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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 operating
seaplanes, such as port and starboard for left and
right, windward and leeward for the upwind and downwind
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 longitudinal
members add strength to the structure and
function as additional keels. The top of the float forms
a 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 surprisingly
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
Keel Chine Step
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
2-3
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, seaplanes
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 sudden
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 buoyancy,
that is, the floats displace a weight of water
equal to the weight of the seaplane. As speed
increases, aerodynamic 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 attitude
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
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