Tidal Diamonds and Stream Atlases

Tidal diamonds are lettered positions on the chart (e.g., diamond A, B, C) that give the set (direction) and rate (speed) of the tidal stream for each hour relative to HW at a reference port. The table on the chart gives this data for both spring and neap rates.

Tidal stream atlases show the direction and rate of tidal streams across a whole area, typically as arrows on a chartlet for each hour relative to HW at a standard port. They give a broader picture than tidal diamonds, which are specific to one location.

At Yachtmaster level, you must be comfortable interpolating between spring and neap rates when conditions fall between the two. Use the ratio of actual range to mean spring and neap ranges to calculate an intermediate rate. Tidal streams are essential for CTS calculations and passage timing.