Anchor Alarm: Computing the Swing Circle

[markpj23]

I thought I understood this, but a rather sleepless night at anchor has me rethinking my approach.

I usually take my anchor scope (always at least 5x the depth) and double it to find the swing circle. Simple diameter = 2r equation. Due to the GPS variance I'll usually add another 50% of scope and set the alarm. For example, with 50ft of chain at the water's edge I set 125ft as the swing circle alarm point.

Nope - way too many false alarms. I increased the circle to 145ft before I got any peace. Checking LOBs and watching the anchor chain a lot gave me confidence we were holding just fine. The wind did shift 270 degrees during the night, in two main shifts vice gradually. Each shift caused an alarm - I guess that's why they built a pilot berth near the helm eh?

I suppose it's possible that the anchor reset itself. We were in a sandy bottom area (anchor is an oversized Mantus, all chain rhode). It's also possible that my little hand-held GPS was not that precise. Or it's possible that my outlook on this problem is wrong.

Try to resist the urge to turn this into another which-anchor-is-best thread

So what method do you use to set your anchor watch?

 

[DavidM]

Are you setting the center of the circle at the point where you drop the anchor?

David

 

[NWSeadog]

I think I see what Mark is doing, which isn't a bad method. Instead of guessing where the anchor sets (since I always forget to set the alarm when I drop the anchor) he is setting it after he has backed down and set the anchor. (i.e. 2r)

Something to consider though is where is the anchor alarm on the boat is when you set it and where is it at night. If you are at the helm when you set it with 50 feet at the water line then you could easily be 70 feet from the anchor. With an aft cabin( if you are using a cell phone) you could be another 30 feet away at the far swing point. 145 feet doesn't look to far off when you use those numbers.

 

[markpj23]

Are you setting the center of the circle at the point where you drop the anchor?

David

Hmm... actually no. My handheld is in the pilot house, which is about 20ft aft of the bow roller. I thought that by adding the additional 50% scope I would compensate for that distance.



You are right though - as far as the GPS is concerned the value for 'r' is the chain scope plus distance from the bow to the GPS. Might be where I'm off here..

 

[RedRascal]

I do the old school method and look out the window when I sense a change worth a look out the window. So yep the wind has shifted and the screen in your bunk shows you are in "your" circle. But it doesn't show you the boat upwind of you is on deck fumbling around with their anchor. I like to see the outcomes of such events.

I am of the camp that it's okay to drag, anchors need a chance to reset and bottoms are not always perfect like Disneyland. You could consider increasing your anchor circle to factor in dragging 25-50' or to a point where you want to stay away from danger. In my mind staying away from danger is the goal more so than if you are dragging. If you are constantly dragging though well then that would be the "which anchor is best thread".

 

[Simi 60]

We use OpenCPN
drop anchor, run a track and make a drink
After an hour or so there is a nice arc drawn on the plotter showing our swing
Set anchor alarm radius to about 10 metres behind the line to allow for tide and stretch back on the gusts.

 

[High Wire]

The center of the anchor drag alarm circle is where the gps antenna is when you turn the alarm on, not where you drop the hook or where you put the waypoint. We typically put out between 60 - 90 ft of chain and set the drag alarm at 120-150 ft. I never get to set the alarm at the anchor drop point. If the lay reverses during the night the alarm will wake me and that’s ok with me. Then I’ll turn it off and back on to reset on that half of the circle.

 

[Jeff F]

I use an app on my phone, and set the anchor position when I drop the anchor from the bow. I also turn on tracking on my Navionics app. The path is useful to know at a glance if anything has changed once you've been swinging for a while.

If there are other boats around I'll often keep the nav system on and mirrored on my tablet to check radar from the berth if I'm nervous.

Sent from my moto g play (2021) using Trawler Forum mobile app

 

[magna 6882]

I hope i am not hijacking the ops question. how do you guys set the alarm when in a crowded anchorage. Do you try figure out the other guys swing radius and if so how?

 

[RedRascal]

Do you try figure out the other guys swing radius and if so how?

If I am in a tight anchorage and I have an opportunity to dinghy over to a neighbor I simply ask them how much rode they have out. I usually start the conversation with "Hello, I wanted to get in sync with you on how much rode to get out tonight. Right now I have X out...." Usually I'll mention something about weather or how much current might run if the tidal change is large. Most folks are really happy to have the conversation and understand it's in both our interests to be in sync. If it's not convenient to ask then I try to get a sense for the angle of their rode coming off their bow. I find most folks are 3:1 to 4:1 with a lesser set doing 5:1 unless there is wind in the forecast or it is windy. Once in awhile you'll get someone with a rope rode at 7:1 on a calm night messing up an anchorage but not often.

I've never used an alarm in a tight anchorage so no tips from me on that. I just look out the window and decide if I am happy or not. After awhile you get a sense for the distances between boats.

 

[markpj23]

.. I find most folks are 3:1 to 4:1 with a lesser set doing 5:1 unless there is wind in the forecast or it is windy. Once in awhile you'll get someone with a rope rode at 7:1 on a calm night messing up an anchorage but not often.....

can't help it but my Navy training ALWAYS has my scope at 5 to 7 times water depth.. I have yet to drag an anchor but maybe I'm just lucky like that

 

[pjtemplin]

I've been enjoying pondering this. I thought about the math of the horizontal component of the radius of the scope, and decided that was mostly pointless, especially above 3:1 scope.



Then I started thinking about dropping a pin on the plotter when you drop the anchor. Gotta factor in the distance from bow pulpit to GPS antenna...but it gets worse. Let's say 20' from pulpit to GPS, and 100' of scope. In theory, you've only moved 100' (the offset is netted out), BUT what if you spin 180 degrees around? Now the pin is 20' the other side of the anchor, AND you're 100' of scope plus 20' of offset away from the anchor, so 140' (and the anchor may need a little distance to reset).


So...I'm thinking scope + (2 x pulpit-to-GPS), plus a little fudge factor for GPS inaccuracy.

 

[markpj23]

I've been enjoying pondering this. I thought about the math of the horizontal component of the radius of the scope, and decided that was mostly pointless, especially above 3:1 scope.



Then I started thinking about dropping a pin on the plotter when you drop the anchor. Gotta factor in the distance from bow pulpit to GPS antenna...but it gets worse. Let's say 20' from pulpit to GPS, and 100' of scope. In theory, you've only moved 100' (the offset is netted out), BUT what if you spin 180 degrees around? Now the pin is 20' the other side of the anchor, AND you're 100' of scope plus 20' of offset away from the anchor, so 140' (and the anchor may need a little distance to reset).


So...I'm thinking scope + (2 x pulpit-to-GPS), plus a little fudge factor for GPS inaccuracy.

This is exactly what I was seeing during my less than restful night on the hook.

 

[sunchaser]

Before cell phones and GPS plotters we learned to anchor the old fashioned way - rode length, charts and visuals. With our cruising grounds involving 6 hour current changes and up to 18' tidal ranges different acquired skill sets come into play. An oversized anchor, all chain rode and a weather eye to set/adjust scope works for us.

Like Simi, we've found the GPS track lines and depth sounder are great tools to watch as the evening wears on

 

[Shrew]

I usually take my anchor scope (always at least 5x the depth) and double it to find the swing circle. Simple diameter = 2r equation.

This approach is just slightly flawed as you're radius is being calculated based on a scope of zero.

A = (Depth + Pulpit height)

C = Rhode Length (A * 5)

D = Swing Radius

E = Diameter

Depth = 10 (ft)
Pulpit Height = 6 (ft)

A = 16
C = 80

D = √(c^2 - a^2)

D = 78.38
E= 156.77 (ft)

You would need to set the anchor on the GPS where you drop the anchor, then calculate the swing radius. You can't set the alarm from where the boat ends up once the anchor is set. you're radius on the GPS will likely not match the actual swing radius. If that is the method being used, you're guessing where the anchor is actually set.

 

[pjtemplin]

This approach is just slightly flawed as you're radius is being calculated based on a scope of zero.

A = (Depth + Pulpit height)

C = Rhode Length (A * 5)

D = Swing Radius

E = Diameter

Depth = 10 (ft)
Pulpit Height = 6 (ft)

A = 16
C = 80

D = √(c^2 - a^2)

D = 78.38
E= 156.77 (ft)

You would need to set the anchor on the GPS where you drop the anchor, then calculate the swing radius. You can't set the alarm from where the boat ends up once the anchor is set. you're radius on the GPS will likely not match the actual swing radius. If that is the method being used, you're guessing where the anchor is actually set.

Not sure what you're trying to say here at all...the referenced post did factor in 5:1 scope. I think you're trying to say that we're treating the rode as already horizontal and that we should instead be looking at the horizontal component of the scope. Yes, except the error is 6% at 3:1, 3% at 4:1, 2% at 5:1, and 1% beyond that. Your math even shows that in this example, the error is 1.62 ft. That's potentially within the error range of GPS accuracy, and did the anchor set in less than 19" from where you dropped the pin?


Seems like we all have to decide what our threshold is. Do you want the alarm to go off as soon as you swing off the original current/wind? Do you want it to go off if you've pivoted more than 90 degrees either side? What's the physical object(s) you don't want to hit, and are they fixed or floating?

 

[AlanT]

We use OpenCPN
drop anchor, run a track and make a drink
After an hour or so there is a nice arc drawn on the plotter showing our swing
Set anchor alarm radius to about 10 metres behind the line to allow for tide and stretch back on the gusts.

+1

Enabling "tracking" on your chart plotter at max enlargement will show you clearly how the anchor is performing - specifically whether it is dragging. What you should see is some initial wavering as the boat settles, followed by a pretty consistent arc or even a circle as the boat is affected by tide/current. Center of the arc is the position of the anchor (if you set a waypoint the moment you drop the anchor that will also help you interpret the track).
~A

 

[cafesport]

Try setting your radius to rode plus boat length for restful nights and no false positives.

 

[Portage_Bay]

Warning: Thread drift. Not the usual topic or subject thread drift. This time it’s geographic thread drift. OP is in Florida, I boat in the PNW. Here’s my technique used for decades on work boats and now on my recreational cruiser. Yes, conditions are different in the two areas but you may find some useful ideas here.

I carry the biggest anchor I can reasonably manage. I use an all chain rode.

I don’t calculate anchor alarm by scope. I don’t correct for the distance from anchor roller to GPS antenna.

The technique is simple and straight forward. One person at the helm, one at the windlass. Lower the anchor and as soon as it touches bottom set a waypoint. Begin backing easy while paying out. When the desired scope is out back until the anchor is well set and the rode pulled as tight as I can without pulling the anchor out or risking damage to the windlass. If it sets quickly and well then I set a second waypoint. If it doesn’t set quickly and well then I pick it up and try again. Add a bit for GPS accuracy, I use 30’. I now have the maximum radius of my swing circle. I say maximum because it would take a lot of wind or current to pull the rode as tight as backing will.

Before relaxing I can use that swing circle to be sure I won’t hit anything I don’t want to hit as the boat swings or I begin to drag.

I leave the plotter on with tracking turned on. Looking at the track and the alarm circle is a good solid check before turning in. If I have any concerns during the night it’s a fast easy check as well. If I have good solid land masses I will also note radar ranges. If I have stabilized radar then bearings as well.

I don’t try to guesstimate what the other boat(ers) have for scope or what they will do. If I’m not comfortable with how close a new arrival is I move.

If I’m expecting or in extreme conditions I’ll set anchor watches. Better to loose some sleep than damage the boat. Setting anchor watches has paid off twice. Once we dragged. Another time the only other boat in

 

[SteveK]

Shrew showed a math calc and it provided a very good radius from the point where the anchor sits.

Setting the alarm at the spot the boat is at after dropping anchor and 5>1 scope gives a radius that will be over top of the anchor. That means if current/wind moves boat 180* you will be outside of the geofence. Doubling the distance from the anchor would prevent a false alarm but likely will have the circle close to land or other boats. And it will allow a lot of anchor dragging before an alarm sounds.

 

[markpj23]

+1

Enabling "tracking" on your chart plotter at max enlargement will show you clearly how the anchor is performing - specifically whether it is dragging. What you should see is some initial wavering as the boat settles, followed by a pretty consistent arc or even a circle as the boat is affected by tide/current. Center of the arc is the position of the anchor (if you set a waypoint the moment you drop the anchor that will also help you interpret the track).
~A

Agreed - and my trusty little handheld keeps a track that shows this. The problem is, I don't want to have to stay up watching the track, nor do I want to be woken up by a false alarm that will have me look at the track and see that all is fine.


I think I'm just being too concerned about a movement of less than 50ft from the anchor circle that I've been calculating thus far.



I'm probably overthinking this... ok likely overthinking this. Just increase the anchor circle a bit and sleep better!

 

[markpj23]

Try setting your radius to rode plus boat length for restful nights and no false positives.

Thanks for this - might be the easiest approach.

 

[Portage_Bay]

Shrew showed a math calc and it provided a very good radius from the point where the anchor sits.

Setting the alarm at the spot the boat is at after dropping anchor and 5>1 scope gives a radius that will be over top of the anchor. That means if current/wind moves boat 180* you will be outside of the geofence. Doubling the distance from the anchor would prevent a false alarm but likely will have the circle close to land or other boats. And it will allow a lot of anchor dragging before an alarm sounds.

If your post is in response to my post #19 perhaps my method was not clearly described. Here's a shorter hopefully clearer version.

1 - Drop the hook and set a waypoint when the anchor touches bottom.
2 - Pay out the desired scope. Back on the anchor to set the hook and pull the rode tight. Set the 2nd waypoint while pulling the rode tight. Unless the anchor drags the boat will not get that far away from the anchor dropped waypoint when using a chain rode. Or even with a rope rode and a length of chain.
3 - Take the distance between the two waypoints and add 30 ft to allow for GPS error. Use less than 30 ft if you have more confidence in GPS accuracy than I do. That is the radius of a watch circle the boat must stay within before an alarm is triggered.

The boat can swing through 180, 360 or any arc. It can cross over the anchor. The alarm will not sound as long as the boat stays within the watch circle. How that watch circle and alarm is set up is plotter / anchor alarm dependent.

This method requires no calculations, introduces no errors. It simply works.

 

[Blissboat]

If your post is in response to my post #19 perhaps my method was not clearly described. Here's a shorter hopefully clearer version.

1 - Drop the hook and set a waypoint when the anchor touches bottom.
2 - Pay out the desired scope. Back on the anchor to set the hook and pull the rode tight. Set the 2nd waypoint while pulling the rode tight. Unless the anchor drags the boat will not get that far away from the anchor dropped waypoint when using a chain rode. Or even with a rope rode and a length of chain.
3 - Take the distance between the two waypoints and add 30 ft to allow for GPS error. Use less than 30 ft if you have more confidence in GPS accuracy than I do. That is the radius of a watch circle the boat must stay within before an alarm is triggered.

The boat can swing through 180, 360 or any arc. It can cross over the anchor. The alarm will not sound as long as the boat stays within the watch circle. How that watch circle and alarm is set up is plotter / anchor alarm dependent.

This method requires no calculations, introduces no errors. It simply works.

Just so. That is how I have always understood an anchor alarm works.

 

[pjtemplin]

If your post is in response to my post #19 perhaps my method was not clearly described. Here's a shorter hopefully clearer version.

1 - Drop the hook and set a waypoint when the anchor touches bottom.
2 - Pay out the desired scope. Back on the anchor to set the hook and pull the rode tight. Set the 2nd waypoint while pulling the rode tight. Unless the anchor drags the boat will not get that far away from the anchor dropped waypoint when using a chain rode. Or even with a rope rode and a length of chain.
3 - Take the distance between the two waypoints and add 30 ft to allow for GPS error. Use less than 30 ft if you have more confidence in GPS accuracy than I do. That is the radius of a watch circle the boat must stay within before an alarm is triggered.

The boat can swing through 180, 360 or any arc. It can cross over the anchor. The alarm will not sound as long as the boat stays within the watch circle. How that watch circle and alarm is set up is plotter / anchor alarm dependent.

This method requires no calculations, introduces no errors. It simply works.

By my napkin, it does introduce errors and falls apart at 180 degrees around. But, my give a darn is busted, so if it works for you, great.

 

[psneeld]

Starting to sound like setting transducer depth. Keel or water depth.

If yours anchor alarm method works... fine, no need to convince others who do or "think" they have it down.

If wondering how to do it, good luck...understanding where the pieces and parts are and how your anchor alarm actually works is important to getting it correct (for you).

Sounds like some have an anchor alarm that only works as a waypoint (hit the button) where you set a radius and others where you can set the waypoint after the fact. If true...of course there will be different techniques if different chartplotters not have different options.

Personally, I slowly evolved to a process that was close to what Portage Bay suggests.

 

[Porgy]

I think this subject has been picked over numerous times and with numerous articles. Here’s but one I vaguely recall. https://www.boatus.com/expert-advice/expert-advice-archive/2014/may/the-ultimate-anchor-watch Figure 2 gives a good visual of what’s going on and describes what some here have said.

 

[catalinajack]

This issue has always challenged my mind. I use the Vesper app anchor alarm. The app accounts for the distance between the boat's GPS location and the bow roller. I release the anchor and immediately invoke the app AT the bow. The anchor will go staright to the bottom. Sure, the anchor may, in extreme conditions, not set exactly at that point but that is not likely. Then I set out anchor line, all chain in my case, to the scope I wish. I can, and do, monitor the position of the boat relative to the anchor. In the morning the app shows a smudge of blue dots all around the anchor point which shows the boat's position throughout the night. Based on the length of rode set out, I adjust the alarm circle to a radius that makes sense for that set. Generally, I set the radius to 50 feet in excess of the length of the rode deployed.

I do seem to have an anchor that sets quickly. Evidence? Here's an anecdote. I once made a rookie mistake in not paying attention (5 second lapse) in a narrow channel in a 30-knot breeze. I was outside the channel in a flash and could not recover in time and was driven against a shoal with no chance of getting free without assistance. We were in a remote location so I chose to notify the Coast Guard of our position. The CG recommended deploying our anchor. We did. It set in sand immediately with virtually no scope. How did I know that? Cuz, as the tide rose we floated free but the boat stayed in place. The anchor prevented the wind and strong current from further driving the boat against the shoal. I remain amazed that that anchor set that way. And, mind you, the wind and the strong current there was still doing its best to pin us against that shoal. Three hours later, late in the afternoon, Towboat US pulled us free but, as dark was approaching, we were forced to anchor for the night in an anchorage with a strong current and those 30-knot winds. Our anchor held us fast.

 

[Portage_Bay]

By my napkin, it does introduce errors and falls apart at 180 degrees around. But, my give a darn is busted, so if it works for you, great.

Here's a graphic of how it works and why it doesn't fall apart at 180. As long as the anchor doesn't drag the boat stays within the watch circle no matter the arc of swing. I'm curious what other errors do you see in this method? Always learning and trying to improve.

 

[Porgy]

Here's a graphic of how it works and why it doesn't fall apart at 180. As long as the anchor doesn't drag the boat stays within the watch circle no matter the arc of swing. I'm curious what other errors do you see in this method? Always learning and trying to improve.

Your graphic indicates that the GPS antenna is directly over the anchor roller and that the anchor sets immediately with zero dragging to bury. There are anchor alarm apps that allow you to move that point but I’ve yet to see a boat with a GPS antenna on the pointy end of the bow pulpit.