SCENARIO:
The vessel is on a passage from WPT001 to WPT002 which is a distance of 19.7 nautical miles and a bearing of 240º magnetic with an estimated boat speed of 5.0 knots. We have determined that there is a current of 2.0 knots in the direction of 130º magnetic (This can be obtained from current tables on the chart, local pilot books or by you own calculations to derive the set and drift).
Step 1
Draw the required course line on the chart from WPT001 to WPT002 in the direction of 240º label this with two arrows and the notation C 240 M 19.7 nm. Set the compass to a distance of 2.0 nautical miles (this is the speed of the current), place the point at Point A and draw an arch. Draw a line from Point A in the direction of 130º until it meets the arch you have just drawn, this will create Point B, add the notation 130 M S2.0 (this is the direction of the current). Your chart work should look similar to diagram below:
Step 2
Now set your compass to a distance of 5.0 nautical miles (this is the estimated boat speed). Place the point of the compass on Point B and draw an arch so it bisects the course line. With your parallel rules draw a line from Point B to this bisection point, creating Point C. With your parallel rules measure the angle from Point B to Point C, this creates our COURSE TO STEER, which has taken into the effect of the 2.0 knot current. Write the angle that was obtained and the boat speed on this new line, in this example, CTS 265 M S 5.0 as in the next diagram:
CONCLUSION:
Our new course to steer (CTS) is 265 M, by maintaing this course we will achieve a Course Made (CMG) Good of 240 M. Be sure to make corrections to the CTS to allow for any leeway that the vessel might make.
