OUPV / Six Pack Captains License Course

PLOTTING - Time, Distance, Speed

Plotting

Time, Distance, Speed

Time, distance and speed can be calculated using the following formulas:

D = S X T /60, T = 60 X D/S, S = 60 X D/T

Dead reckoning uses speed, time, course and distance to find your vessel’s approximate position.

To solve for any of the three variables, the other two must be known. To solve for distance you must know your speed and time.  To solve for time you must know the distance and speed.  To solve for speed you must know distance and time. Often times a mariner will use a time/distance/speed wheel (TDS wheel), which is permitted on the exam.

For our work, time will always be in minutes, speed will always be in knots, and distance will always be in nautical miles. This will save on performing conversions. Military time, or the 24 hour clock, is generally used.  It is important to be able to accurately add or subtract military time. Sometimes an hour must be borrowed and changed to 60 minutes prior to subtracting.  When adding, sometimes if the final answer is 60 minutes or more is converted to hours and minutes. Accuracy is extremely important.

Practical Exercise:  Practice adding and subtracting the following in military time.  Remember that one hour is equal to 60 minutes.

  1. 12 h 30 m
    +2 h 14 m

     

  2. 10 h 15 m
    +1 h 55 m

     

  3. 20 h 15 m
    -10 h 8 m

  4. 20 h 15 m
    -1 h  35 m

  5. You leave port at 8:10 am and arrive at your destination at 10:05.  How long were you underway?

Answers:  1. 14 h 44 m    2. 12 h 10 m    3. 10 h 7 m       4. 18 h 40 m   5. 1 h 55 m

Now you are ready to practice some time, distance and speed calculations.  Remember to use the appropriate formulas above.

Practical Exercise:

You are at Stratford Point Light at 08:42 am.  You are headed to Stratford Shoal Middle Ground Light to go fishing.  Your speed is 14 knots.  What is your estimated time of arrival (ETA)? Also, what is your course to steer (CTS)? Watch the video to learn how ETA and CTS are determined.

You arrive at Stratford Shoal Middle Ground Light and alter course to Old Field Point Light.  You slow your vessel to 7 knots.  What is ETA and CTS?

Distance Off Using A Running Fix

Sometimes large vessels need to stay a certain distance off shore to avoid grounding and use a technique called doubling the bow beam bearing as a method to accomplish this.  The vessel traveling parallel to the shore will take a running fix bearing of an object on shore at a 45 degree angle.  If the vessel continues to travel parallel to the shore, when the bearing is doubled to 90 degrees, the distance the vessel has traveled is the same as the distance the vessel is from the shore.  This will form an isosceles triangle as shown in the diagram below.  While this is not exact, it is a good method to use as a back-up.