It's a conversion factor:

Distance (100's of yards) =
speed (kts) / time (in 3's of minutes) [EQ1]

Let's see how it works out:

Given that.....
Distance in nautical miles = speed (kts) X time (in hrs) [EQ2]

Then.....
Distance (100's of yds)
= distance (NMs) X 6080 (ft/nm) / 300 (ft per 100 yds)
= 20.27 (100's yds/NM) X dist (NM)
[EQ3]

And also.....
Speed (kts) X time (3s of minutes) =
speed (kts) X time (hrs) X 20 (3's of minutes / hr)
[EQ4]


So to get from a questionable equation 1 [EQ1] above, to a known
equation 2 [EQ2}
we try this:

20.27 (100's yds/NM) X dist (NM)
= Speed (kts) X time (hrs) X 20 (3's of mins/hr)

or NM = Kts X Hrs X 20 / 20.27 = Kts X Hrs X 0.99

In other words, the expression you gave is a close approximation -
within one percent.

Brian Whatcott Altus OK

On 27 Mar 2004 15:06:34 GMT, (Gould 0738) wrote:


"Every three minutes, a boat will travel 100 yards x the speed in knots."
The rule of thumb would obviously disregard variables like, wind, current,
tracking error, and so forth..............but as for the premise itself.....

Always true?
Never true?
True only at certain speeds?........(not likely, would probably work at a
variety of speeds since the time and distance are in consistent proportion)