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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
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
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.
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.
5. To calculate 300 x 12
= 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).
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UT
NOTE: Calculations are approximate, and you may prefer to adjust the
number of zeros in the answer by making a common-sense estimate
instead of working through the mathematical reasoning.
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.
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.
EXAMPLE: If you set 75 (mph) on the outer ring
opposite the SPEED INDEX triangle on the inner, you
can determine that in 20 minutes (inner ring) you can
travel 25 miles (outer ring); or that by finding 20 miles
on the outer ring, you can see that it will take you 16
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
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 liters-per-hour).
EXAMPLE: How much fuel do you need to fly 7 hours
and 30 minutes, if you know your fuel consumption rate
is 320 gallons per hour. Set the SPEED INDEX to 32
(320 gallons per hour). Find 45 (450 minutes, or 7 hours
30 minutes) on the inner scale. The number opposite is
the answer: 24, or 2400 gallons. What is your
consumption rate if you used 150 gallons in 50 minutes
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.
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