Effect of shallow water on the hydrodynamic characteristics of a flat-bottom planing surface Page: 4 of 37
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NACA TN 3642
given in the appendix. A drawing of the model used is presented in fig-
ure 1. This model, which was of solid mahogany, was 19.6 inches long
and 2.5 inches wide. It had a flat bottom surface and a longitudinally
curved, circular-arc top surface.
The model was attached to a three-component strain-gage balance by
means of two struts and a crossbar, as shown in the test setup illustrated
in figure 2. The balance was housed in a metal box to protect it from
spray. Adjustment in vertical position was provided by moving the staff
vertically. Adjustment in trim was provided by a pivot in the balance.
Four steel tables, each 10 feet long and 6 feet wide, were placed
end to end in the tank to form a false bottom. A given depth of water
was obtained by adjusting the height of each table with four brass screws
operated by hand cranks through a slot in the table top. A sketch of the
test setup showing the tables is given in figure 3.
The table tops were made of 1/8-inch sheet steel bolted to an angle-
iron framework. The maximum variation from a horizontal plane was
approximately 0.04 inch along the longitudinal center line of each table.
The gaps between adjacent tables were kept to about 1/2 inch or less and
no effects of the gaps were discernible on the forces recorded.
For the determination of wetted lengths, underwater photographs of
the grid inscribed on the bottom of the model were taken by means of an
electrically operated camera housed in a watertight box mounted under
the last table along the run. The photographs were taken through a glass
plate in the table top. The camera shutter, high-speed flash lamps,
and film advance were actuated by a photoelectric unit when a light beam
was interrupted by the carriage as the model passed over the camera.
All the shallow-water runs were made in salt water. The difference
in density noted in the list of symbols was caused by the fact that the
tank had been drained and refilled with fresh water when the deepwater
runs were made.
The estimated accuracies of the test measurements are as follows:
Trim angle, deg.......................... 0.1
Speed, ft/sec . . ........................... 0.05
Lift, lb ... ......................... 0.25
Drag, lb ............................ 0.1
Trimming moment, ft-lb ..................... 0.25
Wetted length, in. ........................ 0.05
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Christopher, Kenneth W. Effect of shallow water on the hydrodynamic characteristics of a flat-bottom planing surface, report, April 1956; (digital.library.unt.edu/ark:/67531/metadc57919/m1/4/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.