Advanced Navigation Flashcards

What is a 'running fix' and when is it used?

Answer: A position fix obtained by taking a bearing of a single object, advancing the position line along the vessel's track, then taking a second bearing of the same or another object

A running fix is used when only one object is visible. The first bearing is taken and the position line advanced (transferred) along the track made good. When a second bearing is taken, the intersection of the advanced line and new line gives a fix.

A depth sounding taken with an echo sounder shows 18.5 m. The tide height is 2.3 m. What is the charted depth at that position?

Answer: 16.2 m

Charted depth = sounding − tide height = 18.5 − 2.3 = 16.2 m. Subtracting the tide height removes the tidal contribution and gives the depth referenced to chart datum.

When planning a cross-Channel passage from the UK to France, which publication gives standard port tidal predictions for English Channel ports?

Answer: Admiralty Tide Tables, Volume 1 (NP201)

Admiralty Tide Tables NP201 (European Waters) provides full tidal predictions for standard ports and differences for secondary ports around the English Channel and Western European coasts.

A vessel is approaching a headland with a tidal race. How should the skipper time the passage?

Answer: Arrive at or near slack water or when the stream is fair, before it builds to maximum rate

Tidal races can produce dangerous overfalls, breaking seas, and confused water. Passage at slack water or during a fair (favourable) stream before it builds is standard practice. Maximum spring rates in a race can be lethal.

What is a 'Secondary port difference' in tide tables and how is it applied?

Answer: A time and height correction listed in tables that, when applied to a standard port's prediction, gives the secondary port's tide

Secondary port differences include time differences (HW and LW) and height differences (HW and LW heights). These are applied to the standard port predictions after interpolating for the appropriate date.

A cross-track error of 0.3 nm is acceptable on an open ocean crossing but not in a narrow channel. Why?

Answer: In a channel, margins are small and 0.3 nm of error could put the vessel on a bank or reef — accuracy requirements scale with the density of hazards

Navigation precision requirements increase as the proximity and density of hazards increases. Offshore, 0.3 nm is negligible; in a 0.5 nm wide channel with shoals either side, it is unacceptable.