PLOTTING THE EFFECT OF CURRENT
Step 1: We’ll use a universal plotting sheet to work out our current vectors. The plotting sheet is marked with units for distance and angle. We’ll use it to draw vectors, lines that represent both angle and distance. To solve problems for tidal current, the trick is to create a plot based on one hour of travel. That way our speed (knots) equals our distance (miles), and the length of each vector directly represents our speed. In this problem, we’re steering a course of 020 true and making 5 knots through the water. Using dividers with a pencil tip on one end, measure off 5 miles on the distance scale and make an arc near our course. Next, using the compass rose, line up parallel rules from the plotting sheet’s center mark through the 020 mark on the compass rose. Draw a line from the center to our pencil arc; this vector represents our course and boat speed. Above the line, label it “C 020 T.” The “C” reminds us that this is our course, uncorrected for current; the “T” reminds us that this is a true course, corrected for compass error. Below the line, label it “S 5.0”; the “S” reminds us that this is our boat speed (through the water, not over the ground).
Step 2: Now we’ll draw a second vector representing the current’s set (direction) and drift (speed). Using NOAA’S Tides & Currents tables, we’ve determined that the current is setting us at 082 true at a speed of 2.9 knots, so we can start by marking off a distance of 2.9 miles with the dividers. From the head of our course/speed vector, scribe an arc near 082. Set the parallel rules at the center of the plotting sheet and at 082 on the compass rose, then walk them to the head of the course/speed vector. Draw a line that intersects our second arc. Label these “SET 082 T” and “S 2.9.”
Step 3: Using parallel rules, draw the vector from the center of the plotting sheet to the head of the set/drift vector. Read the angle off the compass rose, then measure the distance with the dividers. This vector shows that our actual track is 042 true and our speed over the ground is 6.8 knots. Label this vector “TR 042 T” and “SOG 6.8.” We’ve solved for current. A plot on the chart based on this exercise would be called an “estimated position” — more accurate than a simple dead-reckoned position but still not as accurate as a positive fix.