Portland Plotter

What a Portland plotter is

A Portland plotter (the Breton plotter works the same way) is a clear plastic rectangle with a rotating protractor in the middle. The protractor carries a square grid. When that grid lies along the chart’s meridians and parallels, the dial’s N points to true north, so the number at the index mark is the direction of the plotter’s edge. It does the job of parallel rules and the compass rose in one, and it works anywhere on the chart, not only near a rose.

Measuring a bearing between two points

1. Lay the long edge along the line joining the two points.
2. Check the direction-of-travel arrow points from where you are to where you are going.
3. Turn the dial, not the base, until its grid lies along the meridians with N towards the top of the chart.
4. Read the true bearing at the index. For magnetic, read against the variation scale (west on one side, east on the other), or add west and subtract east variation.

Laying off a bearing or a course from a point

1. Set the bearing on the dial at the true index, or at the variation mark if your bearing is magnetic.
2. Put the plotter on the chart and turn the whole plotter until the grid lies along the meridians.
3. Slide it, keeping it square, until the edge passes through the mark or position.
4. Draw the line and label it: a single arrow for a water track, a double arrow for a ground track, the time for a position line.

Distance: always the latitude scale

One minute of latitude is one nautical mile. Open the dividers to the two points, carry them to the latitude scale at the side of the chart, level with what you are measuring, and count the minutes. Never use the longitude scale along the top and bottom: a minute of longitude is shorter than a mile everywhere except the equator. For long legs, set the dividers to a round number of miles and walk them along the line.

Common mistakes

• Arrow pointing backwards, giving the reciprocal (180° out).
• Grid squared to the coastline or a pencil line instead of a meridian or parallel.
• Applying variation the wrong way: going from true to magnetic, add west and subtract east.
• Measuring distance on the longitude scale.
• Turning the dial when laying off, instead of turning the whole plotter.

Use it with the other tools

• Learn the marks on the chart with the chart symbols quiz and the buoyage trainer.
• Work out the height of tide for an under-keel check with the tide calculator.
• Put the bearings and distances you measure into a plan with the passage planner.
• Practise saying a position over the VHF, figure by figure, with the phonetic alphabet and radio numbers trainer.