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From: Bob Young (no email)
Date: Wed Apr 04 2001 - 01:42:54 EDT
I nautical mile is actually 1852 meters, or 6076.12 feet! I still say 6000
feet is a good enough approximation, and better than 1800 meters..
If you think 100 feet is too much line, you can use 50 feet and 30 seconds as
the dividend - or any other length and appropriate time, for that matter. I
don't like trying to count knots. That method was used before accurate stop
watches were available. A sand glass was turned, and when it ran out the
number of knots in the line counted was the speed. Measuring the time for a
standard length to run out is simpler. My method just requires you to manage
the line as it runs out, making sure it doesn't foul. When the line reaches
the end, it's very obvious.
I haven't actually tried this, but here's another idea I have had. It also
uses a line and drogue, but this time just deploy the whole line, which need
not be very long, and measure the tension by extension of a spring. A simple
linear "spring balance" device should work. Of course, it would have to be
calibrated to read knots directly, and the scale would probably be non-linear.
I think I have actually seen such a device for sale in a catalog, so it ought
to work.
On Tue, 3 Apr 2001 17:40:23 +0200, Aharon wrote:
>Have you actually done this?
>I tried something similar with a half filled water bottle (simulated log) on
>the end of a string. Using the idea that 1 nautical mile is about 1800
>meters, I tied knots in the string every 3 meters. Then I let the bottle go
>and started my stopwatch. I counted how many knots passed through my
>fingers during 6 seconds (3meters / 6seconds = 1 knot speed). If 5 knots
>passed in 6 seconds I was doing 5 x 3meter / 6sec x 3600sec/hr = 9km/hr = 5
>knots.
>Drawbacks:
>- You want to use small diameter rope/string cause more than 20 meters of it
>is a lot to manage.
>- Good luck feeding it out at 6 knots without getting it fouled.
>- My results were never repeatable nor anywhere near the speed on the GPS.
>
>Maybe a longer string would reduce error, but increase the likelihood of
>fouling. Thank God for GPS!
>
>-Aharon
>
-- Bob Young, on 04/04/2001
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