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National Advisory Committee for Aeronautics, Technical Notes - A New Method of Studying the Flow of the Water Along the Bottom of a Model of a Flying Boat Hull

Observations in regard to the flow of the water along
the bottom of seaplane floats and flying~boat hulls, par~
ticularly in regard to the-action of the step, are fofind
in many reports of tests of both models and full-size
craft. Most of the data have-been based en observation
of the type and the changes of the waves emerging from
under the hull. Other observations have been made'from
the flow pattern obtained on the bottom of a hull, such
as produced by the action of chemicals on the bottom
paint. In some recent studies in the N.A.G.A. tank, the '
flow pattern on the bottom of a hull has been obtained by; "
applying spots of a semiplastic fluid and recording the '
streaks that proceeded from them. ”This method is labori—
ous and, although it does give a general pieture of the I
flow at specific speeds, it permits no detailed study of
the changes that occur.. Simple transparent forms, such as
the glass planing surfaces tested by Sambraus (reference '1).
have permitted direct observations of the flow over the
bottom surface but, as far as is known, the method of il—
lumination and the study of the flow over the bottom ofma
hull form, as herein described, has not been used prior
to the present investigation.

The purpose of the present preliminary investigation
was to explore the possibilities of directly observing the
flow over the bottom of a model of'a flyinz~boat hull
.through a transparent bottom by means of suitable illumi—
nation arransed to make the flow visible. It was desired
to observe the general behavior of the water flowing alone
the under surface of the hull and, in particular, to ob—
serve the nature.of the action of the step in the transi—
tion from the predominantly displacement condition to the
predominantly planing condition.

The form of the model ofvthe flying-boat hull chosen
for this investigation is generally similar to that of
N.A.C.A. model ll«G (reference 2) with the exception that
the forebody has developable surfaces, resulting in a some—
what fuller bow. The model is constructed in two sections.
a forebody section and an afterbody section Joined at the
step; the depth of the step may be varied by raising on
lowering the afterbody section with respect to the flore—
body section. This arrangement is similar to that used
for the model of reference 3. The tail extension is omit—
ted, the model endinq~at the sternpost. The bottom of the
model is formed of sheets of transparent plastic, 1/8 inch
thick, attached to a heavy wooden shell of the desired
form. It is possible to look directly through the bottom
from a position above the model.