Anchoring in the wind

[CaptTom]

When monitoring for drag, what I always do is just leave a chartplotter or phone charting app on with tracks turned on. If there's any wind or current, you should end up with a very clearly defined "arc" on the screen. Anywhere in that arc, you're OK. Sometimes you'll get a second arc downwind from the first. That could mean the anchor dragged a bit, then grabbed. Bit it could also mean that some of the chain which was laying loosely on the bottom straightened out.

Either way, you only have to glance at the screen occasionally to know exactly what's going on.

The other thing is to look for natural "range markers" on shore. A tree lines up with a rocky outcrop, or a point of land lines up with structure or whatever. Find at least two, or better yet, three ranges, widely separated. As long as they are aligned, you haven't moved. If one is moving, you can tell which way you're dragging. If you're swinging a lot, you'll probably have to allow some amount of leeway, as long as it swings back at some point.

 

[GrandWood]

All I asked 4 pages ago is would I drag as I swing side to side, I was thinking the anchor would saw loose, lol

I’m throughly schooled on anchoring now, thank you all very much. Much appreciated

 

[Mako]

@GrandWood now you learned the lesson of starting anchoring threads! It would be less painful if you started a debate on singles versus twins. Ha

 

[Mambo42]

@GrandWood now you learned the lesson of starting anchoring threads! It would be less painful if you started a debate on singles versus twins. Ha

Anchoring is one of the most important parts of being on the water, at least if you don't go from marina to marina.
And in order to feel and be safe I think it is necessary everyone understands what it is that you are doing and how to do it safe, especially in heavy weather.
I know that there are a lot of people who basically don't care, I see them each year, anchoring in the weirdest places, dropping hardly any chain, not setting their anchor and somehow many of them get away with it. However, others are not so lucky and they end up on the shore or on the sea bed.
In my opinion boat owners should always play it safe, after all, it is your own property.

 

[luna]

If you cannot reach 7 : 1, only can get to 5 : 1 you simply forget that if waves show up you may not have 5 : 1, you will have perhaps 3 : 1. Then you will reach 22 degrees

I have no desire to extend an anchoring thread, but I am perplexed by the above statement.

If your anchor rode was deployed with a scope of 5:1 and the "waves show up" please explain how you suddenly find yourself at 3:1?

The only thing I can think of is that the waves lifted the boat up. There would just be more water under the boat.

I won't bother you with the math but if you were originally anchored in 10 feet (3 metres) of water, the waves would have to lift the boat 6.667 feet (2 metres) to accomplish this feat.

Anchored in 50 feet (15 metres) of water the waves would need to lift the boat about 33.3 feet (10 metres) ......Yikes!

Does this really happen in the Med?

 

[Mambo42]

I have no desire to extend an anchoring thread, but I am perplexed by the above statement.

If your anchor rode was deployed with a scope of 5:1 and the "waves show up" please explain how you suddenly find yourself at 3:1?

The only thing I can think of is that the waves lifted the boat up. There would just be more water under the boat.

I won't bother you with the math but if you were originally anchored in 10 feet (3 metres) of water, the waves would have to lift the boat 6.667 feet (2 metres) to accomplish this feat.

Anchored in 50 feet (15 metres) of water the waves would need to lift the boat about 33.3 feet (10 metres) ......Yikes!

Does this really happen in the Med?

The waves in the Med are indeed quite different then the waves out on the ocean. If you follow Awanui NZ (Nordhavn 51 channel on Youtube) you will know he first was in the Med and then went outside on the Atlantic ocean. He reported a completely different wave pattern (from high waves with a short period to less high and much longer period).
Obviously we don't have waves of 15 mtr, but that is also not necessary. To make it more understandable I made some pictures of vector diagrams in one of the books I am using. The explanation is pretty straight forward. I will see if I can put text in between the pictures, if not then I will put them in separate posts.

In the first diagram you can see the second situation when waves cause the boat to pitch. The starting point is L = 5T, but then with waves the bow pitches up, stretches the chain and since the chain does not get any longer that means the boat will get a forward pull. That results in the angle of the chain/shank with the sea bed changing to 22 degrees, which is a situation in which the anchor has almost lost all holding power, no matter how heavy the anchor is.

In the last diagram you can see that even when the wind and waves put a force on the boat there is always a part of the chain remaining on the sea bed, resulting in the shank remaining horizontal.
In order to keep that chain on the sea bed it automatically means the weight of the chain (in Newton) needs to be more than the forces on the boat (also in Newton). There is no other explanation.

 

[luna]

Thanks for the explanation. It seems to confirm that it was the water, by way of waves, that got deeper which would result in the reduced scope.

The issue I have is the reduction in scope from 5:1 to 3:1 in 3 metres of water means the waves are lifting the boat about 2 metres. A wave over 2 metres in height in only 3 metres of water would more than likely have already broken. Who would be anchored in that spot?

In 15 metres of water the wave would need be about 10 metres high to reduce the scope from 5 to 3:1.
I certainly would not be anchored there.

Thanks again.

 

[DDW]

If you cannot reach 7 : 1, only can get to 5 : 1 you simply forget that if waves show up you may not have 5 : 1, you will have perhaps 3 : 1. Then you will reach 22 degrees, meaning you have almost no holding power left, no matter how heavy or big your anchor is.
....
I will never anchor in an anchorage where I cannot have the required space around me and I will not tolerate anyone coming within that swing circle if storms are forecasted. I may not be the most friendly person then, but I don't care, I care about my boat, my safety and if you want to wrap yourself around a rock...........be my guest, but I am not going to join you.
....
The larger anchor is only going to help you if you can keep that anchor shank on the sea bed, otherwise it is not going to do you any good. The table I posted applies to almost all anchors, at 25 degrees you will have lost all holding power.

You should check your math, 3:1 is 19 degrees, not 22. It is great to have the luxury of unlimited swinging room and unlimited chain in the locker. You are perhaps the only one with that luxury.

Now, the anchor losing all holding power at some shallow angle - this is provably not correct. Steve on Panope has done probably more anchor testing than anyone in the world, ever. He has tested all of the most popular anchors and some off beat ones. His normal test is at 3.5:1 scope, which you say will have "almost no holding power". And yet the better anchors develop near full or full holding power at that scope. Yes, he has also tested the effect of scope. Anyone with an unhealthy interest in anchoring should spend a couple of days watching his 160+ videos.

Math and theoretical analysis and pictures in books will only get you so far, without validation testing. The validation testing has been done, and does not support your theory.

 

[GrandWood]

After reading this thread, I’m dumping 150ft of chain no matter what. I always look for 15 to 20ft of water at low tide, when I’m looking to go somewhere. My GB has 3.5 ft of draft, and anything less the 15ft, I get sketched out.

 

[Mambo42]

Thanks for the explanation. It seems to confirm that it was the water, by way of waves, that got deeper which would result in the reduced scope.

The issue I have is the reduction in scope from 5:1 to 3:1 in 3 metres of water means the waves are lifting the boat about 2 metres. A wave over 2 metres in height in only 3 metres of water would more than likely have already broken. Who would be anchored in that spot?

In 15 metres of water the wave would need be about 10 metres high to reduce the scope from 5 to 3:1.
I certainly would not be anchored there.

Thanks again.

The most important take away is: 'when waves appear the angle of the chain (and thus shank of the anchor) with the sea bed changes. On top of that, waves create a jerking motion, which means increasing and decreasing force on the anchor. That is basically the same as wiggling something loose when it is stuck.

As for the wave heights ?

Here in the Med you anchor on the lee side of an island and that is not necessarily in a protected bay. Reason for this is that the protected bays are usually already taken by marinas and ports of villages. During the season we have about 5000 charter boats out on the water, meaning all marinas and ports will be full, every single day.
So most of the time everybody anchors in open sea and if the wind suddenly changes you can find yourself in very rough water. The anchorage in the videos below is the same bay where we also dragged our anchor, also a sudden, not forecasted storm. Also then a lot of boats dragged their anchors and for me this was the turning point. From that moment on I wanted to make sure it would never happen again. And since I figured it out, we have never dragged anchor anymore, not even in extreme weather with 65+ kts.

Problem with this anchorage, and with amost all anchorages in the Med, is that there is no place to hide, other than the other side of the island. The island in the video is Corfu and if you take a look at Corfu then you will see that there is hardly any protection if the wind shifts from NW to East. You can pick up your anchor, but you can't go anywhere, you won't find any shelter. The only shelter is on the other side of the water, a place called Igoumenitsa, but that is a 2 hr sail, which you don't want to do in the middle of a storm.
So yes, those 2 mtr waves you will have quite quickly if a storm suddenly arrives.

In the first video you hear that all the boats in the anchorage are gone, they have all dragged anchor. The boat that made the video had 50 mtrs of chain out in 5 mtrs of water, they held on with help of their engine. They had 60+ kts of wind.
In the other video you actually witness a mayday call. Petriti is a village halfway on the island of Corfu, has a small port for maybe 10 boats, but in the season there will be about 40 to 70 boats anchored outside.
The Mayday call comes from a sailing vessel that had a lightning strike which took out their engine and then they started dragging their anchor. 8 persons were onboard.

or this one (same boat)

In the last video, which was a freak storm this summer in Rovinj, Croatia, 26 boats were thrown on the rocks. You can see the pitching of the boats in the anchorage in front of Rovinj.

So after seeing this, maybe you understand why I am so deliberate about anchoring properly and thereby saving my boat.

Unfortunately here in the Med we are getting more and more of this extreme weather. Weather forecasting is pretty p*ss poor, so most of the time you are caught by complete surprise, so you need to be ready, at all times.

 

[HeadedToTexas]

All I asked 4 pages ago is would I drag as I swing side to side, I was thinking the anchor would saw loose, lol

I’m throughly schooled on anchoring now, thank you all very much. Much appreciated

As usual, a lively discussion on the physics of anchoring. Excellent stuff. While the physics are interesting, the real question should be "How can I better read the weather to avoid anchoring in conditions that cause sleepless nights?" A couple restless nights and the resulting expansion of my grasp on wind and weather modeling have convinced me that there is no way to always avoid unpredicted wind shifts. The only way to avoid sleepless nights is to more aggressively move to a back up site if and when the wind does shift.

To address the OP's question, I don't think swinging on anchor will saw the anchor's hold loose unless the rode is completely taught, and that is not going to happen at the 25 knot wind you saw.

 

[Mac2]

Good discussion and videos. Once again this thread confirms that your “storm” anchor should be your primary anchor. OP was worried about the anchor dislodging while swinging back and fourth. Totally get it. There is very little reaction time if this happened where the OP was anchored. Now consider if it is nighttime and you can’t see shore. If I started to drag I would start the engine to put it in gear, but then you have to time it to steer towards open water and not land. One consideration I have is to let the rode go after clipping some fenders to the chain rode and retrieving it later. That way I don’t have to worry about getting fouled on the rode and I can just steer towards open water. The second lesson, also confirmed by the videos, was the biggest threat at anchor-other boats. You can have the best anchor setup in the world, but it’s useless when another boat drags across your rode and snags it. This has happened to me several times. I make a point of trying to be the one on the outside-furthest from shore. If someone drags towards me (rare because they should be protected from the land side) we are dragging towards open water.

 

[Mambo42]

As usual, a lively discussion on the physics of anchoring. Excellent stuff. While the physics are interesting, the real question should be "How can I better read the weather to avoid anchoring in conditions that cause sleepless nights?" A couple restless nights and the resulting expansion of my grasp on wind and weather modeling have convinced me that there is no way to always avoid unpredicted wind shifts. The only way to avoid sleepless nights is to more aggressively move to a back up site if and when the wind does shift.

To address the OP's question, I don't think swinging on anchor will saw the anchor's hold loose unless the rode is completely taught, and that is not going to happen at the 25 knot wind you saw.

If you are in an area where the weather forecast is pretty accurate and there are no non forecasted TS or heavy weather or even 180 degrees shifts in wind, your life will be a lot easier.
In the videos I posted you can see the Ionian, which is far less predictable than the Aegean. In the Aegean weather forecasts are also way off, but at least it is only the wind speed. The wind is usually coming all from the same direction, so if you are anchored on the right side of the island you will be safe.
In the Ionian that is not the case. What looks totally safe based on the forecasted wind and weather can turn into a complete nightmare a few hours later (as you can see in the videos).

As for sawing the anchor loose while swinging at anchor, I am not worried about that either. I don't swing around my anchor, I swing around the anchor chain and that makes it a completely different story. I have been lying next to my anchor at 40+ kts wind, meaning the chain was not even stretched. That will give you peace of mind.

 

[HeadedToTexas]

If you are in an area where the weather forecast is pretty accurate and there are no non forecasted TS or heavy weather or even 180 degrees shifts in wind, your life will be a lot easier.

Hehe, hard to argue with that! Western Lake Erie is apparently one of those unpredictable areas.

 

[Mambo42]

Good discussion and videos. Once again this thread confirms that your “storm” anchor should be your primary anchor. OP was worried about the anchor dislodging while swinging back and fourth. Totally get it. There is very little reaction time if this happened where the OP was anchored. Now consider if it is nighttime and you can’t see shore. If I started to drag I would start the engine to put it in gear, but then you have to time it to steer towards open water and not land. One consideration I have is to let the rode go after clipping some fenders to the chain rode and retrieving it later. That way I don’t have to worry about getting fouled on the rode and I can just steer towards open water. The second lesson, also confirmed by the videos, was the biggest threat at anchor-other boats. You can have the best anchor setup in the world, but it’s useless when another boat drags across your rode and snags it. This has happened to me several times. I make a point of trying to be the one on the outside-furthest from shore. If someone drags towards me (rare because they should be protected from the land side) we are dragging towards open water.

I hear this a lot and I made the same mistake the one time I dragged the anchor picking it up or even throwing it overboard.
Since then I changed my approach. I added another 40 mtrs of chain, making it a total of 140 mtrs. When I chose the spot where to anchor I always take into consideration not to be too close to shore.
Reason ?
In the possibility that I would drag anchor, before I am going to pick up the anchor or cut the chain, meaning losing everything, I will first drop everything I have onboard as chain, i.o.w. 140 mtrs of chain in the water attached to my boat. What happens is, that it will further reduce the angle of the chain with the sea bed, ensuring it will pull straight back on the anchor and giving the anchor a chance to reset and regain holding power. In addition, the chain is going to create further friction and holding power itself.

You can obviously not do that when you have the shore right behind your boat, so if you are planning on tying up to the shore, start by already dropping as much chain as you can. If the bay or anchorage won't offer you that solution........don't anchor there in expected bad weather, keep going until you find a better place to anchor.