Advanced Navigation Flashcards

When using a tidal stream atlas, a skipper needs to interpolate between springs and neaps. The spring rate is 3.4 knots and neap rate is 1.6 knots. Today's range is 75% of springs. What is the approximate tidal rate?

Answer: 2.95 knots

Rate = neap + (proportion × (spring − neap)) = 1.6 + (0.75 × 1.8) = 1.6 + 1.35 = 2.95 knots.

A vessel makes a cross-track error of 1.5 nm after 6 hours of sailing. The tidal stream was running athwartships at 0.5 knots for the whole passage. Has the tidal prediction been accurate?

Answer: Yes — 0.5 knots × 6 hours = 3 nm set, but actual error is 1.5 nm, suggesting other factors or the stream was less than predicted on average

Predicted drift = 0.5 × 6 = 3 nm athwartships. Actual error = 1.5 nm. The actual set was half the predicted value, suggesting the stream was weaker than expected or opposing leeway effects reduced the net error.

What is the difference between a 'water track' and a 'ground track' in coastal navigation?

Answer: Water track is the vessel's movement through the water; ground track is the actual path over the seabed, accounting for tidal stream

Water track is determined by heading and speed through water. Ground track (or track made good) is the actual path over the ground, which is the vector sum of water track and tidal stream.

A Yachtmaster candidate uses a three-bearing fix and the resulting cocked hat is large. What does this indicate?

Answer: Errors in bearings, compass deviation, or that the features used are poorly separated in angle

A large cocked hat indicates accumulated bearing errors. These may be from compass deviation, imprecise bearing-taking, or poorly separated features (less than 30–35° apart). The candidate should take new bearings and investigate compass error.

A vessel is navigating in thick fog and cannot see anything. Which of the following gives the most reliable continuous position fix?

Answer: GPS cross-checked with radar ranges to charted features

GPS provides continuous, accurate position. Cross-checking with radar ranges to charted features (headlands, buoys) provides redundancy and validates the GPS, guarding against GPS errors or jamming.

A skipper is on passage and their GPS fails. They have a chart, compass, log, and watch. The best approach is:

Answer: Maintain dead reckoning using course steered, log distance, and tidal vectors applied to the last known position, and seek visible position fixes as opportunities arise

Dead reckoning (DR) using course, speed, and tidal stream allows the skipper to maintain an estimated position. Visual bearings, transits, or soundings should be used to update the EP whenever possible.