Calculating the anchor distance from the boat

[Irish Rover]

I'm using Anchor Pro to monitor my anchor position. I drop my anchor with an anchor buoy attached and put out minimum 3/1. My chain is well marked so I've no difficulty knowing how much I have out and the buoy makes it easy to know the bearing. But what distance to set on Anchor Pro - the length of the chain I've out or the distance to to buoy sitting directly above the anchor? What is that distance and how do you calculate it? I currently have a bridle extending 3/4m beyond the bow and attached to the chain at the 30 metre mark. Depth is 9M and the tablet is in the cockpit about 5 meters aft of the bow. So 30 +4 + 5, say 40 meters.
I know I can set any distance I like and monitor by reference to that but if I was being precise what distance should I set and why?

 

[Arcady]

Basic Pythagorian geometry + the horizontal offset to your tablet. (I’m assuming you have an echo sounder for vertical height above seabed).

Edit - Before someone points it out: I know I’m ignoring catenary in the above.

 

[RunAgroundHard]

Precision fails when the wind and current start acting on your boat. Hence a rule of thumb figure would be better.

You could note lat and long when letting go, then lat and long when settled and calculate departure, if you have a gps.

Modern plotters will do this for you.

Basic Pythagorian geometry + the horizontal offset to your tablet.

He is looking for precision, so Pythagoras won’t work because of the catenary. He only knows depth, not displacement.

 

[john_morris_uk]

Sorry to drift the thread but 3 times depth is a bit on the light side unless it’s really benign.

To answer your question, I’d set the limit to be the scope of the chain deployed plus boat length centred on the anchor and not worry about the catenary. This will allow a small margin for the dithering of your position and it’ll soon say if you’re dragging outside the circle. Trying to be more precise would be a waste of time IMHO.

 

[Irish Rover]

Where I am right now is a bit sloppy but apart from an hour last night the anchor chain isn't even extended. I'm almost on top of the anchor buoy most of the time. I checked the anchor after setting and it's buried deep in the sand.
So I have 30 metres of chain out plus 4 metres bridle plus 10+ metres my boat so you suggest 45M??

 

[Arcady]

He is looking for precision, so Pythagoras won’t work because of the catenary. He only knows depth, not displacement.

Then I guess we need to know the weight of chain, cross sectional area of the boat, strength of the wind, currents, inside leg measurement, and a host of random variables, to be precise ;-)

 

[Boathook]

@Irish Rover I thought that these anchor alarms that use GPS should be 'set' for where the anchor is, so if the boat swings to tide or the wind alters the alarm isn't triggered.

Therefore your 30m of chain plus bridle, plus back to where the 'alarm' is on the boat plus a margin for error possibly comes to around 45m.
Some of the places I have anchored an alarm wouldn't be any good as I would be aground !

 

[john_morris_uk]

Where I am right now is a bit sloppy but apart from an hour last night the anchor chain isn't even extended. I'm almost on top of the anchor buoy most of the time. I checked the anchor after setting and it's buried deep in the sand.
So I have 30 metres of chain out plus 4 metres bridle plus 10+ metres my boat so you suggest 45M??

If you set it to much less and it starts to blow, your alarm will go off unnecessarily. If you’re totally confident it’ll only blow from one direction you could reduce:juggle the figured a bit?

 

[Irish Rover]

@Irish Rover I thought that these anchor alarms that use GPS should be 'set' for where the anchor is, so if the boat swings to tide or the wind alters the alarm isn't triggered.

Therefore your 30m of chain plus bridle, plus back to where the 'alarm' is on the boat plus a margin for error possibly comes to around 45m.
Some of the places I have anchored an alarm wouldn't be any good as I would be aground !

You also set the permitted angle of swing. I've mine set 0 - 359⁰.

 

[dukeofted]

On the anchor pro app I try bring the phone to the bow when dropping the anchor and hit the button when I drop to get the location of the anchor. That way I don't need to calculate anything.

 

[Boathook]

On the anchor pro app I try bring the phone to the bow when dropping the anchor and hit the button when I drop to get the location of the anchor. That way I don't need to calculate anything.

You must be able to set a 'range' otherwise the alarm will sound as soon as you set out a load chain / rope and move back from the anchor position.

 

[Minerva]

I don't often set an anchor alarm, but when I do, I approach the anchor spot upwind, drop anchor and drop back. The snail trail on the GPS will have a distinct peak. Drop the marker on this peak and set the anchor alarm radius to be equal to the amount of chain out. No need to complicate things

 

[dukeofted]

You must be able to set a 'range' otherwise the alarm will sound as soon as you set out a load chain / rope and move back from the anchor position.

Yes, I set the range after I have got all of the chain I'm using out. You can edit the range to be your current position plus a bit of buffer. If you set the alarm when you drop the anchor it will display the range to that point as you drop back.

 

[sailaboutvic]

Rover a very easy way is once the anchor is set , go astern till the chain is tight and then look at the app where it is within the circle then allow what ever you feel you need .
But as already said 3.1 isn't the scope you want we nearly work on 5.1 for lots of reasons and only reduce it in very tight anchorages when we sure it's going to be a quiet night or else we move on

 

[Irish Rover]

Rover a very easy way is once the anchor is set , go astern till the chain is tight and then look at the app where it is within the circle then allow what ever you feel you need .
But as already said 3.1 isn't the scope you want we nearly work on 5.1 for lots of reasons and only reduce it in very tight anchorages when we sure it's going to be a quiet night or else we move on

Thanks Vic. 3/1 is minimum when I'm in a mill pond. 40 metres of chain out right now in 6 metre depth in a bay in SE Mykonos where it's blowing 15- 20 knots.
My original question was just me ruminating with nothing better to do and so far I'm not sure I got an answer to the question I thought I was asking.

 

[sailaboutvic]

Thanks Vic. 3/1 is minimum when I'm in a mill pond. 40 metres of chain out right now in 6 metre depth in a bay in SE Mykonos where it's blowing 15- 20 knots.
My original question was just me ruminating with nothing better to do and so far I'm not sure I got an answer to the question I thought I was asking.

As you know we lived for many many years 9 months a year on our hook and since anchor app turned up and we started using them , that's the best and easy way I found of doing it . Forget all the calculations, leave that to a mathematical guys ,
Keep the range wide so the alarm don't go off drop anchor ,
set, then set the range to where the boat is , add whatever and forget , job done .
It's never fail us .

 

[Crinan12]

Yeah me too

On the anchor pro app I try bring the phone to the bow when dropping the anchor and hit the button when I drop to get the location of the anchor. That way I don't need to calculate anything.

 

[alahol2]

I don't often set an anchor alarm, but when I do, I approach the anchor spot upwind, drop anchor and drop back. The snail trail on the GPS will have a distinct peak. Drop the marker on this peak and set the anchor alarm radius to be equal to the amount of chain out. No need to complicate things

You need to drop the marker forward of the 'peak' because your GPS aerial is likely to be several metres behind the bow. Otherwise that's what we do.
Also for completeness... approach upwind or up tide (or some combination of the two)...

 

[Irish Rover]

You need to drop the marker forward of the 'peak' because your GPS aerial is likely to be several metres behind the bow. Otherwise that's what we do.
Also for completeness... approach upwind or up tide (or some combination of the two)...

All very complicated, I have to say. And, as for bringing the phone to the bow when dropping the anchor, forget it - I carry a good amount of spares but not phones. I also deploy the anchor from the flybridge cockpit. I'm happy enough with my system using my chain markers [cable ties] and anchor buoy which is really for recovering a fouled anchor. But for simplicity personified @sailaboutvic wins the biscuit.

 

[noelex]

To work correctly the anchor alarm should be centred over the anchor.

Setting the anchor alarm when the anchor is dropped is not accurate (if no error correction is made) if the gps aerial is located away from the bow, especially if it is near the stern of the boat, as it is in many cases. This will introduce a centring error of 10m plus. This centring error will cause problems (false alarms) if the wind changes significantly and the boat moves to the other side of the swing circle. If you draw out a diagram it shows the error is actually doubled so a 10m error becomes 20m.

If you set your anchor alarm correctly with the centre point over the anchor, the correct alarm distance is:

The length of rode you have deployed + the distance between the bow roller and the GPS aerial + a small allowance for GPS error.

The small discrepancy between the length of the hypotenuse and base of the triangle can be ignored in most cases.

The allowance for errors in the GPS needs only to be small, as modern gps units, providing they have a clear view of the sky, are very accurate. 5m is plenty. Many anchors will slowly drag several meters when near their holding limit. This is a sign that the more typical breakout and fast drag is likely to happen in the near future, so picking up small movements of the anchor is worthwhile.