Navigation & Chart Work Flashcards

What is 'dead reckoning' (DR) in navigation?

Answer: Calculating estimated position using the last known position, course steered, speed, and time elapsed

Dead reckoning (DR) calculates an estimated position by advancing the last known position along the course steered, using the speed made good over a known time period. DR does not account for tidal stream or leeway.

What is an 'Estimated Position' (EP) and how does it differ from dead reckoning?

Answer: An EP accounts for tidal stream and leeway in addition to the DR — it is more accurate than pure DR

An Estimated Position (EP) is the most likely position of the vessel, calculated by applying tidal stream vector and estimated leeway to the dead reckoning position. It is more accurate than pure DR because it accounts for water movement and wind effect.

What is the significance of the depth contour known as 'charted depth' on an Admiralty chart?

Answer: Charted depths are measured from chart datum (Lowest Astronomical Tide) — the actual depth is the charted depth plus the height of tide

Admiralty charts use chart datum (approximately Lowest Astronomical Tide) as the reference level for soundings. The actual depth at any time equals the charted depth plus the height of tide above datum. This ensures charted depths are the worst-case minimum.

What is a 'tidal diamond' on a chart and how is it used?

Answer: A reference point on a chart with a letter identifying its tidal stream data table, giving direction and rate for each hour relative to HW

Tidal diamonds are lettered reference positions on a chart with corresponding tables giving the direction (set) and rate (in knots) of the tidal stream for each hour before and after High Water at the standard port. They allow accurate tidal stream calculations.

What is 'leeway' in coastal navigation?

Answer: The sideways drift of a vessel downwind caused by wind pressure on the hull and superstructure

Leeway is the angular difference between the course steered and the actual course made good, caused by wind pressure pushing the vessel sideways. It is estimated based on wind strength and direction relative to the vessel's heading.

How is a 'course to steer' calculated accounting for tidal stream and leeway?

Answer: Draw the tidal stream vector from the departure point, then the required track direction; the difference from the tidal vector endpoint gives the water track to steer

The vector triangle of velocity is used: plot the tidal stream vector for the passage time, then the required ground track from the tidal vector endpoint. The course from the departure point to where the water track intersects the ground track is the course to steer through the water.