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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Drag Alarm

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Rich G
Navigator

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226 Posts

Initially Posted - 07/15/2009 : 08:28:27

Where do you set your drag alarm limits on your GPS? I can't figure out how to calculate the limit to allow for a full tidal swing, yet still alert me to dragging anchor. It seems like it should be the distance to the boat from the anchor triangulated from depth and rode length, no?

Eg-
Depth: 10'
Rode: 70'

100 + b^2 = 4900

b = approx 69'

So 69' is the radious of the circle you might swing. Double that to get the diameter = 138'.

Set drag alarm to 140'

Am I making any sense? Is my math wrong?

Rich

Cerberus
Hull #4695
1984 SR/FK No Pop-Top

Edited by - Rich G on 07/15/2009 08:32:34

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/15/2009 : 09:12:42

If you swing 180 deg. with the tide or wind, your anchor might move a few feet as it resets to the new direction. It should re-set within 5-10'--if it doesn't, you want to know. In your scenario, I'd probably set the alarm for 150'.

BTW, are you aware that in 10 feet of water with 70' of rode out, you have only 5:1 scope--not 7:1? (I'm figuring roughly 4' for your freeboard at the bow--that might be a little high, but you get the idea.)

Edited by - Dave Bristle on 07/15/2009 09:14:43

delliottg
Former Mainsheet C250 Tech Editor

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Response Posted - 07/15/2009 : 10:52:08

To make the calculation simple, I double the length of my rode and add 10% to allow for a bit of drag while resetting. While you could do more complex calculations for the hypotenuse of the resultant triangle (and it's really a catenary, but I don't remember how to calculate those), by using the actual length of the rode, you've built in a bit of wiggle room to your calculation (if the rode were pulled out straight your swing radius would be larger). That way you've got a reasonable amount of slop set in your alarm to account for changes in depth due to tides (and shortening or lengthening your radius), plus the 10% for drag.

Please feel free to punch holes in my practice, anchoring is one of my least used sailing skills, so I won't be offended.

All that said, I still sleep with my GPS right next to me and tend to wake up every so often to check it. I'll also get up at least once during the night to check my position against the trees or whatever I've noted at the anchorage as a back up to the electronics. I keep a zillion candle power spotlight connected at the stern to allow me to see if it's a moonless or overcast night.

Edited by - on

redviking
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Response Posted - 07/17/2009 : 12:23:58

There is no substitute for experience. A friend calls me AnchorSten after the similarly named beer becasue of my passion for dropping a solid hook. Set the alarm for 150 or so and see what happens. That's usually not enough for me... OT sorta - Blue Water Sailing Mag had a long article on APPS for the Iphone - anchor alarm $9.99...

sten

Edited by - on

Stu Jackson C34
Admiral

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Response Posted - 07/20/2009 : 09:10:45

Anchor alarm? For what? If you set your anchor properly, you shouldn't be going anywhere. See replies #33 and #47 here: http://c34.org/bbs/index.php/topic,2705.30.html

Edited by - on

Steve Milby
Past Commodore

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Response Posted - 07/20/2009 : 11:59:02

Great info, Stu! Thanks for posting it.

Edited by - on

redviking
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Response Posted - 07/20/2009 : 12:29:55

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stu Jackson C34
 Anchor alarm? For what? If you set your anchor properly, you shouldn't be going anywhere. See replies #33 and #47 here: http://c34.org/bbs/index.php/topic,2705.30.html
<hr height="1" noshade id="quote"></blockquote id="quote">

Anyway, none of these folks talk about chain nor do they mention sharpening your hook if you are going to be anchoring in different conditions. The last post is from the guy who supposedly wrote a book, ok good... come hang with me for a few months cruising. If your new Rocna dragged, then why didn't the CQR's in the neighborhood? Don't get me wrong, as soon as I have a freebie or a few extra bucks I will gladly try it - it looks cool. But my 35 lb. CQR on a lot of chain has never dragged moer than a few feet when resetting. I left my wife on the hook in Newport in August for 2.5 weeks with nothing but 35 lb. CQR and some chain. Try that in terms of faith, and then come talk to me...

Also, you should never back down until your anchor sets naturally. Think about it, did big sailing vessels of years gone by back down - hell no. They eased the anchor in and it held. Back down after 10 minutes if you wish, but not a moment before.

But as I have learned the hard way, don't always trust the Catalina 47 owner next to you to know how to anchor. Statistically they don't and I would not give the cumulative data much thought.

AnchorSten out!

Edited by - redviking on 07/20/2009 12:36:26

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 07/20/2009 : 16:17:31

Edited by - on

Stu Jackson C34
Admiral

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844 Posts

Response Posted - 07/21/2009 : 13:47:02

Good point Dave. And you're certainly right about the mud - I'm still wiping it off... We're also "blessed" here in N. California, without thunderstorms and high winds that can come up anytime. Your experiences with those puppies is something I do not envy.

Two or more solutions for me: choice of anchorage -- many of ours are within the currents of San Francisco Bay, but out of the mainstream, in back eddies where the boats don't swing to a current change; Clipper Cove and Horseshoe Cove come to mind. At China Camp and Paradise Cove, where the currents are parallel to the shore, they change every six hours or so. I anchor with my Rocna, back down hard and long to properly set it, check ranges, check again when the current changes, and sleep very soundly.

I believe it's the anchor, as well as the ORIGINAL setting technique.

I only use a small Danforth for a stern anchor, if the particular location suggests one.

I don't depend on an anchor "drag" alarm to tell me anything - I just anchor, firmly.

Sometimes, for grins & giggles, I'll put the track feature on and check it the next morning.

All I'm sayin' is that there are a new generation of anchors out there that bear serious consideration.

Edited by - Stu Jackson C34 on 07/21/2009 13:49:34

piseas
Former Treasurer

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2017 Posts

Response Posted - 07/21/2009 : 15:26:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stu Jackson C34
 All I'm sayin' is that there are a new generation of anchors out there that bear serious consideration.
<hr height="1" noshade id="quote"></blockquote id="quote">

Stu, The reports I have seen do confirm these new high tech anchors are good and bear serious consideration as well as bear serious bucks! Don't think most on our site would consider. We live for the cheap seats!

Steve A

Edited by - on

redviking
Master Marine Consultant

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Response Posted - 07/21/2009 : 18:23:23

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stu Jackson C34
 I don't depend on an anchor "drag" alarm to tell me anything - I just anchor, firmly.

Sometimes, for grins & giggles, I'll put the track feature on and check it the next morning..
<hr height="1" noshade id="quote"></blockquote id="quote">

Now I know why I have only been hit by BendyToys, Catalina's and derelict boats. Anchor long enough and you will drag sometime, somewhere. The resistance to using available technology is remarkable - I mean, do you think you are invincible? I don't and we set the anchor alarm almost every night.

AnchorSten

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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4479 Posts

Response Posted - 07/21/2009 : 19:12:53

I agree with Sten, if the technology exists why not have the electronics on your side? I bought my GPS specifically because it had a drag alarm and my other two didn't. I don't know that I'd ever sleep completely soundly at anchor (unlikely since after my shoulder repair I never have anyway), but I know that I can check my GPS from my rack and be reasonably assured that I'm where my electronics say I am. I have yet to use my new (to me) Fortress anchor, but I have high hopes for it. I know last time we anchored out, we definitely dragged when the tide changed, maybe 15-20 yards south of our original position. My alarm went off twice that night which resulted in a fair amount of time sitting in the cockpit watching the shoreline to see if we were still dragging after letting out more rode.

Edited by - on

britinusa
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5404 Posts

Response Posted - 07/22/2009 : 04:28:49

Makes me think of the Borg!

Resistance is Futile!

I'm with Dave's comments, which came first, an Anchor or Insomniac.

100' of anchor line = 100' of alarm, becuase I know that then the wind changes then the boat will re-hook and I want to be sure it does.

It's fun trying to figure out the actions of the night-time swings around the anchor by viewing the GPS.. Could be a form of Art.

Paul

Edited by - on

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 07/22/2009 : 07:14:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by britinusa
 ...It's fun trying to figure out the actions of the night-time swings around the anchor by viewing the GPS...<hr height="1" noshade id="quote"></blockquote id="quote">Absolutely! From the track, I can see exactly where I dropped the hook, and then from the arc as the boat swings on it, I can imagine intersecting radii to where it actually set. If I swing 180, I can again use radii to see if it re-set in a new location.

Edited by - Dave Bristle on 07/22/2009 07:16:09

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