did my anchor slip? GPS attached

[Exonerated]

I'm attaching a screenshot ("three-am.png") of my GPS tracks while anchored. The track begins on the left when we went to bed (at around low tide). I took the screenshot about 20 minutes after waking up and seeing that the boat had swung to a new spot. To the right (underneath the "ance" in "Distance") is what looks like the circular arc of the boat swinging at its maximum distance from the anchor. But then we have a new arc about 10 ft further out (to the right) from the anchor (under the "H" in "Heading").

I'm wondering: Did the anchor slip (and then recatch) between those two arcs on the right?

My best guess right now is no: It seems more likely that what looks like the arc of a maximally stretched anchor line is actually just the boat drifting a bit inside of the anchor's circle, and/or maybe the wind picked up little to straighten out the anchor line between the two arcs (although I didn't notice any change to the very light wind). But I'm interested in what others think.

More context:

We have 9 ft tides here and are in about 14 ft of water at low tide, which was around when the track begins in the left. I took the screenshot at almost high tide. I think we had put out about 140 ft of anchor line (almost all rope). I set the anchor by running the engine in reverse at medium RPMs for a minute or so. This spot is well known to have great holding.

I wasn't using an "anchor watch" app. It seems like the alarm would be quite prone to false positives, and I don't want to wake up my spouse unnecessarily. Instead I've been looking at the GPS track whenever I woke up a little, and sometimes popping my head out the hatch to look around. At home I'm a sound sleeper, but on the boat I naturally wake up every hour or two to check on where we are. I think next time I'll add the anchor watch app to my arsenal, but just use a setting that will only wake us up if something's definitely wrong (and/or use vibrate-only, so it's more likely to wake up me and not my spouse).

We didn't actually have any issues. (I'm also attaching a screenshot of our track from the rest of the night. The track going off the screen is from when we picked up the anchor and left.) But the worry kept me from getting as much sleep as I would have otherwise. This is only my second time sleeping on an anchor on a boat that I'm responsible for, so I'd love to have any opinions/advice that folks can give.

[Mirage35]

You say winds were light, and between that and what I see on the tracks, it looks to me like the boat was just drifting pretty aimlessly around the anchor. My experience is that dragging will show as an arc at the end of the anchor rode which will move more or less quickly in one direction before setting up another arc. And in enough wind to drag the arc shapes are clearer than what I see here.

If it moves almost instantly to a new location it’s probably a GPS issue - picking up different satellites, etc.

[Exonerated]

Thanks, that makes sense.

[Kelkara]

WHere s the anchor? The question you should be asking is ... Is any point of your track more than 140ft from the place the anchor set? A boat can drift around within this circle with the wind and tide, and not stress the anchor at all ... in fact a boat on rope rode will almost certainly wander around with even the smallest wind and current changes.

[Kelkara]

I've only had an anchor drag twice, but both times the GPS track looked something like this:


If your anchor starts out well set then it should be pretty obvious when it starts dragging. The boat will spend time swinging at the limit of the swing circle before starting to drag, and then it will start to move outside the circle, either quickly if the anchor totally breaks out, or slowly if the anchor is plowing through the substrate.


drifting around inside the swing circle is not a problem ... and very much expected on rope rode.

[thinwater]

Hence, the importance of noting where the anchor was placed. Sure, it moved some during setting, but at least you have a starting point. Failing that, at least note the heading when the rode pulled tight.

[thinwater]

Are any of the anchoring apps actually based on a center point + radius?

[Exonerated]

Kelkara: Yes, I do know the boat will drift around. The reason for my question is that my track looks a little bit like the one in your example: I'm swinging on that arc under the "ance" in "Distance", and later I'm sort of on an arc a little further out. Although I realize that my two "arcs" are both very short and not very arc-like, and that they're very close to each other. So you and other folks said, it does make sense that this is just drifting and not slipping. Thanks!

Do you have a suggested way to mark where the anchor is set? I know of apps that will indicate (and alarm on) the distance from a set point, but I'd also love to be able to mark the spot on the chart.

Or even better, I'd love to see an app that doesn't rely on me having to accurately mark the anchor's spot, but instead calculate an inferred center point of the circle. Maybe that's what Thinwater is getting at.

[Training Wheels]

Originally Posted by thinwater

Are any of the anchoring apps actually based on a center point + radius?

I use ANCHOR! Ideally you set the alarm from the bow when you drop the anchor, if not you can move the anchor icon around to where you believe the anchor to be.

[StuM]

Originally Posted by thinwater

Are any of the anchoring apps actually based on a center point + radius?

Yes.

[Exonerated]

That looks like a nice app, StuM.

[AndyEss]

Originally Posted by thinwater

Are any of the anchoring apps actually based on a center point + radius?

The iNavx is based on center point (anchor) + radius (rode).
It just isn't that easy to set as my version takes the instantaneous GPS heading to position the anchor. By the time I set the anchor (I often solo), set my snubber, and organize my deck, it's been quite a few minutes since I dropped the anchor.
The software doesn't allow me to input a heading - so you have to play with it until your instantaneous GPS heading matches approximately the heading at which your anchor is actually positioned.
Once you have that, the Anchor watch function works well.

[Exonerated]

naive question: Is the reason for the "bearing to anchor" readout so that you can check that against the actual bearing you get from the line in the water?

[AndyEss]

The bearing is so that you can set the center point of the anchor/rode circle.
The anchor is at the center of this circle, the length of the rode is the radius (adjusted for water depth and tidal changes) of the circle.
If your anchor is not dragging, your boat should always be inside this circle while you are anchored.

[Unity]

When I remember ... not as often as I’d like ... I quickly set a waypoint at my position as I start to drop the anchor. I’ve got the windlass control at the helm so it’s pretty easy to do .... if I remember to do it! Of course if you drift before the anchor touches, and if you slide aft while digging it in that position will be somewhat wrong but I find it really valuable especially when there are rapid wind shifts.