Use DR calculations to double check your boat’s position visually and on the chart. Use DR calculations to determine when you should be able to see a particular aid to navigation or landmark based on your boat’s speed and present position. Can you answer the following DR questions? (1) What speed must I maintain to go 45 miles in the six hours before it gets dark? Speed = distance divided by time. (2) The top speed of my boat is about 12 knots; how far can I go in 10 hours? Distance = speed x time. (3) My boat’s average speed is 8 knots; how long will it take me to travel 130 miles? Time = distance divided by speed.
If these little problems are easy for you, it will be a snap to keep a DR plot on the chart or in a log, so you have a backup to your electronics. For the arithmetically-challenged you can buy an inexpensive flat, plastic device called a nautical slide rule, to do the calculations visually. Weems and Plath product #105 is one model. No batteries needed and no formulas to remember!