seaplanes
and weather conditions
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The competent seaplane pilot must be
knowledgeable in the characteristics of water to understand its effects
on the seaplane. Water is a fluid, and although it is much heavier than
air it behaves in a manner similar to air. Since it is a fluid, water
seeks its own level and, if not disturbed, lies flat and glassy. It
yields, however, if disturbed by such forces as winds, undercurrents,
and objects travelling on its surface, creating waves or movements.
Because of its weight, water can exert a tremendous force.
This force, a result of resistance,
produces drag as the water flows around or under an object being
propelled through it or on its surface. The force of drag imposed by the
water increases as the square of the speed. This means that as the speed
of the object travelling on the water is doubled, the force exerted is
four times as great. Forces created when operating an airplane on water
are more complex than those created on land. When a landplane's wheels
contact the ground, the force of friction or drag acts at a fixed point
on the airplane; however, the water forces act along the entire length
of a seaplane's hull or floats with the centre of pressure constantly
changing depending upon the pitch attitude, dynamic hull or float
motion, and action of the waves. Since the surface condition of water
varies constantly, it becomes important that the seaplane pilot be able
to recognize and understand the effects of these various conditions of
the water surface.
Under calm wind conditions, the
waveless water surface is perhaps the most dangerous to the seaplane
pilot and requires precise piloting techniques. Glassy water presents a
uniform mirror-like appearance from above, and with no other visual
references from which to judge height, it can be extremely deceptive.
Also, if waves are decaying and setting up certain patterns, or if
clouds are reflected from the water surface, distortions result that are
even more confusing for inexperienced as well as experienced pilots.
Wave conditions on the surface of the water are a very important factor
in seaplane operation. Wind provides the force that generates waves, and
the velocity of the wind governs the size of the waves or the roughness
of the water surface. Calm water resists wave motion until a wind
velocity of about 2 knots is attained; then patches of ripples are
formed. If the wind velocity increases to 4 knots, the ripples change to
small waves that continue to persist for some time even after the wind
stops blowing.
If this gentle breeze diminishes, the
water viscosity dampens the ripples and the surface promptly returns to
a flat and glassy condition. As the wind velocity increases above 4
knots, the water surface becomes covered with a complicated pattern of
waves, the characteristics of which vary continuously between wide
limits. This is referred to as the generating area. This generating area
remains disarranged so long as the wind velocity is increasing. With a
further increase in wind velocity, the waves become larger and travel
faster. When the wind reaches a constant velocity and remains constant,
waves develop into a series of equidistant parallel crests of the same
height.
Table 1:
weather and wind conditions
Terms used by US
Weather Service |
Velocity (mph) |
Estimating Velocities
on land |
Estimating Velocities
on Sea |
|
Calm |
less than 1 |
Smoke rises vertically |
Sea like a mirror |
Check your glassy
water technique before
water flying under
these conditions |
Light air |
1 - 3 |
Smoke drifts; wind
vanes unmoved |
Ripples with the
appearance of scales
are formed but without
foam crests |
|
Light breeze |
4 - 7 |
Wind felt on face;
leaves rustle; ordinary
wind vane moves by
wind |
Small wavelets, still
short but more pronounced;
crests have a
glassy appearance and
do not break
Large wavelets; crests |
|
Gentle Breeze |
8 - 12 |
Leaves and small
twigs in constant
motion; wind extends
light flag |
Large wavelets; crests
begin to break. Foam
of glassy appearance,
perhaps scattered
whitecaps |
Ideal water flying
characteristics in protected
water |
Moderate Breeze |
13 - 18 |
Dust and loose paper
raised; small branches
are moved |
Small waves, becoming
longer; fairly frequent
whitecaps |
|
Fresh Breeze |
19 - 24 |
Small trees in leaf
begin to sway; crested
wavelets form in
inland water |
Moderate waves; taking
a more pronounced
long form; many
whitecaps are formed,
chance of some spray |
This is considered
rough water for seaplanes
and small
amphibians, especially
in open water |
Strong Breeze |
25 - 31 |
Large branches in
motion; whistling
heard in telegraph
wires; umbrellas used
with difficulty |
Large waves begin to
form; white foam
crests are more extensive
everywhere, probably
some spray |
|
Moderate Gale |
32 - 38 |
Whole trees in motion;
inconvenience felt in
walking against the
wind |
Sea heaps up and white
foam from breaking
waves begins to be
blown in streaks along
the direction of the
wind |
This type of water
condition
is for emergency
only in small aircraft in
inland waters and for
the expert pilot |
shipping
measures wind by the Beaufort scale
Beaufort
Number |
Wind Velocity
(knots) |
Seaman’s Term |
Sea Condition |
Typical**
Wave
Height
Feet |
Typical***
Condition
Attainment
Time |
0 |
0-1 |
Calm |
Glassy-smooth, mirror-like
|
Smooth |
- - |
1 |
2-3 |
Light air |
Scale-like ripples |
Ripples |
1-10 min |
2 |
4-6 |
Light breeze |
Small, short wavelets with glassy
crests |
1/3 |
5-15 min |
3 |
7-10 |
Gentle breeze |
Large wavelets, crests begin to
break, occasional form |
1-2 |
5-20 min |
4 |
11-16 |
Moderate breeze |
Small waves, some whitecaps, more
frequent form |
2-3 |
15-60 min |
5 |
17-21 |
Fresh breeze |
Moderate longer waves, better
formed, many whitecaps, much foam, some spray |
3-4 |
15-60 min |
6 |
22-27 |
Strong breeze |
Large waves form, many whitecaps,
foam everywhere, more spray |
4-5 |
1/4-2 hr. |
7 |
28-33 |
Moderate gale |
Sea heaps up, streaks of foam
spindrift begins |
5-6 |
1/2-3 hr. |
8 |
34-40 |
Fresh gale |
Moderately-high long waves,
crests into spindrift, well-marked streaks of foam |
6-7 |
1/2-3 hr. |
9 |
41-47 |
Strong gale |
High waves, sea rolls, dense
streaks, spray affects visibility |
7-9 |
1/2-4 hr. |
** |
Wave height is dependent
on water depth and length of time that the wind has been blowing.
These are typical heights for lakes, bays and estuaries. The above
conditions and wave heights should prevail after the times indicated.
Don't neglect the effect of large numbers of powerboats on enclosed
bodies of water in estimating wave heights. |
*** |
These attainment times
are for winds of constant or increasing intensities. For decreasing
intensities, surface characteristics will have to be relied upon; for
example, one-foot glassy-smooth waves still indicate BN=0 wind
conditions. |
An object floating on the water surface
where simple
waves are present will show that the water itself does not
actually move along with the waves. The floating object
will describe a circle in a vertical plane, moving upward as
the crest approaches, forward and downward as the crest
passes, and backward as the trough between the waves
passes. After the passage of each wave the object stays at
almost the same point at which it started. Consequently,
the actual movement of the object is a vertical circle
whose diameter is equal to the height of the wave. This
theory must be slightly modified however, because the
friction of the wind will cause a slow downwind flow of
water resulting in drift. Therefore, a nearly submerged
object, such as a hull or float, will slowly drift with the
waves
When the wind increases to a velocity of 12 knots,
waves will no longer maintain smooth curves. The waves
will break at their crest and create foam - whitecaps.
When
the wind decreases, the whitecaps disappear. However,
lines or streaks form which can be used as an accurate
indication of the path of the wind. Generally, it will be
found that waves generated by wind velocities up to 10
knots do not reach a height of more than one foot.
A great amount of wind energy is needed to produce
large waves. When the wind ceases, the energy in the
wave persists and is reduced only by a very slight internal
friction in the water. As a result, the wave patterns continue
for long distances from their source and diminish at a
barely perceptible rate. These waves are known as swells,
and gradually lengthen, becoming less high, but increase
in speed.
If the wind changes direction during the diminishing
process, an entirely separate wave pattern will form which
is superimposed on the swell. These patterns are easily
detected by the pilot from above, but are difficult to see
from the surface.
Islands, shoals, and tidal currents also affect the size of
waves. An island with steep shores and sharply pointed
extremities allows the water at some distance from the
shore to pass with little disturbance or wave motion.
This
creates a relatively calm surface on the lee side. If the
island has rounded extremities and a shallow slope and
outlying shoals where the water shallows and then
becomes deep again, the waves will break and slow down.
This breaking will cause a considerable loss of wave
height on the lee side of the shoal. However, if the water is
too deep above the shoal, the waves will not break.
When waves are generated in non-flowing water and
travel into moving water such as a current, they undergo
important changes. If the current is moving in the same
direction as the waves, they increase in speed and length
but lose their height.
If the current is moving opposite to
the waves, they will decrease in speed and length but will
increase in height and steepness. This explains "tidal rips"
which are formed where strong streams run against the
waves. A current travelling at 6 miles per hour will break
almost all waves travelling against it. When waves break, a
considerable loss in wave height occurs to the leeward side
of the breaking.
Another characteristic of water that should be mentioned
is the ability of water to provide buoyancy and
cause some objects to float on the surface. Some of these
floating objects can be seen from the air, while others are
partially submerged and are very difficult to see. Consequently,
seaplane pilots must be constantly aware of the
possibility of floating debris to avoid striking these objects
during operation on the water.
World Marine weather, including
currents wave heights is available on
http://www.yachting-life.com/ (opens in
new window)
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