Common Method: Compass Swing

Deviation is the error caused by the sailboat’s own magnetic fields affecting the compass. To calculate it, you compare your boat’s compass heading to a known accurate heading and measure the difference.

Basic Formula

Deviation=Magnetic Heading−Compass Heading\text{Deviation} = \text{Magnetic Heading} – \text{Compass Heading}

  • If the compass reads lower than the magnetic heading → deviation is East.
  • If the compass reads higher than the magnetic heading → deviation is West.

Common Method: Compass Swing

This is the standard method used on sailboats.

Step 1: Choose Calm Conditions

  • Minimal wind/current
  • Flat water
  • Electronics and metal objects in normal operating positions

Step 2: Steer Known Headings

You steer the boat on several headings:

  • N
  • NE
  • E
  • SE
  • S
  • SW
  • W
  • NW

Usually every 15° or 30° for accuracy.

Step 3: Compare Compass to Known Heading

You need a trusted reference:

  • GPS course over ground (in calm conditions)
  • Transit ranges
  • Known charted landmarks
  • Harbor range markers
  • Another calibrated compass

Example:

Actual Magnetic Heading Boat Compass Reads Deviation
090° 085° 5° East
180° 184° 4° West

Step 4: Create a Deviation Card

You build a table for the helm.

Example:

Compass Heading Deviation
000° 2°E
030° 1°E
060° 0°
090° 5°E
120° 3°W

This card stays near the compass.

Important Navigation Relationship

The full correction chain is:

Compass+Deviation=MagneticMagnetic+Variation=True\text{Compass} + \text{Deviation} = \text{Magnetic} \\ \text{Magnetic} + \text{Variation} = \text{True}

Often remembered as:

  • “Compass to True: Add East”
  • “True to Compass: Subtract East”

What Causes Deviation on Sailboats?

Common sources:

  • Engine block
  • Batteries
  • Speakers
  • Wiring
  • Stainless steel
  • Tools near the binnacle
  • Electronics
  • Autopilot systems

Even handheld radios near the compass can change deviation.

Professional Adjustment

A marine compass adjuster can:

  • Swing the compass
  • Use compensating magnets
  • Reduce deviation to near zero

Most boats still keep a deviation card because deviation changes slightly by heading.

ASA / Coastal Cruising Context

For ASA navigation training, you’re typically expected to:

  1. Understand deviation vs variation
  2. Read a deviation card
  3. Convert:
    • Compass → Magnetic → True
    • True → Magnetic → Compass

Example:

  • Compass = 100°
  • Deviation = 3°E
  • Variation = 7°W

Then:

100∘+3∘−7∘=96∘ True100^\circ + 3^\circ – 7^\circ = 96^\circ\text{ True}