A small magenta diamond enclosing a letter, printed at a point on the chart where the tidal stream has been measured. A table on the chart gives the stream's direction and its spring and neap rates for each hour before and after high water at a named standard port. Navigators interpolate between spring and neap rates for the day's range to estimate set and drift for passage planning.
Diamond C gives 070 at 2.4 knots springs three hours after HW Dover, so we'll carry a fair stream round the head.
Diamond tables are referenced to high water at the named standard port, not local high water, a common slip in passage planning.
Also heard as
- tidal stream diamond
- diamond
Related terms
Set and drift
The direction a current flows toward (set) and its speed (drift).
Tidal current
The horizontal flow of water that accompanies the rise and fall of the tide.
Fair tide
A tidal current flowing in the same direction a vessel wants to go, adding to its speed over the ground.
Tidal gate
A headland, strait or bar that can only be passed comfortably at certain states of tide, fixing the timing of a passage.