did my anchor slip? GPS attached

[Exonerated]

Nicholson58: Your explanation for the shape of the GPS track makes a lot of sense to me. The rode could even have just been caught in a pile of sand, and then released.

sy_gilana: Can you say more about why you think it slipped / why you disagree with the other responses here? Everyone else had me pretty well convinced that it did not slip.

Interesting idea for an alarm.

[CaptFigmo]

I've got Anchor Alert app on my smart phone.
Set it and forget it.
Sleep like a baby until the alarm sounds.
Then I am definitely dragging.

[JPA Cate]

Its kind of neat how people have such different takes on this. My first thought was that the OP was anchored on a bunch of rope rode, with little chain, perhaps as much as a boat length. To me it looked like you scooted around a bit, but I am not at all certain your anchor dragged. I've seen our tracks when we've dragged, and then snagged again, and it is like the establishment of an arc farther away from the dropping point. Tidal swings also affect where your arcs are.

We've used, first, taking bearings on significant items ashore, to have "ranges" when we set the anchor, so if the bearings change, we've moved. Then, we used our GPS, which had an anchor alarm (it was a Gulf War Trimble). Now, we use our AIS alarm, and it works okay, too. But we always set the anchor by backing down hard on it, and always check a range (sometimes one has to use binoculars.)

Exonerated, when/if you get your own boat, you'll be able to experiment with chain vs. nylon rode, and also with anchors. To that end, I'd like to recommend two CF threads: "Pictures of Anchors Setting," and "Videos of Anchors Setting", for your consideration.

[mstr_cmdr]

Sorry folks if I missed the basics - correctly setting the anchor? If done correctly, then there is little chance of anchor dragging in the conditions stated.
Then there is the problem with both GPS signals and computations; and, using a "smart" phone. DOP (dilution of precision) can fluctuate, causing the historical position to alter, leading to the mis-apprehension of anchor drag.
I must have also missed the other basics - including seamanship (in case of battery failure/loss of cellular signal, or other act of whichever is your preferred deity).
An old aviator friend of mine is used to telling me, "When all else fails, look out the cockpit window and go back to basics."
Instead of relying on the wizardry, rely on eye-ball Mk. I and guesstimate the potential maximum swing radius.

Non sunt multiplicanda entia sine necessitate

[Jim Cate]

An old aviator friend of mine is used to telling me, "When all else fails, look out the cockpit window and go back to basics."
Instead of relying on the wizardry, rely on eye-ball Mk. I and guesstimate the potential maximum swing radius.

With all due respect, relying upon looking out the window to determine if one has dragged is pretty ineffectual, especially at night. And IMO and IME, early warning of a dragging anchor is of great benefit. Such warning is readily available today, and despite your fears of technological failure, has been quite useful to us.

Apparently you posses superior "seamanship" and never drag, so perhaps these thoughts do not apply to you, but for the rest of us mortals, nocturnal anchor dragging is a real (if rare) possibility and GPS based alarms provide some hope of recovery before damage is done. Foolproof? Nope... better than Mk I eyeball? Hell yes!

Jim

[Exonerated]

Sorry folks if I missed the basics - correctly setting the anchor? If done correctly, then there is little chance of anchor dragging in the conditions stated.

I agree with you and others that my anchor didn't drag, and that dragging in this case was unlikely.

But I'm with Jim! I'll take all the electronic help I can get to be more secure.

[Exonerated]

Exonerated, when/if you get your own boat, you'll be able to experiment with chain vs. nylon rode, and also with anchors. To that end, I'd like to recommend two CF threads: "Pictures of Anchors Setting," and "Videos of Anchors Setting", for your consideration.

Yes, I love those pictures! I'd seen them awhile back and actually tried to dive on my anchor the first time I anchored (same place, a few weeks earlier). With the calm conditions and good holding ground, I didn't feel any real *need* to, but I thought it would be fun to compare what I saw with those photos.

The problem was finding the anchor while swimming around! I was anchored in about 20ft with 6x or 7x that much rode (almost all rope, and a little chain) and could see through maybe 5 or 10ft of water, I think. I had no trouble swimming down as far as the bottom, so you'd think I should be able to follow the anchor line, coming back up to breath along the way. The problem was current: I had trouble keeping myself in exactly the same spot while coming up to breath, so when I dove down again I couldn't find the line anymore. I got tired and gave up.

[Nicholson58]

Originally Posted by Exonerated

Nicholson58: Your explanation for the shape of the GPS track makes a lot of sense to me. The rode could even have just been caught in a pile of sand, and then released.

sy_gilana: Can you say more about why you think it slipped / why you disagree with the other responses here? Everyone else had me pretty well convinced that it did not slip.

Interesting idea for an alarm.

We use Vesper Marine Watchmate 850 AIS. It places a point on the screen every time period. The result is a cloud approximating a pie shaped wedge over time. Inspect it anytime and know where you are, where you have been. In the Caribbean, most wind is east, constantly. Rarely, we get a shift. On one occasion we jumped to a new arc after sitting for a while. It didn’t seem like high wind and we have a #121 Rocna, all chain rode. I dove on the hook. It was well set in sand. There was no drag track. There was a coral projecting from the bottom between the anchor and the bow that I guessed might have had the chain restrained before it could straighten. The 850 does a really good job as an anchor watch. It uses only about 8 watts on watch and is dedicated. We shut down all other Nav devices at anchor except depth. It is a very good AIS.

[UFO]

Originally Posted by Jim Cate

With all due respect, relying upon looking out the window to determine if one has dragged is pretty ineffectual, especially at night. And IMO and IME, early warning of a dragging anchor is of great benefit. Such warning is readily available today, and despite your fears of technological failure, has been quite useful to us.

Apparently you posses superior "seamanship" and never drag, so perhaps these thoughts do not apply to you, but for the rest of us mortals, nocturnal anchor dragging is a real (if rare) possibility and GPS based alarms provide some hope of recovery before damage is done. Foolproof? Nope... better than Mk I eyeball? Hell yes!

Jim

Second That

[fxykty]

Originally Posted by Exonerated

Yes, I love those pictures! I'd seen them awhile back and actually tried to dive on my anchor the first time I anchored (same place, a few weeks earlier). With the calm conditions and good holding ground, I didn't feel any real *need* to, but I thought it would be fun to compare what I saw with those photos.

The problem was finding the anchor while swimming around! I was anchored in about 20ft with 6x or 7x that much rode (almost all rope, and a little chain) and could see through maybe 5 or 10ft of water, I think. I had no trouble swimming down as far as the bottom, so you'd think I should be able to follow the anchor line, coming back up to breath along the way. The problem was current: I had trouble keeping myself in exactly the same spot while coming up to breath, so when I dove down again I couldn't find the line anymore. I got tired and gave up.

If you use mostly rope then just pull yourself along your rode on the surface until you get close to the chain and can no longer stay on the surface. Then dive in one go from there. Chain is usually quite visible on the bottom. Good light really helps.

[noelex 77]

I don’t see any problem with the trace and I don’t think you dragged, although I cannot determine the point where you dropped your anchor, which is really the key to solving these issues.

With 140 feet of rode if the wind swung 180° the GPS signal should have moved:

2 x (140 + the distance from the GPS aerial to the bow)

If I assume the GPS aerial was 20 feet from the bow the two arcs would be:

2 x (140+20) = 320 feet.

The two arcs are less than this distance apart.

The other clue is on the screenshot on the left. It looks like the wind has swung back to the original direction before you left. The GPS shows the boat returning to the same position which would not have happened if the anchor had moved.

Originally Posted by Exonerated

Yes, I love those pictures! I'd seen them awhile back and actually tried to dive on my anchor

Thanks. I glad you have enjoyed the photos. A float with a small weight is very useful. Drop this when see the the anchor, or even the chain and it will keep you orientated. You can move the float as you swim along the chain towards the anchor.

I like to always know my anchor position. GPS is very accurate these days. In very bad visibility we use a laser rangefinder and handbearing compass to locate this position and use this as the point to dive. One of us stands at the bow with the hand bearing compass and laser rangefinder and using hand signals directs the diver until they are the correct distance and bearing, as indicated from the GPS position of the anchor. At this stage we dive down, or drop a float and search the seabed from this point. It is usually very close to the anchor.

[Nicholson58]

Originally Posted by UFO

Second That

Third that Our Watchmate has two wake the dead external alarms with selector switch for Off, Below, Cockpit. One does not sleep through it. Very reliable.

[mstr_cmdr]

No Jim, I do not possess "superior seamanship" - an incorrect assumption.
A further incorrect assumption is that I have never dragged an anchor - to be sure, I have.

What I try to do is practice the art of seamanship and navigation, and back it up with some wizardry (not the other way round); I hold no fear of the electronic wizardry failing (and it will, spectacularly and at the most inopportune time (for example, lightning strike)).

Basic seamanship and navigation (don't get me started on rope work ) are but too-commonly being by-passed for reliance on something with a micro-chip and battery.

A question:
How many know of sailors whose total navigational skills are confined to the power button on a GPS-plotter?

Non sunt multiplicanda entia sine necessitate

[noelex 77]

A GPS plot is the perhaps the only technology that will show if your anchor has moved after a significant change in wind direction. It also works in very poor visibility (although here radar can be used).

The anchor alarm when appropriately adjusted provides a further level of protection, as it will keep watch when you are asleep etc.

The anchor alarm has alerted me to dragging on a couple of occasions and I cannot understand the reluctance to use technology that almost every boat has anyway.

The most important aspect of anchoring is to have top quality equipment and the skills to use this effectively, but even if these rules are followed anchoring is never 100% secure.