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Old 21-04-2021, 05:39   #16
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Re: Where/When to set anchor alarm

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Originally Posted by Stu Jackson View Post
If you set it where you lay back to, the alarm circle will have to be much bigger.


Think about it.
Nope.

Lets assume you use a 100 ft radius alarm setting at your anchor.
Some of the anchorages we use are a bit tight. So I do not want to know when I have moved 200 ft (100 ft past my anchor) with a wind change. I want more time so I prefer to know when I am 100ft away from my preferred spot, not 200.

It would be ok to set it at the anchor point if you in a nice clear anchorage and winds are variable and you expect to swing all around.
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Old 21-04-2021, 05:45   #17
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Re: Where/When to set anchor alarm

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Originally Posted by JebLostInSpace View Post
If you set the alarm on top of the anchor, you can give the alarm circle a radius equal to your rode (really, you could do a bit smaller depending on the depth, but in relatively shallow water this is probably close enough). Then, even if the wind shifts 180, you're still inside the circle as long as the anchor doesn't move.

If you set the alarm after lying back on the anchor, you have two choices. 1) You can set a nice small circle that encompasses the area you will wander through in the current conditions. Or 2) you can set a huge circle that will contain the area you'll swing to in a wind shift.

Option 2 clearly sucks. The circle in that case is so big your anchor alarm won't go off until you're already on the rocks. Option 1 sounds like it would be good as long as you're willing to reset the alarm if conditions change. However, the alarm is still not going to go off right away when the anchor drags. Because you want the circle to cover some of the area between the boat and the anchor, and the boat is at the center of the circle, the circle must then also cover area behind the stern of the boat. Your boat will only enter this area if the anchor is dragging, but your alarm won't go off. Better to make the center of the circle on top of the anchor.
It depends on the anchorage. Usually you in a lee shore so the wind is behind i.e, there is room to drag. But should the wind change or a funny blow come through and my stern starts to face the beach, Iíd prefer to know sooner.
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Old 21-04-2021, 05:46   #18
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Re: Where/When to set anchor alarm

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Originally Posted by Apollo19.5 View Post
Depends on what I am trying to achieve and where the perceived risks are for me. If I am in a deep anchorage and more worried about a dragging, then I set the mark when dropping the anchor and the radius equally to my scope (and a little to allow for tide drop). If I am in an anchorage (usually shallower) where my risks are due to an unexpect change of wind direction effecting my lay and putting me closer to an obstacle (ie shallower water or rocks/reef), then I set it at where I lay after setting the anchor and the radius allowing for small variation from that laying position but well short of the obstacle.
Well said. Thats it.
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Old 21-04-2021, 05:48   #19
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Re: Where/When to set anchor alarm

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Originally Posted by Tomodore View Post
It's great that your wife and kids are getting serious education. Sounds like an opportunity to receive input and possibly even admit that they are right and you are wrong. Sailing with a skipper that can bend and even change is a key to a happy boat. Do you want to be right or happy about an issue like this?
Iím right
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Old 21-04-2021, 05:56   #20
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Re: Where/When to set anchor alarm

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Originally Posted by Uncle Bob View Post
Yep, you're wrong.
Nah not wrong!

As Apollo said:

ď Depends on what I am trying to achieve and where the perceived risks are for me.Ē

So in my opinion there is no right or wrong, it depends on the situation.

Thanks for all the responses.
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Old 21-04-2021, 07:15   #21
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Re: Where/When to set anchor alarm

I try and set mine to where the anchor is. My problem is that I don't always remember. ​Then I set it to where I end up and just live with the alarm during the tide change. I am usually on the boat doing a watch for the tide changes anyway.

I need to do some research for an android app that has a late set like SV Jedi mentioned. Setting alert zones would also help if there is an area you don't want the wind to push you in.

I am just happy that people are using an anchor watch systems.
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Old 21-04-2021, 08:01   #22
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Re: Where/When to set anchor alarm

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Originally Posted by noelex 77 View Post
This not quite right, at least not unless you are standing at the bow with the smart phone/tablet.

Using the method suggested, the recorded position will be the position of the smart phone/tablets GPS antenna. If you are in the cockpit this may be 10m or more from the true anchor position. If you swing to the other side of the anchor circle this error is doubled so it can become quite significant. This mistake is a common cause of false alarms.
That is good point, minimizing distance errors will give you better watch performance. We have a center cockpit and the tablet is located more to the center of the boat than usual.

What is your solution adding the missing boatlength?
Is there any anchor apps where you can offset the gps location?
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Old 21-04-2021, 08:11   #23
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Re: Where/When to set anchor alarm

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Originally Posted by OneKoolCat View Post
.....

I need to do some research for an android app that has a late set like SV Jedi mentioned. Setting alert zones would also help if there is an area you don't want the wind to push you in.
We use Anchor Lite - free app (by Peckish Sloth) - it is well made, and allows late setup of the anchor by reporting direction to and distance to the anchor.
If I want to use this function, we sail back downwind from the anchor to the end of the rode under high RPM (it is part of the anchor holding anyway. This gives you the correct direction to the anchor, and the length of the rode out is somewhat larger than the distance (depth of water).
The App also has paid version which I did not try.
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Old 21-04-2021, 08:24   #24
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Re: Where/When to set anchor alarm

For me the simple answer is do I want to know when the anchor moves or when the boat moves? Current, wind etc means the boat always moves. So I figure that’s a given. So I set the anchor as center point. I use a cheap app usually that allows me to move the anchor point after I set it since I usually don’t remember to set it when I drop anchor. Also, in a bigger boat my phone with the app can move quite a bit so if I move the center point based on rode plus boat length with direction obvious when I am backing down hard I have fewer false alarms.
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Old 21-04-2021, 09:02   #25
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Re: Where/When to set anchor alarm

Not only at anchor but also on moorings! Twice I've had a mooring fail over the years!
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Old 21-04-2021, 10:19   #26
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Re: Where/When to set anchor alarm

It really comes down to where you anchor and the tidal swing. If you anchored close to shore or obstructions you will want to know as soon as you began to get too close and in this case you will want to have your app set at the point of your boat after you have drifted back full on the road. You can set your app for an alarm at 20 or 30 feet if need be depending on how close the obstructions. If you are not near obstructions or other boats and expect a tidal change that may put you on the opposite side of where you set your anchor you may want a 200' alarm set to let you know if you are adrift after you have swung 180 degrees. For me, I want to know when we make the swing so I can check the anchor set when we are reversed position. This also should work if you are in a crowded anchorage assuming you are far enough from other boats to all swing together with changing tides and wind.
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Old 21-04-2021, 10:20   #27
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Re: Where/When to set anchor alarm

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Originally Posted by Barbaria View Post
That is good point, minimizing distance errors will give you better watch performance. We have a center cockpit and the tablet is located more to the center of the boat than usual.

What is your solution adding the missing boatlength?
Is there any anchor apps where you can offset the gps location?
The easiest way is to look at the recorded track in. Usually the point where the anchor was dropped is quite obvious. This will be at the apex as the boat is transitioning between forward motion into the wind (or current) and falling downwind.

However, the centre of the anchor alarm radius should be set forward (into wind) of this point to allow for the distance between the gps antenna and the bow.

The diagram below illustrates this method.

There are alternative methods that are sometimes useful. From your finishing position the anchor will be located the distance of the rode length plus the distance between the gps antenna and the bow (you can deduct a little for catanary and the fact that it is the base length of the triangle rather than hypotenuse that is important). The direction will be roughly into wind.

As you swing around at anchor in reasonable or strong wind the trace should be a constant distance from the alarm boundary (as post #12 shows), unless the chain is caught on debris. This is a good indication that you have set the alarm position correctly and can be used to adjust the position if needed.

In many anchorages selecting a tight alarm boundary is not needed, but in some situations an instant notification of even a slight movement of the anchor is nice to have. Modern gps units are very accurate and it is possible to select a tight alarm radius when this needed without any false positive alarms. Like most boating skills, it worth trying and learning how to do this on your equipment before it is needed in anger.
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Old 21-04-2021, 10:29   #28
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Re: Where/When to set anchor alarm

To warn of anchor dragging an alarm should be activated by the position of the anchor and here's how to estimate the anchor position:
1) As the boat swings on the anchor, record a sequence of positions along the swinging circle.
2) Compute the radius and center of the swinging circle defined by the sequence
3) Set the alarm based on motion of the circle center (the anchor position)
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Old 21-04-2021, 10:45   #29
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Re: Where/When to set anchor alarm

Thanks Mark,

I will check it out.

Cheers,
Peter
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Old 21-04-2021, 10:49   #30
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Re: Where/When to set anchor alarm

I think the OP and commenters arenít quite seeing the issue the OP is talking about. And the OPís anchor alarm software is either brain dead or he hasnít studied/learn its details.
I use iNavx for some navigation and my anchor alarm. As the OP notes, during anchoring, there are many tasks going on. Standing at the bow with your notebook as you release your anchor sounds really dumb to me - and increases the chances of dropping your tablet overboard, maybe even floating down on top of said anchor.
I set my anchor singlehanded because 99.8% of the time I am single handed. After dropping I have to run back to the cockpit to reverse engines - I drop when i come to a complete stop, and the wind has just begun to give me sternway.
After Iíve stretched the rode out, I let it sit for a few minutes, then slowly bring up the RPMs in reverse, watching reference points on the shore to confirm the anchor has set.
Then I get myself a cold beer.
I grab my Ipad, go to tge iNavx anchor alarm set up - and set the vector (distance (rode length) and direction (compass heading) of where I dropped the anchor. I bet I get the location of the anchor within 10-13 meters (say 40 ft).

I anchor in waters usyally between 5 ans 10 meters deep, over good holding on sand bottoms. Deeper water will increase my error band in exact anchor location.
The iNavx software has a quirk in setting anchor headings. Their way isnít intuitive to me, but once you figure it out its ok
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