Tidal turbines are the obvious way to tap ocean currents, but they suffer a brutal problem: current speeds are pathetic. The Gulf Stream cruises at just 1.5–2.0 m/s. Since kinetic power scales with V³, a stationary turbine there generates almost nothing. The trick is to move faster than the water. Enter the underwater kite — a tethered hydrofoil that flies in figure-eights through the current at 10× the flow speed, sweeping an enormous virtual area with a small wing.

The concept exists. Sweden's Minesto Dragon 12 (12 m wingspan) produces 1.2 MW in a 1.4 m/s tidal flow off the Faroe Islands. What happens when we scale to a full farm blocking a kilometer of the Florida Straits?

The Loyd equation, wet edition

For a crosswind (or cross-current) kite, Miles Loyd showed in 1980 that power output is:

P = (2/27) × ρ × A × V³ × (C_L³ / C_D²)

Where the C_L³/C_D² term captures the kite's ability to sweep an area far larger than its wing. For a well-designed hydrofoil, L/D ≈ 15 and C_L ≈ 1.2, giving C_L³/C_D² ≈ 270.

For a single 100 m² kite in a 2 m/s Gulf Stream (ρ_seawater = 1025 kg/m³):

P = (2/27) × 1025 × 100 × 8 × 270
  ≈ 16.4 MW per kite

That's ten times what a giant offshore wind turbine averages. Water is 830× denser than air; that density is doing all the heavy lifting.

Blanketing a kilometer of ocean

Space each kite 200 m apart to avoid wake interference. A kilometer-wide, 100 m-deep swath fits maybe 25 kites across × 3 vertical layers = 75 kites, ~1.2 GW continuous. That is baseload — no diurnal cycle, no seasonality worth mentioning. It replaces a full nuclear reactor, from a patch of open ocean.

Where the physics fights back

• Tether drag. A 200 m Dyneema cable at 20 m/s flow has drag force F = 0.5 × 1025 × 20² × (200 × 0.05) × 1.2 ≈ 2.5 MN. That eats maybe 15% of gross power. Manageable.
• Cavitation. At 20 m/s hydrofoil speed, local pressure on the suction surface drops toward vapor pressure (~2.3 kPa). At 30 m depth, ambient is ~400 kPa. The margin is thin — push wing loading too high and the foil implodes bubbles that erode titanium in months. This caps kite speed and forces conservative C_L.
• Anchoring. Each kite pulls on its anchor with ~800 kN of steady force. Multiply by 75 kites and you're driving suction piles into a seabed that already has to survive hurricanes.
• Sargassum. The Gulf Stream carries megatons of floating algae. A kite flying blind at 20 m/s into a mat of Sargassum is a spectacular failure mode. You'd need active sonar avoidance and daily hull cleaning.

The geopolitics of a river you don't own

Extracting 1.2 GW removes about 0.001% of the Gulf Stream's kinetic energy — negligible. But the Gulf Stream flows past a dozen jurisdictions on its way to warming Europe. Even slightly perturbing it invites lawsuits from every country north of Portugal.