Anchor watch
The MOB way of checking on an anchor is fine. And maybe 40ft of error
is something you're willing to accept.>Jeffrey Siegel
I might be missing something here but when I'm in anchor, say in 25' with 200' of chain out, my boat might move up to 150' from the anchor location (MOB) due to tide or wind change without the anchor dragging. I use a Furuno GP-32 to set the MOB location and calculate the distance from the waypoint to the boat. I then transmitt that data via NMEA 0183 connection to a Furuno RD-30 display at the head of our bed, where I can set an anchor watch based on .01 NM increments. This is about 60' increments so on a calm night I may set the alarm to .02 so if the distance from MOB waypoint to boat exceeds 120' then the alarm sounds in our state room.
I don't use a PC because I don't want to run the inverter over night and I don't use the Furuno NavNet chartplotter because when NavNet is on the radar magnetron is on and the additional run time will shorten it's life spand. Anyhow, how do you differentiate between boat movement in different directions and possilbe anchor dragging?
--
Frank Osborne
Discovery N46-37
Home Port: Anacortes, WA
Currently in La Paz MX
Blog:http://mvdiscovery.blogspot.com/
...when I'm in anchor, say in 25' with 200'
of chain out, my boat might move up to 150'
from the anchor location...
I can set an anchor watch based on .01 NM
increments. This is about 60' increments...
so on a calm night I may set the alarm to .02
Excellent! A real world example. Some of the numbers will surprise
you. First, there's one number that's left out - the height of your
bow. I'll use 8' as a good estimate. I'm also not going to consider
tide swing - but that has a pretty small effect with 200' of chain.
First we have to determine the maximum amount of swing that you can
actually experience. This is easy Pythagorian Theorem math based on
the triangle formed by the height off the sea bottom, the chain let
out, and the horizontal distance you can possibly move away from the
anchor. In this case:
(200)^2 = (25 + 8)^2 + X^2
where X is the horizontal distance the boat can move away from the
anchor. The first surprise is that your estimate of 150' is pretty
far off. I know - it seems like it should be 200 - 25 or so. But
work out the math and the answer is 197 feet. It doesn't feel like it
should be that close to 200 but it is.
So first, setting your anchor alarm to 0.02 minutes (not nm) distance
will set it to about 120 feet. Moderate current or wind will most
certainly cause that alarm to go off. So you'll wake up and set it to
0.03 or 180 feet. In a short amount of time, that will alarm too
because you can easily move back 197'. So now you'll be wondering if
your anchor is dragging all because the math wasn't right to begin
with. And we haven't even considered the GPS offset error.
Assuming you've swung around after anchoring in the afternoon (since
any real dragging will probably happen at the most terrifying time of
the night!), and assuming your GPS is set back 20 feet from your bow,
there's another 40' of GPS offset error introduced along with an extra
10' of GPS positional error (assuming WAAS, differential, etc). This
means that a normal MOB/anchor alarm setup can't possibly detect any
dragging reliably until you have moved 250' back.
So now you bump-up your alarm radius to 0.04 or 240 feet. Once again,
it is quite possible that the alarm will go off as it reaches the
error thresholds over time. After hearing the alarm go off three
times at now double your original distance estimate, you'll be moving
the boat in the middle of the night even though it probably hasn't
really dragged one inch.
Instead, using a more sophisticated algorithm, the total error can be
reduced to about 10'. Also, most GPS's can provide 0.001 minute
resolution. Even an iPhone or an AT&T Tilt will do that. That allows
you to take advantage of the 10' error with 6' increment values. The
end result is that the alarm goes off when you're really dragging.
Anyone who has done a fair amount of anchoring with traditional anchor
alarms knows this radius-bump-up dance very well. We've all done it
and we've all wondered whether the anchor was really dragging. After
a few sleepless nights, I played around with understanding the issues
and tried to find a way to make it more reliable. ...even if it does
involve technology.
---===
Jeffrey Siegel
M/V aCappella
DeFever 53RPH
W1ACA/WDB4350
Castine, Maine
www.activecaptain.com
The Interactive Cruising Guidebook
..
i guess we all have our personal preference, there is no right or wrong...
here is how i do it... when i drop the hook i zoom in to near max on the
helm plotter. this quickly shows me whether or not we're holding (yes, i
take bearings too) as the graphical representaion of the swing will tell the
story. if the arc gets too thick, I know we're not holding.
I dont' use an drag alarm on the plotters (too far away at night) but i keep
a portable garmin by my bed. I works well thru the glass deck... I usually
set the alarm between 50 and 100'. This alerts me if we drag and also
alerts me if the wind shifts as the arc increase and exceed the alarm
setting (with the typical shallow water/ 8:1 scope i use.
works great for me...
pascal
miami, fl
hatteras 53MY
on M/Y Charmer anchored jsut north of St Lucie bridge.
live helmcam at www.sandbarhopper.com
Jeffery,
Thanks for the math lesson. Normally we'll anchor so we have at least a
500' radius of clear water around the anchor and if the
anchor alarm goes off when it's set to .02 that means the wind has come
up and I'll want to get up and take a look around.
My Furuno GP-32 will only send out a NMEA 183 sentence with with 2
decimal points of precision for distance from waypoint.
It can output the nmea sentence in 3 different formats; 1) NMEA-REM1, 2)
NMEA-REM2, or 3) NMEA-AP. Part of the RMB nmea
sentence is the distance, in NM such as 1.10 or .01 NM, to the way point.
In the first two formats the distance is given to one
decimal point, only in the NMEA-AP format does it go to 2 decimal
points. Example -
$GPRMB,A,0.00,L,START ,002,5726.898,N,13538.168,W,0.01,357.6,0.1,V*5D
Here the distance is given as 0.01 minutes. That's about 60 feet. My
Furuno RD-30 display requires a RMB, BWR, or BWC
sentence for the anchor alarm and the GP-32 only puts out the RMB. I
would like to have the third decimal point of precision but
can't with this equipment. I normally note the distance to waypoint when
pulling back on the anchor to set it and use that as my
baseline to set the alarm.
We've used this equipment for 2 years from Juneau to La Paz with success
but nothing makes me sleep at night like lots of
space, a large anchor (110 lbs), and plenty of heavy chain out. I was
always amused in BC and Alaska that the yachties would
come in to a bay and spend an hour finding just the right place and
setting the anchor and standing on the bow calculating how close they
were to everything and then a fisherman would come in late, pull straight
in (been there a hundred times) drop his
200 lb fishermans anchor and turn off his light and hit the bunk. All in
about 5 minutes. If you walk the commercial docks, fishing
boats have anchors normally twice the size for their length as yachties
do.
Now if you could only come up with a depth alarm that doesn't give false
alarms every time a school of fish goes under the boat
at 3 AM.
Frank Osborne
Discovery N46-37
Home Port: Anacortes, WA
Currently in La Paz
Blog:http://mvdiscovery.blogspot.com/
Part of the RMB nmea sentence is the distance,
in NM such as 1.10 or .01 NM, to the way point.
$GPRMB,A,0.00,L,START ,002,5726.898,N,13538.168,
W,0.01,357.6,0.1,V*5D
Yes, you're right. But note that the latitude/longitude is given to 3
decimal points:
57 degrees 26.898 N, 135 degrees 38.168 W
So the distance of 0.01 nm has been purposely reduced in precision.
There isn't a very good reason that a more precise and accurate value
isn't calculated from the lat/lon positions instead.
My Raymarine does the exact same thing - it drives me nuts. Yes, I do
set the anchor alarm on that too but I usually way over-compensate for
distance because of all of the known issues with these types of alarms.
We've used this equipment for 2 years from Juneau to
La Paz with success but nothing makes me sleep at
night like lots of space, a large anchor (110 lbs),
and plenty of heavy chain out.
I definitely agree. In no way was I questioning anything about your
anchoring abilities. You obviously know what you're doing. I was
only using the real world example to demonstrate the problem with
existing anchor alarms.
Now if you could only come up with a depth alarm that
doesn't give false alarms every time a school of fish
goes under the boat at 3 AM.
We have 2 depth sounders. My first stab would be to average them and
watch trends. Plotted against ENC/S-57 charts could provide a
"distance to danger" although that sounds very error prone.
Or you could drop a lead line! ;-)
---===
Jeffrey Siegel
M/V aCappella
DeFever 53RPH
W1ACA/WDB4350
Castine, Maine
www.activecaptain.com
The Interactive Cruising Guidebook
..