114 seamanship secrets
electronic bearings or an electronic or celestial fi x). In our example, we are
using visual bearings. Plot a DR at the same time as the second position.- Always measure current set from the DR. Use parallel rules or a
protractor and measure the direction from the DR to the second
position. In our example, we measure the direction from the 1230 DR to
the 1230 fi x. Th e current set is 045 degrees true. - Always measure current drift from the DR. Measure the distance
from the DR to the second position. In our example, we measure from
the 1230 DR to the 1230 fi x. Convert that distance to speed using your
nautical slide rule. In the example, the current has set our boat 0.5
nautical mile in 30 minutes, so the drift is 1 knot.
WHAT IS THE CURRENT’S EFFECT ON YOUR
COURSE AND SPEED?
Your boat follows a course over the seabed called course over the ground, or COG.
To fi nd COG, measure the direction between two or more consecutive positions.
In the example (the dashed line in the accompanying art) this is the actual course
of the boat from the 1200 fi x to the 1230 fi x. Measure this with your parallel rulers
or protractor to fi nd course over ground (COG).
Find your speed over ground, or SOG, the same way. If the distance be-
tween the 1200 and 1230 positions measures 2.5 miles, your SOG is 5 knots.
Which of the six boat/TR relationships do you see?
boat on TR?
boat to the right of TR?
boat to the left of TR?Plot three positions to calculate the set of a current. Because the fi x is obtained from visual
bearings, it is labeled VIS.