|
|
|
|
|||||
|
||||||
Subject: Re: Buoys
From: Lu Abel (lu.abel@XXX.XXX)
Date: Mon Jul 12 1999 - 14:15:28 EDT
I'll certainly defer to a Coastie with direct experience on a buoy tender,
but my impression is that buoys are on chains about 50% longer than the
water depth (scope = 1.5:1).
I learned this in an interesting way. In the early 1980s, Loran C was the
high tech electronic navigation system and, just as GPS prices have dropped
in the past five years, back then Loran prices were dropping out of the
stratosphere. So in 1983 I installed one on my boat.
Loran receivers didn't initially calculate latitude and longitude
(remember, this is 1980 and the microprocessor chip has barely been
invented). Instead they displayed "Time Differences" (TDs) between
received signals. TDs are lines of position. If you look on a 1:80:000 or
smaller scale coastal chart even today you'll see a fine spiderweb of TDs
with designators like "9960-X-25440" printed on them. One was supposed to
take the TDs displayed on one's Loran, plot them on the chart as LOPs, and
cross two to get a fix.
Problem was (and still is, even as Loran fades from the scene) that the TDs
plotted on charts were based on some theoretical calculations which totally
ignored physical phenomena which distorted the position of the TDs. So
plotted Loran positions could be off by 1/4 mile or more.
But even though Loran did a not-so-great job of giving one a position, it
had fantastic repeatability. That is, if one was at a point (say a buoy),
noted its TDs, and then later went back to those exact TDs, one would be
within 50 feet or so of the point. So whenever I ventured someplace new in
clear weather, I kept the Loran on and noted the TDs of important marks as
I went.
(Thanks for staying with me so far, I'm almost back to buoys)
At the time I was sailing out of Naragansett Bay (where our friend is
headed -- good luck and it's sure a beautiful place!). Block Island, which
is about 15 miles SW of Naragansett Bay, is a great destination for a
weekend. To get there, one sails WSW from the mouth of Naragansett Bay
until one clears a reef which extends fairly far north of the island, then
turns south for the harbor on the west side of the island. The end of the
reef is marked by a big sea buoy, "1BI." (If anyone has a copy of the
training chart 1210TR, you can find it just on the left hand edge of the
chart).
Clearing 1BI properly is really important, and given the way fog can
sprout up instantly in that part of New England getting 1BI's Loran TDs was
one of my first priorities. As I ran out to Block Island several times
that first summer of Loran ownership I always passed close by the bouy and
noted its coordinates. Surprise -- they were different each time! Not by
a lot, but by a few hundredths, where each hundreth corresponded to 50 feet
of position. It looked like either the buoy was jumping around by about
100 yards or so, or there was something wrong with my set. But none of my
other my waypoints were jumping around. Then I looked at the chart and
realized that 1BI is in close to 150 feet of water. What I was seeing was
simply the buoy swinging around on its chain!! A review of the TDs I had
accumulated showed the radius of the circle to be about 150 feet, which
would correspond to the 1.5:1 scope I mentioned at the beginning.
With this experience as warning, I noted the TD variations in other
deep-water buoys. Invariably, the circle seemed to have a radius about the
same as the water depth.
The moral (besides that buoys have 1.5:1 scope :-) is that when one sees
weird navigation data (whether celestial or electronic) is to THINK.
Lu Abel
At 08:08 AM 7/9/99 -0700, you wrote:
>On Thu, 8 Jul 1999, Ferrantelli wrote:
>
>> Also, as a
>> coast guard quartermaster speciallizing in aids to navigation positioning I
>> would be intereseted in comments (on or off list) relating to buoys, fixed
>> aids, GPS, DGPS, LORAN -- what's good, what's bad--- etc.
>
>How much does the position of a buoy vary from its charted position as a
>result of tides and currents? Presumably there is some slack in the anchor
>chain, and how much slack would determine how far the buoy could move in
>any direction. I suppose it would depend on the depth of the water and the
>tidal range and perhaps the strength of the current in the area. What is a
>typical radius of the resulting circle of position? Ten yards? Fifty
>yards?
>
>Chuck Taylor
>SEASCAPE GB 32-2
>Everett, WA, USA
>
|