We are now going to learn to calculate back and forth from the compass course to the ground course. We have already studied calculating from compass course to true course in the previous lesson, but we have not given any attention to wind and tidal currents jet.
Under the influence of a cross wind, the boat not only moves straight through the water, but the boat will be also drifts aside, this is what we call drift. An angle is formed between the keel line of the boat (the true course) and the course through water, CTW also called dead reckoning line or water track. The difference between the true course and the CTW is called drift. We can measure this angle in practice by throwing a long line and make a bearing on it with a compass. The drift is maximal if you are beating to the wind and there is no drift in case you are running. High waves boost the drift angle and relatively light ships are more affected by drift.
If there is a tidal current, that means that the water with our yacht sailing in it, will slide over the earth in a certain direction. This creates an angle between the course through water and the course over ground, or simply ground course (= COG). We call this angle that is caused by tidal streams, the set. The set can not be seen from the boat by looking at the water. Only when you see land or a buoy nearby, you see how much set there is. You can calculate it by comparing the CTW and COG. The COG can be seen on the GPS. You can also measure the set by drawing a construction in the nautical chart. We will come back to that later.
A north wind COMES from the north and a north tidal stream GOES to the north.
Calculating the ground course
We can make the following calculation to calculate from a compass course to a course over ground.
CC Compass course
DEV Deviation +
MC Magnetic course
VAR Variation +
TC True course
Drift (wind) +
CTW Course through water, dead reckoning line or water track
COG Course over ground
The drift and set can be positive or negative, Clockwise, ie wind or tidal currents push us to starboard, it is positive (+). If wind or tidal currents push us counterclockwise (ie to the port side) the angle is negative (-). Always make a drawing of the situation to determine if you are dealing with a positive or negative set or drift angle.