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Old 08-24-2016, 09:01 AM   #1
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Anchor Alarm

Interesting article from ActiveCaptain this morning.

Personally I never set an anchor alarm, and after reading this probably even less likely to!

========================

>>> Anchor Alarms >>>

One of the most controversial newsletters we ever released happened
over 3 years ago - it was about the mathematics of anchor alarms. It was
so popular that we're running it again because there are so many new
ActiveCaptain members who missed it.

It should be simple. Pick the spot to anchor; come to a stop; drop the
anchor and set the anchor alarm. Then pull back until the anchor sets.
Now if you pull away further from the anchor set point than the distance
you specified, alarms should go off, right?

Well, not exactly. The mathematics are surprisingly a lot more complex.
We know. It seems easy and obvious. We've been involved in many debates
until the pencil and paper come out and then, "oh yeah" is heard.

Here's the missing magical point. You've got to notice that the point
where the anchor position is set in the alarm is the position of the GPS
and not the position of the bow/anchor. That one small point ends up
bringing a whole bunch of trigonometry into the calculation. When the
boat swings 180 degrees, the error created by that offset equals twice
the distance from the bow to the GPS.

Let's take an example for a typical 42' sailboat with a GPS on the stern
rail. This is the worst case problem but is very typical and
demonstrates what happens very well.

So we're anchoring in 10' of water with a bow that's 5' off the water's
surface. A good scope for a night without much weather expected would be
5:1. This means 75' of rode will be let out and pulled back to set hard
(we call that power setting). The anchor alarm is set at 125', way more
than the 75 put out. And since we power set the anchor, we couldn't
possibly move 50', right?

At 3 am, because these things always happen at 3 am, the anchor alarm
goes off. You're 127' back. You remember that you way over added to the
75' and start planning what you're going to do in the total black of
night with the moderate wind that's now blowing.

What really happened is that the tide changed at 1 am. During the next 2
hours you slowly swung around moving back. Not realizing this new math
for anchor alarms you didn't realize that the GPS displacement caused 84'
of position error in the anchor alarm. Your alarm went off after moving
back only 52'. In reality, your anchor alarm should watch you move back
another 32' without your anchor moving 1 inch on the sea floor. The
anchor alarm should have probably been set at about 75 + 84 + 10 + 10 =
179 feet. The two 10's are for GPS accuracy error and slop since the
anchor doesn't set immediately. Can you imagine setting an anchor alarm
at almost 200' with only 75' of rode out? And yet, that's the right
number.

We haven't found an anchor alarm that compensates for this GPS
positional error. It's one of the reasons we wrote DragQueen (available
for free in the Apple app store and Google Play). Since the anchor alarm
is on a phone, the GPS position is the phone itself. When deploying the
anchor, we stand with the iPhone at the bow to eliminate one half the
GPS position error. There's still another position error based on where
the GPS is located while we sleep at night (25' back in our stateroom).

Remember too that this positional error happens at all angles. Swing
about 90 degrees to the side and the error is about 1 times the GPS
displacement distance. Even that can be significant.

Given a heading/fluxgate sensor and a few configuration settings, 100%
of this GPS positional error could be eliminated. How come not a single
marine electronics manufacturer has done it?

If you're still saying, "wait a second - there's not a 2x error in the
position" - check out this graphic proof of what happens. We'll wait to
hear the "oh yeah":
https://activecaptain.com/articles/m...chorAlarms.php

Happy anchoring!

One recent note: Since running this original newsletter item, we found
that Vesper now has this swing calculation built into their anchor alarm
on their products.
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Old 08-24-2016, 10:20 AM   #2
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Thanks for posting this eye opener!
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Old 08-24-2016, 10:50 AM   #3
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In my opinion, having anchor'd out constantly, over many years, and many boats, this is a non issue.

While factually correct, the whole article is a non issue from a practical sense.

When you set your anchor alarm, it draws a circle around the boat. If the boat goes outside that circle the alarm goes off. Pretty simple.

If you set an alarm circle to the proper "size" then the boat will not go out of the circle and you'll not get false alarms.

You do not determine the "swing circle" size using a calculator, you determine the boats "swing circle" through experience. Really a trial and error process. I have never seen an anchorage where the 40' from my GPS to my bow makes enough difference to be notable from a practical sense.

In my opinion, little email articles like the OP mentioned are advertising, pure and simple. Send out an "article" to your email list to keep people excited about your product. Not a bad concept BTW. Everybody thinks you're company is "helpful" when in reality you are just advertising. Then you can get a bunch of TF members chatting about it online, and yippee, your customer base grows, and you make more $$$ Woo Hoo!!!
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Old 08-24-2016, 12:30 PM   #4
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Anchor Alarms

I have to agree with Kevin Sanders on this one. Yes, I have had numerous false alarms throughout the years but after awhile you learn to set the alarm to eliminate most of these. We cruised for over 10 years along the Mexican Coast line and the piece of mind, especially in the Sea of Cortes with it's very unpredictable winds that pop up in seconds, is well worth the few false alarms we experienced. Frankly better being woken up with a false positive than running into a rocky shoreline or into the vessel behind you.

I also found that there were fewer false positives when I set the anchor alarm after we had the anchor set and took in consideration our scope rather than setting it where we dropped the anchor.

Richard W.
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Old 08-24-2016, 12:46 PM   #5
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Quote:
Originally Posted by ksanders View Post
In my opinion, having anchor'd out constantly, over many years, and many boats, this is a non issue.

While factually correct, the whole article is a non issue from a practical sense.

When you set your anchor alarm, it draws a circle around the boat. If the boat goes outside that circle the alarm goes off. Pretty simple.

If you set an alarm circle to the proper "size" then the boat will not go out of the circle and you'll not get false alarms.

You do not determine the "swing circle" size using a calculator, you determine the boats "swing circle" through experience. Really a trial and error process. I have never seen an anchorage where the 40' from my GPS to my bow makes enough difference to be notable from a practical sense.
Kevin, I think the point of the article is to help those without your years of experience understand why the circle they come up may not be large enough depending on their boats configuration. Sure, to you it may be self evident, but not everyone is you. There are those who would rather learn from others experiences than to learn everything from their own trial and error. There are those that may not be as quick as you to pick up on things. There may also be those who anchor in places where a 40' difference between the gps location on the boat and the actual anchor location may make a difference.

Was the article earth shattering? No.
Did I find it interesting and give me something else to consider? Yes.

I don't use an anchor alarm often but I have never considered the error induced by the distance from anchor to gps antennae. Of course, I'm not the brightest bulb on the tree so it takes me longer than most to get these things.
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Old 08-24-2016, 02:22 PM   #6
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Quote:
Originally Posted by ksanders View Post
In my opinion, having anchor'd out constantly, over many years, and many boats, this is a non issue.

While factually correct, the whole article is a non issue from a practical sense.

When you set your anchor alarm, it draws a circle around the boat. If the boat goes outside that circle the alarm goes off. Pretty simple.

If you set an alarm circle to the proper "size" then the boat will not go out of the circle and you'll not get false alarms.

You do not determine the "swing circle" size using a calculator, you determine the boats "swing circle" through experience. Really a trial and error process. I have never seen an anchorage where the 40' from my GPS to my bow makes enough difference to be notable from a practical sense.

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Old 08-24-2016, 02:36 PM   #7
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Thus .....learn to understand things rather than just following proceedure.

Technology is great, as long as you understand the principles behind it or make dang sure you read all the disclaimers and potential error lists.
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Old 08-24-2016, 03:15 PM   #8
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Kevin, I think the point of the article is to help those without your years of experience understand why the circle they come up may not be large enough depending on their boats configuration.
Actually, I think the article was written because too many people who have their GPS 40' back from the bow don't really understand the implication of the 2X error if they are anchoring in more shallow water. If you're anchoring in 80', then yeah, it will have less than an effect. But if you're anchoring in 10', then an 80 foot error caused by a 40' GPS offset is quite sizeable. The numbers in the article give a good reference. With a 5' bow height, most people would put out 75' of chain giving 5:1 scope. But very few people would then set their anchor alarm at about 200' because it seems way too much.

And yet, 200' is the correct setting.

And how is it advertising? I don't get that. The newsletter is our way of giving back to the community for all their contributions. We attempt to find interesting and technology-based items and write about them. Anyone can easily opt out with no penalty or issue. Why is there something negative to find with that?
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Old 08-24-2016, 03:18 PM   #9
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It would be nice if you could create a circle on your plotter, move it anywhere you want on the plotter and then adjust its radius to your liking once you set its location on the plotter. Then if you move outside the circle you trigger an alarm.

Sort of like a guard zone on your radar but with more flexibility in where you can locate it on the screen.
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Old 08-24-2016, 03:22 PM   #10
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And how is it advertising? I don't get that. The newsletter is our way of giving back to the community for all their contributions. We attempt to find interesting and technology-based items and write about them. Anyone can easily opt out with no penalty or issue. Why is there something negative to find with that?
It can clearly be construed as advertising in a broad sense.

It's not a negative.

And there is no need to defend it.
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Old 08-24-2016, 03:29 PM   #11
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And there is no need to defend it.
It's nothing that I care or need to defend. It just is. But I don't really understand how providing a free newsletter to the users of a free service provides advertising. Advertising for what? For additional users? Who then pay $0? Sounds like a pretty inefficient plan.

Isn't it just more plausible that the simplest reason for the newsletter is the real reason? And the simplest one is that we want to give something back and enjoy writing articles to accomplish that.
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Old 08-24-2016, 03:41 PM   #12
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I like having an anchor alarm capability available.


The "self-induced" error because a fixed GPS isn't where the anchor is.... seemed intuitively obvious, to me (the most casual observer). Guess it's not, for everyone, though.


I can do the math, but I'm not usually into expending that much violent effort when some sighting points and the Mark I* Eyeball works pretty well anyway.


But the cell phone fix -- stand on the anchor platform and set the anchor alarm right above where the anchor went down -- is also dead obvious. At least, after I saw one of Jeff's first notes about it (maybe the original article), several years ago.


I'm more affected by GPS inaccuracy -- i.e., the quality of the fix available from the "current' satellite configuration (whatever my device can see at the moment, especially if that device is located below where we can hear it); that seems to cause more false alarms for us, in the few cases when we've had false alarms.


We can set a total of 6 positional anchor alarms and at least one depth alarm, on the boat at any given time. Overkill. But a combination of two positional alarms (mostly just with a backup for power management purposes) and a depth alarm in places where depth varies enough to make a difference, has come in handy. Once.


That "once" was enough to make futzing with the stuff worthwhile.


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Old 08-24-2016, 04:02 PM   #13
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It's nothing that I care or need to defend. It just is. But I don't really understand how providing a free newsletter to the users of a free service provides advertising. Advertising for what? For additional users? Who then pay $0? Sounds like a pretty inefficient plan.

Isn't it just more plausible that the simplest reason for the newsletter is the real reason? And the simplest one is that we want to give something back and enjoy writing articles to accomplish that.
I did not condem your use of your newsletter as advertising, so you do not need to defend it.

but... don't try to convince me/us that it is something at it is not. it is advertising pure and simple. just because you do not get direct revenue from the folks that read your newsletter does not make it into something benevolent. AC is a business entity and your newsletter is advertising.
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Old 08-24-2016, 04:53 PM   #14
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"current' satellite configuration (whatever my device can see at the moment, especially if that device is located below where we can hear it); that seems to cause more false alarms for us, in the few cases when we've had false alarms.
Gee, not that THAT would never happen.



Personally I set two alarms, one that basically alerted to me when the boat clocked around due to wind or current, because I like to know and do a check if I felt like getting up. The other was set to trigger when we got a certain distance from a specified way point.. which I could determine during or after the anchor setting process by doing simple math or a satisfactory enough guess. Sometimes where the anchor eventually sets is not where you dropped it, so I don't get all pedantic about it. I liked waiting until everything was settled in and done as a last seamanship type of thing before cocktail time. A quick check of the plotter track tells you if you are really dragging, and how fast.
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Old 08-24-2016, 04:59 PM   #15
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Ok boys, don't get this one closed as well "for people to calm down"... There are meds and adult beverages for that.

I for one am more interested in all the thoughts of how to properly gauge a safety zone electronically than anybody. Help keep this particular posting open.......
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Old 08-24-2016, 05:02 PM   #16
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I am sure there are many good ideas out there, some help eliminate guesswork, some minimize errors, etc....

Some may choose old fashion ways over anchor alarms..

I try and pick tips that work for me and work out my own system.

No one system seems foolproof. So those using several at once have the best shot at catching dragging before it is a problem.
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Old 08-24-2016, 05:27 PM   #17
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If I'm receiving valuable, useable information, I don't care if it's advertising or not. Some advertising is informative, some is deceptive. I believe I can discern the difference. You can only fool me once, and then you go to my SPAM folder. I'm in control, the end game is mine.

I do not understand why there is a knee jerk reaction whenever Jeffery posts or emails something. And, if advertising generates revenue that's fine with me. I wish I had developed AC. I think Jeff and Karen deserve all they worked so hard to achieve. Isn't that the American way?
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Old 08-24-2016, 05:54 PM   #18
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If I'm receiving valuable, useable information, I don't care if it's advertising or not. Some advertising is informative, some is deceptive. I believe I can discern the difference. You can only fool me once, and then you go to my SPAM folder. I'm in control, the end game is mine.

I do not understand why there is a knee jerk reaction whenever Jeffery posts or emails something. And, if advertising generates revenue that's fine with me. I wish I had developed AC. I think Jeff and Karen deserve all they worked so hard to achieve. Isn't that the American way?
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Old 08-24-2016, 07:41 PM   #19
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FWIW, the initial post for this thread was by Menzies, not Jeffrey.

BTW, thanks for posting it Menzies. I found it informative.
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Old 08-24-2016, 08:03 PM   #20
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It would be nice if you could create a circle on your plotter, move it anywhere you want on the plotter and then adjust its radius to your liking once you set its location on the plotter. Then if you move outside the circle you trigger an alarm.

Sort of like a guard zone on your radar but with more flexibility in where you can locate it on the screen.
That's exactly how Coastal Explorer works. I set up a boundary circle (that's what they call it) at every anchorage, drag it to my boat position right when my anchor hits the bottom, set the radius, then set it to alarm if I go outside the circle. Works great.
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