Anchoring Technique - Three Questions

[Alaskan Sea-Duction]

Our average depth for the hook this year in B.C. and SEAK was 40 to 60ft. There were times we anchored deeper 80-120ft. 3:1 worked most of the time. Never dragged even in 45kt winds. We have a 77lb rockna with 550ft of chain. It always sticks. Sometimes, you had to back up a bit to allow it to roll over. Sometimes, the current sets it for you.

 

[Mako]

Does anyone hear know of any suppliers (probably commercial-industry) with good pricing on either Poole (type N) or AC-14 stockless anchors? Targeting about 75kg.

 

[Mambo42]

Interesting topic and great to see so many different techniques and opinions.

After a not so pleasant dragging experience in a storm I wanted to figure out why we were dragging and I got almost as many different opinions as there are colors in the rainbow. Being a pilot I knew that mathematics had to be involved, but I could not find any info until the moment I ran into a book called: "Dass ist Motorboot Fahren' written by Joachim Schult. It is in German, but for me that is no problem.

He explained the mathematical part behind anchoring and when you read that you realize that all the different scopes are just quick reference guides for people who don't want to calculate.
After some more searching I actually found a complete calculation program where you can fill in your boat specifics, the weather, the current and the waves. The program will then tell you what the forces involved are and thus you can figure out what you will need to set in anchor and chain.

The most important information I took away from his book are:
1. It is possible to calculate all the environmental forces on the boat (wind, current and waves).
2. It is also possible to calculate the holding power of the anchor and chain
3. As long as the holding power is greater than the total sum of the environmental forces you will not drag. That is of course if the anchor is set well.

This however also means that, if the holding power is less than the environmental forces you will drag, no matter how well you have set the anchor.
And how does the dragging start ?
The chain gets lifted of the seabed, which raises the shank of the anchor and once the shank of the anchor reaches 25 degrees the anchor has lost all holding power and you will drag.
In other words, it is imperative the keep the angle of the shank to the seabed preferably at 0 degrees and for that you need the chain.


Another thing to take away is that there is no magical anchor. What you need to find is the anchor that holds best is the least favorable seabed. In good seabed most anchors hold well, but what we should be looking at is the holding power in seabeds the anchor does not favor. And only then do you start looking at the types of seabed you will encounter most.
That way of reasoning did make a lot of sense to me, since the chances on dragging are greater when the seabed is less favorable to the anchor.

So what determines the holding power of the anchor ?
The total holding power is equal to the anchor weight x the seabed factor. You can find the tables with the seabed factor online for anyone who is interested.
Automatically you will understand that the heavier the anchor is the more holding power it has. So oversizing your anchor does not only look nice in the marina, but also makes sense when it comes to anchoring in adverse weather.

Then comes the question: 'how do you calculate the environmental forces ?'

When it comes to wind it is basically the forces the wind can excercise on the hull that is in contact with the wind. In other words, if the wind comes onto the bow you will need to calculate the frontal surface of your boat. That will give you a number of square meters (or square feet in the US) and that you then have to multiply by an amount of N which is given for each windforce.
E.g. at 4 Bft the force of the wind is around 15 N/m2, but at windforce 10 it is 204 N/m2.
When your frontal surface is 10 m2 you will find 150 N at 4 Bft, but 2040 N at 10 Bft.

Your anchor and chain therefore have to withstand those forces and usually at low windspeeds that is not a problem. But since the forces of the wind are squared when the wind increases you can understand the required holding power increases likewise.

Also the current excercises forces on the hull and those forces are quite high. In fact the density of water is 835 times that of air, so we can expect the forces to be very high quite quickly.
At 1 kt current and e.g. a surface area of 1.25 m2 it is only 70 N, but at 5 kts it is already 1650 N. And the surface area is of course the frontal part that is in the water.

Comparing wind to current we see that a current of 2 kts has the same effect as a windspeed of 6 Bft, so definitely something to take into consideration.

Lastly we have the wave factor. These forces are more difficult to calculate, but basically we have 3 different forces.
The first one is the up and down motion, the pulling on the chain when the boat goes up to the crest of the wave.
Second is the motion of the water in the wave, which acts as a current.
Third is the force of a breaking wave on the hull.

These 3 forces, wind, current and waves determine the total environmental forces and as has become clear the stronger the wind and the current are the environmental forces increase with dramatically. Not linear, but exponentially.

When it comes down to how much anchor and chain you need to throw out it must be clear that, since the environmental forces change drastically when wind or current increases, there cannot be such a thing as a fixed scope.
In fact, the 3 : 1, 5 : 1 and even 7 : 1 scope are all based on calm weather, calm seas and hardly any waves.
None of us starts dragging at calm weather, it happens only when either wind or current, or both increase drastically. The forces on the hull become such that the chain is lifted off the seabed which raises the shank of the anchor and with each degree the shank gets lifted the holding power of the anchor becomes less. At 20 degrees off the seabed the holding power is only 50 degrees and at 25 degrees there is no more holding power.

After reading the book I started calculating the figures for my boat and came to a frontal surface of about 18 m2. The frontal subsurface area is about 3 m2.
Where it becomes interesting is when current and wind don't come from the same direction. Then you have to recalculate everything to figure out how much surface is in the wind and how much surface is in the current.

On my boat I have a 50 kg (500 N) Sarca Excel and 140 mtr of 13 mm chain, which weighs 4 kg/mtr or 40 N.

So how do I decide how much chain to throw out ?
I want to have at least 10 mtr of chain lying on the seabed behind the anchor. After that the chain can be lifted off the seabed in bad weather, which means I calculate the expected wind forces (we have no current here) based on the forecasted weather. For safety purposes and based on experience I add at least 1 Bft and sometimes 2 Bft, depending on the location where we are.
That way I can calculate the expected environmental forces in N and since that is the force that will lift the chain off the ground I divide that number by 40 N. That will give me the length of the chain that I need as a minimum. After that I add 10 mtr, which is the part that always has to stay on the seabed. And I leave the holding power of the anchor completely out of the equasion. That is extra bonus for me.

As an example, let's say the expected wind force is 8 Bft, which is 37 kts.
I add 1 Bft for safety, which makes it 45 kts (takes care of the gusts).
At 45 kts the windforce is 157 N/mtr2 and in my case that comes down to 18 x 157 N = 2826 N
This 2826 N I have to divide by 40 N, which gives me 71 mtrs. To that I add 10 mtr behind the anchor and in total I will therefore throw out a minimum of 81 mtr. And all of this is based on a max angle of 25 degrees between the bow to the seabed.
In shallow water (up to 7 mtr) I don't add extra chain for the water depth, but over that I add the water depth to the bow as well.

Some people will call this overkill, some told me I have to buy a different anchor, but as long as nobody was able to give me a mathematical explanation of what exactly happens when you start dragging and why you start dragging............I basically left all those opinions for what they are.
Here in the Med we do stern to docking and anchoring most of the time, in fact I would say about 95 % of the time. The other 5 % is free anchoring, without a land line. And if there is one thing you cannot afford is to start dragging your anchor when you have a rocky shore 20 or 30 mtrs (or less) behind your stern. You need to know for sure that you won't drag.
Currently we are in the Aegean, which is known for very strong winds most of the time. Yesterday we came into an anchorage with steady 30 kts and gusts to 40 kts. We only wanted to go for a swim, nothing else. Waves were almost none, no current. Water depth was 8 mtrs.
So I dropped 100 mtr in total, stabilizers on and we were not moving anymore. 4 sailing vessels tried to anchor as well, all gave up and left the anchorage. There were 3 other boats anchored, but they were closer in to the shore, less exposure to the wind and shallower water. We stayed for 2 hours, hauled the anchor up and sailed to Kos.

So at the helm I have a page in my checklist that gives me an instant number of chain to put out at certain windspeeds. And if we have the chance we will drop even more. The number in the checklist is only the minimum, not the maximum.

I am glad I read the book, later I found the online calculator and an explanation for the commercial shipping

https://www.ocimf.org/document-liba...environmental-loads-on-anchoring-systems/file

https://www.ocimf.org/anchoring-calculator/

 

[psneeld]

Calculating anchor tackle is the easy part...most rules of thumb aren't bad and when in doubt factor up.

What is not easy is to know the bottom holding and that ultimately is the most important.

That's what makes anchoring in severe weather always a gamble unless you exhibit the same amount of environmentally calculated pull by testing that number with engine power.

 

[SteveK]

I anchored in what I thought was a nice spot and wondered why none of the other boats in the bay thought so. Well, as I usually so after anchoring I pick points of land and spend the next while checking position. Sounded like anchor dragging in gravel. Time to move.

 

[chanprr]

I anchored in what I thought was a nice spot and wondered why none of the other boats in the bay thought so. Well, as I usually so after anchoring I pick points of land and spend the next while checking position. Sounded like anchor dragging in gravel. Time to move.

All good stuff posted. The seabed is a big factor in choosing a spot to anchor, and the type of anchor to be deployed. For efficient setting, choose a flat spot on the seabed to drop anchor. My Bruce fails in rocky bottoms, but excellent in sand, gravel and mud. I use a minimum 3 to 1 scope but increase that depending on wind and current in the anchorage. Oh lastly, the number of vessels already anchored is sometimes a major deciding factor, and swing room.

 

[Mambo42]

Calculating anchor tackle is the easy part...most rules of thumb aren't bad and when in doubt factor up.

What is not easy is to know the bottom holding and that ultimately is the most important.

That's what makes anchoring in severe weather always a gamble unless you exhibit the same amount of environmentally calculated pull by testing that number with engine power.

That is why I keep the holding power of the anchor as a reserve. I will let the chain do all the work. When the environmental forces cannot lift up the chain I don't even get to the holding power of the anchor.
What you do need for that is a heavy chain and a lot of it, luckily I have both.

 

[psneeld]

That is why I keep the holding power of the anchor as a reserve. I will let the chain do all the work. When the environmental forces cannot lift up the chain I don't even get to the holding power of the anchor.
What you do need for that is a heavy chain and a lot of it, luckily I have both.

That's not always possible. Either the anchorage demands a reasonable scope .....and many here attest that their chain lifts in far less than severe conditions, therefore relying on anchor holding power... or you have to have a chain so big or long that your boat or anchorage just can't hold it all.

 

[O C Diver]

.
And how does the dragging start ?
The chain gets lifted of the seabed, which raises the shank of the anchor and once the shank of the anchor reaches 25 degrees the anchor has lost all holding power and you will drag.
In other words, it is imperative the keep the angle of the shank to the seabed preferably at 0 degrees and for that you need the chain.

<Snip>

The forces on the hull become such that the chain is lifted off the seabed which raises the shank of the anchor and with each degree the shank gets lifted the holding power of the anchor becomes less. At 20 degrees off the seabed the holding power is only 50 degrees and at 25 degrees there is no more holding power.

So this raises an interesting question:
If you're going to power set your anchor and you accept the above information regarding shank angle and holding strength, at what anchor ratio (depth to rode length) do you need to be to get an acceptable shank angle / holding strength for the anchor to bury? Clearly 1:4 has lost most of the anchors digging and holding strength.

Ted

 

[BroadCove]

Ted, that is a very astute observation. It reminds me of an old Calvin & Hobbes cartoon, where Calvin asks his dad how they determine the weight limit on bridges, and his dad responds: They drive heavier and heavier trucks over it until it collapses, and then they rebuild it. (I'm not suggesting anyone is recommending power setting such that the anchor fails and then resetting once the fail point is "known", it just gave me a chuckle to remember the cartoon.)

 

[luna]

That is why I keep the holding power of the anchor as a reserve. I will let the chain do all the work. When the environmental forces cannot lift up the chain I don't even get to the holding power of the anchor.
What you do need for that is a heavy chain and a lot of it, luckily I have both.

The notion that the chain does all of the work in day to day anchoring and the anchor itself sees no load is bizarre. If that is the case, why do we need anchors?

For a reasonable reference guide to rode behavior and the forces involved when anchored, you might want to have a read through this.

Tuning an Anchor Rode

Bottom line is, put lots of your rode out to keep the anglulation less than 8º and you should be fine. If not move.

 

[psneeld]

In my experience, being a fair weather anchorer (less than 30 kts), often it is the chain that does all the work.

If I can tuck into a tiny creek with good wind/wave protection (so swing or surge) and good bottom, I would use a fairly short scope (around 4:1) but the current was usually strong and sometimes the boat would hang on the anchor.

But many times in open water anchorages with plenty of room but less desirable bottom, I would let out maybe 100 or more feet of chain (well over 10:1) where the anchor wouldn't dig in at all.

My readings on ship anchoring seem to lean towards they too rely on their incredibly heavy chain and engines running during heavy winds as their anchor size and design are not really high holding like small boats have. So the concept isn't really bizarre.

 

[Mambo42]

So this raises an interesting question:
If you're going to power set your anchor and you accept the above information regarding shank angle and holding strength, at what anchor ratio (depth to rode length) do you need to be to get an acceptable shank angle / holding strength for the anchor to bury? Clearly 1:4 has lost most of the anchors digging and holding strength.

Ted

It is exactly the reason why I wanted to know the mathematics behind anchoring. Then you realize there is no magic scope which will work well for all situations.
A scope of 4 : 1 may work well in 5 Bft, but in 7 to 8 Bft it may just start to drag.
That is why it is so important to make some sort of a calculation of the environmantal forces on the boat. Once you know those forces you can figure out quite easily how much chain you will need to drop. In other words, I am not dealing with scope anymore, I am just dealing with numbers that can be calculated.
It is a completely different mindset when it comes to anchoring. I understand that not everyone will want to do that, but since we encounter more rough weather than calm weather (about 80 % rough to 20 % calm) I don't want to gamble anymore, I want to see the calculation behind it.
I know that some people will make a combination of the holding force of the anchor and the holding force of the anchor chain. For me, I decided to have the anchor chain take almost all of the forces, the anchor is an added safety. As long as I can always keep 10 mtrs of chain on the seabed I know I will have the full holding power of the anchor at all times.
And now I know how to calculate the total amount of anchor chain required to keep those 10 mtrs on the seabed.

 

[Mambo42]

My readings on ship anchoring seem to lean towards they too rely on their incredibly heavy chain and engines running during heavy winds as their anchor size and design are not really high holding like small boats have. So the concept isn't really bizarre.

That is indeed what the larger ships do, they rely for 90 % of the holding force on the weight of the chain.
Once I realized that I changed my approach as well. Before I went also with fixed scopes. 3 : 1 for a calm weather, few hour stop. 5 : 1 for calm weather overnight and 7 : 1 for rougher overnight weather.
It was with the 7 : 1 scope that we found ourselves, all of a sudden (in the middle of the night) in a storm where the wind came out of a direction which was 90 degrees off the forecasted direction. Instead of the forecasted 15 kts we got 40 kts of wind. That brought heavy waves into the anchorage and around 3 AM all hell broke lose in the anchorage. In the end we had to pick up the anchor and just motor around, putting the bow into the waves and wind.
That was the moment I asked myself why is there no normal calculation possible to figure out how much chain you really need.
With the knowledge that I have now I will happily go into that anchorage again and drop the calculated amount of chain, knowing that no matter what happens, we will not drag anymore. We may get hit by boats that do start dragging, but to avoid that as much as possible we anchor out in the deeper parts, where most boats cannot anchor.
I know that my approach is not necessary for boaters who never go out at sea, who just stay in the waterways or for those who only go out at good weather.
This part of the Med is known for adverse weather most of the time, which means gambling will get you in trouble sooner or later. A calculation won't give you 100 %, but 99 % is good enough.

 

[Mambo42]

The notion that the chain does all of the work in day to day anchoring and the anchor itself sees no load is bizarre. If that is the case, why do we need anchors?

I agree with you that it is not standard and it is not for everyone. But in the part of the Med where we are it is quite common to from 2 Bft to 9 or 10 Bft in a matter of minutes. It is called the Meltemi wind during summer and sometimes we have the Sirocco which has identical wind speeds.
If I would just calculate with the holding power of the anchor at e.g. 5 Bft I would find myself dragging when the wind all of a sudden picks up.
Sadly the forecasting system in Greece is still in the days of Odysseus, meaning your guess is as good as ours, so you just have to be prepared for the absolute worst.
My first defense is then the anchor chain and only when that is not enough I have my second line of defense, the anchor itself. But in order to have the full holding power of the anchor I need to know for sure that the shank is at 0 degrees with the seabed. The only way to know that is when I have at least 10 mtrs of chain behind the anchor which is always on the seabed.
Since I can calculate how much chain will be lifted off the seabed at a certain windspeed I know exactly how much chain I need to throw out in order to have the 10 mtrs behind the anchor on the ground.

And as a reference for the holding power of the anchor hereby some figures between holding power and angle of the shank with the seabed:

Holding power 100 % at zero degrees
Holding power 85 % at 5 degrees
Holding power 70 % at 10 degrees
Holding power 60 % at 15 degrees
Holding power 50 % at 20 degrees
Holding power 0 % at 25 degrees

As you can see the loss of holding power goes rapidly down with only a small increase in the angle of the shank with the seabed. It could only take a large wave to reach that 15 or 20 degrees and when you all of a sudden lose 40 to 50 % of your holding power of your anchor you could be in for a very nasty surprise.
I don't like surprises, as a pilot I am trained to be ahead of the power curve, to always be on top. I do like overkill, nobody ever died from overkill or overpreparation.
But I fully understand why others don't want to take this approach, everyone has to decide for themselves what works for them and what doesn't.

 

[Mambo42]

What is not easy is to know the bottom holding and that ultimately is the most important.

The manufacturer of the anchor should be able to give you the seabed factors for that particular anchor. Once you have those numbers you multiply that seabed factor with the weight of the anchor and you have the holding power of the anchor.

 

[psneeld]

Unfortunately the variables again play too much into rules of thumb versus "numbers"

Just because one is in the same anchorage doesn't mean bottom composition holding power is always even nearly the same.

Wind force on frontal area varies wildly at times where wind gusts and boat veering changes minute to minute.

Same with waves in many anchorages as reflected waves in confined areas.

Then there are surges in tidal areas where the current force isn't always the same.

In my experience, that is exactly why I am a fair weather anchorer. I have just been through too many of those variables that I just rather be in a dock or certified mooring.

Granted, even pilings can snap or line chafe through in a slip. But there are multiple piling and lines if I add them as backups...but I only really anchor on one system with lots of variables.

The ultimate test is whether I feel a marina's infrastructure can withstand severe conditions.

 

[psneeld]

The manufacturer of the anchor should be able to give you the seabed factors for that particular anchor. Once you have those numbers you multiply that seabed factor with the weight of the anchor and you have the holding power of the anchor.

I really don't think their "suggestions" are any better than rules of thumb.

They certainly can't or don't cover all possible combinations of bottom material and as I posted before, you really don't have an accurate knowledge of what is exactly where you dropped anchor or what's down below the surface where if the anchor actually gets to.

I too was trained as a pilot, but had a pilot job (and many other do too) that asked you to do many things with a helicopter that there was no previous or comprehensive data on. You had "guidelines" because that's really the best you could get, then there was crowd sourced anecdotal info (just like anchoring)...but in the end too many variables in every situation to "calculate" hard numbers so you just had to rely on those rules of thumb and experience.

The bottom line is when the variables were too many (like sketchy wind forecasting)...sometimes best to not plod ahead with a plan.

 

[rslifkin]

Anchoring large ships has very little to do with anchoring small boats. The dynamics change at that scale, plus they aren't weight constrained for ground tackle and have a full time anchor watch.

With the weight constraints of a typical recreational boat, an extra 20 lbs of anchor adds far more to holding power than an extra 20 lbs of chain. So up-sizing chain for holding power is a waste.

Holding power is reduced at short scope, but if you've got a generously sized anchor and are in a decent bottom (as you've hopefully sized for relatively crappy bottoms), you have lots of excess holding power. So you can afford to throw some away with shorter scope to allow you to fit in better.

There's no one correct answer, and different anchors will behave differently at short scope as well. Deeper water also requires less scope than shallow water, as chain catenary becomes dramatically more effective, even with fairly light chain.

 

[psneeld]

I would argue at some point weight of chain helps a lot. It can take all the work out of the anchor but more importantly in shorter scopes it keeps the pull angle down to keep the anchor buried. Probably why many "experts" recommend a length of change above an anchor with a nylon rode aside from the chafe issue.

At some point the 20 pounds does make a difference, especially at the very small anchor range, but at some point it may reverse or see saw up the combinations.

I pretty well know the practice of ship anchoring dynamics, that's why I often leave anchor threads when more than one start comparing ships to small vessels....even though there are a few useful takeaways like just when and where the amount/weight of chain comes into play.

 

[rslifkin]

I would argue at some point weight of chain helps a lot. It can take all the work out of the anchor but more importantly in shorter scopes it keeps the pull angle down to keep the anchor buried. Probably why many "experts" recommend a length of change above an anchor with a nylon rode aside from the chafe issue.

At some point the 20 pounds does make a difference, especially at the very small anchor range, but at some point it may reverse or see saw up the combinations.

I pretty well know the practice of ship anchoring dynamics, that's why I often leave anchor threads when more than one start comparing ships to small vessels....even though there are a few useful takeaways like just when and where the amount/weight of chain comes into play.

Chain does help performance as mentioned, but until you get to fairly deep water or very heavy chain, the benefit is limited. Pound for pound, a bigger anchor adds far more holding power (at equivalent scope) than the benefit from heavier chain.


In fairly shallow water, you'd need massively oversized chain or crazy long scope (more than 10:1) to avoid pulling the chain pretty much straight in high winds (when holding power is most needed). In deep water you may not pull the chain straight in 50 kts even with 3:1 scope.

 

[SteveK]

........................................

And as a reference for the holding power of the anchor hereby some figures between holding power and angle of the shank with the seabed:

Holding power 100 % at zero degrees
Holding power 85 % at 5 degrees
Holding power 70 % at 10 degrees
Holding power 60 % at 15 degrees
Holding power 50 % at 20 degrees
Holding power 0 % at 25 degrees

As you can see the loss of holding power goes rapidly down with only a small increase in the angle of the shank with the seabed. It could only take a large wave to reach that 15 or 20 degrees and when you all of a sudden lose 40 to 50 % of your holding power of your anchor you could be in for a very nasty surprise.
...........................................

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.

 

[sunchaser]

Sooner or later an anchor, associated ride and said vessel will be at anchor. Water depth, tide rise and fall, predicted wind velocity, bottom conditions and wave action will come into play.

The more one anchors and gains knowledge is called experience. Arguing about the right scope, rode type and anchor itself is good internet fun for some. Hundreds if not thousands of anchoring events in a variety of conditions is a pretty good teacher too, these are the guys to listen to.

 

[O C Diver]

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.

That doesn't mathematically work in shallow water. For my 45' boat anchored in 10' of water, if I used a 4:1 scope (40' of chain below the surface), the entire chain length would be shorter than the length of my boat. Unless you're meaning greater than 3:1 plus a boat length of chain, I couldn't imagine anchoring with only 40' in 10' of water.

Ted

 

[luna]

My readings on ship anchoring seem to lean towards they too rely on their incredibly heavy chain and engines running during heavy winds as their anchor size and design are not really high holding like small boats have. So the concept isn't really bizarre.

Mambo42
Quote:

That is indeed what the larger ships do, they rely for 90 % of the holding force on the weight of the chain.


While I agree that when subjected to significant forces, a heavy chain will maintain a lower angulation than that of a lighter chain, I don't think that is what is meant by the above two statements. Maybe Im wrong.

I do note that the OCIMF anchoring calculator (applicable for vessels between 16,000 and 300,000 dwt.) that Mambo42 provided, does not require the user to input any information regarding the anchor chain whatsoever.

If ships actually heavily rely on the weight of the chain to restrict their movements, please show me where this feature is included in any of the calculations.

 

[psneeld]

Calculate all ya want...but bottom line is.... the higher your chain gets lifted off the seabed, the greater the chance of your anchor dragging.

My experience shows that when I anchored in Florida marl overtop of hard pan that the anchor would never penetrate.... if I put out 100 feet of chain in 7 feet of water, the anchor never bit, never moved and the chain just drug around the anchor in wind shifts.

Granted I may not been in those situations where the boat experienced more than maybe 20 knots of wind because I was sheltered behind obstructions, but the real wind was gusting to over 30.

So the experience Sunchaser was discussing taught me to buy more than enough chain in length and size, a slightly over sized anchor, prefer sheltered anchorages over open, learn wind forecasting well, and know the bottom composition/shelter/ longest rode necessary and possible. The rest falls into place without a "formula".... just rules of thumb.

Until one builds that experience or has had a lifetime of close calls over fails... just head to a protected, well built marina and sleep tight if a great anchorage is not near where you are when you want to stop for the night.

 

[Mambo42]

Unfortunately the variables again play too much into rules of thumb versus "numbers"

Just because one is in the same anchorage doesn't mean bottom composition holding power is always even nearly the same.

Wind force on frontal area varies wildly at times where wind gusts and boat veering changes minute to minute.

Same with waves in many anchorages as reflected waves in confined areas.

Then there are surges in tidal areas where the current force isn't always the same.

In my experience, that is exactly why I am a fair weather anchorer. I have just been through too many of those variables that I just rather be in a dock or certified mooring.

Granted, even pilings can snap or line chafe through in a slip. But there are multiple piling and lines if I add them as backups...but I only really anchor on one system with lots of variables.

The ultimate test is whether I feel a marina's infrastructure can withstand severe conditions.

I agree that there are lots of variables, but by taking out many of those variables I get more on top of the situation.
Unfortunately the marinas are mostly overbooked during the summer season, on top of that it is mostly loaded with charter boats of which the crews have zero experience. Chances on damage are pretty high. And if that is not enough the prices per night are idiotic. Over 400 euro per night is not an exception and then you still don't have water or electricity.
So that is why we choose to anchor out.

It is true that veering of the boat changes the loads, but only in a positive way. As soon as the boat veers you get a sideways component, but it means the force that is pulling on the anchor is getting less. Only if you would have the stern tied to the shore would the force, pulling away from the anchor, increase.
In other words, veering does not worry me too much. Also, in a storm you don't get 180 degrees wind shifts i a matter of minutes. A sudden thunderstorm can do that, but the wind behind the thunderstorm is less than the windfront ahead of the thunderstorm.

Waves are indeed a factor to take into account. When we are in a bay where waves can become high I do throw out more chain. But most of the time we only have to deal with wind. 20 kts is more or less standard here, 30 kts is also quite normal. 40 to 50 kts is also regular in the Aegean, but if there is a chance we will see that in an anchorage we try to look for more protection as soon as we can.

It would be nice if we would be in calmer weather, but the Aegean is not known for calm weather, so if you want to see the islands in this area you have to be prepared. Next year we will first move into Turkey again, where there is more protection and in June we will move from Turkey to Croatia where the wind is usually a lot less, but sudden storms still happen. This year they had quite a few sudden heavy storms in Croatia, but luckily there is also a lot of bays where you can find shelter from at least the waves. The wind however will keep pounding you.

 

[psneeld]

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

 

[Mambo42]

Calculate all ya want...but bottom line is.... the higher your chain gets lifted off the seabed, the greater the chance of your anchor dragging.

That is exactly the problem and also the reason why your anchor drags. The only way to prevent that is with enough chain. If you have a heavy chain you can drop less, if you have a light chain you will have to drop more.

My experience shows that when I anchored in Florida marl overtop of hard pan that the anchor would never penetrate.... if I put out 100 feet of chain in 7 feet of water, the anchor never bit, never moved and the chain just drug around the anchor in wind shifts.

Hope you did not get me wrong, we do set the anchor and we set it well. If it does not hold we will haul it up and start over again. If the seabed is such (eg seaweed) that the anchor does not want to hold we will not anchor in that place, we will find a different spot.
Luckily I have a drone, so I can find the patches of sand relatively easy. If we cannot see them (for what reason it may be) from the boat, we can see them via the drone from about 25 to 50 mtr high.

Granted I may not been in those situations where the boat experienced more than maybe 20 knots of wind because I was sheltered behind obstructions, but the real wind was gusting to over 30.

Sadly in this part of the Med the wind is usually 7 to 8 Bft 90 % of the time. It is called the Meltemi wind and there is nothing you can do about it, you just have to live with it or not come here at all. Since we had to go to Turkey we had to get from West to East and that means going through the Aegean. Now we have spent our allotted time in Turkey (90 days) so we had to get out. However, Turkey is beautiful, so we want to come back next year. Therefore decided to keep hanging around in the east of the Aegean, but even now that it the end of October (Meltemi should be over) we still have 25 to 30 kts of wind every single day.
That means that you must know how to anchor safely with an accuracy of 99 %, otherwise you basically cannot sail in this area of the Med.
Now that it is October the city ports have more space, but the marinas are all filled up to the top, there is no chance getting in there. But even for the city ports you can see a race around 4 to 5 PM. All of a sudden you will see 10 or 20 boats racing at full power to get one of the few spaces in the city ports. It is ridiculous, but that is how it is.

So the experience Sunchaser was discussing taught me to buy more than enough chain in length and size, a slightly over sized anchor, prefer sheltered anchorages over open, learn wind forecasting well, and know the bottom composition/shelter/ longest rode necessary and possible. The rest falls into place without a "formula".... just rules of thumb.

Agree with you on everything, but here in Greece there is one thing you won't be able to do or even get. There is no accurate wind or weather forecast.
Reason ?
Somehow the Greeks don't find it necessary to set up measuring and reporting stations on the islands and thus you cannot get the data you need to be able to make any forecast.
Normally you could look at cloud patterns, but there are no clouds in this area. If I have seen one or two clouds during the last 3 months it is a lot. Is great for the solar panels, but not for forecasting. The wind however has been blowing almost constantly with 25 - 30 kts, sometimes gusting to 40 - 45. In three months we had exactly 4 calm weather days, that was it. And we were actually lying most of the time on the dry in Didim, Turkey.

A long time ago I had to study meteo for my pilot license and had to be able to make forecasts based on available data. I now find out that in the Aegean I am basically sailing in a black hole ! There is no accurate data. What the weather apps predict is based on computer simulations, but that has nothing to do with reality.
And thus I have no other option than to be prepared for the worst.

 

[SteveK]

That doesn't mathematically work in shallow water. For my 45' boat anchored in 10' of water, if I used a 4:1 scope (40' of chain below the surface), the entire chain length would be shorter than the length of my boat. Unless you're meaning greater than 3:1 plus a boat length of chain, I couldn't imagine anchoring with only 40' in 10' of water.

Ted

Ted, yes this is a case of location. We have 10 foot tides so anchoring in 10 feet to start with seldom occurs. So yes anchoring in 10 feet of water I defer to your experience as I am not even sure what I would drop in 10 feet.