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.
Drift / Leeway
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.
Tidal current / Set
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 from compass course to course over ground:
Compass course to steer
Magnetic course to steer
Magnetic water track
True water track
Course over Ground
CC Compass course
DEV Deviation +
MC Magnetic course
VAR Variation +
TC True course
Drift / leeway (wind) +
CTW Course through water / dead reckoning line / water track / course to steer
COG Course over ground / Ground track
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.
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Questions & explanations
Question 1: The compass course is 90° and the wind is North, what will be the drift?
a: To starboard so positive
b: To port so positive
c: Top starboard so it is negative
Question 2: The compass course is north, the tidal currents west, the set is?
Question 3: Variation is the difference between...?
a: compass- and magnetic course
b: compass- and true course
c: magnetic and true course
Question 4: In what direction runs a northern tidal stream?
a: 0 degrees
b: 180 degrees
Question 5: Calculating the compass course, is something we do in case we want to...:
a: make a plan
b: determine our estimated position
c: determine our course over ground
Question 6: When the tidal current is against the ships heading, our speed through the water is...?
a: the same as the speed over ground
b: faster than the speed over ground
c: slower than the speed over ground
Question 7: On tidal waters we would like to enter a port. Our course over ground would be 134°. There is a strong cross tidal stream in the direction of 12°, causing a set of 20°. The southwestern wind causes a drift of 5°. The variation is 4° west and deviation +1. What compass course do you need to steer to sail into the port?
Question 8: You are leaving a port with a compass course of 20°. There is a eastern tidal current and no wind. The set is 10°. The variation is 2° west and the deviation -2. Which coarse over ground do you need to draw in the nautical chart to determine your estimated position (EP)?
Question 9: You would like to enter a port with a course over ground of 91°. You assume that the set will be 6° and the tidal current is south west. Because of a southern wind you will drift 10°. The variation is 4° west and deviation -1. What will be the compass course?
Question 10: you are leaving a port with a compass cours of 156°. There is a western wind, you estimate the drift angle 6°. The eastern tidal currents cause a 8° set. The variations is 3° west and the deviations +1. What course do you need to put into the nautical charts to determine your estimated position?