easy calculations and adjustments 55
- To fi nd speed or time from distance traveled
Set the distance wheel fi rst. Hold it fi rmly in place with your nondominant
thumb.
Turn the small wheel to the other known factor, either speed or time.
Read the window to solve the unknown factor.
For example: Using the chart, you determine that you must travel
13.5 miles to reach the sea buoy ahead. Th e GPS shows a speed over
ground of 9 knots. If the time now is 1445, what time should you arrive
at the sea buoy?
Because you know the distance, set that first. Set the speed window
to 9 knots. Read the time: 1.5 hours. Add 1.5 hours to 1445 to find your
ETA of 1615, which should agree with the ETA your GPS receiver is
giving you. (I can imagine you thinking, “If my GPS receiver will give
If you don’t have a slide rule or cal-
culator, you will still want to work out
time, distance, and speed solutions.
Follow these steps and work through
the examples.- Draw a large triangle.
- Subdivide the triangle with a
horizontal line drawn halfway
between its base and its apex.
Now bisect the lower part of the
triangle with a vertical line. - Place a D in the upper portion.
Place a T in the lower left portion
and an S in the lower right portion.
These represent the factors of
Distance, Time, and Speed. - Cover the unknown factor in order
to find what to do to solve the
problem. For example, if you cover
the D, the S and T are side by side.
Multiply them together to fi nd D.
If you cover the S, you see that you
must divide T into D. If you cover
the T, you must divide S into D. If
time is a known factor, you must first
convert it to hours. For instance,
if the time traveled is 1 hour 36
minutes, convert to hours like this:
36 ÷ 60 = 0.6. Total hours = 1.6.
Example 1
You have 64 miles to go and must
arrive in 5 hours 22 minutes. What
speed must you make?
Cover up the S (speed unknown);
you must divide time into distance.
Convert time to hours fi rst: 22 ÷ 60 =
0.4 (rounded). Total hours = 5.4 hours.
64 nautical miles ÷ 5.4 hours = 11.9
knots (round to 12 knots).
Example 2
You are averaging 5 knots under sail.
The next port lies 38 miles downwind.
It’s 0800 hours. What is your estimated
time of arrival?
Cover up the T (time unknown); you
must divide speed into distance.
38 nautical miles ÷ 5 knots
= 7.6 hours.
Convert tenths of hours to minutes:
0.6 x 60 = 36 minutes; 7 hours 36
minutes.
0800 + 7 h 36 m = 1536,
i.e., 3:36 pm.Time, Distance, and Speed—Manual Method