Dead reckoning

When we are on an open sea passage out of sight of land, at night or in a dense fog, we must be able to keep an accurate track of our position and course at all times.
There are many ways we can do this, and many techniques we can use for finding our position.
The most basic technique is called the Dead-Reckoning Technique.
Let’s assume that we are sailing from A to B, and that A is the last fixed position plotted on the chart at 10:00.
Now it is 11:30 and we have to plot a new position using the Dead Reckoning (DR).
The true course is 045 degrees, and the speed through the water, taken from the speed log is 7 knots
Knowing the course, the speed, and the running time from 10:00 am to 11:30 am, we can find the distance sailed using the formula – D is equal to S times T divided by 60, where D is the distance in nautical miles, S is the boat speed, and T is the time in minutes.
In our case: 7 knots times 90 minutes divided by 60 equals 10.5 nautical miles.
Using the dividers, we span a distance of 10.5 miles on the latitude scale which is closest to the course line, and which runs along the sides of the chart.
We applied this span on our course, placing one leg of the dividers on the 10:00 position and the other leg on the course, marking the new position with an X, recording the time of 11:30am.