Anchoring Technique - Three Questions

[Nick F]

I had the following request for some further numbers on required scopes. Two small charts attached which I hope answer the questions. Note that the values I am calculating are not for "straightening out" the chain (which theoretically would take an infinite force), but for reaching the point where the chain is pulling horizontally at the anchor on the seabed.

Quote
I would like to tighten up some of my assumptions regarding scope calculations for lesser used depths. Unfortunately, XL spreadsheets aren't my specialty, or any experience with them. I was hoping I could impose upon you to work some numbers for me.

My anchor chain is 1.5 pounds per foot.

My solid depth scope number is 15' at a 7:1 ratio.

What I would appreciate you doing for me is to calculate the force required to straighten the chain out at 15' ratio 7:1 with 1.5 lbs. chain weight. Then using the force and chain weight, calculate the scope ratios for:

10'
20'
25'
30'
unquote

 

[rwidman]

I don't have the patience to read ten pages of responses, but I do have the same boat as the OP so I will l say this:

I always set out at least 5 to 1 scope, sometimes more (I have 30' of chain and the rest rope) and I always set the anchor under power. Since I am the one working the bow and my wife is at the helm, I don't know what RPM she is using, but it's more than idle speed by quite a bit.

 

[Mambo42]

Today is a perfect day to explain why I had to make a mathematical calculation and cannot just go for a fixed scope.
Currently we are on anchor in the old port of Kos, Greece. As usual we had to do stern to mooring, which means you drop your anchor, go reverse and tie the stern to the shore.
Not a problem if you have 10 - 15 kts, but today we are having around 25 kts at the moment and forecast for the night is 25 - 30 kts (6 - 7 Bft). With stern to docking you don't have the luxury of dragging even one meter, so it needs to be good 100 % of the time.
For 30 kts I am looking at about 75 N/m2 and for my boat that means 1200 N or 30 mtr of chain. 30 mtr plus the 10 mtr I want to have solid on the seabed means a minimum of 40 mtr of chain. Depth below the boat is 2.6 mtr, near the anchor is 5 mtr.
Since I always assume 1 Bft as safety we have the 50 mtr mark just below the water and that means a 10 : 1 scope. Snubber is on and we are about 1.5 mtr from the shore. Wind is full on the bow, so no sideway forces. 2 stern lines with shock absorbers, 2 spring lines and we will be fine, even if the wind would pick up to 8 Bft.

Some may say that it is better to move outside......not really at this moment. On the north side the wind is already 8 Bft with 2 mtr waves. On the South side, right below the mountains it is 10 Bft, but luckily no waves.

 

[rslifkin]

With a stern tie or med mooring longer scope makes sense. Dragging is a bigger concern, and being that the boat can't swing, there's no issue with longer scope causing a huge swing radius.

 

[Mambo42]

With a stern tie or med mooring longer scope makes sense. Dragging is a bigger concern, and being that the boat can't swing, there's no issue with longer scope causing a huge swing radius.

We have indeed no swing radius, but have seen enough boats in stern to docking that were smashing into the dock due to not enough chain. In some marina's you even cannot throw out more than 20 mtr and those are the ones we avoid.
We would have preferred to throw out more chain, tomorrow is also going to be quite breezy, on Thursday it will die down for a 2 days and then it will pick up again. Luckily not a lot of waves in here, maximum 50 cm.

 

[Marco Flamingo]

I disagree veering lessens the load or even worse .... it might help to break the anchor out.

My experience is that veering both lessens and increases the load. Although it seems that the boat is only going back and forth, it is most likely going in a figure 8 path. I experienced this when resetting the anchor one night. I suspected that I was dragging and needed to increase the scope. I had the chain in a common anchor chain stopper and there was too much tension to allow me to release it. I had to wait until the boat veered to one side and begin its dance to the other side. Tension was way less until it was well on the way to the other side of the figure 8 dance, at which point I had released the stopper and then learned that the force directly on the windlass was enough to cause it to semi-free-wheel in reverse. Fortunately, it free-wheeled sufficient to reduce the tension and then the chain/line connection payed out. That allowed me to grab the line and take a hitch around the Sampson post. Much easier to grab and throw a hitch of rope than dealing with speeding chain.

So for my boat the lesson I learned is pay out all my chain (@100') and tie off using the rode on the Sampson post. That will allow me to control the anchor rode in the middle of a dark and rainy night while buck naked. I now usually drop all the chain plus the depth in line. That 1) keeps only chain on the bottom, 2) gives me anchor line on the Sampson post should I need to increase scope, 3) quiets any chain-on-anchor-roller noise and 4) gives me some nylon stretch.

I often pull anchor and find a 4-5 foot section halfway down the chain that is covered with mud. That is the point around which I rotated all night while the anchor simply sat on the bottom 50 feet away doing nothing. Okay by me.

 

[luna]

Marco.

Your experience with variable loads while veering mirrors mine, but we need to resist the max. load which occurs due to an increase in frontal area as we veer off of the wind. Otherwise we drag.


With respect to this phrase:

Do you really think (like Mambo42) that your anchor is "doing nothing"?

Using your example, 50' of your chain and all of your rope is suspended in a (lets say 25' deep) water column due to the load imposed from the forces acting on the boat.

The remaining 50' of chain lies on the bottom with the first 5' or so stirring up the seabed as you drift around.

In order for the anchor to be "doing nothing" you must believe that the force required to drag this 50' of chain across the seabed, is greater than the force required to lift the other 50' chain of off the seabed.

I would suggest the opposite is true. If it takes 100 lbs. of force to lift the 50' of chain from the seabed, my gut says about 75 lbs is being resisted by the anchor, which at that point can be said to be doing something.

Anybody know the coefficient of friction for 3/8" chain in the well lubricated environment of the seabed?

Perhaps Nick F can develop a graph that lets us all know the loads required to drag various lengths of chain only (no anchor as we don't need one) across the seabed.

 

[Marco Flamingo]

Do you really think (like Mambo42) that your anchor is "doing nothing"?

Absolutely. If I dropped 800 feet of chain, I wouldn't need an anchor attached to the end of it in most anchorages that I use. It all depends on how much tension is put on the rode. In some situations, I don't put enough tension on the chain to ever pull on the anchor. If no tension, no anchor, no problem. I'm not recommending, just stating physics.

 

[luna]

But you said that you only had 100' of chain, 50' of which was suspended because it was being pulled on & therefore not on the bottom (you knew that because it was clean) and your anchor was doing nothing.

Now you have 800' feet of chain, maybe an anchor and are prepared to use all of it.

Good luck with that.

 

[BruceK]

Interesting reading. Sounds like we need a loading device which attaches to the rode linked to a readout screen or phone to know the loading level in real time and whether action is required. Does this exist as yet, or is it in development?

 

[Nomad Willy]

One skipper setting at 3-1 is not the same as another at 3-1.
There are many variables involved.

Primarily there is the “throat angle”. That is the anchor shank to fluke angle. At least that’s what I’ve thought for any years ad it seems to fit our anchoring discussions so I’ve just adopted it and I haven’t been called out.

And then there’s the physical properties of the sea floor. The biggest variable in anchoring. Some mud holds at much higher fluke angles than sand or gravel. And the fluke acts as a lever depending primarily on the shank to fluke angle. If the shank end is too high w a wide throat angle getting a bite will be very difficult kinda like a wing stalling .. or not going deep enough at normal scopes and nearly being prone to breaking out. But anchors come w a certain throat angle and that has a great effect the habitual ways of the anchor. The claw having a good rep. for short scope is one example. Given enough scope and a narrow throat angle the same claw could possibly hold well or better.

Too many variables to say one feature like a large surface area will have high holding power basically most of the time. But some anchor manufacturers will say fluke area basically will determine anchor performance. But you’ve got to burry that big fluke.

 

[Mambo42]

Marco.

Do you really think (like Mambo42) that your anchor is "doing nothing"?

Perhaps I should explain my approach a bit more precise, so that is becomes clearer.

I do calculate the forces on the boat based on the forecasted winds and current and let the chain pick up all those forces. When the wind/current forces on the hull are known I simply divide them by the weight (per meter) of the chain and after that I add 10 mtr which will lay quiet on the seabed plus the depth of the water itself.
So when the wind starts to blow stronger than forecasted I first have the 10 mtr of extra chain, which needs to be lifted off the seabed. And only then comes the holding power of the anchor come into play.

When the wind shifts direction it usually does not do that at full force. Only when you have a TS the wind direction can change quite quickly, but a TS is just a short period of time. When we are talking frontal winds the wind shift is more gradual and never at full force.
Since the anchor chain was calculated for the full force of the wind (plus 1 Bft for safety) a wind shift will not cause my anchor to be pulled around. At maximum I will turn around the 10 mtr mark, still leaving 10 mtr of chain on the seabed and the anchor untouched (but pointing the wrong direction).

In most anchorages when the wind starts to blow I do check the anchor and the chain, so far indeed my calculations have been correct, we pivot around the chain, not the anchor.

Like I said, it is overkill, but since nobody ever died or a boat got beached due to overkill, I will apply this method when I have the chance to use it. In most ports in the Med you have to do the stern to docking, using your anchor, and you won't have the full amount available in space. However since the winds are, in this area of the Med, coming from the North (95 % of the time during the summer) I always look for a berth in port where I have the stern pointing North. That way the forces on the hull are handled by the shore line and the anchor is only used to keep the bow pointing in the right direction.
If they would offer a South berth I will simply decline and anchor outside, using all the chain I have calculated.

In the current port, Kos, we don't have enough for wind force 10, but we do have enough for wind force 8 and the forecast is only wind force 6 to 7. On top of that, the seabed seems to be mud, which means very good holding and lastly.............we can only move 1.5 mtr backwards, then we would be hitting the quay, which would also stop the dragging.

One thing I need to do however is connect a buoy to the anchor (not in ports of course) to show fellow boaters where my anchor is. That way they won't drop over my anchor or pull it out when they are anchoring.
An added benefit of the buoy is that the line that is attached to the buoy can handle 12.000 N of pulling force, which means that if the anchor would be stuck I can simply go there with the dinghy, attach the line to the dinghy and pull the anchor free.
Saw that trick of attaching the buoy (with a self righting buoy that always stays above the anchor) on Youtube and have been using it ever since, works great and no more screaming/shouting or hunking the horn in an anchorage.

 

[SteveK]

Mambo. In reading your posts you are describing scope in a different way. It all comes down to the anchor staying firmly rooted into the seafloor. Whatever way you describe it, as long as the anchor shank is not lifted and the pulling force is horizontal with the seabed this is accomplished.

 

[Nick F]

Interesting reading. Sounds like we need a loading device which attaches to the rode linked to a readout screen or phone to know the loading level in real time and whether action is required. Does this exist as yet, or is it in development?

Offshore drilling rigs which are anchored have exactly this - load indicators on each of the several (typically 8 to 12) anchor lines.

 

[Northern Spy]

When I reconfigured and re-moored a floating breakwater composed of ten derelict ships, I often found that the mooring anchors were not connected. A shot or two of studlink chain buried in the mud was what was holding them (more or less) in position. Don't underestimate the coefficient of friction of chain in firm mud. Especially after a long soaking.

Interestingly enough, many of the galvanized shackles were found to be moused with stainless steel wire. The shackle pins (sometimes bolts) were sharpened like a pencil by galvanic corrosion.

 

[rwidman]

When I reconfigured and re-moored a floating breakwater composed of ten derelict ships, I often found that the mooring anchors were not connected. A shot or two of studlink chain buried in the mud was what was holding them (more or less) in position. Don't underestimate the coefficient of friction of chain in firm mud. Especially after a long soaking.

Interestingly enough, many of the galvanized shackles were found to be moused with stainless steel wire. The shackle pins (sometimes bolts) were sharpened like a pencil by galvanic corrosion.

What wire would you recommend for shackles?

 

[psneeld]

I am sure Northern Spy will give the best answer, but most boaters don't moor for long periods. We anchor for relatively short periods and can inspect and rewire, spray galvanize enough to not worry.

If your anchor spends more time in the water than out, monel might be the better choice. Can always use galvanized wire, but in thin diameters I am not sure that's a great choice.

 

[Nick F]

Interestingly enough, many of the galvanized shackles were found to be moused with stainless steel wire. The shackle pins (sometimes bolts) were sharpened like a pencil by galvanic corrosion.

Are you suggesting that the presence of the stainless lockwire led to the pin corrosion?

 

[Mambo42]

Mambo. In reading your posts you are describing scope in a different way. It all comes down to the anchor staying firmly rooted into the seafloor. Whatever way you describe it, as long as the anchor shank is not lifted and the pulling force is horizontal with the seabed this is accomplished.

That is indeed my approach, I make sure the anchor shank is not lifte, under any circumstance, and thus I will always have the maximum holding power of the anchor.........if I would ever need it. The calculations are based on the anchor chain doing most of the work for me.

It is not an unknown technique for boats, you can even buy an extra weight for your anchor chain (that can also be used to pull your anchor out when it is stuck), which ensures you are pivoting around a certain point in the anchor chain.
https://www.ultramarine-anchors.com/anchor-ring

 

[Mambo42]

The weight of the chain, gravity, laying on the seabed keeps the anchor set. Scope 3>1 in most cases will accomplish this IMO. The anchor, set well holds position as long as the chain lays flat. My guess, if you have a boat lenght of chain on the seabed you are anchored.

I also thought that 3 : 1 or 5 : 1 would be enough, but that one storm proved me hopelessly wrong. We were well set in 5 mtr water, had about 30 mtr of chain out. With the 3 mtr from the water level to the bow we are talking a scope of almost 4 : 1 and then came the wind and the waves.
The anchor held well for about 4 hours and after that it was done, we started dragging.
Already then I had a 13 mm chain and a 50 kg anchor, so no changes there. 30 mtr at 4 kg /mtr is 120 kg of chain which is 1200 N.
Problem was that the wind was putting well over 1600 N on the chain, lifting it up, lifting the shank up and it was bye bye to the holding power of the anchor. The holding power of the anchor is based on pulling straight backwards, so not under an angle. The moment the anchor chain gets lifted of the sea bed the holding power of the anchor goes down real fast.
Add a few waves to that and now you really know the anchor is not going to hold.

We anchor out most of the days and I don't want to sleep not knowing if I have enough chain out to keep us safe through the night, especially here in the Aegean where the wind blows pretty strong almost every single day.
Now that I can calculate how much I need as a minimum and I checked my calculations by verifying underwater, I know that nothing is going to happen, so I sleep perfectly fine

The 3 : 1, 5 : 1 and even 7 : 1 are basically derived from these calculations and overtime people forgot the calculations, just simply went to the fixed scopes, not knowing anymore how they were calculated.
Basically I went back to basics.

 

[SteveK]

The anchor held well for about 4 hours and after that it was done, we started dragging.

But what if the chain remained on seabed, if the anchor drags then the anchor is or could be the reason? To exaggerate, a pleasure boat anchor attached to the chain from a freighter surely will drag in spite of the chain resting on the ground.
The boat became heavier load that exceeded the anchor ability to hold fast.

 

[Northern Spy]

What wire would you recommend for shackles?

I use a zip tie.

 

[Northern Spy]

Are you suggesting that the presence of the stainless lockwire led to the pin corrosion?

Yup. I'll see if I can dig up some photos.

 

[psneeld]

Are you suggesting that the presence of the stainless lockwire led to the pin corrosion?

Google stainless anchor galvanized chain.

Some reports ( because few do it) of the chain or shackle near the anchor wasting at a rapid rate.

I think I remember a thread discussing it here on TF a couple years back.

 

[psneeld]

I also thought that 3 : 1 or 5 : 1 would be enough, but that one storm proved me hopelessly wrong. We were well set in 5 mtr water, had about 30 mtr of chain out. With the 3 mtr from the water level to the bow we are talking a scope of almost 4 : 1 and then came the wind and the waves.
The anchor held well for about 4 hours and after that it was done, we started dragging.
Already then I had a 13 mm chain and a 50 kg anchor, so no changes there. 30 mtr at 4 kg /mtr is 120 kg of chain which is 1200 N.
Problem was that the wind was putting well over 1600 N on the chain, lifting it up, lifting the shank up and it was bye bye to the holding power of the anchor. The holding power of the anchor is based on pulling straight backwards, so not under an angle. The moment the anchor chain gets lifted of the sea bed the holding power of the anchor goes down real fast.
Add a few waves to that and now you really know the anchor is not going to hold.

We anchor out most of the days and I don't want to sleep not knowing if I have enough chain out to keep us safe through the night, especially here in the Aegean where the wind blows pretty strong almost every single day.
Now that I can calculate how much I need as a minimum and I checked my calculations by verifying underwater, I know that nothing is going to happen, so I sleep perfectly fine

The 3 : 1, 5 : 1 and even 7 : 1 are basically derived from these calculations and overtime people forgot the calculations, just simply went to the fixed scopes, not knowing anymore how they were calculated.
Basically I went back to basics.

Not sure precise calculating is the basics.

I think its always been trial and error in determining what your boat and tackle do together.

The " basics" to me are to try to grasp the bottom holding before picking a spot. Then determine if based on water depth and the amount of scope I need is suitable for the conditions and location. Lastly, if my chain starts lifting, let out more if I can, if not consider options.

If dragging and the chain is still on the bottom, letting out more chain may work, but there are soft, slimy bottoms where you might drag no matter what...that goes back to step number one, pick good holding unless you want to consider options at a not very convenient time.

 

[Mako]

"The " basics" to me are to try to grasp the bottom holding before picking a spot."
When I was younger I relied on charts and my flashing depth sounder (I still hate that thing). Later on I really began to value a good fish finder / modern depth sounder. Not for its fish-indicating abilities (my fishing skills are horrible) but for gauging bottom type. Seeing hard surface vs sand vs rocks versus 4 feet of muck is a very helpful feature.

 

[psneeld]

Before you know it a bunch of TFers will decry that underwater drones are an absoute safety necessity to check the bottom, the anchor digging in and the chain laying the anchor flat.

 

[Mako]

Actually an U/W drone would be fun. Plus then you wouldn't need to dive down on your anchor in yucky harbors. It would be cool to cruise down to 200-300 feet, a depth I would never dive down to. Hmm, maybe I'll start checking Alibaba...

 

[psneeld]

Actually an U/W drone would be fun. Plus then you wouldn't need to dive down on your anchor in yucky harbors. It would be cool to cruise down to 200-300 feet, a depth I would never dive down to. Hmm, maybe I'll start checking Alibaba...

Fun is what boating is to me, based on so many TFer inputs, sounds like I am laxidaisical, blowhard and just don't take boating seriously enough.

Yes a robotic underwater drone would be a blast, till some clowns put a speargun on one...

But some will say I am irresponsible if I don't have one to check my anchor or lead me though the Dismal Swamp to avoid logs.

 

[Mako]

A drone with a speargun attachment would be perfect for clearing out the lionfish which tend to live at great depths off Florida. Maybe a stun-gun to kill them off.