Anchor Alarms

[Knowazark]

OK, so its pretty easy to set an anchor alarm on most GPS units, but how many people actually use them?

If you do use the anchor alarm function, what distance do you set as your alert distance? Obviously, you want to set the alarm so that it will alert you to a dragging anchor, but how far is acceptable movement from your waypoint? Setting an alarm distance of 0.1 nautical mile (185 metres or 202 yards) is too far to be useful, but what about 0.01 nm (19 metres or 20 yards ... a bit more than a boat length)? If the alert distance used is too small, then the alarm will go off if the boat swings to a minor wind change or with the tide .... that's gonna be a pain in the rear end.

What alert distances do you use, and does it depend on the situation? Or do you just have a set alert distance and simply activate the alarm when needed?

[cdennyb]

I used to set my waypoint when I dropped the hook, one button push. Then I set the radius at what I stopped at when the reverse motion stopped. That set the circumference distance to the anchor. Then I added an additional 10% of so fo GPS wander if I could allow it. If the boat swung around anywhere outside the diameter I had set plus the pad I built in, the alarm would go off. anywhere inside, I was cool.
Now I use the Anchor Watch app on my droid phone. It wrks so much easier and I can take it with me or have it notify me if something moves too far the wrong way.

[barnakiel]

While at anchor, we will always use the alarms:

- on the GPS,
- on the echo,

GPS - a circle around our current place (where we are, not where we dropped the hook), size depending on how deep it is and how much position change we consider fine,

Echo - shallow alarm, deep alarm.

At times we will mark the GPS alarm around the place where the hook really is. At times we will mark a proximity alarm around an obstacle that we do not want to get to close with.

b.

[svcambria]

Originally Posted by Knowazark

OK, so its pretty easy to set an anchor alarm on most GPS units, but how many people actually use them?

If you do use the anchor alarm function, what distance do you set as your alert distance? Obviously, you want to set the alarm so that it will alert you to a dragging anchor, but how far is acceptable...

What alert distances do you use, and does it depend on the situation? Or do you just have a set alert distance and simply activate the alarm when needed?

My GPS anchor alarm works in units of 0.01 nautical miles - about 60 feet. So the anchoring point becomes the waypoint reference, the scope is covered by rounding up the nearest 0.01 mile (i.e. in 60 feet of water with 200 feet of scope which has 140 feet of horizontal distance equals 0.03 miles) plus 0.01 mile for GPS dither, in 200 feet I would set the alarm at 0.04 miles. And yes, I set it every time, it's the best way to get a good night's sleep. Then, if the alarm goes off, I know I am dragging. On the other hand, it's only happened once in 60 months of anchoring...

Michael

[noelex 77]

Anchor alarm = cheap insurance
Its saved me a couple of times.
GPS next to the bed is also helpful

[Roaring Girl]

As barnikiel, we mark where we ended after setting the anchor. Distance is a function of depth (scope) and worries - but normally ends up being 0.02nM. 0.01 is normally too little and it goes off all the time.

We like to know if the boat swings through 180 degrees whether because of tide or wind. That may mean sleepily getting up and poking your head out to check that the other boats swung the same way you did and then bumbling back to bed, but we prefer that to finding out by a thump!

[sailorboy1]

I set the GPS to a little more than the scope, or a little more than twice the scope depending on the wind and direction. I know if I set at about the scope it will go off if wind changes, but sometines I what that.

[Dockhead]

It all depends! If it is a wide open space without nearby hazards, like St. Vaast in Normandy where we spent a couple of pleasant nights last month, I let out all of our 100 meters of chain, set the alarm with a generous radius, and sleep tight.

If it is a tight spot with hazards, then I set it tight. Often that means so tight that you know the alarm will go off when the tide changes even if the anchor is well set.

That's a small price to pay, I think, and I usually get up a few times at night at anchor anyway.

Keep in mind that if you have a lot of chain out, your boat will move around more due to the chain's sinking and pulling the boat towards the anchor in calms. This calls for a more generous radius.

I LOVE your bedside chart plotter/compass setup, Noelex! That has been on my list for a while; maybe this winter I will get around to it. What a sleep-saver -- just open one eye to see your position, instead of getting up and crawling to the chart table. Love it!

To whomever has gone 60 months without an anchor alarm -- boy, are you lucky! Or set a generous radius! I guess I have an anchor alarm go off every other time anchoring, or at least one in three. Which doesn't mean I'm dragging, but often the radius has to be strict enough so that you catch a drift in one direction out of your swinging circle, while that same radius will set off the alarm if you innocently swing around to the other side of it.

That problem was addressed by some guy here who was developing a black box anchor alarm. He fixed it up so that the center of the radius of the alarm was the position of the anchor rather than the position of your GPS antenna at the edge of your swinging circle. That seemed perfectly logical to me, but more trouble than its worth. I would far prefer to have a function on the chart plotter which lets you shift the center of the alarm circle to where you think the anchor is, or shape it like a guard zone to warn you if you are even swinging, much less dragging, towards some particular hazard.

[Mark Zarley]

For those who have trouble hearing the alarm go off, we use a baby monitor . KISS solution to a simple problem.

[osirissail]

I also use the GPS mounted by the bed. I bought a Garmin 176 on Ebay and an external antenna for it. Then mounted it so I can easy see it when lying in the bed.
- - Over time, the GPS will develop an arch or circle of track dots to form a circle around the location of your anchor. So long as the little "triangle/boat symbol" remains on or in the track circle I close my eyes and go back to sleep. Additionally, you can tell which way you are facing by looking at the relationship of the boat symbol to the center of the track circle. The GPS is set up North Up so if the little symbol is at about 2 o'clock I am facing southwest. The display is also set to show SOG and the Time. Normally, SOG will be zero or maybe 0.1 if the boat is drifting around the circle.
- - Utilizing the anchor alarm has never worked very well as the boat strays out of the confines of the alarm and/or a GPS spike sends the display position out in left field. In other words, too many false alarms.
- - I always memorize what landmarks are around me and their relationships and a peek out the porthole now and then assists in knowing if I am still where I think I should be.

[Dockhead]

Chart plotter by the bed is not to enhance hearing the alarm. It is to be able to glance at the boat's position without getting up and completely waking up. Just like Osiris says -- if your boat is happily lying to a well-set anchor this immediately visible as a dense arc on the plotter from your track function. If your boat is outside of this arc you know you've got a problem. At this point, I have to get out of my bunk, even put on some bloody clothes if I have guests on board, go to the nav table, to check. By this time I'm pretty well awake. Having the plotter by the bed will be fantastic.

I also take transits on landmarks around so that I can pop my head out the hatch and judge position. I've been cruising since before chart plotters and anchor alarms, so this is an old habit. But this traditional method does not provide the precision you get with the plotter.

Another great reason to have the plotter by your bunk is in case you do any ocean crossing or other long passages -- you can monitor your progress when off watch. Especially if your instruments are networked and your plotter shows wind data and speed through the water, as ours does.

[goboatingnow]

Additionally, you can tell which way you are facing by looking at the relationship of the boat symbol to the center of the track circle.

Most GPS's will give errors if interpretated like that, they really need a consistent direction to align the heading properly.

[noelex 77]

Originally Posted by goboatingnow

Most GPS's will give errors if interpretated like that, they really need a consistent direction to align the heading properly.

Thats true for COG, but providing the wind is moderate the boats position in the circle is quite acurate for determining wind direction.
For very light winds the compass is more useful.

[osirissail]

Just a clarification. The GPS will display your position by lying down track dots regardless of your speed or boat direction/movement. You do not need any motion at all to determine which way the boat is "pointed" when lying to anchor and observing the circle/arc of track dots. In most circumstances any whisper or more of breeze will stretch out your anchor rode to a degree and since you are attached to your anchor the bow of the boat will be pointing toward the anchor. Your position on the GPS track/arc circle combined with the center of the arc/circle (your anchor) will show which way your boat is pointing which is normally a result of some whisper or more of wind.
- - Your COG, SOG and other readouts on your GPS are dependent upon movement to calculate these values. At anchor most likely all of these functions will be at or very close to zero. If they are not then you may be dragging anchor.
- - To clarify Dockhead's post I am talking about a GPS with a little "display screen" that shows your track, not necessarily any nautical/land map information. Normally I have the display scale all the way down to 80 ft/100 feet so any nautical map information would not be discernible. These types of GPS's are available on Ebay for very good prices.

[trinescape]

Every overnighter we do i set my anchor drag alarm on the gps ,i also set my depth alarm as well ,i had to fit an external beeper on my gps because we could only just hear it from our bunk. Our gps alarm has gone off twice ,once anchored off a river mouth waiting till the next mornings high tide to enter at 2.30 am the wind had picked up to 25-30 knots and we draged about 15m before the anchor reset itself most scary to be woken up by hearing this alarm the second time we were anchored in a popular bay called port vincent and i set the anchor drag parameter too small and when the wind changed direction we swung around and off went the alarm .I find it gives me peace of mind when going to bed though iam always waking a few times a night and just listen and if it feels ok i nod off again-some times i get up and have a look around and sometimes not cheers andy