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 traveling 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 throughout or on its surface. The force of drag imposed by the water increases at the square of the speed. This means that as the speed of the object traveling 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 center 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 operations. 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 (Fig. 15-1).
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.
An object floating on the water surface where simple waves are present, will show that the water itself does not actually move along with 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, become 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 nonflowing 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 traveling at 6 miles per hour will break almost all waves traveling 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.