Section 4

Correcting the Compass

4.2

The Deviation Table

A deviation table has three columns. The first column lists the magnetic readings and the third the boat's compass readings. The centre column lists the differences between the magnetic and compass readings — the deviation.

An Easy-to-Make Deviation Table

There are a number of ways to make a deviation table. Some methods are more accurate than others. Some are complex and require experience, time, and patience. As a result many boaters never make a deviation table.

This approach will yield a table that is sufficiently accurate for most boats. (Even an imperfect deviation table is better than none at all.)

Two people are required on board. The skipper should watch the steering compass and a crew member should handle the hand-held compass.

A table is drawn up similar to that illustrated above. In this case, there are intervals of 15° degrees between each heading. If, after completing the table, deviation between entries is too large - more than 2° or 3° - the process can be repeated using smaller increments between magnetic headings.

Two people can quickly generate a deviation table by comparing readings from the boat's compass with those from a compass not effected by deviation.

The crew member stands on the centreline of the boat, away from magnetic influences, and sights the hand-held compass through the forestay or straight over the bow.

The boat is slowly steered in a circle and the crew person calls out when the boat is on the desired magnetic heading. The skipper holds the boat on the heading, notes the reading on the steering compass, and enters it in the table's "compass" column. The boat is turned to the next magnetic heading and the process repeated until the table is complete.

If the steering compass heading is less than the hand-held magnetic heading, deviation is east. If the steering compass reading is greater than the hand-held reading, deviation is west. Deviation is annotated E or W as appropriate, for example, 3°W.

A Note About Steel Boats

This method for making a deviation table may not work on a steel-hulled boat because of the large and varying amounts of magnetic influence it generates. In this case you may need to call a professional compass adjuster.

Practice Deviation Table
Print this table and use it for the exercises.

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