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Old 27-02-2010, 16:37   #1
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Any Comments on My Anchor Alarm?

Anyone who spends any time at anchor knows how uncomfortable or impossible it can be to sleep when the conditions are less than perfect. I dragged anchor once in the night and once was enough. I developed my own GPS-based portable anchor alarm for insurance and peace of mind and it has worked very well for me and others who have been testing the device.

If anyone has the time to read about my anchor alarm, I would really appreciate any comments, ideas or suggestions. The Userís Guide offers the best overview. Right now I donít know where I am headed with this project or even if the device could be manufactured at a reasonable cost. But I thought this forum would be a good place for a reality check.

What are you using for an anchor alarm and does it work? Will your current alarm wake you and can you easily verify you havenít dragged? Does my device have any advantages over what you are already using for an alarm? Here's a link where you can read more:

Boathooked.com: Projects Index

- Rick
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Old 27-02-2010, 20:06   #2
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Rick,
How much do you have tied in parts for this alarm?

Paul L
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Old 28-02-2010, 08:57   #3
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I've been experimenting with a variety of ways to implement an anchor alarm on a mobile phone. To be honest, I think it's a better platform for it. You already have a way to keep it charged, it's portable, sits right next to me at night in the stateroom (easily waking me up if it needs to) and has much better graphics and output capability. Plus, the whole thing can be done in software significantly reducing the cost.

GPS connectivity can happen through Bluetooth allowing the phone to work within about 30 feet of the GPS (good enough on boats less than 60' in length or thereabouts). I've been using prototypes of this on my boat for the last 3 years. There are some geometry/trigonometry issues to work out since the GPS isn't at the bow creating a 2x distance error when the boat swings. With a heading sensor, that error can be totally eliminated. Even without a heading sensor, I can remove the error down to 1x the distance of the GPS to the bow.

Anyway...it's just an alternative. But I like it better because I don't want yet another device to keep batteries for, re-charge, or plug in. I already have that solved with my phone.
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Old 28-02-2010, 09:09   #4
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There are some geometry/trigonometry issues to work out since the GPS isn't at the bow creating a 2x distance error when the boat swings. With a heading sensor, that error can be totally eliminated. Even without a heading sensor, I can remove the error down to 1x the distance of the GPS to the bow.
Just by using the normal anchor alarm function of most GPSs at the second the anchor lands on the bottom and the rode is straight up and down (instead of after all is set and stowed), you eliminate almost all the problems you are both describing. The boat will then simply circle around a point, and the watch feature can be set EXTREMELY accurately. What other features would you add to this?
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Old 28-02-2010, 12:47   #5
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I use both the GPS anchor alarm (circle) and the depth (low, high) alarm. None is perfect when the conditions are light but hey both are very good when it is blowing.

I know there is also an alarm that beeps when the anchor drags, but bet it will be pretty expensive.

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Old 28-02-2010, 13:14   #6
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Just by using the normal anchor alarm function of most GPSs at the second the anchor lands on the bottom and the rode is straight up and down (instead of after all is set and stowed), you eliminate almost all the problems you are both describing.
You're right that it's the way that almost all anchor alarms with on every chartplotter and software I've ever seen work. Unfortunately, it creates an error of double the distance between the bow and the GPS location as the boat swings around.

Here's an example. Let's say you're anchored in 8 feet of water and your bow is 4 feet off the water. You decide you'd like 5:1 scope so you put out 60 feet of chain. You're on a 45' sailboat with the GPS on the stern (the worst of all possible conditions to prove the point, but not uncommon). You set the anchor alarm to 90 feet because you certainly shouldn't drift back 30 additional feet. In the middle of the night, the current or wind changes and you swing to the other side. Immediately you have 90 feet of error because of the 2x distance to the GPS. Now you drift back 10 feet (completely normal) and the anchor alarm goes off. In reality, your rode is not even stretched out all the way out and your anchor certainly hasn't dragged an inch.

If you've ever set a traditional anchor alarm, you'll notice that when you swing around, you magically have to increase the alarm distance. This is why. It's quite easy to show how it happens in a diagram.
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Old 28-02-2010, 14:33   #7
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Jeff, I understand where you are coming from, but thats not the right way to use an anchor watch. Hit the watch button when you first drop the anchor, when it's straight up and down. Now, my Manson supreme is gonna set in less than 15' or so, and the anchor alarm will not sound once it's set for the overall radius, no matter which way the boat swings...follow me?
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Old 28-02-2010, 14:39   #8
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I too have used Christian's method for as long as I have had GPS on board and it has worked well. Panbo reported on a nifty mobile alarm a while back, Panbo: The Marine Electronics Weblog: Anchor alarms, apps style
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Old 28-02-2010, 15:40   #9
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Jeff, I understand where you are coming from, but thats not the right way to use an anchor watch. Hit the watch button when you first drop the anchor, when it's straight up and down. Now, my Manson supreme is gonna set in less than 15' or so, and the anchor alarm will not sound once it's set for the overall radius, no matter which way the boat swings...follow me?
It is that simple.
A lot of GPS's have an output for a "noisemaker". A 110 dB noisemaker wakes you up where ever you are. The less the number of parts the less chance of failure.

Should you be in poor holding and a squall blows in with high winds you have a point to aim towards should you believe your anchor may drag and wish to use the engine to reduce the strain.
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Old 28-02-2010, 15:45   #10
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Put the dinghy anchor over the bow, pass the rode through the forward hatch and tie it to an empty jerry jug, If you drag the jerry jug will start banging around in the forepeak and wake you up. Cost =0.
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Old 28-02-2010, 15:49   #11
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Should you be in poor holding and a squall blows in with high winds you have a point to aim towards should you believe your anchor may drag and wish to use the engine to reduce the strain.
Now THAT is smart! Good thinking.
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Old 28-02-2010, 15:54   #12
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Question Looks Very Interesting!

Are you selling PCB & components kit?
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Old 28-02-2010, 16:02   #13
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I don't want the current draw of the main gps unit on all night.

When I use the hand held it eats a set of batts over night.

I don't have a fancy new fangled phone but I have seen some apps that are absolutely amazing.

I may end up "moving up" in the technology one day though.


There is this
Anchor Alarm on iPhone
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Old 28-02-2010, 16:12   #14
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Jeff, I understand where you are coming from, but thats not the right way to use an anchor watch. Hit the watch button when you first drop the anchor, when it's straight up and down. Now, my Manson supreme is gonna set in less than 15' or so, and the anchor alarm will not sound once it's set for the overall radius, no matter which way the boat swings...follow me?
I follow you completely but I'll bet you didn't draw out the diagram to see the real distances. You might think that you won't drift past the radius but that will only mathematically happen if your GPS antenna is on the bow, and I doubt it is.

I'm a software developer and I've been working on this problem for 3+ years. There was just a discussion on Panbo about it and at the end of the discussion, Ben admitted that I was right.

Draw it out with measured distances and you'll quickly see that I'm right. Remember that the GPS position recorded is the position of the GPS and not the anchor.

If you put out X feet of rode and you swing around, your anchor alarm will show you to be X + 2G feet back where G is the number of feet from the GPS antenna to the bow. In most cases, that G distance is significant.
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Old 28-02-2010, 16:33   #15
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Got ya. Believe me, I know you're bright. Your website is fantastic and what you've accomplished wasn't easy. Now I know what you're getting at. When I press the button, I already have an offset of 42', because my antenna is on the stern. So, to be totally accurate, I must take the Lat/Lon and remove this error, then use the alarm set to this position. To be honest, I normally dont split hairs like this when anchoring, but as a technical question, you are infact quite right! I guess I could stern anchor... BTW, I lived in Boot Key for several months a while back...have a great time!
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