The examiner is testing whether you can extract tidal stream data from two sources — the chart and a tidal stream atlas — and apply it correctly to passage planning and pilotage. A pass-standard answer names both sources, explains how to use them (including the interpolation method), and shows awareness of when one source is preferred over the other.
What the examiner is probing Can you actually use these tools, not just describe them? Expect follow-up questions that put numbers in front of you.
Tidal set and rate — definitions
- Set: the direction toward which the tidal stream flows, expressed as a true bearing (e.g. 095°T).
- Rate: the speed of the tidal stream, expressed in knots.
Sources of tidal stream data
1. Tidal stream data on the chart Admiralty charts show tidal stream diamonds (e.g. ◆A, ◆B). Each diamond has a reference port. The table in the chart margin gives set and rate for each hour relative to HW at that port, for both spring and neap rates. To use it:
- Identify which diamond is nearest your position.
- Find HW time at the reference port; determine how many hours before or after HW you need.
- Read off the spring and neap rates for that hour.
- Interpolate between springs and neaps using the actual range compared to the mean spring and neap ranges.
2. Tidal stream atlas Atlases (e.g. Admiralty Tidal Stream Atlases) show arrow plots for each hour referenced to HW at a standard port, with figures printed beside arrows in the format neap rate (spring rate) in tenths of a knot — e.g. 18,31 means 1.8 kn neaps, 3.1 kn springs.
- Turn to the page for the relevant hour before/after HW.
- Find your position; read the nearest arrow for set, and the printed figures for rate.
- Interpolate between spring and neap rates as above.
Interpolation Calculate where today's range sits between mean neap and mean spring ranges for the reference port (found in the tide tables or almanac). Apply the same proportion to the difference between neap and spring rates. This is sometimes called the proportional method.
Which source to prefer The tidal stream atlas gives a broad-area picture useful for passage planning and routing decisions. Chart diamonds give more precise, localised data and are generally preferred for close-quarters pilotage and WO calculations.
Structuring your spoken answer State which source you are using and why. Give the reference port. Walk through: find HW time → identify the relevant hour → read set and rates → interpolate for today's range. Quote a set as a three-figure true bearing and a rate in knots. If the examiner hands you a chart or atlas page, narrate every step aloud.