Math makes my so-called brain ache. I can do it, but it takes a lot of effort.

Easier to ask if anybody else has ever checked this formula than to pencil it
out myself and probably make mistakes.

This would be a handy formula is known to be true, or even a close
approximation.
Useful for short legs when dead reckoning.

"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)

pretty close, if not exact.

3 minutes = 1/20th hour.

100 yards/3 minutes = 20 times (100 yards/3 minutes) = 2,000 yards.


Math makes my so-called brain ache. I can do it, but it takes a lot of
effort.

Easier to ask if anybody else has ever checked this formula than to pencil it
out myself and probably make mistakes.

This would be a handy formula is known to be true, or even a close
approximation.
Useful for short legs when dead reckoning.

"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)

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)

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

Wind, current, and tracking error will offset the acutal results even more, but
99% accuracy is not bad for a memory crutch.

Thanks! (And to Jax who also affirmed the theory)

I have to admit my head started spinning about the second line of your
numerical explanation. (I'm a writer who does words, my wife is a banker who
does math...) But it looks like your process was sound and I've now learned a
handy rule of thumb.

Gould 0738 wrote in message
...

"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)

Want another useful approximation? Try the digital sextant.

My little finger's width viewed at arm's length subtends one
degree. And all its corollaries are useful; my fist subtends 6
degrees, my stretched hand from thumb tip to little finger tip
subtends 15 degrees. Rests on the fact that a degree is
1/57.whatever of a radian.

My hand, outstretched, is a convenient 57 half inches from my
eye; so a half inch at that distance is a degree. Give or take a
few seconds. Calibrate yourself and your crew.

Used as a vertical sextant this is useful (in the absence of
radar!) to discover if you're overtaking the yacht ahead. As a
horizontal sextant it's great for helping other crew members
identify obscure marks. 'See the tall chimney? OK, 18 degrees to
the right and two degrees above the waterline . . .' means 'one
hand span plus three fingers right and two fingers above'. Much
better than just saying 'just right of the shrouds'. Not as good
as putting someone's head in transit with the shrouds and the
mark , though.

Other mental dead reckoning tricks (like the sine clock) are
sadly being made redundant by GPS.

JimB

to other crew members if they kn