Distance. Let’s assume you have traveled
for 22 minutes at 10 knots. How far
have you gone?
S × T 10 kn × 22 min 220
D = —— = —————— = ——— = 3.7 nm
60 60 60
Time. Now let’s assume you are about to
start a 15-nm leg of your cruise. How long
will it take you to get there at 20 kn?
60 × D 60 × 15 nm 900
T = ——— = ————— = —— = 45 min
S 20 kn 20
Speed. Now let’s assume you need to
be at your destination within 30 minutes.
You are 10 nm away. What speed must you
go to reach your destination on time?
60 × D 60 × 10 nm 600
S = ——— = ————— = —— = 20 kn
T 30 min 30
THE 24-HOUR CLOCK
To simplify working with time in navigational
problems, it is convenient to consider
the day as one 24-hour period and to
number the hours in series: 00 to 24. This
is known as the 24-hour clock, and is sometimes
called nautical time (Figure 18-13).
Time of day is shown in four digits;
the first two digits indicate the hour beginning
at midnight, and the second two digits
indicate the minutes past the hour. As an
example: 2015 means 15 minutes past the
20th hour, or 8:15 P.M. Refer to this time
as “twenty-fifteen.”
Always express nautical time simply
in four digits. The terms “A.M.” and
“P.M.” are not used, and the word “hours”
is never used.
Times such as 0900 and 2000 are correctly
referred to as “nine hundred” and
“twenty hundred,” respectively.
1. What was the elapsed time if a boat left
at 0912 and arrived at its destination at
1547?
Arrive: 15 47
Depart: 09 12
Elapsed Time: 06 35 (6h 35m)
2. What was the elapsed time if a boat left
at 1047 and arrived at its destination at
1612?
Arrive: 16 12 (1572)
Depart: 10 47
Elapsed Time: 05 25 (5h 25 m)
(Note the change of 1612 to 1572. Subtract
1 hour and add 60 minutes to allow easy
subtraction of minutes.)
214 Chapter Five
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Figure 18-13. 24-hour clock