UK Admiralty Tidal Atlases show speed in units of tenth of a knot.
For instance the chart for 3 hours after high water is valid from HW+2½ to HW+3½, not from HW+3 to HW+4.
[2] Tidal atlases may provide additional information for areas such as estuaries where it important to calculate tides away from the ports.
[4] To calculate the rate at an intermediate tide between neap and spring, interpolation is required.
Traditionally this has been done using a "calculation of rates" chart found inside tidal atlases.