Question 1

What is the difference between a tidal stream atlas and tidal diamonds on a chart, and when is each used?

Accepted Answer

Tidal diamonds: specific points on the chart (labelled A, B, C...) with a table in the chart border showing tidal stream direction and rate at each hour relative to HW at the standard port — used for precise calculations at specific locations. Tidal stream atlas: a separate booklet showing the stream pattern over a large area at each hour — better for passage planning and understanding the overall flow pattern across a passage TIDAL STREAM SOURCES: both use the same underlying data (UKHO survey data) but present it differently. Tidal diamonds give interpolated data at a fixed point — useful for EP calculations and course to steer. The tidal stream atlas shows the whole picture at once — critical for passage planning (e.g., Portland Race, the Solent, the Dover Strait). In practice, the Coastal Skipper uses both: atlas for planning (when to depart, where the favourable stream runs) and diamonds for the actual course-to-steer calculation. The two can occasionally disagree in complex tidal areas — the diamond data (from the chart) is usually more current.

Question 2

The standard port (Portsmouth HW) is 1800 hrs, height 4.8m. The secondary port time difference is -0020 and height difference is -0.4m. What is the HW time and height at the secondary port?

Accepted Answer

HW time: 1800 - 0020 = 1740 hrs. HW height: 4.8 - 0.4 = 4.4m. Note: differences are applied directly when given as simple offsets. When height ratios are given (not simple differences) use: (secondary HW - secondary LW) / (standard HW - standard LW) × tidal range Secondary port calculation: apply the time difference algebraically (negative = subtract; positive = add). Height differences work the same way. This example is the simple case where differences are given as absolute values. The more complex case (in reed's and almanac) uses ratio calculations interpolating between MHWS/MHWN differences. Always read the secondary port data carefully — some almanacs give "differences" that require interpolation between spring and neap values based on the standard port's predicted height. This is a core Coastal Skipper exam topic.

Question 3

A skipper is relying entirely on a chartplotter with AIS for passage navigation. What are the key limitations that require maintaining traditional skills?

Accepted Answer

Chartplotter limitations: 1) Chart data may be out of date (new wrecks, changed cardinal marks, silting); 2) GPS antenna failure = no position; 3) Power failure = no display; 4) Small-vessel AIS (Class B) is not fitted to all vessels — canoes, kayaks, small fishing boats, naval vessels are all invisible; 5) Chartplotters can show the vessel in a precise but wrong position (datum error, signal multipath). Maintain paper chart, compass, and watch-keeping skills ELECTRONIC NAVIGATION DEPENDENCE: the RYA and MCA explicitly teach that electronic navigation must be backed up by traditional skills. The most dangerous failure mode is overconfidence — the GPS shows a precise position, but the precision may be wrong (multipath, datum error, chart error). AIS Class B is not compulsory for vessels under 300gt recreational — the majority of vessels the Coastal Skipper will encounter (ferries, fishing vessels, RIBs) may have no AIS. A vessel 200m off track in a channel with GPS can be in danger. Paper chart and compass DR must remain a practised skill.