A Breton plotter (French: ), also known as a Portland course plotter or Weems protractor named after later manufacturers producing similar devices, is a

navigational instrument Navigational instruments are instruments used by nautical navigators and pilots as tools of their trade. The purpose of navigation is to ascertain the present position and to determine the speed, direction, etc. to arrive at the port or point o ...

used for

nautical Seamanship is the art, knowledge and competence of operating a ship, boat or other craft on water. The'' Oxford Dictionary'' states that seamanship is "The skill, techniques, or practice of handling a ship or boat at sea." It involves topics a ...

navigation with

chart A chart (sometimes known as a graph) is a graphical representation for data visualization, in which "the data is represented by symbols, such as bars in a bar chart, lines in a line chart, or slices in a pie chart". A chart can represent tabu ...

s. It is a ruler with a rotating rose that enables to plot a course on charts, while aligning the North of the ruler with the North of the chart. The Breton plotter was invented by Captain Yvonnick Gueret, a Breton seaman who developed the plotter during his experiences teaching navigation with a () by

Jean Émile Paul Cras Jean Émile Paul Cras (; 22 May 1879 – 14 September 1932) was a 20th-century French composer and career naval officer. His musical compositions were inspired by his native Brittany, his travels to Africa, and most of all, by his sea v ...

. Gueret was a commander in merchant vessels, served on small fishing boats, delivered private yachts and taught navigation. He later extended the use of the plotter to air navigation for small aircraft.

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