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absence of the force that generated them. The waves
have a uniform and orderly appearance characterized
by smooth, regularly spaced wave crests.
Primary Swell—The swell system having the greatest
height from trough to crest.
Secondary Swells—Swell systems of less height than
the primary swell.
Swell Direction—The direction from which a swell is
moving. This direction is not necessarily the result of
the wind present at the scene. The swell encountered
may be moving into or across the local wind. A swell
tends to maintain its original direction for as long as it
continues in deep water, regardless of changes in wind
direction.
Swell Face—The side of the swell toward the observer.
The back is the side away from the observer.
Swell Length—The horizontal distance between successive
crests.
Swell Period—The time interval between the passage
of two successive crests at the same spot in the water,
measured in seconds.
Swell Velocity—The velocity with which the swell
advances in relation to a fixed reference point, measured
in knots. (There is little movement of water in the
horizontal direction. Each water particle transmits
energy to its neighbor, resulting primarily in a vertical
motion, similar to the motion observed when shaking
out a carpet.)
Chop—A roughened condition of the water surface
caused by local winds. It is characterized by its irregularity,
short distance between crests, and whitecaps.
Downswell—Motion in the same direction the swell is
moving.
Upswell—Motion opposite the direction the swell is
moving. If the swell is moving from north to south, a
seaplane going from south to north is moving upswell.
SEA STATE EVALUATION
Wind is the primary cause of ocean waves and there is
a direct relationship between speed of the wind and the
state of the sea in the immediate vicinity. Windspeed
forecasts can help the pilot anticipate sea conditions.
Conversely, the condition of the sea can be useful in
determining the speed of the wind. Figure 8-1 on the
next page illustrates the Beaufort wind scale with the
corresponding sea state condition number.
While the height of the waves is important, it is often
less of a consideration than the wavelength, or the distance
between swells. Closely spaced swells can be
very violent, and can destroy a seaplane even though
the wave height is relatively small. On the other hand,
the same seaplane might be able to handle much higher
waves if the swells are several thousand feet apart. The
relationship between the swell length and the height of
Ch 08.qxd 8/25/04 11:27 AM Page 8-1
8-2
the waves is the height-to-length ratio [Figure 8-2].
This ratio is an indication of the amount of motion a
seaplane experiences on the water and the threat to capsizing.
For example, a body of water with 20-foot
waves and a swell length of 400 feet has a height-tolength
ratio of 1:20, which may not put the seaplane at
risk of capsizing, depending on the crosswinds.
However, 15-foot waves with a length of 150 feet produce
a height-to-length ratio of 1:10, which greatly
increases the risk of capsizing, especially if the wave is
breaking abeam of the seaplane. As the swell length
decreases, swell height becomes increasingly critical to
capsizing. Thus, when a high swell height-to-length
ratio exists, a crosswind takeoff or landing should not
be attempted. Downwind takeoff and landing may be
made downswell in light and moderate wind; however,
a downwind landing should never be attempted when
wind velocities are high regardless of swell direction.
When two swell systems are in phase, the swells act
together and result in higher swells. However, when
two swell systems are in opposition, the swells tend to
cancel each other or “fill in the troughs.” This provides
a relatively flat area that appears as a lesser concentration
of whitecaps and shadows. This flat area is a good
touchdown spot for landing. [Figure 8-3]
Sea surface smooth and mirror-like
Scaly ripples, no foam crests
Small wavelets, crests glassy, no breaking
Large wavelets, crests begin to break, scattered
whitecaps
Small waves, becoming longer, numerous whitecaps
Moderate waves, taking longer form, many whitecaps,
some spray
Larger waves, whitecaps common, more spray
Sea heaps up, white foam streaks off breakers
Moderately high, waves of greater length, edges of crests
begin to break into spindrift, foam blown in streaks
High waves, sea begins to roll, dense streaks of foam,
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