Anchor Alarms

Tim R. · Jul 28, 2009

I don't use mine any more unless it is supposed to really blow up at night. Set it after you set you anchor. That way it will alarm if you swing too far.

As for other boats, you really just have to guess. Shouldn't be a problem if no one swings.

The only anchorage that I frequent near home that can sometimes get very crowded has a piling I tie to when the low tide isn't too low.

Ron20324 · Jul 28, 2009

If you have all-rope rode, when you power in reverse as far as possible to set the anchor, you will then come forward when the engine is off and settle into your normal position, hit the alarm "set". I back up again under full power to make sure the alarm does not sound. Of course you may swing farther than the alarm allows and, again, you'll have to change the range to accept how far you swing.

If you have a "save track" option, start to save the track. It will show a track as you swing, and that is okay. If the arc starts moving rearward, you're dragging.

Ross · Jul 28, 2009

I swing with the tide and or the wind. Remember one second of lat. or lon. is about one hundred feet. With only fifty feet of rode out you can swing through a one second change in position. I just record my position and check it every now and then. I can sit in my driveway with my hand held GPS and drift around according to the gps.

Ron20324 · Jul 28, 2009

Bob S said:
Good idea Ron, I like the save track feature. If I let out a 100' would i set the alarm at 200' allowing a complete swing?

Bob, I had to re-do my reply. Slight error. I would not allow 200 feet, as you would want to know of more than say, a 90-degree swing/the 100 foot rode. Just in case you need to check for anchor pullout.

Stu Jackson · Jul 28, 2009

After a discussion of this same subject last week, in which I was "roundly defeated" on my insistence that it makes little sense to set anchor alarms, I went out and anchored over night.

Ron's suggestion for using the TRACK feature is the very best advice I can give.

It shows you where you were and where you are.

I awoke at 0400, checked it and went back to sleep.

Other boats who come in later...?

That's another story...

Ron's right.

This ALL assumes you've set YOUR own anchor properly.

This also suggests that I get to go out, yet again, and use you guys as an excuse to my wife that I HAVE to use the boat again to try out the nifty anchor alarm system on our Garmin GPS Map76Cx.

Please, do like my physical therapist says: "It is GOOD for your broken leg to go out sailing more often!"

For a small price, I will give you the name of my physical therapist.

Ross · Jul 28, 2009

Sometimes I anchor in ten feet of water about two hundred feet from shore. My bow roller is four feet above the water so a 7:1 scope is about one hundred feet of rode. This is very sheltered water so I still sleep well. The anchor alarm would go off about the time my keel hit the bottom. Each time is a judgement call. How much sea room do you have, how reliable is the holding, how reliable is your ground tackle and finally what is the weather forecast?

Craig Smith · Jul 28, 2009

Bob S said:
My hand held Garmin has an anchor alarm feature. Is there a rule of thumb when setting it? If I let out a 100' of anchor rode how would you set it? My wife & I spent Saturday night on the hook but the wind was steady S SW all night. I wanted to try this feature but didn't want it going off all night if I set it wrong.

Some of the responses on this thread are a tad off.

GPS anchor alarms work by alerting you if the transponder strays outside of a given circle, defined by the initial set position and a radius. This is designed to equate to your anchor and swing radius.

Many GPS units do not allow you to manually set the position of the anchor, which means that you need to have somebody hit the "anchor" button at the location you drop the anchor. Ideally, for exact accuracy, the unit will also know where on your boat the transponder is (presumably not on the bow) so as to account for this offset. (Conversely you may expect the anchor, depending on type, to drag some distance before setting.) You then set the radius to the length of the rode (which is longer than the horizontal distance to the anchor, but conveniently provides a small margin of error). Straighten the rode without the alarm sounding to check you haven't underestimated the rode length.

GPS may not be centimeter accurate, but this doesn't really matter because it doesn't "wander" much over a short time frame, and for checking that we're not moving we only need an accurate relative coordinate system. So, a properly set GPS alarm can be extremely effective. It can alert you if the boat moves so much as a few meters outside the possible rode swing circle, which in turn says the anchor has moved or the rode has failed.

Read your GPS manual to cross-reference the above comments.

jackhartjr · Jul 29, 2009

I would think you would 'set' it at the location the handheld be resting at.
If you 'set' it at the bow and then walk back to the rear would that not affect the alarm?
Jack

Craig Smith · Jul 29, 2009

Bob S said:
Thanks Craig, I had to read this a few times to digest it but it makes sense. Great advise!
Seeing I'm using a handheld I could carry it to the bow when setting out the anchor.

Quite right, but try to predict where the anchor will end up. And don't try to set it too restricted, most anchors (even the Rocna

) will move a little before re-setting in some circumstances.

jackhartjr said:
I would think you would 'set' it at the location the handheld be resting at.
If you 'set' it at the bow and then walk back to the rear would that not affect the alarm?
Jack

No - you just add the distance, from the bow to the GPS location, to the radius. All the GPS needs to know is 1) where the anchor is and 2) how big a swing radius it has to play in.

Tim R. · Jul 29, 2009

Why risk dropping it overboard. Have the person at the helm set it. For most units, once you get to the appropriate screen you just press a button to activate.

Ron20324 · Jul 29, 2009

Right, Jack and Tim... Craig, it's just a matter of personal preference, but your saying " Some of the responses on this thread are a tad off." is out of line. The GPS will track from wherever you hit the "set" button. If you set it where the boat comes to rest, it will sound if you swing more than roughly 90 degrees, which probably is more than the typical sailing at anchor- especially if you have an anchor riding sail.

For me, if my boat was 200 feet from where I left it and facing in the opposite direction, I would want to know it so I can see where I am, if my anchor let loose, and see where any other boats around me are. That's MY preference.

Tim R. · Jul 29, 2009

Tim R. said:
Set it after you set you anchor. That way it will alarm if you swing too far.

Hmmm, sounds familiar

Dick McKee · Jul 29, 2009

Bob... I'm not going to get in the alarm discussion, but as to others in the anchorage, I motor over to any that will be close, say within 100 yards, and ask them. We have been doing this 30 years and I am always suprised that about 25% of the folks don't know how much rode they have out!

As to the alarm, just remember there is no replacement for a well set heavy anchor. Our boat would be ok with a 50# anchor, but we have a 75# on 300' of chain and a 60# with 150' of chain and 150" of 3/4 rode. We also use a 25' snubber, and sleep very well at night.

Rick Webb · Jul 29, 2009

Make Sure You Can Hear it.

I was on a cruise with some older guys who had no idea the anchor alarm was going off. We where anchored off of Captiva and a front was coming through we decided that we should stand watches through the night. I was sleeping in the salon and was wondering why there was nothing happening when the alarm went off. I got out of my rack and asked my shipmate he said "What alarm?". We had swung a little bit but where OK we reset it and I crashed again. Two hours later we had swung back and the next guy on watch was equally oblivious and gave me the same reply, "What alarm?".

Ross · Jul 29, 2009

Re: Make Sure You Can Hear it.

I remember reading that we should take bearings on three distant points and record them to be able to check for anchor drag. Pre GPS days. We can get too dependant on modern toys.