Photos of Anchors Setting

[JPA Cate]

Originally Posted by Sandy Feet

Hi Ann it's 85lb (38.5kg)

Thanks for getting back to me SF. I bet you're going to be happy with that! Looking forward to pics of your Mantus setting, soon.

Ann

[noelex 77]

We had a great overnight sail. A nice moon and loads of dolphins.

However, by the end of the trip the wind picked up to 40 knots instead of dying, as was forecast to do. Putting too much faith in the forecast, we carried excessive sail and blew out one of the heavy duty Lewmar blocks. A reminder of the forces we deal with.

In the protection of the anchorage the wind had dropped to 30 knots or so. It is hard in this sort of wind to set the anchor with a nice progressive force. I am not sure what those who advocate giving the anchor time to "settle" do in these circumstances .

This was just enough wind to make power setting with reverse unnecessary. So the anchor was set with just the wind force alone.

Anyway, this was the Mantus the next morning. You can see it has set in less than a shank length despite the more abrupt setting force. BTW, this anchorage is listed as poor holding. I am not sure why.

[noelex 77]

The wind was forecast to change to the north and strengthen so we have moved. It was blowing a light westerly when we arrived so from this set we should see the anchor rotate 90 degrees.

It was a bit dark and stormy when I took the photo yesterday, but you can see the Mantus is well set in what looks like sand with a very fine weed cover. The weed roots from this sort of very fine weed give the sand a different texture. It causes the sand to clump and stick together. If you look closely there is an interesting depression or dug out section of seabed just next to the shank extending forward and back. It could be seen better from the bathyscope, but it was a bit hard to judge the depth. Often what looks like slight depressions or mounds from the surface turn out to be more significant when you dive down and take a close look.

I only saw the anchor from the surface, but this sort of scooped out area is typical of the drag mark left behind by a large anchor. In hard sand these marks can persist for a long time especially without any wave action. The other more concerning possibility is in seabeds with large sheets of rock covered in a sand layer, there are often gaps between the rocks and these sometimes display these strange sort of depressions.

The anchor is well set and is not showing any signs of being restricted by a rock layer. The wind will rotate the shank away from the depression.

These sort of anomalies together with debris, rock etc are quite common. They can be enough to effect the setting or the rotation of an anchor, especially a small anchor rotating into a depression left by a large anchor.

It is a reminder that even with a good anchor and sound technique we still don't have complete control of the biggest variable: the seabed.

This is the wind forecast for today and tomorrow:

[noelex 77]

We usually get a genuine 50 knots plus average at anchor at least once a year. Up until now we have just avoided this. The wind experienced in post # 302 was significant, but my best estimation was just under 50 knots.

We have had a few threatening forecasts this winter, but in each case the wind was significantly less than predicted. However, last night with a mean wind forcast of about 40 knots over 48 hours turned out to be a 50 knot average for a few hours in the middle of the night. I can only estimate this as our wind instruments are still not fixed. A local weather station that was inland and I think more protected recorded an average wind of 48 knots at 3am.

At some stage during the night we broke the snubber. I always have a short additional snubber rigged which takes the load off the windlass, but was too short for any elasticity. 50 knots without the elasticity of a long snubber is not very good. You need some elasticity in the system. I have had a few snubber breaks, but they have always gone with a loud unmistakable bang that would awaken the dead. We did not hear this one.

The failure occurred at the knot (a bowline with a Yosemite finish attached to the chain with a soft shackle). In these conditions I should have rigged two long snubbers. My only excuse is complacency from the previous pessimistic forecasts.

A splice is also better than a knot as knots do have a reasonable reduction in breaking strain. For a snubber you need the maximium stretch so you want the thinnest diameter that won't break. I do seem to have a lot of problems with breaking snubbers. I thought I had solved the problem moving to some good quality name brand nylon, but it seems not. This particular snubber had gone through 4 days of 40 knots plus and probably a couple of weeks worth of 30 knots so perhaps just accumulated fatigue is taking its toll?

Anyway the anchor never moved according you the GPS, despite what must have been some significant loading especially without the benefit of elasticity. I will try and get some photos of it.

A short video of the wind although this was mild compared to midnight to 3am wind:

https://www.youtube.com/watch?v=IbEIhXPeQyw

[Tayana42]

Good thing the fetch wasn't any longer. Even so that is a lot of stress on the whole system without your snubber. How much scope did you have out? Another good job of holding by the Mantus.


S/V B'Shert

[BigBeakie]

" For a snubber you need the maximium stretch so you want the thinnest diameter that won't break. I do seem to have a lot of problems with breaking snubbers. I thought I had solved the problem moving to some good quality name brand nylon, but it seems not. This particular snubber had gone through 4 days of 40 knots plus and probably a couple of weeks worth of 30 knots so perhaps just accumulated fatigue is taking its toll? "

Noelex, I've been thinking about this issue of what is the best rode for a snubber, as I listen to several accounts of nylon rodes breaking, either as anchoring snubbers or parachute anchor rodes.

It seems that although nylon has the best stretch/length properties, there are some downsides of nylon to consider, which are that it loses about 30% strength when wet, and under strong cyclic loading, the fibers can heat up, weld together and become very brittle, then break. Steve Dashew had this happen to him and investigated by speaking to a number of rope manufacturers.

The solution appears to be to use polyester rope of thinner diameter that is still adequate breaking strength for max load. But you must use more length to compensate for polyester's less stretch. One way to do that is attach the ends towards the back of the boat and run the rode along the length of the hull (through the stanchion bases?) and then through bow blocks. So this long sided bridle in polyester has better cyclic loading endurance.

What are your thoughts on this idea?

[noelex 77]

I agree Big Beakie. Running the snubber along the deck is a great idea. It avoids the snubber abrading on the seabed in light wind, eliminates and tangles with the chain and if you want to let out more rode you do not have to retrieve much, or any, rode to unhook the snubber. This latter advantage means that you can let out more rode without starting the engine even in strong conditions.

The only problem is not many boats can attach the snubber at the rear without creating a lot of chafe points. I would also worry about the strength of the some of the hardware. I don't think stanchion bases would be strong enough. They are not taking the full load, but they need to resist the sideways force as the snubber tries to take the shortest path between the rear cleats and the chain attachment. You can work out the force based on the change in the angle of pull at each of the attachment points, but it hard to know a reasonable WL of the stanchion bases.

If you can overcome these difficulties, a snubber rigged from the rear is an excellent solution. Even better one along each side gives you a back up. By making one just slightly longer, two independent snubbers could be used routinely with no extra effort.

In terms of material, nylon has a lot more stretch and it seems intuitively to be the best material. In bad conditions a 10m + length of nylon is needed. It would have need a long length of polyester to achieve the same stretch. The distance from the rear to the bow would not be long enough, so you gain the disadvantages of both systems with line along the deck and extending some distance into the water. I am prepared to be convinced polyester is better if someone wants to the maths looking at % of stretch wet and dry breaking strain etc.

The ultimate snubber would seem to combine nylon for stretch with Dyneema in short lengths where there is risk of chafe. Estarzinger has done a lot of great work in this area.

[JPA Cate]

At the risk of being thrashed, I'd like to question the real need for a snubber to stretch great distances. Its real jobs are to remove stress from the windlass and to avoid shock loads to anchor and all the other ground tackle. I wonder if the smaller stretch of using polyester line would be such a big deal?

The snubber that I have used for the past decade or so is 16 mm double braid nylon... way bigger than many advise for our ~12 tonne (fully loaded) boat. I normally deploy around 5-8 metres and it appears to stretch a few hundred mm in gale force winds. It is difficult to accurately assess the actual stretch, since the chain hook is usually well out of sight. At any rate, we have not seemed to suffer any adverse effects from this less than usually stretchy snubber, and this suggests to me that use of polyester wouldn't be such a bad idea. I think that even this limited stretch does an adequate job of removing shock loading from the gear.

What think you?

Jim, using Ann's computer

[noelex 77]

The water visibility is still not great following the storm, but these were the best photos I could get without diving. Some are taken from the surface and some from my still evolving "dangle the camera below the tender" technique.

The Mantus has rotated about 60 degrees in response to the wind change from the setting direction. It has barely moved, other than rotating, despite the rotation and burying much deeper. You can still see the depression in the seabed just behind the roll bar. There is a large clump of loose broken weed that has accumulated in the hole caused by the Mantus initially digging in.

It is a oversized anchor, but it kept us safe in strong winds despite the loss of the snubber.

The setting scope in post #1293 was 5:1 in 7.3 m. I extended the scope to 10:1 for the stronger wind. When I eventually realised the snubber was broken, I rigged another and let out a further 10m of scope.

In this photo you can see the Mantus has rotated about 60 degrees. The depression in the seabed behind the roll bar is still visible and helps pinpoint the location. The shank and chain are not visible but are leading off to the left:




The fluke and shank are a long way down with less than half the rollbar visible. The shank and chain are leading off to the top of the picture:

[BigBeakie]

Hi Jim,

Yes the whole idea is to find a workaround to the vulnerabilities of nylon. I do not have the stats that Noelex questioned re % stretch and WLL to polyester diameter, but from memory you need about double to triple the length of polyester cordage of the same nylon diameter to provide same stretch in the 8mm to 15mm range. But it is much superior in the endurance stakes with repeated cycling under load, as is known from the lifting applications.

The figures are in some rope manufacturers websites. Dashew researched this quite thoroughly, I believe. I spoke with Jonjon about this who is using this system successfully on his cat for quite awhile.

I wonder if plaiting 3 smaller diameter polyester would give us more stretch and adequate WLL performance? I can't find any data on that yet.

[noelex 77]

Originally Posted by Ann T. Cate

The snubber that I have used for the past decade or so is 16 mm double braid nylon... way bigger than many advise for our ~12 tonne (fully loaded) boat. I normally deploy around 5-8 metres and it appears to stretch a few hundred mm in gale force winds. It is difficult to accurately assess the actual stretch, since the chain hook is usually well out of sight. At any rate, we have not seemed to suffer any adverse effects from this less than usually stretchy snubber, and this suggests to me that use of polyester wouldn't be such a bad idea. I think that even this limited stretch does an adequate job of removing shock loading from the gear.

What think you?

Despite the success (ie we did not drag or break anything) of my inadvertent experiment of no snubber, I still think some reasonable elasticity in the system is important in strong wind.

Normally the boat motion is noticeably improved and the forces particuarly at the end of yaw are noticeably less. There have been some theoretical calculations done on this and this link is a great read:

Rode - Dynamic Behavior

The anchor manufacturers also strongly endorse the use of snubbers. For example this is Knox Anchors' thoughts on the subject:
Knox Anchors - The top performing New Generation Anchor for safe and secure anchoring - Springs

Knox Anchors - The top performing New Generation Anchor for safe and secure anchoring - More about wind forces and snubbing


However Jim, I think we should question all these beliefs. There are still some puzzles.

Very few boats use what I would regard as an adequate snubber (say 10m of nylon of a just adequate diameter). In strong wind lots of boats drag, but if this simple piece of equipment is so vital why do so few boats use it? It is particuarly telling when an experienced cruiser like Jim can manage with a snubber that I would consider was marginal in its ability to stretch.

I did not notice the snubber breaking on this occasion - if it so vital why not? After a couple of days at 40 knots+ you get accustomed to a lot of motion and noise. I was in the rear cabin when the snubber broke, but the bottom line is I did not notice.

[hoppy]

Why not just use a rubber mooring line spring on the snubber to absorb the shock?

[Hydra]

What causes a snubber to melt is kinetic energy from waves, turned into heat. I think that we need other means of dissipating this kinetic energy.

A possible solution would be a sea anchor deployed from the stern but I don't see how to prevent it from tangling with the rudder.

Alain

[BigBeakie]

Hoppy: the issue is the weakening of nylon with repeated cycles of stretching, so if that is nylon going through the rubber thingamabob, it doesn't solve the problem, seems to me.

Hydra: Sorry, I don't follow your thinking at all Arn't we discussing being at anchor, as in stuck to the bottom anchor as opposed to parachute anchor? Are you proposing we anchor stern to???

BTW, the issue for parachute anchoring on a nylon rode is the same, but even more so. The stretch on 300 to 600 feet of nylon lying to a parachute anchor is very significant depending on how big the seas are. I have seen a piece of 15mm nylon rode fused together inside the rope from the stretching. That's why I'll be using good quality polyester rode which does not melt like that.

And deploying out the back of a boat would be called a drogue, by convention, and that should be on a non-stretch rode so you maintain your position in the wave train and the drogue stays in the wave body.

[Hydra]

My thinking is to anchor from the bow, as usual, and deploy a large sea anchor from the stern. The purpose is to severely restrict the surging of the boat when there is a lull in the incoming waves, by dissipating energy as a shock absorber does in a car suspension. IMO, an ordinary snubber behaves like a spring: it absorbs kinetic energy by stretching, then gives it back.

I refer to a sea anchor because a drogue is only sized to reduce headway when running downwind. Here, I intend to *cancel* the low-frequency / high amplitude / high energy motion, as when anchored bow and stern. The idea is not to deploy a stern anchor, because it would be impractical when the winds veers and it would not dissipate kinetic energy.

Unfortunately , I avoid anchoring in exposed places, so I never found the opportunity to try this idea.

Alain