The Master's Obligation
Compasses are primary navigation instruments — the Master is responsible for ensuring they are maintained, corrected, and used correctly throughout a passage. An error undetected is an error that compounds.
Magnetic Compass
Why it matters first: The magnetic compass is the statutory compass on most yachts under 3000 GT. It requires no power, no gyroscope, no software — it will function when everything else has failed. That is why it retains primacy as the compass of last resort, and why the Master must know its current deviation and variation at all times.
Deviation and variation. Variation is the angular difference between true and magnetic north at a given location; it is obtained from the chart or almanac and changes slowly with geographic position and over time. Deviation is caused by the vessel's own magnetism — the steel hull, engines, electrical cables, speakers, and loose ferrous items near the binnacle all contribute. Deviation varies with the ship's head. A deviation card must be posted and kept current. The Master should know how to swing the compass and construct a deviation card, or arrange for a compass adjuster to do so.
What degrades a magnetic compass. Placing electronic equipment, tools, or speakers close to the binnacle introduces temporary or permanent deviation. High-power DC cables running nearby, or parking a steel tender on the coach-roof above, can distort the field significantly. Heeling error on sailing yachts changes deviation as the vessel rolls. The Master must brief crew on compass exclusion zones.
Checking the compass. Regular checks using transit bearings, celestial azimuths, or comparison against a GPS COG on a steady heading allow the Master to monitor accumulated error. Any change from the deviation card must be investigated and recorded.
Gyro Compass
A gyrocompass seeks true north by using gyroscopic inertia and Earth's rotation. Its advantage is that it reads true, eliminating the variation correction. Its limitations are significant: it requires power, takes time to settle after start-up (up to several hours depending on latitude and type), is sensitive to rapid changes of speed or course, and becomes progressively less accurate at high latitudes where the horizontal component of Earth's rotation diminishes. Speed and latitude corrections must be applied where the manufacturer specifies. The gyro must be cross-checked against magnetic and GPS data regularly on passage.
Repeaters distribute the gyro heading around the vessel. A repeater error relative to the master gyro must be identified and logged — watch officers must know which instrument they are steering by.
Autopilot
The autopilot steers to a reference — either the gyro, magnetic compass, or GPS track. It does not maintain a lookout, does not react to current set, and does not make routing decisions. The Master must ensure:
- The helmsman (or officer of the watch) understands which reference the autopilot is using, and has verified that reference is accurate.
- Autopilot is never used as a substitute for an officer of the watch: a competent person must remain on the bridge with full situational awareness.
- Manual steering is tested and crew are proficient; over-reliance on autopilot degrades manual steering skills. Planned regular periods of hand steering maintain competence.
- Autopilot gain and response settings are adjusted for sea state — a badly tuned autopilot in heavy weather will hunt, oversteering and causing fatigue on machinery and rig.
- The system is immediately disengaged and manual steering taken if any doubt arises about the reference signal, in congested waters, or in reduced visibility requiring close-quarters decision-making.
All three systems must be cross-referenced continuously. A heading discrepancy between gyro and magnetic that cannot be accounted for by known variation and deviation is a fault requiring investigation before proceeding.