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)

Seamanís 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)