# A Simplified Guide to Small Marine Craft Navigation.

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#### Book by: Sergeant Walker

## Chapter 14 (v.1) - Calculating Navigational Time, Speed, and Distance.

**Calculating Navigational Time, Speed, and
Distance.**

Among many problems frequently encountered at sea for the seaman and the navigator are those involving time, speed, and distance. One such problem would be if after having travelled a certain distance in a certain time, what was the speed of the craft?

Another would be if given a certain speed, how long would it take to travel a certain distance. Alternatively, after having spent a certain amount of time travelling at a given speed, what distance was covered?

The answers to the above questions can be supplied rapidly and accurately using the following formulae.-

To determine distance when speed and time are known then the formula is.-

Distance =

__Speed x Time in minutes.__

__(over)__

*60*

…………………………………………………

To determine time, when distance and speed are known, then the formula is.-

Time in minutes =

__Distance x 60__

__(over)__

*Speed*

…………………………………………………

To determine speed, when distance and time are known, then the formula is.-

Speed =

__Distance x 60__

__(over)__

*Time in minutes.*

…………………………………………………

In formulae 1, 2, and 3 above it is assumed that the time involved is short enough to be
reckoned conveniently in minutes. When such is not the case, the factor 60 is dropped, and the time is obtained in hours and decimal fractions of hours, to convert the decimals to minutes
*multiply* them by 60.

When seconds are involved they should be converted to decimal fractions of a minute; that is
to say they should be *divided* by 60. In most cases, it will be acceptable to work to the nearest tenth of a minute. Thus, 23 seconds, which is 0.383 of minute, would be written as 0.4
minute.

Also in formulae 1, 2, and 3, distance is in all cases stated in nautical miles and decimal
fractions of miles. For the most accurate results distances used in 2, and 3, should be measured along the *ground track*, and the speed used in l, and 2, should be the speed made good along
the *ground track*, not the boat's speed through the water.

Example using formula 1.

What distance was covered by a craft that travelled at 6 knots in 43 minutes?

Distance =

__6 x 43__

__(over)__

*60*

*= 4.3 miles covered*

…………………………………………………

Example using formula 2.

How many minutes and seconds did it take for the craft to travel 6.9 miles at 7.6 knots?

Time =

__6.9 x 60__

__(over)__

*7.6*

*= 54.474 minutes.*

*.474 x 60 = 28 seconds*

Time taken to cover 6.9 miles at 7.6 knots = *54 minutes- 28
seconds.*

…………………………………………………

Example using formula 3. What was the speed of a craft which covered 4.3 miles in 48.3 minutes?

Speed =

__4.3 x 60__

__(over)__

48.3

*= 5.35 knots*

…………………………………………………

Occasionally it may be useful to determine speed over a measured mile. In this instance the time taken to pass between the marks is measured in seconds, and the formula used is ground speed over measured mile.

…………………………………………………

In Example, What was the speed through the water of a craft, which took 11 minutes 13 seconds to cover a measured mile against an adverse tidal stream running at l knot?

Ground speed =

__3600__

__(over)__

11 x 60 + 13 = 5. 35 knots over ground. + 1.00 knot adverse stream.

Speed through water = *6.35 knots*

…………………………………………………

As a final example, if the next leg of' a voyage is 45.2 miles and you are *making
good* 4.7 knots, how long will it take to cover this leg? In which case you would use formula number 2 to determine time but omit the factor 60 since the time will obviously run into
hours:

Time in hours =

__45.2__

__(over)__

4.7 = 9.617 hours . 617 x 60 = 37 minutes.

Time to cover 45.2 miles = *9 hours 37 minutes.*

© Copyright 2019 **Sergeant Walker**. All rights reserved.

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