Tidal Overfalls and Boils

The Airfoil Underwater

We usually think of current moving horizontally. But when fast water hits a sudden rise in the seabed (a ledge or shelf), it is forced upward.

This is identical to how air flows over a wing—when horizontal flow encounters an obstacle, it deflects vertically. Underwater, this creates powerful upwellings that destroy the laminar flow your foil depends on.

The Deflection

The water hits the rock face and shoots vertically. It breaks the surface tension, looking like a pot of boiling pasta. We call this a "Boil."

Boils are not actually boiling—there is no heat involved. The name comes from the visual appearance: mushroom-shaped upwellings, circular ripples, and bubbling turbulence on the surface. They indicate powerful vertical water movement beneath.

A strong boil can push several thousand liters of water per second to the surface. This creates a dome or hump in the water level, sometimes 30-60 cm (1-2 feet) higher than surrounding sea.

The Danger for Foils

A foil relies on smooth, laminar flow. Inside a boil, the water is aerated (full of bubbles) and moving vertically.

If you ride your foil through a boil, your wing loses lift because the water density drops (bubbles). You also lose stability because the water is pushing your fuselage up or down, not just front to back.

The effect is instantaneous and violent. Riders report feeling like they "hit a hole"—the foil drops suddenly as it enters the low-density zone. Recovery is nearly impossible before impact with the water surface.

Where to Find Boils

Boils commonly occur:

• Over underwater rock ledges during strong tidal flow
• At the edges of channels where deep water meets shallow
• Downstream of underwater obstacles like reefs or wrecks
• In narrow passages where current accelerates

Use bathymetric charts to identify potential boil zones. Look for:

• Sudden depth changes (10+ meters over short distance)
• Underwater pinnacles or seamounts
• Channel edges and shelf breaks
• Rocky outcrops in current paths

Reading Bathymetric Charts

Bathymetry is the underwater topography. Learning to read depth contours helps predict boil locations:

• Closely spaced contours: Steep underwater slope—high boil risk
• Widely spaced contours: Gradual slope—lower risk
• Isolated depth changes: Pinnacles and boulders—create localized boils
• Current direction: Boils form on downstream side of obstacles

Download nautical charts for your riding area. Study them before your session. Mark known boil zones and avoid them during strong tidal flow.

Contour Spacing Analysis

Use the "3-contour rule" to identify dangerous slopes:

• Safe gradient: 3 contour lines (e.g., 10m, 20m, 30m) spaced >500m apart = gentle slope, minimal turbulence
• Moderate gradient: 3 lines spaced 200-500m apart = watch for boils in strong current (>2 knots)
• Dangerous gradient: 3 lines spaced <200m apart = expect active boils even in moderate current (>1 knot)

Example: If 15m, 25m, and 35m depth contours are all within 150 meters of each other, that's a 30-meter rise over 150m—extremely steep. Avoid this zone during any significant tidal current.

The Physics of Upwelling

When current encounters a ledge, conservation of mass requires the same volume of water to pass the obstacle. With vertical obstruction, water must deflect upward:

Upwelling Velocity = Horizontal Current Speed × (tan θ)

Where θ is the slope angle of the ledge. A 45° ledge with 3-knot horizontal current generates 3-knot vertical velocity—creating explosive surface boils.

The Overfall Phenomenon

A "tidal overfall" is an extreme form of boil that occurs when very fast current (3+ knots) encounters a steep underwater ledge. The water literally "falls" over the edge, creating a hydraulic jump similar to a river rapid.

Overfalls are characterized by:

• Breaking waves on the surface (even in deep water)
• Loud roaring sound from turbulence
• Visible white water and foam
• Extreme danger to all small craft

Famous overfalls include the Portland Bill (UK), Hell Gate (New York), and Pentland Firth (Scotland). These locations are no-go zones for wing foiling during any significant tidal flow.

Summary

Avoid patches of water that look like they are boiling. The density is low, and the currents are vertical. You will fall. Use bathymetric charts to identify underwater topography that creates boils. During strong tidal currents (2+ knots), even small ledges can generate dangerous upwellings. The visual signature—smooth, mushroom-shaped water domes or active surface bubbling—is your warning to steer clear. When in doubt, ask local riders or fishermen about known boil locations.