Module 8 of 17
Drawing Instruments
Use of parallel rulers, dividers, and proprietary plotting instruments.
Essential Plotting Instruments
Accurate chartwork requires proper instruments. The three essentials are: parallel rulers (or a proprietary plotter like a Breton plotter or Portland plotter), dividers, and a sharp pencil with a rubber. Everything plotted on the chart should be in pencil so it can be erased and the chart reused.
Key Points
- Parallel rulers or proprietary plotter (Breton/Portland)
- Dividers for measuring distance
- Sharp pencil (2B or HB) and rubber
- Always plot in pencil — charts are reusable
Parallel Rulers
Parallel rulers are two hinged rulers that can 'walk' across the chart while maintaining a constant angle. Place the ruler along the line whose bearing you want to measure, then walk it to the nearest compass rose, keeping one ruler firmly pressed while moving the other. Read the bearing from the outer (True) ring.
They take practice to use accurately — the key is pressing firmly on one ruler while sliding the other. On a moving boat, this can be challenging. Some people prefer proprietary plotters for ease of use.
Key Points
- Walk across the chart maintaining a constant angle
- Press one ruler firmly while moving the other
- Read bearings from the compass rose (outer = True)
- Practice on a stable surface first
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