Boating and Sailing.pdf - Moja ladja

Chapter 14: Nautical Radio Gear and Procedures and Your First Longer Cruise
Dead Reckoning Is Alive and Well
Dead reckoning is the art of estimating position by calculating course, speed, and time,
without the aid of electronics.
It’s not as accurate or as simple as turning on the GPS chart-plotter, to be sure, but it’s a
backup for when everything breaks, and it will usually get you within sight of the marker
or the harbor you’re looking for.
On a long cruise, particularly when you’re traveling
at night, it’s standard procedure to track your
course on the chart via dead reckoning to make
sure your electronics have not gone bonkers. And,
if you don’t have a GPS or a LORAN, you can
use dead reckoning to calculate about how long it
will take you to get from point A to point B.
The basics are the relationships of distance (D),
time (T), and speed (S) you learned in elementary
school:
Distance = Speed × Time
Speed = Distance ÷ Time
Time = Distance ÷ Speed
So, for example, if you are traveling 30 mph for two hours, you can easily calculate distance
covered thus:
D = 2 × 30 = 60 miles
To calculate using minutes, the equations look like this:
D = S(T) ÷ 60
S = 60D ÷ T
T = 60D ÷ S
Maybe you’re going from your marina to another at the other end of a big lake and you
want to know if you’ll arrive in time for lunch. The distance is 40 miles and your boat
does 15 mph.
T = 60 (40) ÷ 15 = 160 minutes = 2 hours, 40 minutes
Bet You Didn’t Know
It’s not called “dead” reckoning
because you’ll be dead if you
don’t do it right. The term comes
from “deduced” reckoning,
meaning that you must deduce
your position from indirect information.
It takes a bit of doodling to keep track of your location with a pencil rather than electronic
memory bytes, but after a few trips it becomes second nature, and one more interesting
skill you can use as skipper.
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