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National Advisory Committee for Aeronautics, Technical Notes - Tank Tests to Determine the Effect of Varying Design Parameters of Planing Tail Hulls - I - Effect of Varying Length, Width, and Plan Form Taper of Afterbody

Tests were conducted in Langley tank no. 2 on
models of an unconventional flying-boat hull called a
planing- tail hull to determine the effects on resistance ,
of varying a number of afterbody parameters. The effects
of varying length, width, and plan—form taper of the
afterbody are presented. Tests were made with after-
bodies of two widths, two lengths, and two tapers. In
the tests the depth of step and the angle of afterbody
keel were held constant.

In general, the planing-tail hulls had much lower
resistance than conventional hulls. A typical con—
ventional hull compared with a planing-tail hull had
LO percent greater resistance at the hump speed and from
75 percent to more than 100 percent greater resistance
near the get- away speed; but in an actual application of
the planing- -tail hull the center of gravity would have to
be located aft of the step in order to obtain the reduc-
tion in resistance at hump speed.

It was concluded that decreasing the width of the
afterbody of a planing-tail hull increased the resistance
at hump speed, decreased the trimming moments required
to obtain best trim, and moved forward the location of
the center of gravity required to give best trim at the
hump speed.

Increasing the length of the afterbody of a Planing-
tail hull decreased the resistance over almost the
whole speed range, reduced the variation of trim with
a need, and moved aft the location of the center of gravity
required to obtain bestmtrim at the hump speed. Tapering
the plan form of the afterbody reduced the resistance
over the.lower half of the speed range and had little
effect on the resistance at high speeds. Plan—form
taper also moved forward the location of the center of
gravity required to obtain best trim at the hump speed.

The NACA flying-boat hull with a pointed step
(reference 1) was introduced-as a configuration that
would have low water resistance at high speeds because
of its inherently deep step. The results of preliminary
tests made on models with a hull similar to the type
used in reference 1, called a planing—tail hull, are
nresented in reference 2.