Anchor Alarm Boundary Radius Setting

[Michtsui]

Hi all,

This is a general question for sailing yachts (NOT just Bavaria specific).

Recently, I started to use the app "Anchor Alarm" in my iphone, in which, I can pre-set a safety radius (in meters) around an anchor point (ie. an iphone X location pre-set). Then auto-alarm will keep on being sent out to an phone (which could be the same iphone X or another phone) IF the app (GPS based) detects the iphone X has drifted away from the pre-set location more than the safety radius (in meter) pre-set.

My question is how many meters should be usually set for the safety radius for anchor purpose. I originally set 5 m which seems too short and the alarm keeps beeping. Any idea? Thanks.

[IslandAlchemy]

First rule is to push the set button as you drop the anchor, so your radius centre is over the anchor.

I always set mine to twice the scope (for if you have 20m out, set it to 40m, if you have 50m out, set it to 100m).

[Rampage]

On most anchor apps you don't have to set the anchor point by pressing the button as you drop the anchor: you can move the position of the anchor by dragging it to the correct point.
What I do is to drop the anchor, let out the right amount to chain for the anchorage and get the anchor set by reversing onto it. I then take the phone, set the anchor position (which is usually in the cockpit as I'm at the helm), then adjust the anchor position on the app so as to match the scope, plus length from where I am to the bow, plus a suitable margin (usually about 30m or so).
That stops most alarms but give me enough warning time to do something if the anchor does start to drag.

[Michtsui]

Thanks guys - the replies r helpful.

[Yngmar]

Depends on where we're anchored. If we're alone off a beach, I set it quite large, as there's nothing really to worry about, I just want to be woken up if the boat is drifting off the beach entirely.

In a full anchorage with other boats close by and things to hit all around (Cala Teulada), it has to be set quite tight or it's useless. You may have to live with some false alarms.

False alarms are usually caused by GPS losing accuracy, which can happen occasionally, and more often if you're near tall buildings/cliffs/forests as they tend to obstruct part of the GPS satellite signals (or reflect them). Make sure your GPS sensor is in the right location (clear view of the sky and no metal overhead). I've seen boats with the GPS mounted on the deck under the pushpit and bimini rails and unsurprisingly, they had lots of problems with losing GPS signal entirely all the time.

[Rampage]

I use an app on the iPhone, placed by my bunk at night (I'm deaf so need the alarm nearby to be able to hear it). The secret I found was to make sure that the app uses an averaging position fixing algorithm. That is, it takes a position fix every few seconds but will not alarm until the position averages outside the alarm zone until say 10 position fixes are outside the zone.
Early apps didn't have this feature and this resulted in lots of false alarms as Yngmar says: my current app (Anchor!) use the averaging setup and is remarkably resistant to false alarms yet does alarm quickly once the anchor trips.