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

[Neeves]

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.

This should not be a problem.

Your engine is running as you power set the anchor, so the anchor is at the other end of the rode - in a straight line. You know what length of rode was deployed. You motor along the bearing of the rode for say 40m and you are as near as makes no difference over the anchor, set alarm.

Or if you are recording a track simply motor on a reciprocal course to the point indicated by the track where you dropped the anchor, set alarm.

Its not difficult.

And as most MFDs are at the helm, set the alarm as the anchor is deployed.

Jonathan.

 

[noelex]

This is a diagram that I hope makes things clearer. Notice that after you have dropped you cannot motor forward by the length of your anchor chain and mark that spot on your chart plotter.

There would be an error of 2x distance between the GPS aerial and the bow roller at one side of the swing circle. On many boats this error will be over 20m.

 

[[163233]]

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

I don't know the area, but if the orange is the target position then it looks like you were aiming for a spot awfully close to a narrow channel.

 

[MagicalArmchair]

This is a diagram that I hope makes things clearer. Notice that after you have dropped you cannot motor forward by the length of your anchor chain and mark that spot on your chart plotter.

There would be an error of 2x distance between the GPS aerial and the bow roller at one side of the swing circle. On many boats this error will be over 20m.

Thank you for the detailed diagram.

I don't know the area, but if the orange is the target position then it looks like you were aiming for a spot awfully close to a narrow channel.

The channel at this point is 450 foot across and was deserted but for a motor boat and one other sailing boat. If you go a bit further to the north side of Brownsea, the whole channel (same width) is stuffed with moorings and there is not enough space to swing a cat (I had to concentrate navigating them on the way back in the morning). It was a safe and proper place to anchor.

 

[TernVI]

I don't know the area, but if the orange is the target position then it looks like you were aiming for a spot awfully close to a narrow channel.

That's because the channel is where the water is. Most of Poo Harbour is just damp ground!

 

[LittleSister]

The alarm radius has to be at least the length of the rode plus the distance between the bow and the GPS antenna.

Plus an allowance for variation in the GPS accuracy, and another for some movement of the anchor (digging in deeper, turning etc.), I'd suggest. Of course, all this could be accommodated in the 'add 50%' some have suggested.

 

[noelex]

Plus an allowance for variation in the GPS accuracy, and another for some movement of the anchor (digging in deeper, turning etc.), I'd suggest. Of course, all this could be accommodated in the 'add 50%' some have suggested.

Yes, absolutely. When setting the alarm you do need to add a factor into the system so errors do not produce false alarms.

Often there is no need for a tight alarm radius. In many anchorages you could drag miles with the likely wind direction before encountering any hazards. However, there are some occasions when it is helpful to be alerted as early as possible.

It is worth knowing how your system works. The accuracy of the GPS is the biggest limitation and this depend on the ability of the GPS to have a clear view of the sky, whether your area has satellite differential GPS, etc.

The other factors you mentioned depend on the anchor. The convex plough anchors, for example, can take a long distance to set so this certainly needs to be factored into the equation. On the other hand, unless the substrate is very soft the better anchor designs set almost immediately. Anchors also move backwards very slightly as they dig down deeper in response to stronger wind, but this movement should be very slight. When coming close to the limit of their holding ability most anchors will start to creep back a considerable distance, before they breakout completely. It is worth trying to detect this behaviour, as it will often provide some warning before the anchor drags.

This is a photo of my Mantus anchor. The red arrow shows the mark in the sand where the anchor landed. It is remarkable that good anchors can dig down so quickly. The Manus will rarely take any longer than this example and even shorter is more common.

When turning in response to a change in wind direction, people imagine their anchor breaking out and having to reset. Providing the substrate is adequate and the anchor has been reasonably set, a good anchor will almost never do this. Instead it should “shuffle” around, rotating about the fluke without otherwise moving.

The biggest error in the anchor alarm is not centering the alarm radius directly over the anchor, generally because the distance between the GPS antenna and the bow has not been allowed for. If you can avoid this issue and your GPS can provide a consistent and accurate position, the alarm will not give false positives, but it can still alert you quite small anchor movements.

 

[[163233]]

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?

Mobile phones are often much newer than your plotter. The E7 for instance doesn't work with Galileo, glonass, or Beidou. But your phone probably does. It also uses an internal aerial, but it is a good quality gps.

I've also got an aged standalone raymarine gps and that's poor next to the phone.

 

[sailaboutvic]

@noelex (Yes, absolutely. When setting the alarm you do need to add a factor into the system so errors do not produce false alarms.

Often there is no need for a tight alarm radius. In many anchorages you could drag miles with the likely wind direction before encountering any hazards. However, there are some occasions when it is helpful to be alerted as early as possible.
)
I wish that was the case around here John in south of France and places like Ionian and Croatia , you can find the most isolated spot and everyone will anchor next to you .
Even tho there not many boats about now the last few days boat will Insist in anchoring two boat length from me ,

 

[Jamie Dundee]

Mobile phones are often much newer than your plotter.

Not my plotters but point taken. I just have an aversion to using non marine electronics in a marine environment.

 

[[163233]]

Not my plotters but point taken. I just have an aversion to using non marine electronics in a marine environment.

I had a couple swimming with an iPhone 10 the other day, seemed weird to me.

 

[Jamie Dundee]

I had a couple swimming with an iPhone 10 the other day, seemed weird to me.

I never got over getting my 5S ‘slightly damp’ whilst launching a boat. Some things are just WRONG ?

 

[MagicalArmchair]

Not my plotters but point taken. I just have an aversion to using non marine electronics in a marine environment.

Chortle, I always thought adding "Marine" to any piece of electronics was just a license to add 150% to the price.....

 

[Jamie Dundee]

Chortle, I always thought adding "Marine" to any piece of electronics was just a license to add 150% to the price.....

Many years ago when I had caravans I thought that anything preceded by the word ‘caravan’ was extortionate. Then I got into boats - and ended up buying caravan stuff as it was half the price ?

The exception I make is electrics and electronics, tinned cabling and decently waterproof/sun readable instruments. I’d hate the Mrs to die thinking “I wish he’d spent another £50 for a decent one”.....

 

[MagicalArmchair]

Many years ago when I had caravans I thought that anything preceded by the word ‘caravan’ was extortionate. Then I got into boats - and ended up buying caravan stuff as it was half the price ?

The exception I make is electrics and electronics, tinned cabling and decently waterproof/sun readable instruments. I’d hate the Mrs to die thinking “I wish he’d spent another £50 for a decent one”.....

All joking apart, you get what you pay for, so I won't scrimp on safety gear, electronics, rigging, critical systems, anchoring (ha, as with this thread! The anchor certainly wasn't cheap...). There is a lot to be said for not scrimping in other places too: I needed some microfibre cloths and had to buy them from the chandlers as I had my stash at home. I was used to spending pennies on Amazon to bulk buy loads of very functional, and passable microfibre clothes. The Chandlery had a pack of 5 of a brand I had never heard of for £20 I think? I threw my hands up in despair, grumbled, and purchased them reluctantly. They are amazing!! Worth twice that price! I wash them and reuse them religiously, and they make every deck cleaning job much quicker and easier.

At the risk of contradicting my previous comment, you buy cheap, you buy twice...

 

[RobbieW]

...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...

One comment on this specifically, you've more than enough replies on anchor alarms. Think about having a brew before running in astern to dig in the anchor, it lets the whole system settle a bit before you do that. Its something I remember reading here in the past and now do myself.

 

[GHA]

One comment on this specifically, you've more than enough replies on anchor alarms. Think about having a brew before running in astern to dig in the anchor, it lets the whole system settle a bit before you do that. Its something I remember reading here in the past and now do myself.

I do the exact opposite, and do little to make the anchor's life easier. Ditch a load of chain (but not on the hook) , bang it in reverse and watch the bow dip when it bites. If the anchor can't cope with that and needs kid gloves then I'd really like to know about it then rather than the first big gust of a squall at 3am when the boat shoots off in a different direction. Invariably it copes fine, since the cqr was thrown out a long time ago anyway

 

[RobbieW]

I do the exact opposite, and do little to make the anchor's life easier. Ditch a load of chain (but not on the hook) , bang it in reverse and watch the bow dip when it bites. If the anchor can't cope with that and needs kid gloves then I'd really like to know about it then rather than the first big gust of a squall at 3am when the boat shoots off in a different direction. Invariably it copes fine, since the cqr was thrown out a long time ago anyway

Treat 'em mean to keep 'em keen eh Where I can I use the wind to set the anchor, drop it to the bottom and pay out chain as the wind blows the bow off

 

[GHA]

Treat 'em mean to keep 'em keen eh Where I can I use the wind to set the anchor, drop it to the bottom and pay out chain as the wind blows the bow off

Sometime do the same, but always with a really good long blast in reverse - nice to have some evidence the boat will be there later when you jump in the dinghy

 

[MagicalArmchair]

One comment on this specifically, you've more than enough replies on anchor alarms. Think about having a brew before running in astern to dig in the anchor, it lets the whole system settle a bit before you do that. Its something I remember reading here in the past and now do myself.

do the exact opposite, and do little to make the anchor's life easier. Ditch a load of chain (but not on the hook) , bang it in reverse and watch the bow dip when it bites. If the anchor can't cope with that and needs kid gloves then I'd really like to know about it then rather than the first big gust of a squall at 3am when the boat shoots off in a different direction. Invariably it copes fine, since the cqr was thrown out a long time ago anyway

I like option 2 quite a lot from GHA! My pathetic stab of 2k revs astern pales in comparison. How long do you run it in astern? And how much 'welly' (rpm) do you give it?