RE: Speed

Cosens, Eric D (ecosens@indiana.edu)
Wed, 21 Jul 1999 08:45:00 -0500

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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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In addition to the bottom and water speeds differing if a significant
current exists, civilian GPS is also quite inaccurate when measuring such
slow speeds due to selective availability (SA). Total distance divided by
time would be a more accurate method of determining speed assuming you know
the distance (which you could obtain fairly accurately with the GPS,
especially if you let the beginning and ending points "average" for 10-15
minutes). I'd probably measure the distance with my GPS and divide it by
the time from my watch to get average speed. Of course this is speed over
bottom.

-----Original Message-----
From: TillyLucy@aol.com [mailto:TillyLucy@aol.com]
Sent: Monday, July 19, 1999 11:29 AM
To: gatorjj@mindspring.com; wwpotter@tscnet.com
Subject: Re: Speed

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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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I wouldn't be surprised if the knotstick and the GPS don't agree. The
knotstick measures your speed through the water while the GPS measures your
progress over the bottom. Picture your self motoring at 5 knots directly
into
a 4 knot current going in the opposite direction. Under those conditions,I
would expect that the knotstick would read a speed of 5 knots and the GPS to

indicate a speed of 1 knot.

Best regards,

Dave Kautz
P-15 #1632 Tilly Lucy
Palo Alto, CA

In a message dated 7/19/99 9:05:54 AM Pacific Daylight Time,
gatorjj@mindspring.com writes:

<< I know there's been a lot of discussion on the speed of Potters, and
measuring speed, etc. Wanted to throw in my 2 cents from this past
weekend's
trip.

I used both a knotstick and a GPS, and had a fixed distance I covered. The

distance between Cape Lookout and Beaufort Inlet is about 6.5 nm, which I
covered in about an hour and a half (I know, real scientific so far!). In
rough terms, I averaged about 4 knots. During this run, the knotstick
(which
claims to be accurate to .1 or .2 knots) almost never read below 5 knots,
and
sometimes came off the scale (past 6 knots), while the GPS would read in the

4-5 knot range.

I would have expected one of 2 things--either the GPS was slow and I would
eventually see 6 or 7 knots on it as compensation (as eventually, if I'm
doing 5.5 knots the GPS would error the other way), or the knotstick would
bounce up and down due to waves, between 4 and 7 knots). Neither happened.

I suspect the reasoning behind all this is we were sideways to the waves,
and
keeping a reasonably steady course, we went sort of straight at about 4.5
knots, but also back and forth in tiny increments about a knot? Does this
make sense?


J.J. Falkanger
Cary, NC
P-19# 792 "Fozzguppy" >>