ARC’ing off the boat speed
To complete the plot we need to use the dividers to arc off one hour of boat speed so that the arc crosses the proposed ground track.
If we have travelled for 1 hour and the boat speed was 6 knots, we would measure 6 miles from the nearest point on the latitude scale and transfer this distance, as boat speed to our plot (Fig 8.3).
If we were working up a plot for 2 hours, we would have two tidal vectors joined on to each other with two hours worth of boat speed, 12 miles at boat speed of 6 knots.
When we work up a plot for a course to steer it is always done with whole hours and never with multiples or half hours. This new line, is equivalent to the heading for the boat. The bearing of this water track is the heading that the vessel must make to counteract the tide and remain on the straight line of the ground track. We basically need to point the vessel in this direction to counteract the effect of the tide.
It is important to note that the boat is not moving along the water track, its moving along the ground track, but is pointing in the direction of the bearing of the water track. So if there was a buoy at the end of the ground track we would see it on the port bow due to the angle that we approach. it.