disk attributable to the downstream body. The details of these wake sur- 
veys will be reported in a future DINSRDC report.* 
DATA ACQUISITION AND ANALYSIS 
Data were collected, stored, and analyzed on-line by using a Model 
70 Interdata Digital Computer. A special-purpose computer program was 
written with options for analyzing each of the three basic types of runs: 
(1) steady ahead, (2) dynamic hull pitching, and (3) unsteady acceleration. 
These types of runs have already been discussed in detail. 
The program allowed the propeller blade force and moment data to be 
sampled and stored on magnetic tape as a function of shaft position. 
Sampling was triggered by external pulses generated by a Baldwin Digital 
Encoder mounted on the propeller shaft, as discussed earlier. Pulses were 
generated as a function of shaft angular position; hence, the sampling of 
blade force and moment data was related to shaft position. There were two 
outputs from the shaft encoder; a single pulse per revolution and multi- 
ile (OO pulses per revolution for the current experiments). 
When the experimental condition was achieved, the computer operator 
initiated the data collection cycle. The program "waited" until the sin- 
gle pulse occurred; when the single pulse occurred, the computer again 
"waited" for the occurrence of the first following pulse of the 90 pulses; 
data were then sampled for all channels through an analog-to-digital con- 
verter and stored in computer memory. This process was repeated for 180 
pulses, or two shaft revolutions. At the same time, the program "read" 
two frequency counters into core memory which measured model velocity V 
and propeller rotational speed n. The values of V and n were measured by 
counting the pulses from geared wheels attached to the towing carriage 
drive system and to the propeller shaft, respectively. The values of V and 
n were averaged over two shaft revolutions. Thus, there was an average V 
and n corresponding to each pair of two consecutive revolutions. 
After two revolutions of data were sampled and stored in core memory, 
the data were transmitted from core to a nine-track digital tape recorder. 
The transfer time was small and no pulses were missed during the transfer. 
* 
These wake surveys were conducted by R.F. Roddy, DINSRDC Code 1524. 
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