also has horizontal and vertical lines:
these marks will make it easier to draw at right angles.
Depending on the intended mode of use, there are different models of the Jean Cras protractor ruler.
Only the rules bearing the mention Jean Cras are approved Aviation and Marine.