116 seamanship secrets
Some electronics allow current data
input for crossing areas like the Gulf
Stream. For peace of mind, make this
simple calculation to ensure the com-
puter doesn’t lead you astray.
Find a clear area on the chart. I
find it much less confusing to do my
calculations well away from the plotted
trackline. Many navigators fi nd the cen-
ter crosshairs of a convenient compass
rose ideal.
Find a scale to use for speed.
Choose any convenient scale for your
calculations. It isn’t necessary—and it’s often impractical—to use the latitude
scale. Use a ruler, graph paper, or the tenths increments on a chart. Just be sure
all increments are the same size, and use this scale consistently throughout the
calculation. Make sure the plotting compass opens up to your vessel’s speed. In a
22-knot power vessel, you’ll need a scale with twenty-two increments. In a 6-knot
sailing vessel, you’ll need a scale with six increments.
THREE EASY STEPS TO SOLVE THE MYSTERY
- Draw the set and drift vector. From any crosshair, draw a vector showing
1 hour of set and drift. In the example above, this vector measures exactly
1 nautical mile long in the direction of 45 degrees true. - Draw the TR vector. Convert the trackline (TR) from your chart to a true
direction (if plotted in degrees magnetic). Next, go to the crosshair and
draw a long line that represents the true direction of this TR vector. We’ll
shorten this line later, but make it long for now. - Draw in the third side and solve the CTS triangle. Use your plotting
compass to measure boatspeed through the water from your scale. Power
vessels can calculate speed from engine RPMs; sailing vessels can use
speed averaged from a distance or speed log. Place the needle point of the
compass in the end of your set and drift vector, and sweep an arc across the
TR vector. Use a straightedge to join the end of the set and drift vector to
the TR vector. In the illustration, the dashed line shows the true course to
steer in order to compensate for current and progress along your TR. Apply
variation to convert this true course to a magnetic course to steer. - Solve for the boat’s speed of advance along the TR. Will boatspeed remain
the same, increase, or decrease in this current? To fi nd out, measure the length
of the TR vector. In our illustration you will fi nd that the TR vector is longer
than the course-to-steer vector. Th is means that our speed increases in this case.
Tools Needed to Build a
Course-to-Steer (CTS)
Triangle
X any convenient compass rose
X plotting compass
X a consistent scale (see the
section on fi nding a scale,
below)
X direction measuring tool
(Weems plotter, parallel rulers,
protractor)