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National Advisory Committee for Aeronautics, Technical Notes - The Effect of the Angle of Afterbody Keel on the Water Performance of a Flying Boat Hull Model

N. A. C. A. model ll—C was tested in the N. A. C. A. tank
according to the general method with the angle of afterB
body keel set at five different angles from 2-1/20 to 9°
but without changing other features of the hull. The re»
sults of the tests are expressed in curves of test data
and of nondinensional coefficients.

At the depth of step used in the tests, 3.3 pércent
beam, the smaller angles of afterbody keel give greater
load—resistance ratios at the hump speed and smaller at
high speed than the larger angles of afterbody keel. Cozy
parisons are made of the load—resistance ratios at’Several
other points in the speed range.

The effect of variation of the angle of afterbodf ’m‘”" ‘%
keel upon the take—off performance of a hypothetical fly— ”
ing boat of 15, 000 pounds gross weight liaving a hull of
model ll—G lines is calculated, and the calculations show“»
that the craft witho the largest of the angles of after-
body keel tested, 9°, takes off in the least time and dis~
tance.

The afterbody and forebody of the hull of a flying
boat act together to produce the total hydrodynamic lift
and resistance. At rest, and at low speeds, the forebody
and afterhody together supply the buovancy'required to
keep the hull afloat. With increase in Speed, the bow
rises and the afterbody surface runs in the water at a..
positive angle of attach favorable for lifting. After the
hull rises on the step and planing begins, the afterbody
no longer plays an important part in providing lift, but
it may be a source of considerable resistanéfihif it runs
near enough to the surface of the water to be struck by
spray thrown back from the planing forebody.

The general effect of change in the angle_of after—
body keel upon the water performanbe of models of flying~
boat hulls had been observed in testing a number of mod—
els, but it could not be-deternined quantitatively in
those tests because it could not be separated from the
effects of other changes. These tests have been made with
a model of generally conventional form in which the angle
of aftorbody keel can be changed without changing the rest
of the model.