Nav in a nutshell: How to use a Breton Plotter

How to use a Breton Plotter
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Breton Plotters are simple to use in practice, but tricky to explain on paper – it’s much easier if you have one in front of you as you follow the explanation. But here goes…

If we want to draw a line at 115°M, set the compass rose to that position allowing for variation (see photo below).

Set the compass rose to 115°– we’re lining it up with the 5° west variation

Set the compass rose to 115°– we’re lining it up with the 5° west variation

Then move the whole plotter around until north on the compass rose points to the top of the chart and align the grid with any handy perpendicular chart lines (see photo below), then draw your line.

Line up the ‘north’ grid with any perpendicular chart lines

Line up the ‘north’ grid with any perpendicular chart lines

To read off a bearing between two points is the reverse procedure: line up the edge of the plotter with the two points, align the grid to north and read off the bearing – simple.

To shape a course use the graduations on the plotter edges. On some models these equate to miles at different chart scales, but I find they can be confusing so just use them as ‘units’.

We can then draw a small scale-drawing of the vector triangle on the chart using ‘units’ and not great big sea miles off the latitude scale. The angles will still be the same.

  • Draw a line where we want to go
  • Measure the angle of the relevant tidal arrow in the tidal atlas with the Breton Plotter
  • Mark off an hour’s worth of tidal ‘units’ on the chart, at that angle
  • Instead of swinging off an arc of boat speed ‘units’ with a compass use the edge of the plotter. Pivot it against a pencil point if that helps
  • Align the grid and read the course in magnetic from the variation scale on the plotter
  • Adjust for leeway

Hopefully the ship’s compass is free from deviation, caused by metal or electronics on board: most are, but it pays to check occasionally. If it can’t be corrected out, create a ‘deviation curve’ as explained in any nav textbook.

Then the corrected course will be in °C (degrees compass).

Later in the series we’ll see that doing some chart work helps to deal with changing tidal streams, tacking to clear headlands and checking waypoint positions. GPS is great, but it still pays to know the basics.

This Nav in a Nutshell article by Dick Everitt was published in the February 2012 issue of PBO. For more useful archive articles explore the PBO copy shop.