 14 MEASURED MILE CALIBRATION A. CALIBRATION OF LOG ON MEASURED MILE 14A1. 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. The scale used is derived from the theoretical values obtained from the following equation: H=(V2 X C) / 2g H=height of water V=velocity C=a factor determined by the specific gravity of sea water and the flow about the dynamic orifice g=acceleration due to gravity C is assumed to have a value of 1.02 considering only the specific gravity of sea water; g has a value of 32.16 feet per second. Since there has been no consideration of the flow of water about the rodmeter at factory calibration, it is necessary to calibrate, or adjust, the log for the particular ship on which it is installed. This calibration is performed during a trial run over a measured distance, usually 1 mile in length. 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 derived from an ordinary cruise are worthless for calibrating the log system. Similarly, checking distance indications by comparison of the log readings with the 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 14-1). Since the conditions can rarely be duplicated, record the conditions under which any trial run is made in a table similar to the following: U.S.S. ............................. DISPLACEMENT .................... PROJECTION OF RODMETER ........................... RELATION OF UNDERWATER SOUND GEAR TO RODMETER ......................... DATE ................................. PLACE ................................ LENGTH OF COURSE ............................ DEPTH OF WATER ............................. WIND DIRECTION ....................... DIRECTION OF SEA ..................... STATE OF SEA.......................... 185 Figure 14-1. Measured mile course. 14A2. Preparations for checking the log. If the ship is not near a standard measured mile course, it is possible to use fixed points 1 mile apart as shown in Figure 14-1 If possible, a location should be chosen in which the current effect is small and which has a direction parallel to the axis of the ship's course. The depth of water should be, if possible, greater than 40 fathoms. Shallower water will prevent the ship from 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 and its screws. 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. Vent the system as described in Section 11A5. All hose connections must be tight. The 3Y circuit of the master transmitter indicator must receive controlled 60-cycle alternating current from the constant frequency supply. A frequency less than 60 cycles will result in a proportional positive error in the distance reading. For example, if the frequency is 59 instead of 60 cycles, the distance indication will be 1.66 percent low at all speeds. If the frequency is 61 cycles, the distance indication will be 1.66 percent high at all speeds. As the speed is not affected by the 3Y frequency variations, this distance error may be detected by timing with a reliable stop watch. Check the operation of the instruments before operating on the measured course as follows: Run the speed indication on the master transmitter indicator up to 10 knots by manually moving the main balance arm to the right. When exactly 10 knots has been indicated, place a slip of clean note paper under the moving contact to maintain the speed indicator at 10 knots. At one of the distance counters, measure the time required to indicate 1 nautical mile. This time should be 6 minutes, plus or minus 3 seconds. If the time required exceeds the 3-second allowance, follow the method described in Section 13G49 for adjusting an error between the speed indicator in the master transmitter 186 indicator and the distance indicator of the speed and distance indicator. A member of the crew should be stationed at the speed and distance indicator (repeater) in the conning tower, and another man, usually the navigator, stationed where he can observe the entrance and exit from the measured mile course. Each man should be equipped with an accurate stop watch. Make certain that the A1, A2, and B adjustments are set to the same values as the factory adjustment values listed for each particular log on the plate mounted in the lower right-hand corner of the main mounting plate. 14A3. Operation and calculation. The ship should make two double runs, one at a low speed (approximately 5 knots), and one at a high speed (approximately 15 knots). During a double run, the same shaft rpm should be maintained throughout. At the exact moment the ship enters the measured mile course, the navigator should start his stop watch, and simultaneously signal to the crew member stationed at the speed and distance indicator that the ship has entered the course. At this instant the man at the speed and distance indicator will also start his stop watch and measure the time required for the distance counter to indicate exactly 1 nautical mile. The navigator will measure the time required. to run the exact measured mile. In all probability, the two stop watch readings will not agree. Therefore, it is necessary that the ship continue on the same course and maintain the same propeller shaft rpm until both men complete their timing for each individual mile; that is, the true mile, and the log-indicated mile. Repeat the run on the measured mile course in the opposite direction in the same manner. Time indications of these runs should be noted and recorded. Repeat the double run at the higher speed, and record the time indications in the same manner as described at low speed. List the data as follows:
 Observed Calculated RunNo. Ship'sCourse Length ofCourseNauticalMiles Time toTravelMeasuredMileMin.-Sec. Time toTravelIndicatedMileMin.-Sec. TrueSpeedKnots LogIndicatedSpeedKnots AverageTrueSpeed AverageLogIndicatedSpeedKnots PercentError 1 311 1 11:30.2 11:03.4 5.22 5.43 5.29 5.49 +3.78 2 131 1 11:10.3 10:49.7 5.37 5.55 3 311 1 4:10.6 3:56.5 14.93 15.23 14.95 15.29 +2.30 4 131 1 4:02.4 3:54.7 14.97 15.35
 The following equations are used for calculating the above data: True speed = (3600 X distance in nautical miles) / (time in seconds to cover measured distance) Time in seconds = (Minutes X 60) / seconds Log indicated speed = (3600 X distance measured on counter) / (time in seconds to record log distance) Average true speed = (Run No. 1 true speed + Run No. 2 true speed) / 2 Average log indicated speed = (Run No. 1 log speed + Run No. 2 log speed) /2 Per cent error = ((Average log indicated speed - Average true speed) X 100 %) / Average true speed Note: The error is positive when the log speed is greater than the true speed. The error is negative when the log speed is less than the true speed. 187 Figure 14-2. Adjusting diagram for calibrating the Bendix log.
 188 14A4. General instructions for plotting adjustments. (See Figure 14-2.) Adjustment diagrams (calibration charts) for each particular Bendix underwater log are furnished with the log by the Bendix Aviation Corporation, Marine Division, Brooklyn, New York. The A-calibration line for each particular log is drawn on the diagram at the factory after the log mechanism has been tested and adjusted at the factory. Note the curved black line in Figure 14-2. Charts are shipped in the spare parts box, and within the master transmitter indicator. Additional charts may be obtained from the service department of the manufacturer. When ordering, always indicate the log's serial number and the hull number. 14A5. Plotting true speed in knots adjustment line. (See Figure 14-2.) Using the figures for the average true speed of the first double run (in this case 5.29), and the percent of error of the first double run (in this case +3.78), locate these points on the graph as shown in Figure 14-2. The figures in the vertical column in the center of the chart represent percent error in this manner: Each full division, that is, the distance between figures, represents 1 percent. Divisions above the 100 line represent positive error and divisions below the 100 line represent negative error. Figures in the horizontal line at the lower right side of the chart represent the true speed in knots. Plot the point on the graph for the high-speed double run as shown in Figure 14-2. Draw a line through these two plotted points on the graph as shown by the solid blue line in Figure 14-2. 14A6. Obtaining value for A1-adjustment. (See Figures 14-2 and 14-3.) Draw a horizontal line through the point where the adjustment line intersects the zero knot line from the A-calibration curve to the 25-knot line on the chart as shown by the solid red horizontal line in Figure 14-2. Draw a vertical line from the point of intersection of the horizontal line just drawn, and the A-calibration curve, downward to the base line as shown by the solid red vertical line in Figure 14-2. Read and record the figure at the point at which this vertical line intersects the base 1. A1 ADJUSTMENT KNOB 2. AXLE 3. A2 ADJUSTMENT RING 4. A SPRING (MAIN SPRING) 5. CONNECTOR SCREW 6. MAIN BALANCE ARM 7. RED DOT Figure 14-3. The A-adjustment assembly. line (in this case 3.84) as shown in Figure 14-2. This is the value at which the A1-adjustment knob must be set. Turn the A1-adjustment knob to this value and note the direction in which the knob was turned. 14A7. Obtaining value for A2-adjustment. (See Figures 14-2 and 14-3.) Count vertically the number of major divisions at the zero knot line from the 100 line to the adjustment line. In this case it is 4 1/2 divisions as shown in Figure 14-2. This is the value at which the A2-adjustment must be set. Turn the A2-adjustment ring the number of divisions counted, in the same direction that the A1-adjustment knob was previously turned. We now have completed setting of the A1- and A2-adjustments. Observe the reference line shown on the axle in Figure 14-3. During factory calibration, a red dot is put above this 189 Figure 14-4. The B-adjustment assembly. reference line on the A2-adjustment ring to indicate the zero setting of the A2-adjustment. Shipboard adjustments are made thereafter from this reference mark. 14A8. Obtaining value for B-adjustment. (See Figures 14-2 and 14-4.) Using dividers at the 25-knot line, measure the distance along this line which separates the adjustment line from the previously drawn horizontal line as shown in Figure 14-2. Transpose this distance by means of the dividers to the scale of diagram at the right side of the chart. This reading from the scale of diagram is the numerical value at which the B-adjustment must be set (in this case 3.85). If the adjustment line slopes downward to the right (as shown in the diagram), the B-adjustment is to be set on the negative side of the B-adjustment scale on the instrument. Conversely, if the adjustment line slopes upward to the right, 1. A1 ADJUSTMENT KNOB 2. A2 ADJUSTMENT RING 3. B ADJUSTMENT GEAR KNOB 4. GEAR LOCK KNOB 5. B SCALE 6. WEIGHT ARM 7. INDEX SCREW 8. MAIN BALANCE ARM 9. AUXILIARY BALANCE ARM

Figure 14-5. Weight arm attached to instrument.
 190 1. A1 ADJUSTMENT KNOB 2. A2 ADJUSTMENT RING 3. B ADJUSTMENT GEAR KNOB 4. GEAR LOCK KNOB 5. B SCALE 6. WEIGHT ARM 7. WEIGHT 8. INDEX SCREW 9. MAIN BALANCE ARM 10. AUXILIARY BALANCE ARM

Figure 14-6. Weight arm attached to instrument.
the B-adjustment is to be set on the positive side of the B-adjustment scale in the instrument. Set the B-adjustment from the value determined above as follows: Loosen the gear lock knob. Turn the B-adjustment gear knob until the pointer is at the 3.85 position on the negative side of the B-scale. Tighten the gear lock knob.

14A9. Recording the A1-, A2-, and B-adjustments. After setting the A1-, A2-, and

B-adjustments to the values obtained in Sections 14A6, 14A7, and 14A8, record the values obtained in the performed adjustments table in the upper left-hand corner of the adjustment diagram as shown in Figure 14-2.

14A10. Checking A1-, A2-, and B-adjustments with weight and arm. (See Figures 14-5 and 14-6.) After setting and recording the A1-, A2-, and B-adjustments as described in Section 14A9, check the recorded adjustment

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values as follows: Set the maneuvering cocks and vent cocks to the zero position. Set the instrument to zero position by means of the C-adjustment as described in Section 13A4. Hang the weight arm only on the index screw of the main balance arm as shown in Figure 14-5. Note the speed indication obtained, and record it at the upper left-hand corner of the adjustment diagram. Repeat the operation using the weight arm and weight together, as shown in Figure 14-6. When the log is checked at some future time to see if the adjustments have changed due to temperature, spring tension or for some other reason, use the following procedure: Hang the weight arm alone, and then the weight arm and the weight together on the index screw as previously described. Compare   the speed values obtained with the recorded values in the performed adjustments table. If the values are the same within 1/10 knot, the adjustments have not been changed. If the values have changed, check the instrument carefully. See that the contacts are clean, that all connections and screws are tight, that the adjustments have not been changed, and that the instrument generally is in good condition. If the above checks indicate that the instrument is in a satisfactory condition, it will be necessary to make a recalibration run at the earliest possible opportunity, unless the navigator is satisfied with the speed and distance indications obtained. See Section 1413 for information on recalibration.

B. RECALIBRATION OF THE MASTER TRANSMITTER INDICATOR

14B1. Purpose. The purpose of the recalibration run is to correct the log when it is in error. The log may be in error due to one or more of the following reasons: The condition of the hull has been radically changed; the previous run was not properly made, or the instrument and/or rodmeter has been changed in some way.

14B2. Preparation. The preparations described in Section 14A2 for making a measured mile run should be made in this case with the important exception that the A1-, A2-, and B-adjustments are to be set at the same values as last recorded in the performed adjustments table at the upper left-hand corner of the adjustment diagram (Figure 14-7).

14B3. Operation and calculation. (See Figure 14-7). Make the trial runs and calculations as described for the measured mile runs in Section 14A3. Make one additional set of calculations as follows: On the adjustment diagram there appears an adjustment line for the previous trial run. This dotted blue line on the sample diagram (Figure 14-7), is the line from which all plotting and calculations will now be made. At the true speed of 5.20 knots, and at the true speed of 15.40 knots, adjustment factors of 103.8 and 102.25 respectively are obtained. This will then give values which are to be recorded in the following additional table:
 AverageTrueSpeedKnots PercentError AdjustmentFactor CorrectedPercentError 5.20 2.2% 103.8 2.28% 15.40 3.4% 102.25 3.48% Corrected percent error = (Percent error x adjustment factor) / 100

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14B4. Plotting the new adjustment line. (See Figure 14-7.) Plot the new adjustment line as follows: Add algebraically, the corrected percent errors to the adjustment factors at the true speeds obtained, and plot on the diagram as before (see Figure 14-7). In this example, the corrected percent error of 2.28 added to the adjustment factor of 103.8 equals 106.08, and the corrected percent error of 3.48 added to the adjustment factor of 102.25 equals 105.73. Draw the new adjustment line through the plotted points obtained as shown by the solid blue line in Figure 14-7.

14B5. Obtaining A1-value. (See Figure 14-7.) Obtain the new A1-adjustment value by drawing a straight line horizontally across the diagram through the point at which the new adjustment line intersects the zero knot line, and extending it to the A-calibration curve as shown by the solid red horizontal line. Draw a vertical line (solid red) downward from the point at which the previously drawn solid red horizontal line intersects the A curve, to the base line. The reading at this base line point is the value at which the A1-adjustment is to be set (in this case, 4.15).

14B6. Obtaining A2-value. (See Figure 147.) Obtain the A2-adjustment value by counting the .number of divisions between the points at which the old and new adjustment lines intersect the zero knot line on the diagram; in this case 1.5 divisions. That is, the A2-adjustment is to be turned 1.5 divisions from the previous setting in the same direction in which the A1-adjustment knob was turned.

14B7. Obtaining B-value. (See Figure 147.) Obtain the value at which the B-adjustment is to be set in the following manner: Using dividers, measure the distance between the intersection of the solid red horizontal line with the 25-knot line, and the intersection of the new adjustment line (solid blue) with the 25-knot line. Transpose this distance to the scale of diagram. This gives the percentage (value) that the B-adjustment is to be set away from the previous setting. In this case the B setting is minus 0.9. Therefore, the B-adjustment is to be set at minus 4.75.

C. CALIBRATION OF THE LOG WHEN PERCENTAGE ERROR EXCEEDS 8 PERCENT

14C1. Purpose. This method of calibration is to be used only when the log error, determined during trial runs, exceeds plus or minus 8 percent.

14C2. Preparation. Trial runs have been made, and the percent error at certain true speeds determined. Carefully check all the data and calculations before proceeding with this method of calibration.

14C3. Operation and calculation for setting B-adjustment. Set the maneuvering cocks and drain cocks to the zero position. Attach the weight arm, provided in the spare parts box, to the index screw on the main balance arm, as shown in Figure 14-5. Make and hang weights, preferably lead, on the arm so that the speed indications can be run up to the log indicated speeds obtained in the

193 Figure 14-7. Adjusting diagram for recalibrating the Bendix log.

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calculations for the trial runs. Mark each weight for ready identification. Subtract the percent error at a true speed of approximately 5 knots from the percent error at a true speed of approximately 15 knots, and divide this value by 0.4. Then set the B-adjustment to this value on the plus side of the B-scale; if the percent error at 15 knots is greater than the percent error at 5 knots, in a positive direction. Set the B-adjustment on the negative side of the scale   is high at 15 knots, turn the B-adjustment toward a positive, or more positive, reading on the B-scale and try adjusting again.

NOTE: When 5 and 15 knots are mentioned in the above text, they are used merely to indicate the low and high speed runs at which the trial runs were made.

14C5. Calibration when percent error exceeds 8 percent. The following is an example of calibration when the percent error is greater than 3 percent:

 AverageTrueSpeedKnots AverageLog IndicatedSpeed Knots PercentError 5.20 4.65 -10.5% 15.12 13.87 - 8.3%
if the percent error at 15 knots is less than the percent error at 5 knots in a negative direction.