weather and water wisdom 263
a 10-second period gives a much smoother ride on both power and sailing
vessels.
If you are caught out in high, short-period waves, slow the boat and change
direction to prevent pounding. Sailing and power vessels can tack across waves
for better control.
Significant Wave Height (SW) and Wavecasting
Waves increase to a fully developed state—called signifi cant wave height, or
SW—after about 24 hours of wind. Significant wave height represents the
average height of the highest one-third of all waves in the area, and it will vary
according to the average wind strength over the previous 24 hours.
- Find SW. Use the highest height of waves given during a marine forecast or
from a weather text message. If the forecast says “Seas 4 to 6 feet,” use 6 feet
as SW. If it says “Seas 4 to 6 feet, building to 8 feet,” use 8 feet as SW. - Correct SW to give the full wave spectrum. Use the accompanying wave
spectrum formulas and apply corrections to SW. Th is gives you the range of
wave heights you can expect to encounter.
Wave Spectrum Formulas
Signifi cant Wave Heights (average of one-third of the higher seas): SW
Average Wave Heights (average of two-thirds of the smaller seas): SW × 0.6
Th ree Higher Wave Heights (these seas will run higher than SW):
10% of seas encountered: SW × 1.3
1% of seas encountered: SW × 1.7
Highest occasional wave: SW × 2.0The forecast calls for 3- to 4-foot seas, building to 6 feet during the afternoon
and into the night. What conditions should you expect to encounter? Below is
the spectrum of expected seas based on a SW of 6 feet:Wave period.