Chartwork

Dead Reckoning (DR) and Estimated Position (EP)

Start from the latest accepted position and time. Advance the course or track through the water by distance through the water to obtain the DR. In the convention used here, tidal stream a...

• DR advances movement through the water from the accepted start
• EP adds estimated leeway and tidal movement

Position Fixing

A bearing to a correctly identified charted object creates a line of position. Two suitably separated lines can intersect; a third can expose disagreement. A transit may provide a strong...

• A fix is observation-derived, not error-free
• Choose unmistakable objects and useful angles of cut

Running Fix (Transferred Position Line)

Take and time the first bearing to an unmistakable charted object, then plot its line of position. Between observations, estimate the vessel's ground displacement from course, distance, l...

• The observations must be brought to one common time
• Transfer the earlier line parallel by estimated ground displacement

Course to Steer (CTS)

Draw the desired ground-track line from the starting position. For one stated time interval, plot the tidal set towards its true direction and drift equal to rate multiplied by time. From...

• Use one time interval for tidal drift and boat distance
• Plot the tidal vector from the start

Multi-Hour CTS with Changing Tidal Streams

Choose the proposed passage interval and identify the tidal stream for each relevant hour from the correct source and reference-port time. Convert every rate into a displacement for the p...

• Use the correct tidal hour and partial-hour displacement
• Stack the three hourly stream displacements head to tail in this worked example

Worked CTS: Complete Two-Hour Calculation

A fictional yacht must make good 090°T for two hours at 6 kn through the water. A steady stream sets 180°T at 2 kn. Leeway is ignored for this training example. The boat-distance arc is 2...

• Two hours at 6 kn gives a 12 M boat vector
• Two hours of 2 kn set south gives 4 M tidal drift

Worked DR to EP: Coordinates and Assumptions

Start from the fictional fix 50°00.0′N 001°00.0′W. The yacht travels on 090°T through the water at 6 kn for two hours with no leeway, so its DR displacement is 12 M east. Near 50°N, one m...

• 12 M east at 50°N changes longitude by about 18.7′
• The DR is 50°00.0′N 000°41.3′W

Waypoints and GPS Navigation

GNSS receivers can provide position, COG and SOG; plotters can show bearing and distance to a waypoint and XTE from an active route leg. Before use, verify the coordinate, datum, route di...

• Validate waypoint coordinates and inspect every route leg
• Use a display scale that reveals the relevant hazards

Leeway

Wind and sea can move a vessel sideways to leeward so the track through the water differs from heading. The amount depends on vessel type, loading, point of sail, speed, wind, sea state a...

• Leeway moves water track to leeward of heading
• Its amount varies with vessel and conditions

Clearing Bearings and Danger Angles

During passage planning, draw a boundary that leaves the required margin from a charted danger. From an unmistakable charted object, derive the bearing of that boundary and label the safe...

• Build the bearing from a charted danger boundary
• Label the safe side instead of memorising less-than or greater-than

Yachtmaster Offshore Standard

The RYA Yachtmaster Offshore examination includes navigation and safety, while the theory scope includes position fixing, course shaping and plotting, tidal knowledge, publications and el...

Advanced Scenario Check

A fictional yacht is approaching a headland in reduced visibility. Its chartplotter and tablet show the same GNSS position, electronic chart and copied route. A correctly identified radar...