Mathematic approach to anchoring scope

[wrwakefield]

Originally Posted by davefromoregon

Almost any depth or wind. The point is that at least in my experience, the chain is attached to the anchor and the rope is attached to the chain. Until 130’ of chain has been deployed, no rope will be deployed. Under no circumstance will I put out rope until the chain is fully deployed. The app shows 50’ of chain and 20’ of rope being used( more or less). If I’m going to put out 70’, it will all be chain.

however, I assumed 50’ of depth and 20 knots.

I assume I’m making a mistake on the app.

I found it very worthwhile to read the brief instructions/ explanations embedded in the app, and then moved on to absorbing the detailed online [app specific] material maintained by the author. [Links are embedded within the app.]

There is a lot going on behind that simple user interface.

The question of your anchor is independent; the app calculates the load on the anchor [and boat] for the conditions you enter. It is up to you to choose an appropriate anchor- and set it properly. [Panope’s summary charts can be useful when pondering which anchor for what conditions and bottom type.]

To me the app is all about rode and snubbers to an anonymous anchor that can [or cannot] handle the loads calculated.

It is very quick to use once you have an understanding of its operation.

HTH

Cheers, Bill

[davefromoregon]

Originally Posted by wrwakefield

I found it very worthwhile to read the brief instructions/ explanations embedded in the app, and then moved on to absorbing the detailed online [app specific] material maintained by the author. [Links are embedded within the app.]

There is a lot going on behind that simple user interface.

The question of your anchor is independent; the app calculates the load on the anchor [and boat] for the conditions you enter. It is up to you to choose an appropriate anchor- and set it properly.

To me the app is all about rode and snubbers to an anonymous anchor that can [or cannot] handle the loads calculated.

It is very quick to use once you have an understanding of its operation.

HTH

Cheers, Bill

Fine but the app is producing a result that makes no sense. In every configuration I’ve seen, you put out chain until either you have enough rode deployed or you have deployed all of your chain and begin to deploy rope.

[wrwakefield]

Originally Posted by davefromoregon

Fine but the app is producing a result that makes no sense. In every configuration I’ve seen, you put out chain until either you have enough rode deployed or you have deployed all of your chain and begin to deploy rope.

It is calculating what you ask for… Take the time to read and understand…

One free hint: I’ll wager you have a value in the fixed length variable. [Typical new user mistake with an all chain rode.] Blank that to see the calculated rode length when trying to use all chain. [Added: I read the built-in instructions for rope/chain combination rode and it says it the rope option is checked, to put your chain length in the Fixed field… I don’t use a combi rode so you will have to experiment both ways. My quick takeaway is all chain = fixed blank; chain and rope fixed = chain length— If I understand the instructions correctly…]

[When in all chain mode anyway] the fixed value will be re-entered automatically if you use the wind + / - buttons as they are to help you determine how much more wind your current rode length can handle.

There is no instant gratification with something as complex as this. When you can, take the time to learn the workflow/logic.

HTH

Cheers, Bill

[sv_pelagia]

Set to "chain", enter total possible chain into "Fix".



Then set to "rope", and enter "0" into Fix.



Now, under rope, enter depth (at high tide), bow roller height, snubber, and wind.



Calculate (under "rope" section) ... this will first use up to maximum of chain, then add rope as needed.

[MathiasW]

Originally Posted by davefromoregon

So, I downloaded the app. I must be doing something wrong since it suggests putting out about half my chain and all of my rope in a given scenario.

I’m also a bit surprised that it doesn’t ask for the weight of the anchor.

What am I doing wrong?

35’ boat
13500lbs
20kg Bruce anchor
Monohull sailboat
130’ of 5/16s”
140’ of rope

Hi, I am afraid the parameters you supply are not complete. Wind strength and anchor depth are missing at least. Without constraints in the Fix input field, the app will calculate the length of rode needed for a perfect horizontal pull at the anchor. This can mean a lot of rode, but it normally does not make a big difference to use less rode.

The anchor does not enter the equation at all. What I am calculating is the pull at the anchor exerted by the combined forces of wind and swell. This value is obviously independent of the anchor itself. The anchor will have to take this load and lead it into the seabed. Whether it is successful in doing so or not will depend on the quality of the seabed as well as anchor weight and design. The app does not know about any of these parameters. I would open a big can of worms if I were to try and model all this as well. You can look at anchor tests to see what load your anchor can cope with.

My focus is on the anchor load and the angle of pull at the anchor, and leave the anchor & seabed quality to anchor testing. I find this the cleanest cut in the design of the app.

Cheers, Mathias

[MathiasW]

Originally Posted by davefromoregon

Fine but the app is producing a result that makes no sense. In every configuration I’ve seen, you put out chain until either you have enough rode deployed or you have deployed all of your chain and begin to deploy rope.

Sorry to hear about these troubles!

The app is perhaps a bit difficult to use at first with all the parameters, but once you understand the different input fields, it is a very robust and reliable tool.

Perhaps sharing a screenshot here can help?

Cheers, Mathias

[MathiasW]

Originally Posted by sv_pelagia

Set to "chain", enter total possible chain into "Fix".



Then set to "rope", and enter "0" into Fix.



Now, under rope, enter depth (at high tide), bow roller height, snubber, and wind.



Calculate (under "rope" section) ... this will first use up to maximum of chain, then add rope as needed.

Correct! But there is no snubber in the rope mode, and whatever snubber you had provided in the chain mode is ignored whilst in this mixed rode mode.

Cheers, Mathias

[sv_pelagia]

Originally Posted by MathiasW

Correct! But there is no snubber in the rope mode, and whatever snubber you had provided in the chain mode is ignored whilst in this mixed rode mode.

Cheers, Mathias

Hi Mathias,

The revised app threw me for a minute too, but quickly got it right.



"no snubber" only if you go past the max chain limit and use rope... we have 230 ft of chain then 130 ft of rope. We rarely get to the rope (but occasionally we do!) so need the snubber for, say, 200 ft of chain out.

[davefromoregon]

Originally Posted by MathiasW

Sorry to hear about these troubles!

The app is perhaps a bit difficult to use at first with all the parameters, but once you understand the different input fields, it is a very robust and reliable tool.

Perhaps sharing a screenshot here can help?

Cheers, Mathias

[davefromoregon]

Ok. I get it now. You need to leave the rope field blank. I’m now getting numbers that make sense.

Now, would people here anchor the boat listed above in 90’ of water with 30 knots of wind with 130’ of chain(all of it) and 30’ of rope?

[MathiasW]

Originally Posted by sv_pelagia

Hi Mathias,

The revised app threw me for a minute too, but quickly got it right.



"no snubber" only if you go past the max chain limit and use rope... we have 230 ft of chain then 130 ft of rope. We rarely get to the rope (but occasionally we do!) so need the snubber for, say, 200 ft of chain out.

OK, understood. It may not be entirely clear from the description of the app, but when you are in rope view, the snubber you may have specified in the chain view is NEVER considered, even when no rope is used at all in the end and it is all chain. If you end up not using the rope, but instead install a snubber, you need to switch back to the chain view for the app to include it, I am afraid.

Cheers, Mathias

PS: And sorry for the hiccup with the revised version... When adding features, I try to keep the interface and workflow the same for all features that were already available before, but this is not always possible.

[MathiasW]

Originally Posted by davefromoregon

Ok. I get it now. You need to leave the rope field blank. I’m now getting numbers that make sense.

Okido, excellent! I guess you had used the +/- 5kn buttons at some point, as those will fill the Fix field with the current value for chain and rope. After that it is fixed until you change it manually again.

[sv_pelagia]

Originally Posted by davefromoregon

Ok. I get it now. You need to leave the rope field blank. I’m now getting numbers that make sense.

Now, would people here anchor the boat listed above in 90’ of water with 30 knots of wind with 130’ of chain(all of it) and 30’ of rope?

Dave from Oregon: I entered your original parameters (including 2° seabed angle) :

Answer: 130' chain (all of your chain) and 100.8 feet of rope.


Increasing to 90' of water (plus bow height) and 30 kn wind: increase rope to 192' (But because you only have 140' rope, this limit results in worrisome 4.7° anchor angle)


(FYI: You should switch your depths so that bow height is on left and depth is on right)

[davefromoregon]

Originally Posted by sv_pelagia

Dave from Oregon: I entered your original parameters (including 2° seabed angle) :

Answer: 130' chain (all of your chain) and 100.8 feet of rope.


Increasing to 90' of water (plus bow height) and 30 kn wind: increase rope to 192' (But because you only have 140' rope, this limit results in worrisome 4.7° anchor angle)


(FYI: You should switch your depths so that bow height is on left and depth is on right)

Realistically, I’d be looking for a much shallower anchorage. The worst I’ve anchored in was about 30-40’ and maybe 30 knots. I deployed all of the chain and perhaps 100’ of rope.

In 30 knot winds, I shouldn’t be in more than about 55’ of water. Intuitively, that makes sense though I’m not likely to get a great nights sleep.