by time” (S=D/T). Flipping the expression, “distance equals speed multiplied by
time” (D=Sx T) and “time equals distance
divided by speed” (T=D/S).
In our example, we know our position
at 2: 10 pm, or 1410; we also know that
our distance to the light is 5. 4 nautical
miles. Assuming our boat speed is 8. 1
knots, divide 5. 4 by 8. 1, and we get 0.66.
Two-thirds of an hour is 40 minutes, so
our calculation tells us we’ll arrive at the
light at 2: 50 pm. Currents, sea conditions,
and winds may affect this, but we still
have a good idea of the time it’ll take.
Figure 5. With your parallel
rules, draw a line from your
position to your next way-point; walk the rules through
the compass rose inner circle
to find the magnetic course.
Write it above the line. Next,
measure the distance; write
that below the line.
Follow these steps for each leg of
your trip, and you can be confident about
where you are and where you’re going –
with or without the chartplotter.
BoatU.S. Magazine contributing
editor Tim Murphy is a Cruising World
Boat of the Year judge and coauthor
of Fundamentals of Marine Service
Technology (ABYC, 2012). He sails
Ave Marina a Vineyard Vixen 29,
on Narragansett Bay.
Learn more. www.BoatUS.com/
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