How much snubber / bridle is too much?

[singlespeed]

Interesting, but I have found snubber length is actually dictated by how far the snubbed attachment to the anchor chain goes under the boat in an adverse tide/wind direction issue. A small catamaran anchored near us in the Bahamas, was using a snubber about ten feet longer than overall boat length. During the night the wind died, but the current caused the snubber to pass under a keel and rudder. Current reversed before dawn, causing the boat to swing across the direction of the current. Not only did the snubber's metal attachment rub against the hull, but they spent a lengthy period removing the snubbed and anchor chain. I use a snubber the length of my bow attachment point to the mast. Never had a problem, even in a Cat 1 anchored in a river.

[MathiasW]

Originally Posted by singlespeed

Interesting, but I have found snubber length is actually dictated by how far the snubbed attachment to the anchor chain goes under the boat in an adverse tide/wind direction issue. A small catamaran anchored near us in the Bahamas, was using a snubber about ten feet longer than overall boat length. During the night the wind died, but the current caused the snubber to pass under a keel and rudder. Current reversed before dawn, causing the boat to swing across the direction of the current. Not only did the snubber's metal attachment rub against the hull, but they spent a lengthy period removing the snubbed and anchor chain. I use a snubber the length of my bow attachment point to the mast. Never had a problem, even in a Cat 1 anchored in a river.

Now, that makes a lot of sense! I have often been wary when I saw the bridle disappearing between the hulls towards the stern... I know they cannot reach it, but still...

[MathiasW]

Originally Posted by Pillroller

I always thought the snubber was the shock absorber. The first line of resistance is raising the chain off the ground, the snubber is not under major strain until the rode and chain are in a straight line? The snubber needs to be fairly short, so that it does not exceed its breaking strain, before the rode takes up the load from the attachment point of the snubber to the rode made off to the boat. Which is what one of the previous replies was hinting at, that his snubber is smaller diameter than the rope rode. It should never have to take the full load.

It will be a gradual transition until the snubber takes over most of the load.

I would definitely not advise to make the snubber short, as it will then not be able to store enough energy.

Not sure what you mean by needing the snubber to be short not to exceed its breaking strain. By making it longer you do not affect the breaking strain adversely, on the contrary.

[thinwater]

Originally Posted by MathiasW

...And the only place it can go is the chain and the bridle / snubber. So, once their storage capacities for energy are exhausted, the loads will go up. But it is a race between these two phenomena, and as long as your nylon can cope with it and keeps stretching more and more, there will be no harm done, I agree.

Also water friction, which is likely material, particularly if the boat is moving at an angle.


I've always been intrigued by deploying a devise looking something like a Davis RADAR reflector at he bridle apex point. It could reduce both yawing and surging. But it seemed awkward and I did only a little testing (I have tested drogues for yaw reduction--helps within limits).


One problem with a strongly negative derivative material is that it must be closely matched to the force. On the other hand, a climbing rope snubber is gentle in light winds on poor holding bottoms, and firm in stronger conditions. So the conclusion is that the damping and the spring must be adjustable. By solution was to carry two snubbers, one thinner than the other, and they were tuned as much to holding conditions as weather.

[thinwater]

Originally Posted by singlespeed

Interesting, but I have found snubber length is actually dictated by how far the snubbed attachment to the anchor chain goes under the boat in an adverse tide/wind direction issue. A small catamaran anchored near us in the Bahamas, was using a snubber about ten feet longer than overall boat length. During the night the wind died, but the current caused the snubber to pass under a keel and rudder. Current reversed before dawn, causing the boat to swing across the direction of the current. Not only did the snubber's metal attachment rub against the hull, but they spent a lengthy period removing the snubbed and anchor chain. I use a snubber the length of my bow attachment point to the mast. Never had a problem, even in a Cat 1 anchored in a river.

A long snubber can be attached to the stern cleats and led over the bows in a manner that minimizes friction and chafe. I've done it for years. In this way you can use a long snubber in shallow water, which as it turns out, is when it is most needed (minimal catenary).

[thinwater]

Originally Posted by Pillroller

I always thought the snubber was the shock absorber. The first line of resistance is raising the chain off the ground, the snubber is not under major strain until the rode and chain are in a straight line? The snubber needs to be fairly short, so that it does not exceed its breaking strain, before the rode takes up the load from the attachment point of the snubber to the rode made off to the boat. Which is what one of the previous replies was hinting at, that his snubber is smaller diameter than the rope rode. It should never have to take the full load.

A problem with loading a 3-strand snubber hard with minimal lazy loop of chain is that the rope rotates (spins) underload and the chain wraps around it. If the chain stays wrapped, even one pass, it can chafe the snubber nearly through over night. I've seen people test this theory and nearly lose the snubber.


The problem with using a short snubber is that it only protects the windlass, not the ground tackle and anchor. It is too short to absorb meaningfull energy. That is a different purpose for the snubber, not what the OP was discussing.



Really, long and short snubbers serve completely different purposes and should not be discussed in the same thread. For example, I know people that use very long bridles, that also have a short snubber for setting anchors and breaking them out. Different.

[s/v Jedi]

Originally Posted by thinwater

A problem with loading a 3-strand snubber hard with minimal lazy loop of chain is that the rope rotates (spins) underload and the chain wraps around it. If the chain stays wrapped, even one pass, it can chafe the snubber nearly through over night. I've seen people test this theory and nearly lose the snubber.


The problem with using a short snubber is that it only protects the windlass, not the ground tackle and anchor. It is too short to absorb meaningfull energy. That is a different purpose for the snubber, not what the OP was discussing.



Really, long and short snubbers serve completely different purposes and should not be discussed in the same thread. For example, I know people that use very long bridles, that also have a short snubber for setting anchors and breaking them out. Different.

What you call a short snubber, I call a chain stopper. Again, I have never seen proof that snubbers longer than 20’ are an improvement. People who claim that are using snubbers that are too thick, i.e. they don’t get the stretch right.

[Just Another Sa]

Originally Posted by MathiasW

Warning - possibly only for nerds...

I have heard a number of folks advise against having too much snubber / bridle whilst anchoring, and starting with my analysis at https://trimaran-san.de/die-kettenku...atiker-ankert/, I have now put some thoughts together. Interested in feedback and comments:

- I guess the origin of this warning is to avoid doing some kind of horizontal bungee jumping at anchor, when the energy starts oscillating between kinetic energy of the vessel and the energy stored in the chain / snubber / bridle.

- So, sufficient inelastic damping is paramount, where energy gets dissipated, leading to the infamous 'boiling of the ocean'.

But then it struck me that there is another factor to take into account, which I believe most folks are not aware of. At least I was not until middle of last year, after a discussion with Bjarne.

The point is that when the vessel gets pushed downwind further away from the anchor, like in a gust or swell, some energy gets transferred from wind to vessel. It is simply the wind force times the distance the vessel gets pushed back further. It is work done, very classical.

So, this work needs to get stored somewhere - in the anchor chain and the snubber / bridle, leaving less precious room there to store the energy of the gust / swell itself.

Consequently, this is an effect one would want to minimise. But how? Obviously, the distance travelled by the vessel in the gust / swell should be as small as possible. But then, we want to keep the maximal load on the anchor also as small as possible, and maximise the energy stored in the snubber / bridle.

Now, Hook's law for an ideal spring, F = c * s, where F is the applied force, s the elongation of the spring, and c the spring constant, leads to the energy stored in such a spring to be E = 1/(2*c) F^2 = 1/2 * s * F.

From this we see that if we keep F constant and require a fixed energy to be stored, the elongation s is completely determined. Nothing we can do about it.

One can easily verify that re-routing the path the spring gets loaded will not change this picture at all.

So what can we do?

If we HAD a different spring, which is not linear in its Hook's law, but perhaps follows F = c' * s^2, one finds it gets even worse: E = 1/3 * s * F. So, to reach the same energy storage in the spring with the same force as above would require 50% more elongation... Thus it would require even more work done whilst moving the vessel downwind. No good at all!

Going the other direction of non-linearity, F = c'' * sqrt(s), one finds: E = 2/3 * s * F. So, this is better, as for the same energy and force an elongation 25% less would be required.

Admittedly, this is all rather academic, but perhaps somebody has an idea what materials to use / how to change the snubber / bridle so that it is closer to the 2nd case?

And in any case, our currently used snubbers / bridles are anything but linear, I am sure. Are there any differences in the assortment we currently use, which would make some superior to others when looked at from this particular aspect?

Thoughts?

PS: If somebody says this is a small effect, I'd argue it is not: After all, when the wind picks up very slowly from 0 to a final value, it will push out the vessel further away from the anchor, and the total work done, in terms of force times distance, is exactly the same as one finds stored in the catenary shape of the chain as potential energy.

You seem to be hinting there is 2 works or energy to be absorb by the snubber. The gust itself is not a form of energy, and does not need to be transmitted to the boat in addition of the work done, most of it just remains as kinetic energy of the passing air with no effect on the boat.

There is not. The only one is the work done on the boat causing it to have kinetic energy.

If the horizontal component of the load on the anchor rode is initially F1 and windage is -F1 with no current or wave action accounted for in this analyses.
Then a gust hits with new windage force -F2, the accelerating force at that instant is F1-F2, and in general F(x) - F2, where x is the distance measured horizontally from the anchor position to the boat. At the time the boat has moved far enough that load on the rode (F(x)=F2) balances out new windage value during that gust (-F2), all the work done on the boat by the accelerating force (F(x)-F2) has been converted to kinetic energy, and have to be absorbed by the rode while stopping the boat. (The part of energy dissipated by the drag of the hull in water is just too small to consider in the time interval analyzed, and is thus ignored. Some relevant amount of energy is dissipated later in water in longer time period.)
To minimize that work done, the movement of boat up to the balance point should be minimized, while the movement past that point should be as large as possible to absorb the remaining kinetic energy without excessive loads.

The amount of energy for the same conditions and boat would not be the same for the same gust, but depends a lot by the rode used!

One of the worst cases is, when the exact opposite happens. Before balance point catenary of the chain is large, but chain becomes almost stout by the balance point, resulting a lot of boat movement, and thus large amount of kinetic energy to be absorbed later. Fortunately most of the time that still can be done by a nylon snubber or bridle. But it turns out using a dynema line with no snubbers or chain as anchor rode leads to less kinetic energy, while having more capability to absorb that energy after balance point, as the stretch vs load is closer to linear line, while nylon and chain are both much worse. None are better than linear. Polyester also can be reasonably close to linear, but some are much worse. It depends on how the fibers have been layed out in the rope construction, and presumably all dynema lines are made in a method resulting closest to linear properties, because that also results least stretch that most uses of dynema intend to have.

There was a

video

on one of these anchor threads where some experts suggested using a short buoyant line closest to the anchor (at time 38:00), and no chain at all for different reasons (all up weight and possibly cost savings). But it seems dyneema also works when it is long enough and anchor is heavy enough to have enough holding capacity when rode is pulling upwards the amount resulting from the scope used. (no catenary for dynema line in water)
Don't bet that being the case if there is also chop & waves, in that case more stretch is probably needed and polyester rope is then a better choice at least for part of the rode.

[Just Another Sa]

Originally Posted by MathiasW

Ops, just noticed... Got my signs all wrong. Please change positive for negative and vice versa above... ^42

Yes, and the catenary of chain is by far the worst kind of non linear function F(stretch) with a positive second derivate (F''(stretch) in common use, followed by common large stretch nylon rope.

Non recoverable F(stretch) of steel well past yield point would have negative second derivate, but obviously not a real choice due to being non-recoverable.
I'm unaware of any material having that property in recoverable range of strain.
I wouldn't be surprised if it didn't exist. Better when just aim for as close to linear function as is practically possible. A rope with aligned fibers becomes close and is cost effective, if the stress/strain of the fibers themselves are close to linear as well, unlike nylon with significant hysteresis.

[rondelais]

I wonder if a spring/shock absorber combination located at the anchor would be an interesting solution. The problem with snubbers is that they are elastic, but not very lossy - thus the problem of going back, bouncing forward, having the bow fall off to one side or the other, then being driven back hard by the increased windage, and repeat. Rope just doesn’t have he right mix of spring constant and conversion of energy to heat, although climbing rope does pretty well. I’d want the device to be at the anchor for two reasons - its weight would act as a kellett, and having it there would make adjustment of scope easy. The snubber could then be a short bridle to take the tension off the chain stopper. It might be fun to do some calculations. I guess a real practical problem would be that all the automotive/motorcycle stuff that might be usable would not fare well in salt water....

[CarlF]

I'm stumped on the elasticity difference of a cat bridle vs a single snubber on a monohull

I apologize for not doing the calculation myself but it's been a long time since I've done this sort of thing and it's a Sunday night after a good portion of a nice bottle of wine.

If I wanted snubbers of adequate size with the same energy absorption in a) the bridle on a 25 ton catamaran with 20ft legs or b) a single 20ft bridle on an equal displacement beamy pilothouse monohull - what diameter lines would I use? 1/2" on the cat and 5/8" on the mono? 5/8's on both?

[MathiasW]

Originally Posted by Just Another Sa

You seem to be hinting there is 2 works or energy to be absorb by the snubber. The gust itself is not a form of energy, and does not need to be transmitted to the boat in addition of the work done, most of it just remains as kinetic energy of the passing air with no effect on the boat.

There is not. The only one is the work done on the boat causing it to have kinetic energy.

If the horizontal component of the load on the anchor rode is initially F1 and windage is -F1 with no current or wave action accounted for in this analyses.
Then a gust hits with new windage force -F2, the accelerating force at that instant is F1-F2, and in general F(x) - F2, where x is the distance measured horizontally from the anchor position to the boat. At the time the boat has moved far enough that load on the rode (F(x)=F2) balances out new windage value during that gust (-F2), all the work done on the boat by the accelerating force (F(x)-F2) has been converted to kinetic energy, and have to be absorbed by the rode while stopping the boat. (The part of energy dissipated by the drag of the hull in water is just too small to consider in the time interval analyzed, and is thus ignored. Some relevant amount of energy is dissipated later in water in longer time period.)
To minimize that work done, the movement of boat up to the balance point should be minimized, while the movement past that point should be as large as possible to absorb the remaining kinetic energy without excessive loads.

The amount of energy for the same conditions and boat would not be the same for the same gust, but depends a lot by the rode used!

One of the worst cases is, when the exact opposite happens. Before balance point catenary of the chain is large, but chain becomes almost stout by the balance point, resulting a lot of boat movement, and thus large amount of kinetic energy to be absorbed later. Fortunately most of the time that still can be done by a nylon snubber or bridle. But it turns out using a dynema line with no snubbers or chain as anchor rode leads to less kinetic energy, while having more capability to absorb that energy after balance point, as the stretch vs load is closer to linear line, while nylon and chain are both much worse. None are better than linear. Polyester also can be reasonably close to linear, but some are much worse. It depends on how the fibers have been layed out in the rope construction, and presumably all dynema lines are made in a method resulting closest to linear properties, because that also results least stretch that most uses of dynema intend to have.

There was a

on one of these anchor threads where some experts suggested using a short buoyant line closest to the anchor (at time 38:00), and no chain at all for different reasons (all up weight and possibly cost savings). But it seems dyneema also works when it is long enough and anchor is heavy enough to have enough holding capacity when rode is pulling upwards the amount resulting from the scope used. (no catenary for dynema line in water)
Don't bet that being the case if there is also chop & waves, in that case more stretch is probably needed and polyester rope is then a better choice at least for part of the rode.

Most interesting comment! It is kind of late here now and I need more time to digest it fully. And the next few days we will be sailing at last, with unknown quality of internet, so it may take a while before I respond more fully.

But in brief. Yes, you are correct, when it is a gust, the work done by the wind field on the boat - so its change of position further away from the anchor - is what is "charging up" the chain and snubber. There is only this one contribution.

My motivation was more intended to address the case when there is swell as well. Swell is something I had modelled as kinetic energy having been 'somehow' transferred to the boat. When you do the analysis of a moving vessel, moving in the direction of the wind field, but not because the wind is pushing it there, then you do need to study these two contributions. It would be wrong to just take this kinetic energy and 'plug' it into the potential energy of chain and snubber and be done with it. One also needs to add the energy gained further in the wind field as the vessel decelerates.

One could, of course, argue that one should treat swell and gusts in the same way. Both accelerate the vessel after all. Fair enough. So far, my preference had always been to deal with gusts as (maximal) forces, but treat swell as energy transfers, which I can roughly estimate by looking at my typical max SOG away from anchor. Just to make things not too complicated...

Overall, you seem to be saying that my desire of having a negative 2nd derivative for the spring equation would indeed be advantageous, but alas, it is impossible to achieve and linear is as good as it possibly can get for us. And most of the time we do not achieve even that! Correct?

But as said, I need to look at your comment more closely, and watch that video...

[AJ_n_Audrey]

Originally Posted by Pete7

Interesting, the Pardey's had the same approach to using a para anchor in storm conditions. They would make off one line to the bow and another to one of the forward quarters so the yacht sat at an angle of 20 degrees or so rather than sailing back and forth.

Pete

I remember that bit from the Pardeys' book. I may give that a try the next time I'm anchoring in a breeze and see how it works. Just thinking about it, though, it seems that the angle might cause the boat to sail a bit more to the anchor because of increased exposure of freeboard to the wind, but it would presumably sail in one direction only, increasing stability. If you have plenty of room to swing it might make for a more stable and better night's sleep. I might have had some success with this anchoring on the Bahamas Bank (and have had a better night's sleep).

[s/v Jedi]

Originally Posted by CarlF

I'm stumped on the elasticity difference of a cat bridle vs a single snubber on a monohull

I apologize for not doing the calculation myself but it's been a long time since I've done this sort of thing and it's a Sunday night after a good portion of a nice bottle of wine.

If I wanted snubbers of adequate size with the same energy absorption in a) the bridle on a 25 ton catamaran with 20ft legs or b) a single 20ft bridle on an equal displacement beamy pilothouse monohull - what diameter lines would I use? 1/2" on the cat and 5/8" on the mono? 5/8's on both?

We are a 25 metric ton ketch, 16’ beam, 64’ length and we managed to break 1/2” nylon but not 5/8”. This is a single line 20’ snubber.

[Just Another Sa]

Originally Posted by CarlF

I'm stumped on the elasticity difference of a cat bridle vs a single snubber on a monohull

I apologize for not doing the calculation myself but it's been a long time since I've done this sort of thing and it's a Sunday night after a good portion of a nice bottle of wine.

If I wanted snubbers of adequate size with the same energy absorption in a) the bridle on a 25 ton catamaran with 20ft legs or b) a single 20ft bridle on an equal displacement beamy pilothouse monohull - what diameter lines would I use? 1/2" on the cat and 5/8" on the mono? 5/8's on both?

For the same capability to absorb energy and same length, double ½" rope needs to be replaced with 0.7" single snubber which is something in between 5/8 and 3/4 ". Of course in practice there is no need for exactly matching capability to absorb energy.

But the elasticity would be vastly different, single snubber being much stiffer with much higher load on the anchor rode. How much so depends on the angle between bridle legs or the distance between attachment points for the 20 ft snubber legs, I'm assuming here it is less than 35 ft. If you wanted to avoid that with single nubber, you need to use a longer one (25ft for 5/8") than each leg of the bridle (20 ft), and can then use smaller diameter (5/8") without reduction of energy absorption.
Using 3/4" snubber results same energy absorption when 18 ft long. At 20 ft it has 12.5 % increase in energy absorption.

In all cases I have assumed rope strength to be directly related to cross sectional area, that being proportional to diameter squared. And assumed that max strain at breakage is the same for all. For each actual rope type there is some variation from this theoretical approach.