Anchor Alarm Radius?

[noelex 77]

This is how you may centre the alarm using the GPS track as you drop the anchor. Note the centre of the alarm needs to be displaced by the distance between the bow and the GPS aerial.

[svmeanderer]

Originally Posted by noelex 77

Here is a diagram showing if the anchor alarm is centred on the anchor (as it should be) the swing is a circle the radius is:

R+G

Ummm...no. You didn't show any of the data collection compared to the reporting. The attached document lays it all out. Be prepared to be surprised. The issue is that you're not considering what happens when the GPS is not at the bow. Your drawing showed none of the issue.

You'll never look at your anchor alarm again once you follow the attached document. You'll also realize that 2G is correct.

[noelex 77]

Originally Posted by svmeanderer

The issue is that you're not considering what happens when the GPS is not at the bow.

I have considered this. It is easily corrected. Simply move the centre of the alarm point so the GPS aerial is centred over the anchor. (In your link displace the green dot so that it corresponds with the red dot. This will be about 10m (or whatever the distance from the bow to GPS aerial is) towards the direction the boat is facing when the anchor drops. It is a simple matter to move a waypoint like this)

I do this every time I anchor.

You seem convinced that the centre of the alarm point has to be where the aerial is when you drop the anchor ie 10m back from the anchor. This is not the case. Many marine GPS units and all tablet apps allow you to displace the centre of the alarm point so it coincides with where the anchor is lying.

This must be done for an accurate alarm.

Problem solved.

Note: This is not possible with all marine GPS units. I can only think you have not used a GPS unit like this hence your difficulty appreciating this technique.

Without doing this your GPS anchor alarm has about 10m of extra error which is totally unnecessary. There are times when this discrepancy can be important.

[monte]

he has anchor alarm app
heres what I do.
1 turn on the app as you approach the anchorage do it has an accurate signal.
set the range ring at say 20m more than you expect so it doesn't go off while you are anchoring
2 Slow to a stop over the place you will drop the anchor, walk forward with the iphine and press the drop anchor button as close to directly over where it lays on the bottom.
3 Slowly back down on the anchor, letting out rhode, attach bridle, then reverse at 2000rpm. The chain should be taught, check bearings, watch seabed for movenent, check to see if you are dragging into the yacht anchored behind etc. You have a minute to do all this while the engines at 2000prm in reverse are giving you peace of mind for a good nights sleep.
4 Now that you are set you can adjust the range ring to a couple of meters larger than where the vessel is indicated on the app.

I usually put a waypoint on the plotter at this point as well, and one when I drop the anchor, but I dont leave the plotter on overnight

You may find if you move the phone forward or aft too much you will want to adjust the range ring to suit. I find it is accurate to a few meters so no need to add many meters to compensate for GPS inaccuracy.

[67Therapy]

Too much math. Just use all chain + Rochna.


j/k people...

[Littlechay]

Wind it in guys, it's an anchor alarm!

Sometimes you don't have room to swing so you set it really tight so that if there is a wind change it goes off.

Sometimes there is loads of room to swing 360 so you set it to road length plus a couple of fudge factors based on your experience.

Sometimes you have shorelines on so you don't expect to move at all; in this case you set it to 10m and live with an occasional false alarm.

You are all right and you are all wrong in some aspects.

Some software allows you to move the centre of the alarm circle to centre it on the anchor. Mucking about with an app, plotter, GPS, intrument as you drop your anchor is not practical unless it's so quiet that you don't need an anchor alarm anyway. So do it after the fact, you will know your heading and your scope so it's easy to move the mark.

Watchmate doesn't have a heading sensor built in - great unit though I use one all the time.

You need a heading sensor, not necessarily a fluxgate, if the unit is to automatically compensate for GPS position as below about 3 knots COG means nothing.

Usually you'll notice that you are dragging about the same time as the alarm anyway. Motion of the boat, the noise of the chain rumbling across the bottom, the bang as a shoreline parts in a gust.. whatever.

Dont worry about it too much. Just play with what works for you. SLEEP WITH AN EYE, AND AN EAR, OPEN

[MarkJ]

Originally Posted by gbanker

what radius circle people have found works best

50 meters.

By then the boat has actually moved, not just drifted about a bit.

Stuffed if I can get any of those iPad aps to work so I do it on a GPS handheld.


Mark

[Surrymark]

And if you anchor in 20 feet of high tide, and are sleeping when the tide drops to nine feet?

It would be great if we could be certain about dragging as soon as it starts, but unless anchored in a tight spot with a Danforth, I think it's reasonable to not to get exercised over an apparent few feet of movement. Any warning at all is a good thing; it can only cost some sleep and having you scanning the dark. If an app told me that the boat had moved ten feet beyond the maximum low-tide swing, I'd get hustling (at something). You hundred thousand-milers think this is too casual?

I talked to one fellow at one of my favorite anchorages, and we thought of putting a few led lights on shore, but it encompassed far too many variables and, not having any extra leds anyway, we had a beer.

(Ella came with a 45lb Bruce copy, which dragged several times. A 45lb Manson has been fine, as was a 22 lb Rocna during many nights on 27' Tula.)

[BigBoater917]

So far I feel the safest anchored near Littlechay. I can't speak to the app the OP is using but most plotters allow you to offset the antenna position. This allows you to place the center of the circle on the anchor and rest in peace.

[Capt.Don]

Use a handheld GPS instead of an app. You'll find that your phone/tablet's batteries burn through in about 3 hours. I've gone back to using my handheld GPS with track and anchor alarm. You can easily see your radius and whether you've drifted outside the circle. Can take to the v-berth at night (no 12vdc connection required). My GPS batteries last a couple days!

The apps are cool, but in practice, I don't find them that practical.

[Cheechako]

Originally Posted by Littlechay

Wind it in guys, it's an anchor alarm!

Sometimes you don't have room to swing so you set it really tight so that if there is a wind change it goes off.

Sometimes there is loads of room to swing 360 so you set it to road length plus a couple of fudge factors based on your experience.

Sometimes you have shorelines on so you don't expect to move at all; in this case you set it to 10m and live with an occasional false alarm.

You are all right and you are all wrong in some aspects.

Some software allows you to move the centre of the alarm circle to centre it on the anchor. Mucking about with an app, plotter, GPS, intrument as you drop your anchor is not practical unless it's so quiet that you don't need an anchor alarm anyway. So do it after the fact, you will know your heading and your scope so it's easy to move the mark.

Watchmate doesn't have a heading sensor built in - great unit though I use one all the time.

You need a heading sensor, not necessarily a fluxgate, if the unit is to automatically compensate for GPS position as below about 3 knots COG means nothing.

Usually you'll notice that you are dragging about the same time as the alarm anyway. Motion of the boat, the noise of the chain rumbling across the bottom, the bang as a shoreline parts in a gust.. whatever.

Dont worry about it too much. Just play with what works for you. SLEEP WITH AN EYE, AND AN EAR, OPEN

[belizesailor]

My approach is really basic. If the boat moves significantly then I want to know, regardless of why. So, I set mine to about 90'. Enabling GPS averaging/dampening tends to eliminate false alarms due to GPS fix drift at that range...shorter I get more false alarms.

For example, if the boats swings in a wind shift then that could indicate an approaching squall. I would rather be up and prepared ahead of time...than as we are already dragging. I tend to wake up on weather shifts anyway, but do think I sleep better knowing the alarm is set. I don't feel the need to wake up and look around periodically as much.

I currently run My Anchor Watch Pro on my Samsung phone. Very well done app, with battery friendly features, and the batteries easily make it thru the night. It also has advanced features if you want to get more technical as discussed. And, a variety of averaging/filter/dampening settings to reduce false alarms.