Navigation and Chartwork Flashcards

A vessel observes a lighthouse bearing 270°(T) at 2000hrs and 315°(T) at 2030hrs while maintaining a course of 000°(T) at 10 knots. What is the approximate distance off the lighthouse at 2030hrs?

Answer: 5 nautical miles

Using the running fix technique, the vessel has run 5 nautical miles in 30 minutes at 10 knots. The bow angle changes from 90° to 45° (relative bearings). Applying the doubling the angle on the bow principle or trigonometry, the distance off at the second bearing is equal to the distance run, which is 5 nautical miles.

What is the purpose of applying the 'cocked hat' principle when taking a three-bearing fix?

Answer: To identify potential systematic errors and estimate the most probable position within the triangle formed

When three position lines do not intersect at a single point, they form a triangle called a 'cocked hat'. The navigator's most probable position lies within or near this triangle. A large cocked hat may indicate systematic error such as compass deviation, while a small one suggests good accuracy.

In UK waters, what do the abbreviations 'MHWS' and 'MLWS' stand for on Admiralty charts, and what is their significance for navigation?

Answer: Mean High Water Springs and Mean Low Water Springs — the average spring tide levels used for clearance calculations

MHWS (Mean High Water Springs) and MLWS (Mean Low Water Springs) are the average heights of the highest and lowest spring tides respectively. MHWS is used to define the coastline on Admiralty charts, while chart datum (approximately LAT) is used for depth soundings. These values are essential for calculating under-keel clearance and overhead clearance.

A vessel's compass heading is 130°(C). The variation is 4°W and the deviation for that heading is 2°E. What is the true heading?

Answer: 128°(T)

To convert compass to true: True = Compass + Deviation + Variation (applying W as negative and E as positive). So: 130° + (+2°) + (-4°) = 128°(T). The mnemonic 'Cadbury's Dairy Milk Very Tasty' (Compass → Deviation → Magnetic → Variation → True) helps remember the order, adding easterly and subtracting westerly errors.

What is the primary purpose of a passage plan as required by the MCA and SOLAS regulations?

Answer: To plan the intended route from berth to berth, identifying hazards, abort points, and contingencies

Under SOLAS Chapter V Regulation 34 and the MCA's requirements, a passage plan must cover the entire voyage from berth to berth. It must identify navigational hazards, establish safe passing distances, mark abort points and contingency anchorages, note reporting requirements, and account for tidal and weather conditions.

When approaching a port in poor visibility, the vessel's ECDIS shows the route crosses an area marked with a magenta 'T' symbol. What does this indicate?

Answer: An area where VTS (Vessel Traffic Service) reporting is required

The magenta 'T' symbol on ECDIS and Admiralty charts indicates a VTS reporting point where vessels are required to report to the Vessel Traffic Service. In poor visibility this is especially important as VTS can provide traffic information and navigational assistance to vessels in the area.