A slide rule operates based on the relationship between two scales: in this case, the numbers on the movable ring and the numbers and markers on the fixed ring inside it. Note that the numbers on the two rings, 10 through 90, are not the same distance apart. Note also that the inner ring has, besides the numbers, a large arrow or triangle called the SPEED INDEX, and the outer ring has three markers that help with the calculations: arrows labeled NAUT. and STAT. at the top of the watch face, and an arrow labeled KM at the right of the face.
Compass Directions: The outer bezel is rotated in a counterclockwise direction. This outer bezel includes two rings of markings, including one that can serve as a compass. By orienting the watch so that the N
NOTE: Calculations are approximate, and you may prefer to adjust the number of zeros in the answer by making a
Conversion Among Statute Miles, Nautical Miles, and Kilometers: Set any of the three pointers on the outer ring – STAT, NAUT, or KM – to the number on the inner ring that you wish to convert. The two corresponding values will be opposite the other two pointers.
EXAMPLE: Set 40 on the inner ring opposite the NAUT arrow on the outer ring. By reading the numbers opposite the STAT and KM arrows, you will see 45.3 miles and 70.7 KM.
points north, you can determine any desired direction, including a heading measured in degrees.
Multiplication: The number on the outer ring placed opposite 10 on the inner ring, multiplied by any number on the inner ring equals the adjacent number on the outer ring.
EXAMPLES:
1. To multiply 30 x 12: Place 30 on the outer ring next to 10 on the |
Time, Speed and Distance Calculations (Tachymeter):
The number on the outer ring opposite the SPEED INDEX will be the speed, and any other pair of numbers on the inner and outer rings will be the corresponding time in minutes (on the inner ring) and distance (on the outer ring). You can use statute or nautical miles for distance and speed, as long as you use the same units for both.
| T | 70 |
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12 | 02 | 19 | 81 | 0 | ||
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| inner ring. Opposite 12 on the inner ring is 36 on the outer ring |
| (multiply by the inner ring 10 which equals 360). |
2. | To multiply 30 x 25: Opposite 25 on the inner ring is 75 (multiply |
| this by inner ring 10 which equals 750). |
3. | The same outer ring position is used to calculate: 3 x 12 = 36. The |
| 30 opposite 10 is divided by 10 to become 3, which cancels the |
| multiplication by 10 of the 36 opposite 12. |
EXAMPLE: If you set 75 (mph) on the outer ring |
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opposite the SPEED INDEX triangle on the inner, you |
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can determine that in 20 minutes (inner ring) you can |
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travel 25 miles (outer ring); or that by finding 20 miles | STAT | 70 | 80 |
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55 |
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on the outer ring, you can see that it will take you 16 |
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minutes (inner ring) to go that far. If you move the outer ring so that 35 is opposite 30 on the inner ring, you can determine your speed if you travelled 35 miles in 30 minutes. Looking on the outer ring above
4.To calculate 30 x 120 = 3600: The number opposite 12
(36)is multiplied by 10 since 30 was opposite 10, then it is multiplied by 10 again since 12 was mulitplied by 10 to arrive at 120.
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12 | 02 | 19 | 81 | 71 |
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the SPEED INDEX, you find the answer is 70 MPH (or 70 knots if you consider the 35 miles to be in nautical miles).
Mileage and Fuel Consumption Calculations: The number on the outer ring opposite the SPEED INDEX is your consumption in gallons per hour. Any other pair of numbers will correspond, minutes on the inner scale and gallons on the outer. (You can calculate liters instead of gallons as long as you match liters with
EXAMPLE: How much fuel do you need to fly 7 hours |
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and 30 minutes, if you know your fuel consumption rate |
| 30 | 3 |
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55 |
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5. To calculate 300 x 12 | TUAN 55 |
=3600: The number
opposite 12 (36) is
multiplied by 10 since 30 was opposite 10, then it is multiplied by 10 again since 30 was mulitplied by 10 to arrive at 300.
Division: Line up the number you want divided on the outside ring with the number you want it divided by on the inner ring. The answer is then the number on the outer ring opposite 10 on the inner ring divided by 10.
EXAMPLES: Leaving the ring in place with 30 opposite 10:
1.36 on the outer ring, divided by 12 on the inner ring, equals 3 (the 30 that is on the outer ring opposite 10 on the inner ring, divided by that 10).
2.75 divided by 25, 750 divided by 250 and 48 divided by 16 – all equal 3 (the 30 that is on the outer ring opposite 10 on the inner ring, divided by that 10).
is 320 gallons per hour. Set the SPEED INDEX to 32 | 50 |
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(320 gallons per hour). Find 45 (450 minutes, or 7 hours |
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30 minutes) on the inner scale. The number opposite is |
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the answer: 24, or 2400 gallons. What is your |
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consumption rate if you used 150 gallons in 50 minutes | 2 | 1 | 0 |
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of flight time? Set 15 (150 gallons) on the outer scale opposite 50 (minutes) on the inner. Opposite the SPEED INDEX is the answer; 18 or 180 gallons per hour.
SECTION 8 – ELAPSED TIME RING
If your watch is equipped with a rotatable outer dial with numbers and indicators, this Elapsed Time Ring may be used to indicate the length of time remaining for an activity or the starting time of an activity.
Time Remaining – Set the outer ring so that the triangle is pointing to the time the activity is to end.
Starting Time – Set the triangle on the time the activity is to begin.