264 seamanship secrets
Average wave height = 3.6 feet as follows: SW= 6; 6 × 0.6 = 3.6 feet
10% of highest waves = 7.8 feet as follows: SW= 6; 6 × 1.3 = 7.8 feet
1% of highest waves: 10.2 feet as follows: SW= 6; 6 × 1.7 = 10. 2 feet
Extreme possibility: 12.0 feet as follows: SW= 6; 6 × 2 = 12 feetSEA-CRET TIP
X Estimate signifi cant wave height (SW) by squaring the wind speed
over the past 24 hours and then multiplying by 2%. For example, a
15-knot wind blowing for 24 hours develops a SW of about 4^1 / 2 feet.
A 30-knot wind develops an 18-foot signifi cant wave height after
24 hours.Swell Secrets
Swell—A long crestless wave or series of crestless waves in the open sea.
—Merriam-Webster’s DictionarySwell (also called swell waves) form aft er the wind in a specifi c area ceases to blow.
For instance, if the wind blows from the north for 48 hours, wind waves form
during that period. Th ese waves build in height and move with great speed to
leeward. When the wind dies, the water continues to move through momentum as
a swell. Th e swell may travel hundreds or even thousands of miles before it makes
contact with some obstruction: a shoal, an island, coastal beach, or a cliff. If the
land causes the swell to break, it loses much of its energy. But if the land off ers
only modest resistance, the swell continues past with much of its energy intact.
SWELL AND BREAKER FORMATION
Swell waves form as the environment surrounding the wind waves changes.
Three factors give birth to swell waves:
Dying wind
Wind waves that are moving into a calmer area
Wind waves that are moving into a region with diff erent wind and wind-
wave directionsSwell moves slightly faster than the wind-driven waves that created them.
Deepwater swell have well-rounded tops and gentle motion. Th eir height de-
creases by about half every 24 hours, and their periods increase considerably.
Swell slow, and cluster together, and begin to steepen as they approach land.
If the sea bottom rises to within one to two times of their height, the crest of