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Old 02-20-2013, 01:04 PM   #1
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Setting the GPS Anchor Alarm

I read a good article today on Active Captain on this. Still a little confused. Good graphics on the link.


>>> Anchor Alarm Surprises >>>

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/misc/anchorAlarms.php

Happy anchoring!
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Old 02-20-2013, 02:04 PM   #2
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I sure wish SOMEONE would come up with a DEDICATED piece of equipment that is only an anchor alarm. It wouldn't take much to get a device that could account for all of this, with the power of multi-location alarms and simple one-button set. When I have to go thru the anchor motions, I don't always have the time to mess with a lot of scrolling and paging over to the anchor alarm screen. Hell, I can hardly remember to do it and have to wait until it's set. Then, the flybridge GPS doesn't have an alarm in the stateroom to hear when we are sleep and running Drag Queen all night off of DC power kills the battery.

It's crazy the big boys don't have such an easy device yet.

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Old 02-20-2013, 02:10 PM   #3
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I've used GPS pretty well since it became available. I've used it for guidance, I use it for navigation and I occasionally set a GPS anchor alarm. I've come to the conclusion though that there are just too many errors that all add up if the critical distance is under about 100 feet. In other words, if 100 feet of movement is going to put you on the rocks then you need to keep a physical anchor watch or have some other system to back up the GPS. Jeff does a good job of explaining most of the errors. What he doesn't discuss is that there are also system errors. With WAAS you can get pass to pass guidance to about 6" if you come back over an adjacent track within under an hour but you can easily be 4 feet out if you come back the next day. That's why surveyors and increasingly guidance users go to some kind of RTK system. 4 feet doesn't sound like a big deal but +/- 4 feet turns into an additional 10 feet of error that you need to add to all the other errors that Jeff outlined.
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Old 02-20-2013, 02:23 PM   #4
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I am going to have to study this some more. We are going to the San Juans this summer and we will be on the hook several times. Setting the GPS alarm will allow me to sleep easier. Wish it was simple.....
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Old 02-20-2013, 02:57 PM   #5
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You need to take as many errors as you can out of the system because some of them multiply - particularly any error in where you mark your initial position. If that is out by 10 yards then if everything else is 100% perfect you can never be sure of your position within 20 yards. So do everything possible to get the initial mark right.

We've got a prototype device that someone sold us a few years ago. It allows us to take a mark from where we drop the anchor and then keep the display by the bed but that's not as perfect as you might think. Similarly Jeff's smartphone app allows you to mark the spot when you drop the anchor but again, that's not as perfect as you might think.

When we drop the anchor we try to be drifting slowly back so that we don't cascade all the chain on top of the anchor. That movement alone introduces an error. Then what happens if the anchor drags for 10 feet before it hooks up? I think the ideal situation is you drop the anchor, get it set and then use your chart plotter to move back as close as you can estimate to directly over the hook. Finding that spot isn't all that difficult - you know your scope and your track will show where you stopped forward motion. You have to make some guesses but no matter what you do you won't have a perfect spot. Then you set whatever GPS you are using at that point. When you do it that way you can use your chartplotter as long as it doesn't have some ridiculous current draw and as long as it can wake you. You just need to include all the errors that Jeff outlined when you set your swing distance. IOW you need to add your scope, plus the distance from the pulpit to your GPS antenna plus the tide plus whatever fudge factor you decide to use. Personally I use at least 50 feet error because anything less is going to have me up looking out the window in the middle of the night. BTDT many times.
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Old 02-20-2013, 07:14 PM   #6
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I used my Furuno system anchor alarm on my first anchoring adventure last summer. She went off three times my first night before I widened the watch circle...I can't remember the numbers, but I added about 30%.

Anyway, what saved me was my golfing range finder. Ironically, it was in the car after a battery replacement so I took it along and ranged all the boats near me after I set the anchor. In the middle of the night, I ranged all the boats again and they were all very close to the original numbers. We hadn't drug, only swung.

I have drag queen but I was worried about the phone dying or if it was still working when the phone went to sleep. Would be a good solution since my Furuno sucks down the juice.
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Old 02-20-2013, 07:23 PM   #7
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Re-reading my post I don't think I was clear enough. The key to removing the antenna offset error is that you need to have the GPS antenna directly over the hook when you set the alarm. If you can do that then you have removed the most problematic error. It doesn't matter whether you are using a phone app, a dedicated alarm or your plotter - you want the GPS antenna directly over the hook when you make the mark.
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Old 02-20-2013, 08:26 PM   #8
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Bob's right but you have to include in your calculations your settled distance from the hook. Drop the anchor and standing at the bow you hit your smart phone app. Now you let out 100' of chain in a standard 7-1 scope. Well now your 85' or so from your hook in a fresh breeze. Now you walk back to the salon and set your phone in it's charger. You have just added (fill in the blank for your boat). Your now 100' +- from the hook. When you swing your swinging on a 100' radius. Just set your alarm to include these factors and you should be fine. Our anchor dance during Hurricane Sandy is the picture below. You get the idea.
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Old 02-20-2013, 09:45 PM   #9
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I wake up every hour or so and check whether I've dragged. Tides (currents up to two to five knots) here change several times a day, and each reversal has the potential for stuff to happen. Is that why I prefer tying up at a marina?

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Old 02-20-2013, 11:08 PM   #10
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With all of the concerns of accurate electronic positioning/alert errors mentioned above, and including the use of fathometer alarms, alarms meant to wake you up during the night to indicate possible anchor dragging are sure to impact your circadian rhythms!

I note that after 50+ years of boating (many of which occurred before GPS), the best overall solution is to get a good spot, get a good set, and utilize that oversized anchor, rode and windlass you bought so you could sleep nights.

Of course, sometimes you get stuck with crowded anchorages and storm winds, so every now and then some sleep is invariably lost. As I look back over thousands of anchorings, I probably had some sort of alarm (GPS or fathometer) set less than 1% of the time. In 2012, we anchored around 125 times and I can only recall setting an alarm twice. Once in a crowded anchorage with other boats and fresh wind, and once in an uncrowded anchorage with 45 knots.
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Old 02-20-2013, 11:08 PM   #11
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Quote:
Originally Posted by markpierce View Post
I wake up every hour or so and check whether I've dragged. Tides (currents up to two to five knots) here change several times a day, and each reversal has the potential for stuff to happen. Is that why I prefer tying up at a marina?
If you anchor out a lot you may get to where you are comfortable sleeping a little longer at a stretch.
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Old 02-20-2013, 11:11 PM   #12
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I note that after 50+ years of boating (many of which occurred before GPS), the best overall solution is to get a good spot, get a good set, and utilize that oversized anchor, rode and windlass you bought so you could sleep nights .
So true. I do appreciate the info on how to set the offset on the alarm but this is great wisdom from Jay.
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Old 02-20-2013, 11:41 PM   #13
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The calculations are wonderful, but completely unnecessary.

We anchor out almost every night on the boat.

I have an external alarm connected to the Furuno, with the alarm being in my stateroom.

Set the anchor watch, look at it a couple of times over the course of dinner, and other evening stuff, making it big enough not to false alarm.

Pretty simple.

BTW its easy to see if you're dragging. Just look at your track on the plotter. It should look like a portion of a circle
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Old 02-21-2013, 05:20 AM   #14
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Quote:
Originally Posted by Jay N
"I note that after 50+ years of boating (many of which occurred before GPS), the best overall solution is to get a good spot, get a good set, and utilize that oversized anchor, rode and windlass you bought so you could sleep nights".


So true Jay. I sleep like a top on my Sarca, well not literally on, but you know what I mean. My Lowrance has an anchor alarm function, but I'd have to look it up so set it. My Sarca has never dragged, so I just rely on my senses to wake me if something changed. A significant change in weather or water acts as a pretty good alarm in my experience. I think if I set an alarm, I'd lie there all night worrying if I set it right.
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Old 02-21-2013, 07:08 AM   #15
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I just set the alarm for double my scope or a little more. If it goes off in the middle of the night, I get up and look to see if I'm dragging. If not, I just add 25' or so to the alarm and go back to bed.

So far, I've never dragged.

BTW: I added an external buzzer to my Garmin plotter.
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Old 02-21-2013, 09:27 AM   #16
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I sure wish SOMEONE would come up with a DEDICATED piece of equipment that is only an anchor alarm. It wouldn't take much to get a device that could account for all of this, with the power of multi-location alarms and simple one-button set. When I have to go thru the anchor motions, I don't always have the time to mess with a lot of scrolling and paging over to the anchor alarm screen. Hell, I can hardly remember to do it and have to wait until it's set. Then, the flybridge GPS doesn't have an alarm in the stateroom to hear when we are sleep and running Drag Queen all night off of DC power kills the battery.

It's crazy the big boys don't have such an easy device yet.

Tom-
Tom, I think there is stuff out there for phones. Here is an android app, if you have a smartphone.
https://play.google.com/store/apps/d...horalarm&hl=en
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Old 02-21-2013, 10:19 AM   #17
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I sure wish SOMEONE would come up with a DEDICATED piece of equipment that is only an anchor alarm. ...................
It's crazy the big boys don't have such an easy device yet.
The trend is to combine functions in fewer discrete pieces of equipment and for good reason. You wouldn't want (or have room for) individual GPS, depth sounder, fish finder, AIS display, radar display, anchor drag alarm, and the various other functions of a modern MFD that slip my mind at the moment. And you would hate to have to duplicate them for multiple operating stations.

An anchor drag alarm is something you use once a day, if that, so why not combine it with the GPS rather than duplicating this device?

I believe most MFDs allow installation of a remote buzzer that can be installed wherever you might need it. Actually, you can install multiple buzzers if needed.
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Old 02-21-2013, 11:03 AM   #18
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An anchor drag alarm is something you use once a day, if that, so why not combine it with the GPS rather than duplicating this device?
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Old 02-21-2013, 01:20 PM   #19
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I use Polarview on a lapto as a backup and just click the anchor button as I drop the anchor and set the distance later. The laptop draws way less than my GPS and is easier to use.

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Old 02-21-2013, 01:58 PM   #20
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I will be in the San Juans this summer for two weeks so I am sure when I can't get a mooring buoy or the dock, I will be on the hook. Fun times ahead....
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