Even though you may be an inland lake sailor, it is still a good idea to learn about dead reckoning. It is a way to have knowledge of where you are on a chart.

In order to calculate where you are, you must first have the definite knowledge of where you were. So, before you start on a journey, determine where you are on the chart.

To calculate you must use the following three equations: (D = Distance in miles, T = Time in hours and S = Speed in knots or mph):
D=ST or S=D/T or T=D/S

However, you might want to calculate in minutes instead of hours. In that case the formulas are:
D=ST/60 or S=60D/T or T=60D/S

Some examples:

1) You are sailing at 14 knots. How far will you go in 40 minutes?
To find the Distance use the formula of D=ST/60. You would multiply the Speed of 14 by the Time of 40 and divide by 60, giving you a distance of 9.33 nautical miles.

2) Determine your speed. You leave a known sea buoy and arrive at another known buoy. It took you 40 minutes and you measured on the chart and know the distance is 11 miles.
Using S=60D/T, simply multiply the Speed of 11 by 60 and divide the result by the Time of 40 minutes and you then determine you are traveling at 16.5 mph

3) You have a 9.5 mile reach to your home port and you are sailing at 6.5 knots. How long before you get home?
Using T=60D/S, multiply 60 by the Distance of 9.5 and divide by the Speed of 6.5 and determine you will in port in 88 minutes.

Oh Sure! You are asking, “Why learn to do all this when I can use a GPS?” Good question, but learning dead reckoning is a basic that should not be overlooked.., just like learning your A, B, Cs.

This helpful boating pointer is provided by Catamaran Sailors Magazine, http://www.catsailor.com/.These tips are useful to boaters of all types. ABA assumes no responsibility or liability for events that occur due to actions you or others on your behalf take based on the information given. You are proceeding at your own risk.

Source: americanboating.org