Parallel Rules for Marine Navigation: Plotting Courses and Measuring Directions on Nautical Charts

Introduction: an essential tool for traditional chart navigation

A parallel rule is a traditional marine navigation instrument used to plot a course, transfer a direction and work accurately on a paper nautical chart. It allows a line to be moved parallel to itself while preserving its orientation. This makes it easier to transfer a course or bearing between different points on a chart, particularly between a plotted course line and the compass rose.

Used for generations by recreational sailors, offshore navigators and maritime professionals, parallel rules are part of the traditional equipment found at a chart table. They help navigators visualise directions, plan passages and understand the principles of chart plotting.

Parallel rules are available in several designs. A conventional model generally consists of two straight rules connected by hinged arms that keep them parallel. Other instruments, such as rolling parallel rules, maintain a direction through a different mechanism. The most suitable choice depends on the navigator's preferences, chart size and navigation method.

What is a parallel rule used for in marine navigation?

The main purpose of a parallel rule is to transfer a direction across a nautical chart without changing its angle. It can therefore be used to work with course lines, bearings and directional references shown on the chart.

A parallel rule is particularly useful for:

• plotting a course between a departure point and a destination;

• transferring a direction to the compass rose to read its angle;

• determining the orientation of an existing course line;

• extending a course line while preserving its direction;

• plotting a bearing observed from a fixed landmark;

• planning a coastal or offshore passage;

• practising chartwork and traditional navigation techniques.

A parallel rule is primarily used for the geometric construction of a course. It does not directly measure the distance between two points and does not replace a pair of dividers. Complete passage planning therefore requires other instruments and the information provided on the nautical chart.

How does a parallel rule work?

A conventional parallel rule consists of two rules of equal length connected by hinged arms. This mechanism allows the rules to move closer together or farther apart while keeping their edges parallel.

When one rule is aligned with a course line, the other can be moved towards a reference point on the chart without changing the original orientation. The navigator can then transfer the direction to the compass rose and read the corresponding angle.

The principle is straightforward: the direction remains constant during the transfer. Accuracy nevertheless depends on the instrument's stability, the quality of its mechanism and the way it is handled.

Parallel rules vary in length, material, hinge design and graduations. Some are transparent, making it easier to see chart details beneath the instrument. Others prioritise rigidity and durability.

A conventional parallel rule should be distinguished from a rolling parallel rule. The former generally uses two hinged rules, whereas the latter moves on rollers while maintaining its orientation. Both instruments serve a similar purpose, but their operating methods differ.

How to use a parallel rule on a nautical chart

Using a parallel rule involves a few straightforward steps. Practising the technique helps reduce plotting errors and keeps the chart easy to read.

1. Plot the course between two points

Identify the departure point and destination on the nautical chart. Draw a fine pencil line between these positions, taking charted hazards and the intended route into account.

Place one edge of the parallel rule along the line. If the course has already been plotted, ensure that the instrument is accurately aligned with the relevant segment.

2. Transfer the direction to the compass rose

Move the parallel rule towards the compass rose printed on the chart. Depending on the distance, the instrument may need to be moved in several stages, alternately holding one rule in position while advancing the other.

The hinged mechanism allows the original orientation to be maintained throughout the transfer. Check the alignment regularly to avoid angular drift.

3. Read the course angle

Once the rule reaches the compass rose, use the appropriate graduations to determine the angle corresponding to the plotted direction. The precise method depends on the instrument's design: some parallel rules are primarily intended for transferring directions, while other models include angular graduations or a protractor function.

It is important to distinguish true course, referenced to geographic north, from magnetic course and compass course. Reading a direction from the chart alone does not determine the course that should be steered using the vessel's compass.

4. Check the plotted direction

Before recording a value, check the direction of travel, the graduations and the alignment of the rule. Reversing the direction or positioning the instrument incorrectly can introduce an error of several degrees.

This check is particularly important when the route passes through an area containing shoals, narrow channels or isolated hazards.

Practical example: planning a coastal passage

A navigator is preparing a passage between two ports shown on a nautical chart. After identifying both positions, the navigator draws a line representing the intended route.

The parallel rule is aligned with this line and moved towards the compass rose to determine its orientation. The navigator can then record the direction indicated by the chart and continue planning the passage.

Other factors must also be assessed, including magnetic variation, any deviation of the vessel's compass, currents, wind, tides, available depth and charted hazards. The passage may need to be divided into several legs to avoid a dangerous area or follow a channel.

This example illustrates how a parallel rule supports chartwork without replacing the wider passage-planning process. Selecting a safe route and determining the operational steering course require additional information and calculations.

Parallel rules, rolling rules and Cras rules: what are the differences?

Several instruments can be used to plot directions on a nautical chart, but they do not all work in the same way.

Conventional parallel rule

This instrument generally consists of two rules connected by hinged arms. It transfers a direction while preserving its orientation and is well suited to navigators who use the traditional method of moving course lines towards the compass rose.

Rolling parallel rule

A rolling parallel rule moves on rollers while remaining parallel to its original orientation. It transfers a direction without using the hinged-arm mechanism of a conventional parallel rule. Depending on the model, it may also include angular graduations.

Cras rule

The Cras rule is a navigation instrument with an integrated double protractor, widely used in French navigation training. Its design allows users to plot courses and bearings and read angles using its graduations. Its operating method differs from that of a conventional parallel rule.

These instruments are therefore not interchangeable in every working method. The choice depends on the navigator's habits, the exercise being performed and the compatibility of the instrument with the training materials in use.

How to choose a parallel rule for marine navigation

Choosing a parallel rule involves several practical considerations. The instrument should suit the size of the charts being used and the available space at the chart table.

Key factors include:

• Length: a longer rule may make it easier to transfer directions across a large chart, while a compact model is easier to store and handle in a confined space.

• Material: transparent acrylic or similar materials make it easier to see chart details beneath the rule. Rigid materials can provide stability and withstand frequent use.

• Mechanism: the hinges should allow smooth movement while maintaining the parallel alignment of both rules.

• Graduations: markings should be easy to read and suitable for the navigation method being used.

• Handling: ease of movement and the ability to hold the instrument in position contribute to plotting accuracy.

• Storage: on board, the instrument should be protected against impacts, distortion and accidental movement.

For training purposes, clear graduations and ease of use are particularly important. For regular use on board, the durability of the mechanism and compatibility with the chart table deserve particular attention.

The parallel rule in passage planning

The parallel rule is part of a coordinated set of traditional navigation instruments. Used with a nautical chart, a pair of dividers and appropriate nautical publications, it helps navigators plan a route and understand its orientation.

Charts produced or distributed by hydrographic organisations such as Shom, Admiralty and NGA contain the cartographic information needed for chartwork. Depending on the chart and geographical area, compass roses, meridians, parallels and other reference features help determine directions.

The parallel rule allows navigators to work with these references, but it does not replace chart interpretation or consultation of up-to-date nautical information. Navigators must check the applicable chart edition and corrections and take actual navigation conditions into account.

Traditional navigation instruments remain useful even on vessels equipped with GPS or a chartplotter. They help navigators understand plotted courses, check directions and maintain an additional working method in the event of electronic difficulties.

Conclusion

A parallel rule is a navigation instrument designed to transfer directions and preserve the orientation of a line as it is moved across a nautical chart. Its operation relies on a hinged mechanism that keeps the two rules parallel. It facilitates course plotting, direction reading and the preparation of coastal and offshore passages.

Choosing a suitable model mainly depends on its length, material, graduations and ease of handling. Used with a nautical chart and other appropriate navigation instruments, a parallel rule remains a useful tool for recreational sailors, experienced navigators and nautical students.

FAQ – Parallel Rules for Marine Navigation

What is a parallel rule used for on a nautical chart?

It transfers a direction without changing its orientation, particularly between a plotted course line and the compass rose. It helps with course plotting and chartwork.

What is the difference between a parallel rule and a rolling rule?

A conventional parallel rule generally consists of two rules connected by hinged arms. A rolling rule moves on rollers while maintaining its orientation. They serve similar purposes but use different mechanisms.

What is the difference between a parallel rule and a Cras rule?

A parallel rule transfers a direction by maintaining parallel alignment. A Cras rule has an integrated double protractor for plotting directions and reading angles using its graduations.

Can a parallel rule be used to determine a course?

It can transfer a course direction and help determine its angle when the instrument and method allow it. Converting that direction into the course to steer using the vessel's compass also requires the appropriate magnetic corrections.

Is a parallel rule useful for offshore navigation?

Yes. It can be used to work on large-format nautical charts and plan offshore routes, provided the chart is appropriate for the geographical area and scale of navigation.

How do I choose the length of a parallel rule?

Choose a length suited to the chart size, the distances over which directions are normally transferred and the available chart-table space. A compact model is convenient in confined spaces, while a longer model can be helpful on large charts.

Does a parallel rule replace a pair of dividers?

No. A parallel rule is mainly used to transfer directions. Dividers are used, among other things, to transfer and compare distances on a nautical chart.

Can I navigate using only a parallel rule?

No. A parallel rule is a plotting instrument. Safe navigation also requires appropriate charts, up-to-date nautical information and the other instruments needed to determine positions, distances and courses.