Anchor Alarm

[LONG_KEELER]

I was enjoying a really nice dream this week and the anchor alarm went off and I hit my head on the deck head.

What distance do you input into the alarm ? I'm wondering if the GPS signal is more variable than we think.

I'm thinking length of scope plus length of boat plus a tiny bit more .

 

[Neeves]

It depends on where you have placed the anchor on the chart plotter, and where your GPS aerial is located. Rather than using yacht length you might want to use distance from bow to aerial. But you could use yacht length as this might include your 'little bit more'. You already have a little bit more if you use rode deployed - which will vary with depth, the deeper the water the more pronounced the catenary and the larger the 'error' between rode length deployed and distance from and anchor. If you set the location as being when you drop the anchor the GPS will record that position as the location of the GPS aerial, not the anchor - but maybe you can set an offset on your alarm.

And you thought this was an easy question to ask (and have answered)

Maybe the distance you need to increase is the space between you and the deck head

You will lose signal accuracy if you are in an anchorage surrounded by high steep hills.

You might also decide to increase your 'little bit more' if you have lots of sea room and vice versa

Our alarm on the MFD is pretty quiet - we introduced a piezo electric alarm (instructions were in the manual) now it also stimulates heart beat. Having said that we seldom set the anchor alarm (don't do as I do)

Sincere apologies but I have made this very complicated for you. If you have a decent anchor, are using a sensible scope and the seabed is known to be sound (holding good) - don't worry, just make a rough guess, drink some malt whisky and enjoy the night at anchor.

Jonathan

 

[Graham376]

Usual setting for us on 5:1 if not too crowded is 0.03NM but we set alarm as anchor hits bottom, not when we've dropped back.

 

[mainsail1]

I have found that so long as I give the alarm 40 meters swinging circle I get a good nights sleep. The trick is to make the anchorage suit rather than the other way around.

 

[GHA]

I was enjoying a really nice dream this week and the anchor alarm went off and I hit my head on the deck head.

What distance do you input into the alarm ? I'm wondering if the GPS signal is more variable than we think.

I'm thinking length of scope plus length of boat plus a tiny bit more .

Usually 2 or 3m more than distance to the hook when it was dug in.

 

[LONG_KEELER]

It's encouraging by the feedback that GPS positions are reliable and steady enough to
use for anchoring.

Many I am sure have depended on the upper and lower settings on echo sounders but I was determined to have a go with the new technology this season.

You are probably going to get notice quicker than depth data when anchoring close to shipping movements or harbours . The 40m setting sounds like a good compromise for most yachts and I will be upping mine to that figure.

 

[Graham376]

You are probably going to get notice quicker than depth data when anchoring close to shipping movements or harbours . The 40m setting sounds like a good compromise for most yachts and I will be upping mine to that figure.

One point to remember is that gps mushroom is often mounted on taff rail, in our case 12m from bow. When the tide turns, gps senses chain length + 12m change in position.

 

[RupertW]

One point to remember is that gps mushroom is often mounted on taff rail, in our case 12m from bow. When the tide turns, gps senses chain length + 12m change in position.

Exactly - it’s usually chain length plus boat length (minus a bit for depth of course but I’d leave that in anyway) plus something for those big GPS glitches of 10 metres or more I occasionally see. So for 30m chain out I would usually have 60 or 70m circle with best guess for the anchor position. If the anchorage allows it - if I have to be more precise due to proximity of rocks or shore in one direction I will tend to keep the wide circle but move what it thinks is the anchor position away from the obstacle.

 

[dunedin]

I have switched to using the Anchor Alarm app on my iPad, as
- it allows you to draw any shape of warning zone you want - so generally give it a banana shaped arc, but can make full circle if expecting wind or tide change, or circle with a chunk removed if one isolated danger. So much more flexible
- uses much less power than leaving instruments and plotter on overnight

Regarding GPS accuracy, ran the Anchor Alarm for 48 hours during a period of strong winds / near gale, and the anchor held perfectly and over the 48 hours never had any position more than 10m away from the arc we were swinging to.

 

[MathiasW]

One feature I would not want to miss from an Anchor Alarm is the history of vessel movements. You can then easily see whether the alarm is because of more wind shift than you had expected, or a real drift alert. Seeing the track history also allows for re-arming the anchor alarm once it is clear where the vessel is hovering. So, I usually check before going to bed how the track history relates to my alarm perimeter and adjust, if needed...

 

[Bathdave]

What is your preferred iPhone/iPad app ?

 

[dunedin]

See

What is your preferred iPhone/iPad app ?

See #9

 

[LadyInBed]

I'm thinking length of scope plus length of boat plus a tiny bit more .

OK if you set it as your anchor hits the water.
I tend to set mine when the anchor is set and use about twice the above distance to allow for tide turn when the weather is settled, but when the wind is gusting I set the alarm short and reset it when the tide turns as I would want to be awake then.