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Lesson 6 ETA | Coastal Navigation

Lesson 6 is for Day Skipper only

Difference boat speed en SOG

De boat speed through the water is measured by the speed transducer. This speeds belongs to the course through water, CTW. Wind has already influenced the boat speeds the you can read on the instrument. Speed will be higher when you are running with the wind. SOG = speed over ground = ground speed and belongs to the course over ground. Tidal current is of influence on the course over ground.


The estimated time of arrival (ETA = estimated time of arrival) is calculated with the following formula:

Estimated time of departure ETD + Travel time. The travel time is the distance over ground / speed over ground SOG.

If the distance is 10 Nm and the SOG is 5 knots,leaving at 12.00, the ETA = 12.00 + 10/5 = 14.00. Watch out: If the result of the calculation distance / SOG is for example 2,1 hour, The ETA will be 02.06, not 02.10.


Linking tidal stream vectors

We also could create 1 construction to measure the average tidal stream for a couple of hours. Plot all the tidal stream vectors, one after the other in the nautical chart. In the example below, the red, yellow and green vectors are the tidal streams during hour 1, 2 and 3. You can measure the average tidal stream direction by measuring the course from beginning to the end point. The blue line in the example. The average tidal stream rate is de length of the vector divided by the number of hours. So the length of the blue line divided by 3.

linking tidal stream vectors

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Questions and answers

Question 1: What will be the average tidal stream direction in the next 3 hours if you will experience these 3 tidal streams?

  1. 1st hr 94°, 1.5 kn
  2. 2nd hr 151°, 2.0 kn
  3. 3rd he 228°, 0.7 kn

Question 2: What would be the average tidal stream rate in the next 3 hours, in the example above?

Question 3: What will be the average tidal stream in the next 3 hours? From 1 hr before high water until 1 hr after HW?

  • Tidal current HW  -1: 000° 0,3kts
  • Tidal current HW   0: 022° 1,0kts
  • Tidal current HW +1: 022° 1,4kts

Question 4: What will be the average tidal stream rate in the next 3 hours in the previous question? So from 1 hr before high water till 1 hr after HW?

Question 5: You don't need the nautical chart for this question but this photo gives you an overview of the situation. You can just plot the leading lights on a white sheet of paper for these exercises.


Your estimated position is very close to the buoy "Schouwenbank" (51°45,0'N and 003°14,3'E). The buoy "Schouwenbank" is situated on the leading lights. The leading lights true bearing, you can find in the nautical chart. You are planning to sail to the Noorderhoofd (a lighthouse that is part of the leading lights). The tidal stream in the next 3 hours will be like this:

  • 225° and 2 kts
  • 200° and 1,5 kts
  • 40° and 1,8 kts
What will be the average tidal stream direction and rate in the next 3 hours?

Question 6: From the the buoy "Schouwenbank" situated on the leading lights, you would like to know the average CTW to end up on the leading lights again in 3 hours (during the 3 hours you are allowed to move away from the leading lights). What CTW do you need to steer in case the boat speed is 5 knots?

Question 7: What will be the ground speed in the previous question?

Question 8: To calculate the ETA you need to add the travel time to the ETD. How do you calculate the travel time?

Question 9: At 14.00 you are sailing exactly on the leading lights to next buoy Middelbank. That distance is 4,8 Nm. You will have a ground speed of 6,5 knots. What will be your ETA?

Question 10: You are again on the leading lights and sail on those leading lights to the Noorderhoofd. Imagine that your ground speed is 6,5 knots. What will be the ETA at the buoy Kaloo if the distance is 10,8 Nm?

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