An overfall in sailing refers to an area of extremely turbulent, rough, and often breaking water, typically created when strong tidal currents flow over a shallow, uneven seabed, such as a ridge, shoal, or reef. They often appear as steep, breaking waves, or "white water," and are particularly dangerous for small vessels.

Key Aspects of Overfalls
Formation: Overfalls happen when a fast tidal stream is forced upward by a submerged obstruction or when two opposing currents meet.

Location: Common near coastal headlands, islands, or narrowing channels, appearing as localized, choppy zones even on calm days.

Severity: The intensity depends on the speed of the current, water depth, and seabed topography, often creating "standing waves" (waves that stay in one spot).

Hazards: They can capsize or damage boats, cause severe pitching, and lead to heavy water coming over the deck.

Chart Indication: Often marked on navigation charts with a "wiggly line" symbol or noted as "overfalls".