Course to steer for over an hour
In this second section we’ll take a look at working up a course to steer for more than a single hour.
A single hour
We have so far examined and calculated the course to steer for one hour of boat speed and one hour of tide plotted from our fix or estimated position.
In the next example the distance to go is estimated to take nearer two hours (Fig 8.11). If we have to travel 10.5 miles and the boat speed is 5 knots, we will have to travel for for roughly two hours to reach the destination.
We normally work our CTS course to steer in whole hours, 1hours, 2 hours, 3 hours etc. Its much easier working a plot in this way as it relatively simple calculating the hours of tide we need. However to reiterate what was said before, there are always exceptions to the rule. One of them being traveling in a fast boat or using and working near the edges of a small chart.
The golden rule to remember is that the water track in hours matches the hours of tide used in the plot which matches our estimated journey time or ground track.
The second hour
The tidal stream is rarely constant in rate and set (direction). If we have estimated to take two hours to get to our destination we must work up the plot using 2 hours of tide.
We do this by plotting the 2nd hour of tide on the end of the first hour as shown in the Fig 8.12. If we are plotting a long course to steer over many hours we add all the hours of tide onto the end of the last tidal vector. For accuracy we plot each individual hour of tide as it is extremely bad practice to try and average hours of tides
ARC’ing off the boat speed
To finish the plot off we need to use the correct amount of boat speed. So in this case we would use two hours of boat speed which would be 10 miles (2 x 5 knots). Remember the arc must cross the proposed ground track (Fig 8.13).
We measure 10 miles from the adjacent latitude scale, place one end of the dividers at the end of the 2nd hour of tidal vector and arc the dividers where they meet the ground track.
We can now measure the heading from the water track, which is the direction we steer the vessel to maintain position along the ground track. In this case Fig 8.13 we are getting a small lift from the tide and would approach the buoy on the port side of the vessel.