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National Advisory Committee for Aeronautics, Technical Notes - Tank Tests of Two Models of Flying Boat Hulls to Determine the Effect of Ventilating the Step

The results of tests made in the N.A.G.A. tank on
two models of flying—boat hulls to determine the effect
of ventilating the step are given graphically. The step
of N.A.C.A. model ll—C was ventilated in several differ—
ent ways ani it was found that the resistance cf the nor-
mal form is not appreciably affected by afti'ficial venti-
lation in any of t.he forms tried. Further tests made With
the depth of t_he _step of model 11- C reduced likewise show
no ap.preciable effect— on the resistance from ventilation
of the step. Tests were made on a model o.f the hull of
the Navy PBM—l flying-boat hull both with and without ven—
tilation of the step. It was found that the discontinu_ity
which is obtained in the resistance curves of this model
is eliminated by ventilating the step.

The designers of early flying-boat hulls were often
forced to provide means for "ventilating" the step in or—
der to obtain reasonable performance on the water. With
imprdvements in the design of hulls, however, this prac—
tice became unnecessary and has been practically abandoned.
The construction is still revived occasionally. however,
and designs incorporating ventilation of the step have
been presented at the tank quite recently. The forced
ventilation of the step is usually proposed by some one
who has been in touch with small motorboat practice, where
special means for ventilating the step are often provided.

The freouent citina of successful applications of ar—
tificial ventilation to motor boat hulls led the Bureau
of Aeronautics of_the Navy Department to request an invesr
tiaation of the possibilities of improving the take~off
performance of present flying—boat hulls by providing ar—
tificial means of-ventilating the step. Because of the
hiaher priority of other projects the tests were made as
feasible betWeen tests of other models and were scattered
over a period of about two—years.