Anchor Drag Alarm

[Etesian]

We anchor out alot and sometimes the holding is not that good. Twice last year, the anchor started to drift. So this year we came prepared and brought a Garmin 76Cx GPS handheld that has an anchor alarm feature. The documentation on how it works is incredibly bad.

Does anyone have any experience with anchor alarms? Here are the questions we have.

Is it better to mark the waypoint when/where you drop the anchor or mark the waypoint after letting out the appropriate length of anchor chain?

What is the recommended waypoint radius (nm) setting?

In either case, what distance (m) do you recommend setting the anchor drag alarm?

Recommendations, input and experience is greatly appreciated.

 

[NB Willawaw]

Put the anchor watch on (insert waypoint if that is the way it works on your device) once the boat has settled at the end of her cable.

When setting the limits, think about what you want to do and what the tolerances are for headlands, shallows, etc around you. It also depends on how much cable you have out.

Obviously, its better to have a higher limit to avoid false alarms, but not if it means you are aground by the time it alarms.

You can also set a short term alarm by putting a waypoint on say a headland or feature if a dragging anchor means that you would be put on it by the wind/tide.
Use waypoint and the waypoint arrival alarm for this. Its goes without saying that this would need to be reviewed every few hours, so not much use for overnight anchorages.

I normally use 0.1nm/1 cable as a default setting for anchor watch.

 

[Etesian]

0.1nm / 1 cable?

Not sure what you mean by 1 cable - 1 meter of chain let out?

 

[noelex]

If it safe to swing 360 deg around your anchor and you do not want to be woken up with a change of wind/tide, its much better to set an anchor alarm waypoint that is directly over the anchor.
Note when you drop the anchor the GPS antennae will be almost a boat length back. Its easy to make the compensation with a graphical GPS, providing it allows adjustment of the position of the anchor alarm
The sensitivity setting depends on the amount of scope you have out. To convert nm to m roughly you multiply by 1000 then double it. So 0.03nm is about 60m.

As an example at the moment I have 40m chain out. Because the GPS is near the back of the boat the GPS unit will swing through an arc of about 50m. So I have set the anchor alarm at 0.03nm from the anchor. With a modern GPS unit particularly with EGNOS enabled there is very little error in the signal.

 

[NB Willawaw]

1 Cable = 0.1nm

Not sure what you mean by 1 cable - 1 meter of chain let out?

 

[Swagman]

If it safe to swing 360 deg around your anchor and you do not want to be woken up with a change of wind/tide, its much better to set an anchor alarm waypoint that is directly over the anchor.
Note when you drop the anchor the GPS antennae will be almost a boat length back. Its easy to make the compensation with a graphical GPS, providing it allows adjustment of the position of the anchor alarm
The sensitivity setting depends on the amount of scope you have out. To convert nm to m roughly you multiply by 1000 then double it. So 0.03nm is about 60m.

As an example at the moment I have 40m chain out. Because the GPS is near the back of the boat the GPS unit will swing through an arc of about 50m. So I have set the anchor alarm at 0.03nm from the anchor. With a modern GPS unit particularly with EGNOS enabled there is very little error in the signal.

Well said. I'd second this method.
JOHN

 

[Etesian]

Great input

Thanks everyone for your advice and expertise. Very much appreciated.