Advice on Setting Anchor Drag Alarm

[UFO]

I was looking at an anchor drag (Anchor Alert) app last night and accidentally left it running in the background and had a rude awakening at 3AM due to the back portion of my house moving into the neighbors garden - we'll according to the app it did.



What is a good distance to set for the alarm to go off for an anchor drag app to avoid false alarms yet keeping safety in mind.

[Jim Cate]

Depends on the situation. If there are serious hazards nearby, the alarm should go off in time for you to take remedial action. This can lead to false alarms, but is a good trade off IMO. If there is nothing for miles downwind, we leave something like 30 m beyond the swinging circle.

It is difficult at times to get the anchor point in the alarm to agree with the actual position of the hook. Our Vesper alarm allows one to move the position around until the agreement is good enough, and when it is critical, we spend some time getting that right.

IMO, a good anchor alarm is one of the truly useful developments in marine electronics.

Jim

[noelex 77]

Anchor alarms need a very good GPS signal.

If you have an external aerial with satellite differential fix, and the centre of the alarm point is correctly set at your anchor, then an alarm radius of 5m greater than the radius when you set your anchor should not give any false alarms.

If the GPS aerial is inside in a fiberglass boat often around 10m is needed. If you are trying the app at home inside the house, the poor GPS fix and multipath distortion mean these type of numbers will not work, but most houses have good anchors .

As Jim has indicated, in many anchorages there is a lot of room to drag and these tight limits are not often used. 10m for an external aerial and 15m for an internal aerial are more common numbers and these distances allow for some error when marking the centre of the circle. However, anchors (especially plow designs in strong wind) often very slowly drag, moving back a few inches in each gust without setting any deeper. These type of movements do not normally put the boat in any danger, but they do indicate the anchor is struggling to cope. These slow drags are often followed by the more rapid drag where the anchor lets go completely.

Recognising these relatively small movements of the anchor is difficult. With stronger wind the chain catenary will be eliminated and the snubber will stretch, so the radius from the anchor will increase slightly even if the anchor does not move. In addition, as the anchor buries deeper in response to the stronger force, it will move slightly, especially in softer substrates. Setting tight limits on the anchor alarm, it is just possible to be alerted to slow drags without any false alarms, but the alarm needs to set accurately, especially if there are wind shifts and the boat is rotating around the swing circle.

[Bean Counter]

We use drag queen phone app that lets you set the position of the anchor and also set the error levels of GPS and time for error conditions to be ignored. We usually allow some distance for anchor reset when swing can be 180 degrees. Depends on how close the other boats are or rocky shore. We measure the distance to the other boats with radar, it is always much further than it looks (to us). The phone uses a lot less power than the nav system and is more user friendly. Hasn't let us down yet in 4 years. Mind you we nearly always wake up before the alarm goes off unless it is a GPS signal error. If in a really tight and potentially dangerous anchorage with predicted wind or current shift we keep anchor watch until the shift, use two phones and the plotter as an abundance of caution. If the holding is a bit dodgy I might start up the engines and back down on the anchor to make sure it has reset after a shift.

[noelex 77]

Originally Posted by Bean Counter

We use drag queen phone app that lets you set the position of the anchor and also set the error levels of GPS and time for error conditions to be ignored. We usually allow some distance for anchor reset when swing can be 180 degrees.

Good anchors that have been set well in a reasonable substrate should almost always “shuffle” when subject to a new direction of pull. This means the anchor will rotate, but other than the rotation it will not move.

If your anchor breaks out and drags before resetting it is certainly something I would want my anchor alarm to alert me to.

[ozsailer]

G'day UFO,
currently cruising and our setting of anchor alarm is as follows. As soon as I have stopped to drop the anchor I set a mark/waypoint. This then forms the centre of my anchor swinging radius. I then let out the appropriate rode and dependant on how much I have let out I then set the distance for the anchor alarm. If I have let out 25 metres of rode the distance I set is 60 metres ( fifty metres swing radius plus some fudge room)
This has worked well for me to date using the anchor alarm on the Garmin chartplotter. Amazing how well the brain becomes acustomed to the sound of the alarm at all hours of the day or night.


Greg H
Blue Pointer
Lagoon 440.

[belizesailor]

Originally Posted by Jim Cate

Depends on the situation. If there are serious hazards nearby, the alarm should go off in time for you to take remedial action. This can lead to false alarms, but is a good trade off IMO. If there is nothing for miles downwind, we leave something like 30 m beyond the swinging circle.

It is difficult at times to get the anchor point in the alarm to agree with the actual position of the hook. Our Vesper alarm allows one to move the position around until the agreement is good enough, and when it is critical, we spend some time getting that right.

IMO, a good anchor alarm is one of the truly useful developments in marine electronics.

Jim

Some have the ability to set different distances for different arcs of the swing circle. I use MAW (My Anchor Watch) which has this ability. Very handy if you have a potential hazard closer on one sector.

MAW also has an LPF (Low Pass Filter) function which smooths out GPS drift and cuts down on false alarms.

As far as personal preferences on alarm settings, I prefer to know when the boat moves significantly even if its not dragging. This alerts me to potential changes in weather etc, like an approaching squall or big current shifts. Such changes usually wake me up anyway, but I like having the alarm too. So, I set my alarm after the hook is set and set the swing distance relatively short (depends on anchorage depth).



...on the hard PanaMarina, Panama...no worries about dragging here! [emoji16]

[noelex 77]

Originally Posted by ozsailer

If I have let out 25 metres of rode the distance I set is 60 metres ( fifty metres swing radius plus some fudge room)

This means your anchor can drag about 35m before the alarm goes off !

[fatherchronica]

How we set ours is different depending on where we are and how crowded it is. One night in a shallow bay a pretty good local named wind came up leaving us clise to a sandy, lee shore. We had 2 foot breaking waves hitting every once in awhile. The rest of the vessels were all power boats more than twice our length and outside us. Several dragged anchor and several motored all night into the wind to take strain off of their rodes. I reset the alarm for a very short leash and slept the rest of the night on the settee so I was close to look out everytime it went off. In the dawn I found we had not dragged at all, and the alarm was just going off as the wind veered back and forth. Not the most restful night, but far from my scariest nights. We love the anchor alarm like we love our Monitor and our auto pilot.

[philiosophy]

If I am worried about dragging I keep a anchor watch. The alarm would be the secondary alert.

[akprb]

I do not like the apps but prefer the alarm on the ships GPS or Chartplotter.

This is an INVALUABLE TOOL.

I’ll set a fairly small radius to start then over the first hour our so move it out if it goes off.

If you desire a truly good nights sleep at anchor this is one other function to become intimate with ;-)

[roland stockham]

I will often use a distance of about 25m total not plus the swinging circle. If the wind or tide changes I like to know and make sure the boats settles to the new conditions. However I have only dragged once in over 40 years and frequently leave the boat at anchor for long periods. The one time was when I had to reduce the chain to a ridiculous scope (less than 3-1) to avoid a mored boat and got hit by a 35kn squall.

[sailcrazy]

We use our cockpit-mounted on-board chartplotter, and set a mark when we drop the anchor. Then we set another mark after we've set the anchor, activate the anchor alarm, and will use a distance of about 120% of the rode deployed as an alarm distance, depending on wx and proximity of other boats. Invariabley, we see an arc of where the boat has moved back and forth. But knowing where the anchor is and where we started from, it's easy to tell where we are and if we've moved out of our "arc". We keep the o'head hatch in our aft stateroom open at night, right behind the cockpit, so if it alarms it will wake me up and I can respond accordingly.

[akprb]

That is one of the problems with Raytheon plotters, the alarm is not the loudest.

I contacted Raytheon regarding this and if they had an external speaker. The answer was no.

Anyone else bump into this?

Thanks!

[Cadence]

Originally Posted by UFO

I was looking at an anchor drag (Anchor Alert) app last night and accidentally left it running in the background and had a rude awakening at 3AM due to the back portion of my house moving into the neighbors garden - we'll according to the app it did.



What is a good distance to set for the alarm to go off for an anchor drag app to avoid false alarms yet keeping safety in mind.

Did the alarm on your houses depth finder go off also?