Davis Instruments

Davis 15" Parallel Rules

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  • Regular price $35.99
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Parallel Rules

Parallel rules are the most basic plotting instrument. Davis Parallel Rules are completely transparent with a clear optical finish for exact plotting. Each 15" (38
cm) blade features large, easy-to-grip handles. Precision molded for the most accurate straight line plotting money can buy. All Davis plotting tools feature sharp printing process, high gloss finish on front and back, and over-lamination for durability.


To Measure Course Bearing:

  1. Place outer edge of one rule on the course line. Holding that rule down firmly against the chart, swing the second rule out away from the first.
  2. Hold the second rule firmly against the chart and swing the first rule up to meet it.
  3. Alternately holding and swinging each rule, “walk” he rules across the chart to the nearest compass rose.
  4. With the outer edge of one rule at the center point of the rose, read the bearing at the rose.


To Draw a Course or Bearing from a Given Point:

  1. Place the parallel rules on the chart so that the outer edge of one of the rules passes through both the center point of the compass rose and the desired bearing.
  2. “Walk” the rules across the chart as above until the outer edge of one of the rules passes through the given point. Draw the line against the edge of the rule.