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.