A seaplane is defined as "an airplane
designed to take
off from and land on water." Seaplanes can be generally
classified as either flying boats or floatplanes. Those that
can be operated on both land and water are called amphibians.
The floatplane is ordinarily understood to be a conventional
landplane equipped with separate floats instead of
wheels, as opposed to a flying boat in which the hull
serves the dual purpose of providing buoyancy in the
water and space for the pilot, crew, and passengers. The
float type is the more common seaplane, particularly those
with relatively low horsepower.
It maybe equipped with
either a single float or twin floats; however, most seaplanes
are the twin-float variety. Though there is considerable
difference between handling a floatplane and
handling a flying boat, the theory on which the techniques
are based is similar. Therefore, with few exceptions, the
explanations given here for one type may be considered to
apply to the other.
In the air the seaplane is operated and controlled in
much the same manner as the landplane, since the only
major difference between the floatplane and the landplane
is the installation of floats instead of wheels.
because of the float's greater weight, replacing wheels
with floats increases the airplane's empty weight and thus
decreases its useful load, and rate of climb.
On many floatplanes, the directional stability will be
affected to some extent by the installation of the floats.
This is caused by the length of the floats and the location
of their mass in relation to the airplane's CG. To help
restore directional stability, an auxiliary fin is often added
to the tail. The pilot will also find that less aileron pressure
is needed to hold the floatplane in a slip and holding some
rudder pressure during in-flight turns is usually required.
This is due to the water rudder being connected to the air
rudder or rudder pedals by cables and springs which tend
to prevent the air rudder from streamlining in a turn.
Research and experience have improved float and hull
designs throughout the years. Figure 1 and Figure 2 illustrate
the basic construction of a float and a flying boat. The
primary consideration in float construction is the use of
sturdy, lightweight material, designed hydro-dynamically
and aerodynamically for optimum performance.
Fig 1: Float Components
All floats and hulls now being used
watertight compartments which make the seaplane virtually
unsinkable, and prevent the entire float or hull from
becoming filled with water in the event it is ruptured at
Both the lateral and longitudinal
lines of a float or hull
are designed to achieve a maximum lifting force by diverting
the water and the air downward. The forward bottom
portion of the float (and a hull) is designed very much like
the bottom surface of a speedboat. The rearward portion,
however, differs significantly from a speedboat.
A speedboat is designed for travel at an almost constant
pitch angle and, therefore, the contour of the entire
bottom is constructed in approximately a continuous
straight line. However, a seaplane float or hull must be
designed to permit the seaplane to be rotated or pitched up
to increase the wing's angle of attack and gain the most lift
for takeoffs and landings. Thus, the underside of the float
or hull has a sudden break in its longitudinal lines at the
approximate point around which the seaplane rotates into
the lift off attitude. This break, called a "step," also provides
a means of interrupting the capillary or adhesive
properties of the water.
The water can then flow freely
behind the step, resulting in minimum surface friction so
the seaplane can lift out of the water. The steps are located slightly
behind the airplane's centre
of gravity, approximately at the point where the main
wheels of a landplane are located. If the steps were located
to far aft or forward of this point, it would be difficult, if
not impossible, to rotate the airplane into a pitch-up attitude
prior to planing (rising partly out of the water while
moving at high speed) or lift off.
Although steps are necessary, the sharp break along
the float's or hull's underside causes structural stress concentration,
and in flight produces considerable drag
because of the eddying turbulence it creates in the airflow.