SEAPLANE CHARACTERISTICS
There are two main types of seaplane: flying boats (often
called hull seaplanes) and floatplanes. The bottom of a
flying boat
’s fuselage is its main landing gear. This is
usually supplemented with smaller floats near the
wingtips, called
wing or tip floats
. Some flying boats
have
sponsons
, which are short, winglike projections
from the sides of the hull near the waterline. Their pur-
pose is to stabilize the hull from rolling motion when
the flying boat is on the water, and they may also pro-
vide some aerodynamic lift in flight. Tip floats are
sometimes known as sponsons. The hull of a flying
boat holds the crew, passengers, and cargo; it has many
features in common with the hull of a ship or boat. On
the other hand,
floatplanes
typically are conventional
landplanes that have been fitted with separate floats
(sometimes called pontoons) in place of their
wheels. The fuselage of a floatplane is supported
well above the water’s surface.
Some flying boats and floatplanes are equipped with
retractable wheels for landing on dry land. These aircraft
are called
amphibians
. On amphibious flying boats, the
main wheels generally retract into the sides of the hull
above the waterline. The main wheels for amphibious
floats retract upward into the floats themselves, just
behind the step. Additional training is suggested for any-
one transitioning from straight floats to amphibious
aircraft. [Figure 2-1]
There are considerable differences between handling
a floatplane and a flying boat on the water, but simi-
lar principles govern the procedures and techniques
for both. This book primarily deals with floatplane
Figure 2-1. Flying boats, floatplanes, and amphibians.
Ch 02.qxd 8/24/04 10:33 AM Page 2-1
operations, but with few exceptions, the explanations
given here also apply to flying boats.
A number of amphibious hull seaplanes have their
engines mounted above the fuselage. These seaplanes
have unique handling characteristics both on the water
and in the air. Because the thrust line is well above the
center of drag, these airplanes tend to nose down when
power is applied and nose up as power is reduced. This
response is the opposite of what pilots have come to
expect in most other airplanes, and can lead to unex-
pected pitch changes and dangerous situations if the pilot
is not thoroughly familiar with these characteristics.
Pilots transitioning to a seaplane with this configuration
should have additional training.
Many of the terms that describe seaplane hulls and
floats come directly from the nomenclature of boats
and ships. Some of these terms may already be
familiar, but they have specific meanings when
applied to seaplanes. Figures 2-2 and 2-3 describe
basic terms, and the glossary at the end of this book
defines additional terms.
Other nautical terms are commonly used when operat-
ing seaplanes, such as port and starboard for left and
right, windward and leeward for the upwind and down-
wind sides of objects, and bow and stern for the front
and rear ends of objects.
Research and experience have improved float and hull
designs over the years. Construction and materials have
changed, always favoring strength and light weight.
Floats and hulls are carefully designed to optimize
hydrodynamic and aerodynamic performance.
Floats usually have bottoms, sides, and tops. A strong
keel
runs the length of the float along the center of the
bottom. Besides supporting the seaplane on land, the
keel serves the same purpose as the keel of a boat when
the seaplane is in the water. It guides the float in a
straight line through the water and resists sideways
motion. A short, strong extension of the keel directly
behind the step is called the
skeg
. The
chine
is the seam
where the sides of the float are joined to the bottom.
The chine helps guide water out and away from the
float, reducing spray and helping with
hydrodynamic
lift. Hydrodynamic forces are those that result from
motion in fluids.
On the front portion of the float, midway between the
keel and chine, are the two
sister keelsons
. These lon-
gitudinal members add strength to the structure and
function as additional keels. The top of the float forms
deck
that provides access for entering and leaving the
cabin. Bilge pump openings, hand hole covers, and
cleats for mooring the seaplane are typically located
along the deck. The front of each float has a rubber
bumper to cushion minor impacts with docks, etc.
Many floats also have
spray rails
along the inboard
forward portions of the chines. Since water spray is sur-
prisingly destructive to propellers, especially at high
r.p.m., these metal flanges are designed to reduce the
amount of spray hitting the propeller.
Floats are rated according to the amount of weight they
can support, which is based on the weight of the actual
volume of fresh water they displace. Fresh water is the
standard because sea water is about 3 percent denser
than fresh water and can therefore support more
weight. If a particular float design displaces 2,500
pounds of fresh water when the float is pushed under
the surface, the float can nominally support 2,500
StepChineKeel
Wingtip Float
Bow
Spray Rail
Forebody Length Afterbody Length
Stern
Figure 2-2. Hull components.
Ch 02.qxd 8/24/04 10:33 AM Page 2-2
diverting the water and the air downward. The forward
bottom portion of a float or hull is designed very much
like the bottom of a speedboat. While speedboats are
intended to travel at a fairly constant pitch angle, sea-
planes need to be able to rotate in pitch to vary the
wings’ angle of attack and increase lift for takeoffs and
landings. The underside of a seaplane float has a sud-
den break in the longitudinal lines called the
step
. The
step provides a means of reducing water drag during
takeoff and during high-speed taxi.
At very low speeds, the entire length of the floats
supports the weight of the seaplane through buoy-
ancy, that is, the floats displace a weight of water
equal to the weight of the seaplane. As speed
increases, aero dynamic lift begins to support a certain
amount of the weight, and the rest is supported by
hydrodynamic lift, the upward force produced by the
motion of the floats through the water. Speed increases
this hydrodynamic lift, but water drag increases more
quickly. To minimize water drag while allowing
hydrodynamic lift to do the work of supporting the
seaplane on the water, the pilot relaxes elevator back
pressure, allowing the seaplane to assume a pitch atti-
tude that brings the aft portions of the floats out of the
water. The step makes this possible. When running on
the step, a relatively small portion of the float ahead of
the step supports the seaplane. Without a step, the flow
of water aft along the float would tend to remain
attached all the way to the rear of the float, creating
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 consid-
erations 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 fill-
ing 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 com-
partment 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 CompartmentsDeck
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
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 air-
planes, 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
rudders may tend to prevent the air rudder from
streamlining in a turn.