Why Continuous Position Monitoring Matters
A passage plan is only a statement of intent. The sea introduces leeway, tidal set, helmsman error, and current — all of which push a vessel progressively off the planned track. Without systematic position monitoring, small errors compound silently until the vessel is in danger. The purpose of position monitoring is not record-keeping; it is early detection of deviation so that corrective action is taken while sea room still exists.
The oral examination syllabus underpinned by MSN 1858 assesses whether OOWs can demonstrate competent use of all available means to determine and monitor position continuously throughout a passage.
What Constitutes a Fix and How Often
A fix requires at least two independent position lines. On a modern yacht the primary tool is GNSS (GPS/GLONASS), but GNSS alone is a single-source fix. Cross-checking against an independent method — visual bearing, radar range, depth contour — is best practice and expected in an oral exam. The frequency of fixing depends on proximity to hazards, speed, visibility, and available sea room. In open water an hourly fix may suffice; in coastal or confined waters fixes should be taken and plotted far more frequently.
Tools and Methods
- GNSS: fast and accurate but susceptible to spoofing, jamming, and datum error — always confirm chart datum matches receiver datum
- Visual bearings: compass or pelorus bearings on charted objects; three bearings give a cocked hat which itself indicates quality of the fix
- Radar: range rings and VRM give highly accurate distance-off; radar bearings are less accurate than visual
- Depth: echo sounder cross-referenced against the charted depth (accounting for tide) provides a rapid sanity check
- Celestial: relevant on ocean passages; confirms independence from electronic infrastructure
Monitoring the Track
Once underway, the OOW plots fixes at the agreed interval and compares each fix to the planned track. Cross-track error (XTE) shown on a chartplotter is useful but must be checked against the paper or electronic chart to ensure proximity to hazards is assessed, not just whether the line looks tidy. Log the fix, the time, and the course made good (CMG) at each plotting interval.
Action When Off Track
- Establish the facts: determine the magnitude and direction of the deviation and identify any immediate danger.
- Identify the cause: set, leeway, steering error, or a combination. This informs the correction.
- Assess the danger: is there adequate sea room? Are there hazards on the side to which you have been set?
- Apply a correction: alter course to return to track, or if conditions warrant, establish a new safe track. Do not blindly steer back to the original waypoint if that line now passes through shoal water.
- Inform the master: any significant deviation should be reported immediately.
- Verify the correction is working: take a new fix promptly after the alteration to confirm the vessel is responding as expected.
- Record: note the deviation, cause (if known), action taken, and person informed in the deck log.