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6
MEASURED MILE CALIBRATION
 
A. CALIBRATION OF LOG ON MEASURED MILE
 
6A1. Purpose of calibrating log. Each complete log system is calibrated at the factory with a standard U-tube mercury manometer, having a scale graduated in knots. This scale is based on a constant determined by towing-tank experiments on a standard rodmeter. The rodmeter used with each log system has physical dimensions identical with that used in towing-tank experiments. Thus it is possible to calibrate each log system by means of the standard manometer mentioned above. The pump unit contains an orifice plug as shown in Figure 6-1, the position of which determines the calibration of the log. During factory tests this orifice plug is rotated to a position that will cause the log to indicate a speed equal to that indicated by the standard mercury manometer. The centerline on the orifice plug is then extended to the orifice plate and marked as reference point O. While

Figure 6-1. Pump orifice plate.
Figure 6-1. Pump orifice plate.

  maintaining the mercury deflection at a steady value, the orifice plug is rotated to either side of O for the purpose of making calibration lines at 2, 4, and 6 percent, plus and minus. The equipment is shipped from the factory with the pump setting at 0 percent. When the equipment is installed in a ship, a factory representative checks the operation of the entire system and set the pump orifice at a value that was determined during the measured mile trials of a ship of the same class. When the correction for a particular type or class of ship is unknown, the setting is left at 0 percent. No changes in speed calibration settings should be made on the shaft rpm knot data alone. The data upon which the rpm-knot table, or curve, are based were in most cases determined during the measured mile trials of the ship, or on another ship of the same class. Trial conditions can rarely, be duplicated; and even with suitable corrections for foul bottom, variations in displacement and trim, and effect of wind and sea, speed indications thus derived are worthless for calibrating the log system. Similarly, checking distance indications by comparison of log readings with distances traveled between ports cannot be used for recalibration purposes since the current effect cannot be determined with the degree of accuracy required. Thus the most accurate and suitable method of checking the calibration of a log under actual operating conditions is to run the ship over a measured course 1 mile in length (Figure 6-2).

6A2. Preparations for checking log. If the ship is not near a standard measured mile course, it is possible to use fixed points one mile apart as shown in Figure 6-2. If possible, a location should be chosen in which the current effect is small and having a direction parallel to the axis of the ship's course. The depth of water should be at least 20 fathoms. Shallower water will prevent the ship from

 
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Figure 6-2. Measured mile course.
Figure 6-2. Measured mile course.

  developing proper speed for a given propeller shaft rpm, and the log will tend to read high when checked in shallow water because of wave systems established by the ship. Before making the runs over the measured mile, the log system should be checked for proper operation as follows:

The hydraulic system should be entirely free of air. The system should be vented as described in Section 3A3. All hose connections must be tight. Leakage at the drip fitting on the pump must not exceed three drops per minute. If leakage is in excess of this amount, the pump must be disassembled and the rotary seal inspected and replaced if required. The 3Y circuit of the master speed indicator must receive controlled 60-cycle a.c. from the constant frequency supply. A frequency greater than 60 cycles will result in a proportional negative error in speed reading. A frequency less than 60 cycles will result in a proportional positive error in speed reading. For example, if the frequency is 59 cycles instead of 60 cycles, the speed indication will be 1 division high in 60, or 1.66 percent high at all indications. This would amount to a 0.25 knot over-reading at 15 knots, and a 0.4 knot over-reading at 25 knots. If the frequency is 61 cycles, the speed indication will be 1.66 percent low at all indications. As the log distance is not affected by the 3Y frequency variations, this speed error may be detected by aiming, with a reliable stop watch the registering of 1 mile on the master speed indicator distance counter while the ship is maintaining a steady speed. (This particular test need not be made on a measured mile course.) Readings should be taken of the log speed indicator during this test. The average speed calculated from the stop watch reading should check with the average of the pointer reading within 1 percent.

6A3. Operation and calculation. The ship should make three runs at each speed selected. One run should be made in each direction over the measured course, and then the first run should be repeated. As many speeds should be selected as time will permit. In general, three typical speeds - low, medium, and high - are the minimum for proper log calibration. A signal system should be used to permit observers to read and record simultaneous readings

 
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at the propeller shaft revolution indicators, and the master speed indicator. The crew members stationed at the propeller shaft revolution indicators will read and record indications on each revolution counter at the beginning and end of each run. They should also read and record indications on the rpm pointer every 15 seconds throughout the run.   compensate for this error. Let us assume that the log averages 1.0 percent high at all speeds. Adjust the orifice plug by loosening the six screws in the orifice plate, and rotating the orifice plug clockwise, to subtract a value of 1.0 from the original setting. In the example given, the orifice plug would be moved from 0 percent to a point halfway


Calibration of Pitometer Underwater Lag on a Measured Mile
 
U.S.S................
Displacement: 1300 tons
Condition of Bottom: Clean (1 month out of dock)
Setting of Pump Orifice: 0%
Projection of Rodmeter: 36 inches
Relation of Underwater Sound Projector to Rodmeter: Rodmeter 3 ft. to Stbd. of Stbd. Sound Ball
  Date: April 14, 1941
Place: Provincetown, Mass.
Length of Course: 1 mile
Depth of Water: 20 fathoms
Wind Direction: 180 degrees (T)
Wind Intensity: 5 knots
Direction of Sea: 180 degrees (T)
State of Sea: 1

Record and Average Data as follows:
Run
No.
Ship's
Course
Elapsed
Time
Min. Sec.
Knots
Over
Ground
Log
Knots
Log
Distance
Average
rpm
13113:54.015.3915.201.01200.0
21314:06.014.6315.17.99201.0
33113:52.715.4715.191.01200.5
 
Average = Average of Run No. 1, No. 2,
No. 3 and No. 2
  Average:
15.03 Knots over ground
15.18 Log knots
 1.00 Log distance
200.5 Average rpm

Log Percent Error = (15.18 -15.03)/15.03 X 100 = (+) 1.0%
Crew members stationed at the master speed indicator should read and record the indications of the distance counter at the beginning and end of each run. They should read and record the speed pointer every 15 seconds throughout the run. The data shown above should be summarized and entered in the machinery history.

If, after measured course trials are held, the recorded data positively indicate that the log has a definite error, the orifice plug in the pump unit (Figure 6-1) may be adjusted to

  between 0 and (-) 2.0 percent. This would be a setting of (-) 1.0 percent. Tighten the orifice plate screws securely. The orifice plug is not to be adjusted each time the log appears to be in error. Once a setting has been properly made there should be no further occasion for changing it unless some structural alteration is made in the ship's hull forward of or near the rodmeter. A structural change may affect the pressures at the rodmeter, thus necessitating a new correction.
 
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