In article om,
says...
On Oct 27, 2:41 pm, Skip Gundlach wrote:
... I wasn't able to find any author by that name. However, I have,
now, read a 21-year-old text by that name and which had Earl
Hinz, as I offered in my reply before, as the author. ...

Earl R Hinz is one of my heroes. He is an extraordinarily talented
writer and researcher and has produced the seminal books on cruising
in Oceania based on his own trailblazing travels. You may disagree
with him, and there are a few thoughts of his that I take issue with,
but he deserves more than an ordinary amount of respect. The last
edition of _The Complete Book of Anchoring and Mooring_ came out in
2001. I have an older edition and I don't know if the new one
mentions the newer anchors but even if it doesn't the fundamentals
haven't been changed by them. I've been using a Spade for a few years
now and it is a good anchor, but it is used in the same way as the
Delta from which it was evolved. Not that anyone can learn to anchor
by reading books alone, but the Hinz book on anchoring is worth a read
and his articles and books on Pacific cruising are very good, indeed.
Of course, I think Bob is being silly, but I hope that won't put you
off Hinz.

On a more or less unrelated topic, I use my GPS for anchor watch all
the time. I've got a Furuno GP-31 and it has a simple graphic page
that displays a "bread crumb" trail. I find that I can see where I
dropped the anchor on that screen and put a goto point there. The
anchor watch alarm is then set to go off if I go outside a circle
around that point. While we sit at anchor the gps continues drawing
the track on the screen and pretty soon a thick arc is drawn. This
makes it very easy to see if we are dragging even if it is pitch black
and raining as it so often is when a front passes by in the night...

After reading other posts citing an apparent lack of precision in GPS
positions (probably related to weak signal strength in a forest), I
decided to do an experiment to determine GPS precision with a modern
WAAS-enabled GPS chip set.

Hardwa Prototype board from an oceanographic data buoy using a
UBLOX TIM-4A gps module. (12-channel receiver, WAAS enabled) Data was
stored on SD card on the board. GPS signals were received using an
outdoor antenna with a 25-foot cable connected to in indoor GPS
reradiator The GPS antenna connected to the UBLOX module was about 1ft.
from the reradiator. A Garmin GPSMAP 76C placed near the UBlox antenna
showed 8 satellites in view with near maximum signal strentgh on 6 out
of the 8 satellites. At that time the indicated precision on the GPSMAP
76C was about 10 feet.

Setup: GPS data was collected at 1-minute intervals for about 23 hours
starting at 1816Z 27 Oct. 2007. The data point collected was the most
recent of the 1Hz NMEA outputs from the GPS module. The data was NOT
averaged over the 1-minute interval.

Data Processing: The collected minutes North and Minutes East
were each averaged. Degrees North was 44 and Degrees East was 123
for all data points. The difference between each data point
and the appropriate average was computed for each minutes North and
minutes East point. The distances North and East of the average
position were computed using the relationships that 1 minute of
latitude equals 1 nautical mile and 1 nautical mile = 6000 feet. The
distances East or West of the average position were corrected
by multiplying the difference in degrees times the cosine of
the latitude. I believe the overall systematic errors in this
approach are less than a few tenths of a foot for each data point.

Once the North and East positions were calculated, the distance
of each point from the average position was calculated using
d = sqrt(PN * PN + PE * PE). (All calculations were done
using an Excel spreadsheet).

The position errors were then examined to see what percentage
of the positions fell within a specified distance from the
average position.

Results:

Maximum Position Error: 32.0 feet.
Average Position Error: 9.6 feet.

Percentage with error 10ft 60.2%
Percentage with error 15ft 83.9%
Percentage with error 20ft 96.4%
Percentage with error 25ft 99.5%
Percentage with error 30ft 100.0% (1366 out of 1367)


Conclusions:

1. With a modern GPS and a well-placed antenna, GPS precision
should not be a limiting factor for anchor alarms if you
are in an area with good WAAS corrections. You can
find some information about the precision increase with
WAAS he http://gpsinformation.net/exe/waas.html

2. Anchor alarm radius has to account for swinging
to the anchor in normal winds and for the possibility
that the winds or tidal currents may set your boat
all the way to the other side of the anchor. These
factors may limit the usefulness of anchor alarms
in smaller anchorages or tight quarters.

3. Waking to the alarm with a wind or tide change is probably
preferable to setting the alarm radius so large it
will accomodate such changes.

4. GPS anchor alarms tell you nothing about the
position of other boats in the anchorage. In fairly
calm conditions, boats may respond differently to
the combination of wind and current. Bumps, thumps and
curses can ensue!

Mark Borgerson