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Old 28-02-2010, 16:35   #16
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Originally Posted by Therapy View Post
I have iPhones and have tested the apps that have anchor alarms. I'm also an iPhone developer. The iPhone's GPS has very poor accuracy inside a boat and especially below deck. I've found it to be completely worthless as an anchor alarm - I'm a liveaboard and anchor out often. If Apple allowed Bluetooth GPS's to connect to the iPhone, it would solve the proble but so far they don't allow it. I have roqyGPS now and have been testing that - it allows BT GPS on a jailbroken iPhone. That's getting outside normal though...
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Old 28-02-2010, 16:38   #17
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I didn't see any of the well deserved cheers for Dana-Tenacity's superb low tech solution. I use only a slight variation of the plan. The line from my dinghy anchor at the bow leads into my forward head where it's tied to a sauce pan. When the pan hits the head liner, I'm up! I only add this alarm in questionable situations. Usually I'm content with the gentle "Beep" from my Garmin. 'take care and joy, Aythya crew
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Old 28-02-2010, 16:42   #18
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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!
For the life of me, I can't see how that works. How exactly does the GPS know what direction the bow is to apply the 42 foot offset?

Yeah, Boot Key is great...just getting into the swing of things here after a week. I can see how you can lose a few months here! Now just give me some of that good Florida warmth!
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Old 28-02-2010, 16:46   #19
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I didn't see any of the well deserved cheers for Dana-Tenacity's superb low tech solution.
How well does it work of the current changes during the night and swings you around? That's sort of a common thing when anchored.
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Old 28-02-2010, 17:03   #20
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Jeff,

I think you over-conceptualized the concept. Just set the alarm's center for the place where the anchor is, then set the alarm's circle to just slightly more than the distance to the anchor at mid-tide.

Alternatively, if you cannot swing freely, set the alarm center for where the GPS unit is and set the alarm circle from here.

b.
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Old 28-02-2010, 17:30   #21
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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.
Just a thought.
If your anchor takes 40 ft to set and your GPS is on the stern does that not mean the anchor is where the stern was when the anchor was dropped, on a 40 ft boat.

So if anchor setting distance is S then
your anchor alarm would show you at X+2G-2S
if S is 40 and G is 40 then
your anchor alarm would show you at X,

It all depends on the bottom and a factor called dumb luck which is prevalent in the actual act of anchoring.
All equations suffer from the same problem, indentifing all variables and even then a Fudge factor is usually added as the real world is not perfect.
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Old 28-02-2010, 17:53   #22
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Rick,
How much do you have tied in parts for this alarm?

Paul L
Paul,

My cost of materials for an anchor alarm is currently $175. I know that could certainly be reduced if it was redesigned for manufacturing but I've only built the device in a few small batches.

- Rick
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Old 28-02-2010, 17:55   #23
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I think you over-conceptualized the concept. Just set the alarm's center for the place where the anchor is, then set the alarm's circle to just slightly more than the distance to the anchor at mid-tide.
It just doesn't work that way. Again, if your GPS is on your stern of a 45' boat, your error is 90 feet without your anchor dragging 1 inch.

If you drew it out on graph paper you'd see it. I know it doesn't feel like it could be that far off but it really is.
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Old 28-02-2010, 18:12   #24
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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.
There are certain merits to your solution. However, I specifically set out to develop a single-purpose self-contained portable device. I did not want a solution that relied on the boat's GPS and I did not want to have to power any other electronics. As we all know, things fail on a boat. My device only draws about 125mA and it is not dependent on any other hardware or software. And, as you've noted, the geometry is simple because I take it to the bow while anchoring and mark the anchor position just as it touches bottom.

Of course, the anchor doesn't set exactly where I've marked but I immediately know the difference because my device always shows the distance between the anchor mark and its current position. While it is still draped over our bow flag, we read the anchor alarm's calculated distance as we back down to set our anchor. If I have 100 feet of rode out and the distance reads 120 feet while backing down to set, I know my "anchor mark error" is 20 feet. I simply add that distance to the rode length plus boat length (I move the alarm to our aft stateroom) plus GPS error for the radius of my circle of swing. The geometry is simple.

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Old 28-02-2010, 18:24   #25
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Are you selling PCB & components kit?
To date, I have made just two small batches of the anchor alarms. In exchange for comments and feedback, I have been selling the alarms for my cost of materials. Although I do still have a few units available, that was not the purpose of this thread. Current users have had very good comments so I need to decide whether to keep this a hobby or redesign so they can be built faster and cheaper.

I really want to learn what others are using for an anchor alarm, whether it works and what is the cost. I understand many chartplotters have an anchor drag feature. But as far as I know, chartplotters aren't cheap. And what are their power requirements?

- Rick
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Old 28-02-2010, 18:32   #26
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I really want to learn what others are using for an anchor alarm, whether it works and what is the cost. I understand many chartplotters have an anchor drag feature. But as far as I know, chartplotters aren't cheap. And what are their power requirements?

- Rick

That is my deal and like I said the handheld uses up a set each night.
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Old 28-02-2010, 18:39   #27
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However, I specifically set out to develop a single-purpose self-contained portable device.
I think what you've done is quite cool. I've tinkered with hardware and know how rewarding it feels to see something real working (www.activecenter.com).

I'm just a fan of making my phone do more and then just working out the ways to power it - since I need to keep it alive already. You don't need to rely on the boat's GPS either. I often use a Bluetooth one or one that's built into most phones today. The built in ones usually don't work well below deck though.

I anchor out a lot and love to find ways to make it safer, allow me to sleep better, and become aware of issues before they happen.
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Old 28-02-2010, 18:47   #28
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That is my deal and like I said the handheld uses up a set each night.
I can plug my Garmin handheld into 12V power so I'm not using the internal batteries. But I have a number of problems with the anchor drag feature of the Garmin. First, I can't easily see the lat/long of the marked anchor position and there is no way to see my current distance to that position. My Garmin gets poor satellite reception in my aft cabin where I sleep. And finally, it's audible alarm is not very loud. I wanted a better solution.

- Rick
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Old 28-02-2010, 19:02   #29
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For the life of me, I can't see how that works. How exactly does the GPS know what direction the bow is to apply the 42 foot offset?

Yeah, Boot Key is great...just getting into the swing of things here after a week. I can see how you can lose a few months here! Now just give me some of that good Florida warmth!
Sorry...I wasn't clear. I meant I would do the correction. As in, on paper. I was mostly just kidding...I'm never that precise about anything!

I hope the warmth comes back in spades soon. Ah...those Boot Key harbor nights...I can hear the high tension lines along the highway buzzing now...
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Old 28-02-2010, 19:22   #30
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I'm just a fan of making my phone do more and then just working out the ways to power it - since I need to keep it alive already. You don't need to rely on the boat's GPS either. I often use a Bluetooth one or one that's built into most phones today. The built in ones usually don't work well below deck though.
I think phones are an interesting platform. But would you build an anchor alarm application for a Windows-based PC? I'm a software engineer and I would not for fear of crashes, viruses, or some other hardware/software incompatibility that might halt my application. How long before phone operating systems inherit the same problems and complexity? I like programming a microcontroller to do one thing and one thing only with no operating system or other software to interfere.

- Rick
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