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unnecessary drag.
The steps are located slightly behind the airplane’s
center of gravity (CG), approximately at the point
where the main wheels are located on a landplane
pounds. A seaplane equipped with two such floats
would seemingly be able to support an airplane
weighing 5,000 pounds, but the floats would both be
completely submerged at that weight. Obviously,
such a situation would be impractical, so seaplanes
are required to have a buoyancy of 80 percent in
excess of that required to support the maximum
weight of the seaplane in fresh water. To determine
the maximum weight allowed for a seaplane equipped
with two floats, divide the total displacement by 180
percent, or 1.8. Using the example of two floats that
each displace 2,500 pounds, the total displacement of
5,000 pounds divided by 1.8 gives a maximum weight
for the seaplane of 2,778 pounds. Many other considerations
determine the suitability of a particular set of
floats for a specific type of airplane, and float
installations are carefully evaluated by the Federal
Aviation Administration (FAA) prior to certification.
All floats are required to have at least four watertight
compartments. These prevent the entire float from filling
with water if it is ruptured at any point. The floats
can support the seaplane with any two compartments
flooded, which makes the seaplane difficult to sink.
Most floats have openings with watertight covers along
the deck to provide access to the inside of each compartment
for inspection and maintenance. There are
also smaller holes connected by tubes to the lowest
point in each compartment, called the bilge. These
bilge pump openings are used for pumping out the
bilge water that leaks into the float. The openings are
typically closed with small rubber balls that push
snugly into place.
Both the lateral and longitudinal lines of a float or hull
are designed to achieve a maximum lifting force by
Step
Skeg
Retractable
Water Rudder
Mooring
Cleat
Mooring Cleat
Bumper
Bumper
Internal Bulkheads Dividing
Watertight Compartments
Deck
Hand Hole Covers
Chine
Chine
Sister Keelson
Sister Keelson
Keel
Keel
Bilge Pump
Openings
Spray Rail
Bow
Stern
Figure 2-3. Float components.
Ch 02.qxd 8/24/04 10:33 AM Page 2-3
2-4
with tricycle gear. If the steps were located too far aft
or forward of this point, it would be difficult, if not
impossible, to rotate the airplane into a nose-up attitude
prior to lifting off. Although steps are necessary, the
sharp break along the underside of the float or hull
concentrates structural stress into this area, and the
disruption in airflow produces considerable drag in
flight. The keel under the front portion of each float is
intended to bear the weight of the seaplane when it is
on dry land. The location of the step near the CG would
make it very easy to tip the seaplane back onto the rear
of the floats, which are not designed for such loads. The
skeg is located behind the step and acts as a sort of
chock when the seaplane is on land, making it more
difficult to tip the seaplane backward.
Most floatplanes are equipped with retractable water
rudders at the rear tip of each float. The water rudders
are connected by cables and springs to the rudder
pedals in the cockpit. While they are very useful in
maneuvering on the water surface, they are quite
susceptible to damage. The water rudders should be
retracted whenever the seaplane is in shallow water or
where they might hit objects under the water surface.
They are also retracted during takeoff and landing,
when dynamic water forces could cause damage.
SEAPLANE FLIGHT PRINCIPLES
In the air, seaplanes fly much like landplanes. The
additional weight and drag of the floats decrease the
airplane’s useful load and performance compared to
the same airplane with wheels installed. On many airplanes,
directional stability is affected to some extent
by the installation of floats. This is caused by the
length of the floats and the location of their vertical
surface area in relation to the airplane’s CG. Because
the floats present such a large vertical area ahead of
the CG, they may tend to increase any yaw or sideslip.
To help restore directional stability, an auxiliary fin is
often added to the tail. Less aileron pressure is needed
to hold the seaplane in a slip. Holding some rudder
pressure may be required to maintain coordination in
turns, since the cables and springs for the water
　
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