The Bay of Biscay has a fearsome reputation, but with modern weather forecasting and a patient approach to timing, the crossing is manageable for a well-prepared crew on a modern catamaran.
The Key Principle
Wait for the window. The single most important factor in a safe Biscay crossing is patience. A good weather window means:
- Winds below 20 knots for the full crossing duration (60–70 hours)
- Swell under 2 metres
- No fronts expected for at least 72 hours
- A stable or building high-pressure system
Reading GRIB Files
GRIB (GRIdded Binary) files are the foundation of offshore weather routing. Key sources:
- PredictWind — best for routing optimisation, paid subscription
- Windy.com — free, excellent visualisation
- SailDocs — email-based GRIB delivery for offshore (via satellite)
What to Look For
- 500mb charts: Show the position and movement of highs and lows
- Surface pressure: Identify fronts and convergence zones
- Wind at 10m: Your actual sailing conditions
- Wave height and period: Crucial for comfort on a catamaran
The Biscay High
In July and August, a semi-permanent high-pressure system often sits over or near Biscay. When it’s established:
- Light to moderate northwesterlies prevail
- Seas are 1–2 metres
- Visibility is typically excellent
The ideal scenario: Leave La Rochelle on the back side of a high as it moves east. You’ll have NW winds for the first 24 hours, then light and variable as you approach the Spanish coast.
Timing Your Departure
A typical July/August weather window lasts 3–5 days. With a 60-hour crossing, you need at minimum a 72-hour window (crossing time plus safety margin).
Decision Framework
- 5 days out: Identify potential windows in the models
- 3 days out: Models should agree — commit to provisioning
- 1 day out: Final go/no-go based on latest GRIB
- Morning of: Check for any rapid changes, then depart
Route Options
| Route | Distance | Notes |
|---|---|---|
| Direct (La Rochelle → A Coruña) | ~350 nm | Standard route, fastest |
| Coastal (via Brest → Ushant) | ~400 nm | More shelter options, longer |
| Southern arc | ~380 nm | Angled route, better wind angle |
For a catamaran, the direct route is usually best — multihulls handle offshore conditions well, and minimising time at sea reduces exposure to weather changes.
Safety Considerations
- Shipping lanes: Cross the Traffic Separation Scheme between Ushant and Finisterre. Cross at right angles, maintain a sharp radar watch at night.
- Orca zone: Biscay is within the orca interaction zone (Jul–Oct). See our separate guide.
- Fatigue: 60+ hours at sea is demanding. Plan watches carefully, eat well, stay hydrated.