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National Advisory Committee for Aeronautics, Report - Tank Tests to Determine the Effect on Planing Tail Hulls 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 ejects on resistance of varying a number of
afterbody parameters. The efl’ects of varying length, width,
and plan-form taper of the afterbodg are presented. Tests
were made with afterbodies of two undths, two lengths, and two
tapers. In the tests the depth of step and the angle of afterbody
lceel were held constant.

I n general, the planing—tail hulls had much lower resistance
than conventional hulls. A typical concentional hull compared
with a planing-tail hull had 40 percent greater resistance at
the hump speed and from 76 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 reduction
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 speed, and mated aft the location of the center of gravity
required to obtain best trim 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 efiect 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 presented in reference 2. The NACA
planing-tail hull has a pointed-step forebody in combina—
tion with a very long afterbody that extends back to the
region where the tail surfaces would be attached; thus no
t-ail extension is required.