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FOUNDED BY WILLIAM H. WEBB
CRESCENT BEACH ROAD GLEN COVE. NEW YORK 11542
NOTE ON
HYDRODYNANIC PRESSURE ON
SHIP SECTION IN WAVES
by
Dan Hoffman
Hydrodynam.tc Pressure on Transverse
Ship Section La Waves
by Dan Hoffman
The purpose of this note La to sunemrize some recent research on
hydrodynamic pressure on a ship model bull in waves, carried out at
Webb Institute of Naval Architecture, as part of a more extensive project
sponsored by the
American
Bureau of Shipping related to the longitudinal,transverse and local strength of ships. The work is an extension of the
author's work at
Trondheim.(1).
Ship Form and Test Conditions
A feasibility study using a fivefoot model was first
carried out in
both
regular and irregular waves (2) The model was towedat full load and even keel conditions at a speed equivalent to 19 knots,
corresponding to the ship's usual service condition0 Though the primary
purpose of the above tests was to
check
the prediction methods of bowemergence and submergence9 pressure meâsurements at a single
location on
the keel at O.2L aft of the F. P. were recorded along with additional
supporting iniormation such aa the slope of the wave (relative to the keel)
at that location
and the relative motion.The
tests vere performed for a range of regular waves between 0.5L and 2.25L at an approximately constantheight to length ratio h/L value of 1/480 The irregular sea was set to
simulate
the 40-knot Pierson-Moskowits spectrum9 equivalent roughly toBeaufort No. 8.
Results of
the abovetests
indicated the possibility of the use of small models for pressure measurements and a more systematic series oftests
was then performed using a 5°-0" model of the Wolverine State, forwhich fulisca1e data are presently available0 Pressure measurements were
recorded at the following five locations, using a Statham PM 222 TC
0.11 aft of F P. on the keel. .2L " " ' .2L " ' " " " bilge 02L " " " " " side ,45L a st " keel
The model vas tested at full load and at a reduced Loading condition
corresponding to the usual service condition. For each of these conditions
o speeds were used equivalent to 16.5 and 12.0 knots, i.e., Fn .22 and
.16, respectively0 The test program included the same range of regular waves as before and the same irregular wave conditions,. The wave slope and
the relative motion vere recorded at 0.2L aft of F. P. In addition, wave
heights and model speeds were recorded.
Test Results
The data were analyzed to give the response amplitude opeators (BAO)
of the hydrodyis&mIc pressure at all 5 locations, and the pressure spectra in the irregular sea are being calculated. Meanwhile, the Mariner spectrum was
compared with results predicted from the regular wave BAO's and showed
excellent agreement as shown in
Fige
1. Thus, the linearity of the pressurevariations with wave amplitude has been checked with previous findings as
given in (1).
The effect of change of. draft on the BAO pressure curves was found to
be a shift of frequency of the maximum valUe of pressure amplitude variations,
with no change in magnitude. This result vas to be expected because of the
change in the ship's natüral heave and pitch frequencies 1th change in draf t.
Slams of considerable magnitude (0.4 psi) were often recorded in the
hiéht draft condition and are presently being analyzed for correlation with
the relative vertical velocity and the relative wave slope0
The theoretical pressure will next be calculated, using essentially the
method given In (1), with some more recent refinements in the mapping method
and in the coefficients of equations of motion incorporated, Comparison
with full'scale data should theù indicate the reliability of both pressure
measurement on small models and the theoretical method of calculation.
The
above studies will be associated with future work at Webb concerningtransverse structural strength of large tankers. A more detailed progress
report is presently being prepared, and it is hoped that this will be
I
References
D0 Hoffman, "Distribution of Wave Caused
Hydrodynamic
Pressures andForces on a Ship Hull", Norwegian Ship Model Tank Publication No. 94,
October 1966.
W. Maclean and D. Hoffman, "A Note on Instrumentation and Facilities
for Seakeeping
Research at the Robinson Model Basin, WébbInstitute,
8isPJL SEC.
