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3-1
CHARACTERISTICS OF WATER
A competent seaplane pilot is knowledgeable in the
characteristics of water and how they affect the seaplane.
As a fluid, water seeks its own level, and forms
a flat, glassy surface if undisturbed. Winds, currents,
or objects traveling along its surface create waves and
movements that change the surface characteristics.
Just as airplanes encounter resistance in the form of
drag as they move through the air, seaplane hulls and
floats respond to drag forces as they move through
water. Drag varies proportionately to the square of
speed. In other words, doubling the seaplane’s speed
across the water results in four times the drag force.
Forces created when operating an airplane on water
are more complex than those created on land. For
landplanes, friction acts at specific points where the
tires meet the ground. Water forces act along the
entire length of a seaplane’s floats or hull. These
forces vary constantly depending on the pitch attitude,
the changing motion of the float or hull, and
action of the waves. Because floats are mounted
rigidly to the structure of the fuselage, they provide
no shock absorbing function, unlike the landing gear
of landplanes. While water may seem soft and yielding,
damaging forces and shocks can be transmitted
directly through the floats and struts to the basic
structure of the airplane.
Under calm wind conditions, the smooth water surface
presents a uniform appearance from above, somewhat
like a mirror. This situation eliminates visual references
for the pilot and can be extremely deceptive. If
waves are decaying and setting up certain patterns,
or if clouds are reflected from the water surface, the
resulting distortions can be confusing even for
experienced seaplane pilots.
DETERMINING SEA CONDITIONS
The ability to read the water’s surface is an integral
part of seaplane flying. The interaction of wind and
water determine the surface conditions, while tides and
currents affect the movement of the water itself.
Features along the shore and under the water’s surface
contribute their effects as well. With a little study, the
interplay between these factors becomes clearer.
A few simple terms describe the anatomy and characteristics
of waves. The top of a wave is the crest, and
the low valley between waves is a trough. The height
of waves is measured from the bottom of the trough to
the top of the crest. Naturally, the distance between two
wave crests is the wavelength. The time interval
between the passage of two successive wave crests at a
fixed point is the period of the wave.
Waves are usually caused by wind moving across
the surface of the water. As the air pushes the water,
ripples form. These ripples become waves in strong or
sustained winds; the higher the speed of the wind, or
the longer the wind acts on them, the larger the waves.
Waves can be caused by other factors, such as underwater
earthquakes, volcanic eruptions, or tidal
movement, but wind is the primary cause of most
waves. [Figure 3-1 on next page]
Calm water begins to show wave motion when the
wind reaches about two knots. At this windspeed,
patches of ripples begin to form. If the wind stops, surface
tension and gravity quickly damp the waves, and
the surface returns to its flat, glassy condition. If the
wind increases to four knots, the ripples become small
waves, which move in the same direction as the wind
and persist for some time after the wind stops blowing.
As windspeed increases above four knots, the water
surface becomes covered with a complicated pattern of
waves. When the wind is increasing, waves become
larger and travel faster. If the wind remains at a constant
speed, waves develop into a series of evenly
spaced parallel crests of the same height.
In simple waves, an object floating on the surface
shows that waves are primarily an up and down motion
of the water, rather than the water itself moving downwind
at the speed of the waves. The floating object
describes a circle in the vertical plane, moving upward
as the crest approaches, forward and downward as the
crest passes, and backward as the trough passes. After
each wave passes, the object is at almost the same place
as before. The wind does cause floating objects to drift
slowly downwind.
While the wind is blowing and adding energy to the
water, the resulting waves are commonly referred to
　
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