First trip out with my new anchor - but did I drag?

[MagicalArmchair]

The anchor, for fairness sake, can remain anonymous. Its a next generation anchor, that's not a Rocna that I had on my old boat, and with which I never dragged.

For our first adventure to dip new purchase, we went for an anchor off Brownsea here for the night:

At high water I supposed I would have around five meters under me at most. I missed the marker at 35 meters, and ended up with 40 meters of chain out, so a fair old overnight scope of 8:1 at HW, with a 10 meter snubber to take the load from the windlass. When we arrived, it was blowing a little, perhaps gusting 15 knots, and overnight it was dead calm. I put my anchor alarm app on, and put the alarm radius at 40 meters to match the chain. I had one spurious GPS reading that set off and alarm, but on the whole, our position stayed in the watch area.

Then, in the morning, it started blowing a little again (again, nothing more than 15-20 knots); the tide was nearly fully in. The alarm went off: It appeared were dragging.

I upped the alarm radius to 57 meters to shut the thing up from buzzing every two minutes. That cluster in the bottom left were the recordings (that get logged every ten minutes) as I made the decision to up anchor and leave. The position of the boat at that time is as I was pulling the anchor up.

Do we think I dragged? Perhaps on weed here? The anchor seemed pretty stuck fast as the windlass loaded up on retrieval, and I had to drive over the anchor to pull it out. It came out covered in soft mud on its flukes, the kind you would expect to get very good holding from.

App fail? Or Anchor fail? At 57 meters away from the initial position, it appears like I had dragged by at least 17 meters? Or could most of this be account for by my wandering up and down the (12m) boat!? (On waking, I moved about 5 meters aft I suppose - bunk, to breakfast)

 

[prv]

If you had to break out the anchor to leave, I doubt you dragged.

How accurately did you peg the original anchor position while setting it? Drifting while motoring it slowly down on a windlass? Pressed the button a couple of minutes after dropping? Pressed it on a phone in the cockpit of a 40-foot boat while dropping it from the bow?

I see enough inaccuracy in my GPS anchor watch one way or another that I always set the alarm radius larger than the amount of chain, usually about 50% more.

Pete

 

[pawl]

Surely it depends on precisely where you were when you set the alarm. If you had 40m of scope out plus about 10m of boat you could, in theory, move 100m.

 

[RupertW]

Also even if you did drag you don’t explain how you knew the anchor was actually set. Unless you have really tested the set then the anchor could have bee on unsuitable seabed or rubble or garbage then set nicely later on decent substrate a few years away.

 

[Spirit (of Glenans)]

Before we had electronics we used to take a transit.

 

[sailaboutvic]

One thing I tend to do is look where I am on the app while I setting the anchor and the chain if fully stretch if it corresponds with the amount of scope dropped I know I'm in the right ball park then it depend how much room I have around me on how bigger margin I allow , if there are boats near by 10 m but much more the rest of the times of cause your back in the uk and also have to allow for the tided different where we are we don't .
The problem with given little safety margin is it can go off when the gps isn't accurate ,
I seen mine being as much as 8 m at times on my iPad .
These app are great but can also be a pain at times , once I am woken with a false alarm it takes me ages to get back asleep .

 

[duncan99210]

I always set the alarm zone well beyond the actual scope: say I put out 40m scope then the alarm zone will beset to 55m from the anchor. That removes much of the spurious alarms but still gives more than enough warning if the anchor pops out. Also make sure the alarm uses an averaging system, so that one bad gps position doesn’t set it off: it take several out of zone fixes before it alarms.

 

[TernVI]

How much does your GPS position wander about when the boat is strapped to the pontoon?
The phone screen seems to show a lot of scattered positions, not a progression of the boat moving down tide?

 

[MagicalArmchair]

Thanks for the responses

Surely it depends on precisely where you were when you set the alarm. If you had 40m of scope out plus about 10m of boat you could, in theory, move 100m.

The app talks constantly about Radius, which means the diameter on screen will be 80m (the snubber is attached to the last 10m of chain, and isn't in 'series' with the chain).

Also even if you did drag you don’t explain how you knew the anchor was actually set. Unless you have really tested the set then the anchor could have bee on unsuitable seabed or rubble or garbage then set nicely later on decent substrate a few years away.

I set the anchor by stopping the boat first, dropping the anchor until it touched, and then chugging backwards whilst letting out chain. At 20m of chain, I asked the 1st mate to gun her astern, she gave a couple of bursts of astern, and I saw the line go taught and we were going nowhere. It wasn't sustained "digging in" (one of the things I've reflected on), next time I'll let her run in astern for a little time at higher revs, say 2,000 rpm. Once all the chain was out, I didn't bother going astern again to 'stretch out' the chain (the tide and wind were pulling us back). In future, for completeness sake, I might run the engine astern a second time.

Before we had electronics we used to take a transit.

When we arrived at anchor, I was showing my eldest transits and how to be sure we were not dragging. Very hard to do a transit from your bunk though .

I always set the alarm zone well beyond the actual scope: say I put out 40m scope then the alarm zone will beset to 55m from the anchor. That removes much of the spurious alarms but still gives more than enough warning if the anchor pops out. Also make sure the alarm uses an averaging system, so that one bad gps position doesn’t set it off: it take several out of zone fixes before it alarms.

That's interesting, and in this case, if you look at the red dots in the bottom left, it doesn't look like I am wandering much farther that 60m, if I were to set it to 150% of the rode length...?

How much does your GPS position wander about when the boat is strapped to the pontoon?
The phone screen seems to show a lot of scattered positions, not a progression of the boat moving down tide?

She was sheering about a little in the wind and take, which accounts of the scatter, and the iPhone X I've got is pretty bang on once its got a fix (in my old boat, I used it plenty for navigation). When its firing up the GPS however, it takes a while to get a precise fix.

I did set the alarm after the anchor was set, so the anchor on the plot is definitely not accurate. What concerned me was the distance between the furthest two readings which are further than 80m. Hmm

 

[Robin]

Before we had electronics we used to take a transit.

And developed a sixth sense for any minute change in sounds or whatever. The tide flow around Brownsea can be quite strong and wind over tide can start the maypole dancing , enough to confuse any App whereas a quick head out of hatch might well suss the situation better. Relying on a phone whilst under a duvet is not my idea of anchoring.

 

[fredrussell]

Nowt to add except to say thank God we’re all talking about anchors again. I mean that sincerely, in case there’s any doubt.

 

[MagicalArmchair]

And developed a sixth sense for any minute change in sounds or whatever. The tide flow around Brownsea can be quite strong and wind over tide can start the maypole dancing , enough to confuse any App whereas a quick head out of hatch might well suss the situation better. Relying on a phone whilst under a duvet is not my idea of anchoring.

Ha, I got to that point with my old boat, however, I am still very much in anchor anxiety mode with the new one. Every movement has me poking my head out of the hatch, which doesn't lend itself to a happy first mate or a restful nights sleep.

Nowt to add except to say thank God we’re all talking about anchors again. I mean that sincerely, in case there’s any doubt.

There hadn't been one for a while - I consider it my public service duty! This thread is still missing a good, red blooded, punch up over anchor brands, shapes, sizes, colours and efficacy - watch this space, said brawl will hopefully arrive soon...

 

[noelex]

The position that is recorded by the GPS is the position of the antenna not the position of the bow.

The alarm radius has to be at least the length of the rode plus the distance between the bow and the GPS antenna. If using a marine GPS alarm, the GPS is frequently mounted near the stern. If using a phone/tablet app it obviously depends where the phone tablet is located.

In addition, when dropping the anchor the centre of alarm radius has to be displaced by the distance between the bow and the GPS antenna. In other words you cannot just activate the alarm as soon as you drop the anchor using the position reported by the GPS unless the GPS unit is at the bow.

Failure to make this correction (and many make this mistake) will double the error at the other side of the swing circle.

As an example, if your GPS is 10m from the bow and you activate the anchor alarm when you drop the anchor with 40m of chain, then at one side of the circle the anchor alarm will show that you are 40m away, and at the other side of the circle you will be shown as 60m away. All without the anchor moving. I suspect this was the cause of the problem in this case and the anchor did not drag.

I have drawn a diagram which may help, but with my poor internet connection at the moment it will not load. I will see if I can post it later.

 

[Jamie Dundee]

As a matter of interest, why do people use phone apps rather than the system built in to most chart plotters? Surely a plotter with dedicated gps antenna will be more accurate than a mobile phone?

 

[noelex]

As a matter of interest, why do people use phone apps rather than the system built in to most chart plotters? Surely a plotter with dedicated gps antenna will be more accurate than a mobile phone?

This is a good point. A GPS anchor alarm really needs a high quality, reliable and accurate GPS fix.

An external antenna with a modern, high quality GPS chip makes a significant difference.

Not only does this eliminate false positive alarms, it will also allow you detect small movements of the anchor. Anchors often slide slightly just before they let go completely. If you can detect these small movements, it is a useful sign that your anchor is struggling.

 

[duncan99210]

OK, I use a phone app because the accuracy on that is 5m +/- which is about the same as the plotter and AIS accuracy. I use the phone because the power consumption is tiny compared to the instrument suite, with the app it is very easy to set the anchor position (and the chart plotter isn’t, needing to go through a series of menu choices) and the alarm on the plotter wouldn’t disturb my sleep even if I were sleeping next to it (which I don’t, it’s 4 or 5 metres away).
So, when I consider the alarm is needed, I set it by moving the anchor on the alarm to my best estimate of its position in relation to where I’m standing holding the phone: that might be at the bow or more likely in the saloon. Having checked outside for any obstructions which might mean a smaller alarm distance, I’ll set the alarm ring to somewhere 10-15m outside where I might swing to. As I said earlier, that minimises false alarms due to a bad gps position yet still give ample time to start sorting things out if I do get a real alarm.
The phone lives on a charger by my bunk and has an alarm that will wake the entire crew if it goes off. I‘ve been using it for about 5 years now and it’s not let me down with false alarms so far.
All that said. If we do get nasty conditions blowing up unexpectedly, then we mount an anchor watch and make use of transits to get early warning of anything going wrong but that more because it’s a sensible way to approach things rather than lack of trust in the alarm. We’ve had one instance where the alarm went off as we had the engine starting because the transits looked dodgy: cue the remark ”I told you we were moving” from the Admiral.

 

[GHA]

As a matter of interest, why do people use phone apps rather than the system built in to most chart plotters? Surely a plotter with dedicated gps antenna will be more accurate than a mobile phone?

Plotter draws too much power. Plus android anchor app let's you have more than just a radius so it can let you know if you drift out of a channel. Can't remember actually having an alarm go off due to inaccurate GPS position from the phone in an awful lot of time on the hook. No need to be up and down to the cockpit anymore to take transits. technology works

 

[MagicalArmchair]

The position that is recorded by the GPS is the position of the antenna not the position of the bow.

The alarm radius has to be at least the length of the rode plus the distance between the bow and the GPS antenna. If using a marine GPS alarm, the GPS is frequently mounted near the stern. If using a phone/tablet app it obviously depends where the phone tablet is located.

In addition, when dropping the anchor the centre of alarm radius has to be displaced by the distance between the bow and the GPS antenna. In other words you cannot just activate the alarm as soon as you drop the anchor using the position reported by the GPS unless the GPS unit is at the bow.

Failure to make this correction (and many make this mistake) will double the error at the other side of the swing circle.

As an example, if your GPS is 10m from the bow and you activate the anchor alarm when you drop the anchor with 40m of chain, then at one side of the circle the anchor alarm will show that you are 40m away, and at the other side of the circle you will be shown as 60m away. All without the anchor moving. I suspect this was the cause of the problem in this case and the anchor did not drag.

I have drawn a diagram which may help, but with my poor internet connection at the moment it will not load. I will see if I can post it later.

Yes, I believe you are correct and the anchor didn't drag. If we properly "geek it up", we can see that the maximum distance is 96 meters between plots. if we take the 12 meter length of the boat from this (and the iPhone was in my pocket at times), we get down to 84 meters - this is well within the margin for error of the GPS. If we remove the 'outliers' , which could be to do with the GPS 'warming up' on the phone, pre hard fix, I suggest the actual anchor radius was well within 80 meters.

As a matter of interest, why do people use phone apps rather than the system built in to most chart plotters? Surely a plotter with dedicated gps antenna will be more accurate than a mobile phone?

This is a good point. A GPS anchor alarm really needs a high quality, reliable and accurate GPS fix.

An external antenna with a modern, high quality GPS chip makes a significant difference.

Not only does this eliminate false positive alarms, it will also allow you detect small movements of the anchor. Anchors often slide slightly just before they let go completely. If you can detect these small movements, it is a useful sign that your anchor is struggling.

I hadn't figured out the setting, to my shame, on the Raymarine e7 (new boat!). I do like turning all instruments off when at anchor to conserve battery power, I'll look into the e7 alarm.

So the conclusion is, we probably didn't drag, and it was the anchor anxiety speaking... wasn't it nice to have a lovely new anchor thread though (sorry, and thanks for allaying my worries). All good news though, I need to anchor through a couple of nice teethy gales, with an anchor watch, and not drag, then I'll finally be able to relax at anchor and be sure my kit is up to scratch. ?

Which anchor apps do you use? (to 'drift' the thread... pun intended)...

 

[Neeves]

When you sort out which app to use....

Why not use the anchor alarm on your e7 and the phone app - and compare the 2, setting both and setting the anchor position on both in the same place. This way you will define the difference in sensitivity - which will mean you might sleep more soundly - or not. I do appreciate that the e7 will use power, and more power - but given that you started a thread because of your uncertainty - it might save you sleepless nights in the future (or use the e7 instead - then you can worry about power.

You can also tell us how the 2 systems compare - and none of us will need to guess, or worry - either

Jonathan

 

[noelex]

I hadn't figured out the setting, to my shame, on the Raymarine e7 (new boat!). I do like turning all instruments off when at anchor to conserve battery power, I'll look into the e7 alarm.

Nice analysis.

I have not tried the e7, but for some strange reason plotters have primitive anchor alarms that do not allow for the necessary displacement of the centre of the alarm circle.