weather and water wisdom 265
the swell breaks; this creates extremely hazardous conditions in inlets for small
craft. For example, a small 3-foot swell passing through an inlet begins to break
when it passes over a sandbar 3 to 6 feet below the surface.
Some harbors are open to the sea, with little protection from natural or man-
made breakwaters. A ground swell may roll into such areas, straining mooring
lines or ground tackle.
AVOID CROSS-SWELL MADNESS
A cross swell results from high wind waves blowing across the top of an undeveloped
swell. Th is creates havoc for power and sailing craft alike.
Plan any Gulf Stream transit with great care in the fall, winter, or early
spring. Every few weeks, cold fronts march across North America, bound for
the Atlantic coast. As the front approaches, southerly winds begin to blow, and
within 24 hours the waves are fully developed. Just as that sea starts to develop
into swell, the cold front passes. Winds shift northwest, velocity increases, and a
steep northwesterly sea builds on top of the undeveloped southerly swell. Th row
in some northerly Gulf Stream current fl owing against that northwesterly gale
and you have complete chaos. Many seasoned mariners have lost their lives
under these circumstances.
If a cold front is headed your way, wait to make the Gulf Stream crossing.
Delay departure 12 or more hours aft er the front passes before casting off.
BEWARE OF ROCK AND ROLL AREAS CAUSED BY SWELL
You’re island-hopping in the Lesser Antilles and expect to make your next
stop late this aft ernoon. Last night, an uncomfortable groundswell turned your
idyllic anchorage into a stay at the Hard Roll Café. You were on the protected
lee side, so why did this happen? How can you use the topography of an island
to warn you of this in the future?
As swell approach a fl at or continuous coastline, rising bottom contours
slow their forward motion. Swell approaching a beach obliquely have diff erent
speeds on their off shore and near-shore sections. Th e off shore section, travel-
ing in deeper water, experiences less friction with the seabed and is therefore
faster than the near-shore section. As the swell approaches the beach, therefore,
its crestline becomes less oblique and more nearly parallel with the shore. In
general, swell will crest and break parallel to the coast.
Refraction of a Swell
Roundish islands that are surrounded by gently rising bottom contours,
however, present another problem. As swell roll into the beach, they meet
little resistance from the bottom. Instead they bend, or refract, on each side
of the island and then continue wrapping around the back side. When the