Height Is Not Power
Wing foilers often look only at wave height. This is a mistake. A 3-foot wave can be a gentle bump or a powerful wall of water. The difference lies in the period.
Wave period is the time in seconds between two wave crests passing a fixed point. It measures how much energy is packed into the swell.
The Energy Relationship
Wave energy is proportional to height² × period. Doubling the period quadruples the energy. A 2ft/12s wave carries more power than a 4ft/6s wave.
Longer period = deeper energy, faster travel, more power
Period Categories
Wave period determines wave characteristics and riding quality:
Wave Period Classification
Short Period
4-8 seconds
Local wind chop, messy, weak
Avoid
Mid Period
8-12 seconds
Regional swell, usable
Moderate
Long Period
12-20+ seconds
Groundswell, powerful, clean
Best
Short Period (4-8 seconds)
These are wind waves—locally generated and messy.
- Close spacing: Waves stack tight, 20-50m apart
- Shallow energy: Power concentrated near surface
- Fast decay: Lose energy quickly over distance
- Riding feel: Bumpy, chattery, like riding over road bumps
- Breaking: Weak crumble, no punch
Long Period (12-20+ seconds)
This is groundswell—waves that traveled hundreds or thousands of miles.
- Wide spacing: 150-300m+ between waves
- Deep energy: Power extends meters below surface
- Slow decay: Travel across entire ocean basins
- Riding feel: Long rolling hills, smooth glide windows
- Breaking: Powerful, hollow, heavy water movement
Even a 2ft long-period swell carries immense energy. It moves faster, breaks harder, and provides superior glide.
Wave Speed and Wavelength
Period determines how fast waves travel:
Speed Comparison
6 second period
Short period chop
~9 m/s
12 second period
Typical groundswell
~19 m/s
18 second period
Epic swell
~28 m/s
Formula: Speed (m/s) ≈ 1.56 × Period (seconds)
The "Spacing" Factor
Period creates spacing between waves. This spacing is critical for foiling:
- 6s period: 30-50m gaps. No glide window. Constant impacts.
- 10s period: 100-150m gaps. Brief glide window. Manageable.
- 15s+ period: 200-350m gaps. Long glide window. Downwind paradise.
Longer period creates a cleaner, more predictable surface for the foil to glide on.
Practical Tips
For downwinding, target 12s+ period for easier bump connections
For surf foiling, 10-15s provides best wave quality
Ignore short period swells (<8s) unless desperate
Multiple swells? Ride the longer period direction
Summary
Don't judge the forecast by height alone. A small wave with a long period offers a better ride than a tall wave with a short period. Look for 10-12 seconds minimum, 15+ seconds for epic sessions. Period is power.
Related Articles: