STEPS 1-5
Step 1: Mark the departure and destination points on a chart. I will refer to these as A and B respectively.
Sharpen your pencil. You will need it for the accuracy
Step 2: Measure AB (= the course distance) to enable you to start gathering data on the currents. It need not be measured very accurately at all unless you need to know your predicted SMG for some reason (the time taken is computed separately and does not require this distance to be accurately known).
Step 3: Divide the course distance by the expected
boat speed. This gives time of the journey if there is no current or leeway.
= 6.8 / 4 = 1.7 hours for this example
Step 4: You know the passage will take longer than that if there is total current against you (and shorter if it it with you), so make an educated guess how much longer by looking at the current along the way (this may actually be difficult, but that is a whole different topic).
This gives you a starting point for how many hour lots of current you need to consider (or half hour lots if this current info is provided). For simplicity I will describe hourly data here, but the technique works equally well for any segment you care to look at.
Divide the course into an equal number of sections as per the number of hours you estimate and determine the current in each segment for the specific hour you will be there.
This is only an approximation as the current will not be constant over the hour.
For this example you can guess that the journey will take over 1.7 hours, but probably less than 2.5. As the first hour is partial, you will need a couple of extra hours of tidal stream data.
Step 5: For this next step, it is important to draw all distance
displacement vectors to the same scale as the chart.
Determine what proportion of the first hour the first lot of tidal stream data will apply and
work out how much the boat would be displaced by this.
In this case departure is at 09:00 so the average of the last half an hour of tidal data applies (estimate 2.65 knots) for half an hour.
So the boat
displacement due to current during this time is 1.3 nm.
Draw 1.3 nm starting from A heading south, the direction the current will take you.