Rate (Tidal Stream)

The speed at which a tidal stream flows, typically measured in knots.

The rate of a tidal stream is the velocity at which the water moves due to tidal forces, usually expressed in knots (nautical miles per hour). Tidal streams are horizontal movements of water caused by the rise and fall of the tide, and their rate can vary significantly depending on location, time of day, and the phase of the tide. The rate is a crucial factor for navigation, as it affects a vessel’s speed over the ground and can influence course planning, fuel consumption, and safety. Tidal stream rates are often published in tidal stream atlases or tidal diamonds on nautical charts, showing expected speeds and directions at various times relative to high water at a reference port. The rate typically increases as the tide moves towards mid-tide (about three hours after high or low water), reaching a maximum known as the maximum rate, and then decreases as the tide turns. Mariners must account for the rate of tidal streams when plotting courses, especially in narrow channels, estuaries, or areas with strong currents, to avoid being set off course or encountering hazardous conditions.