How much snubber / bridle is too much?

[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.

[tkeithlu]

Hmmm. Horizontal bungee jumping. I like that.

The same situation exists in an automobile's suspension on a bumpy road. Lots of energy gets stored very briefly in the springs, and returns as the springs expand after the bump. Some becomes heat. You can't change that any more than you can reduce the windage (and water friction) of the boat. But, you don't want the car bouncing up and down until all the energy becomes heat in a less than perfectly elastic spring.

The answer in a car is to use a shock absorber, damping the return of the spring by making oil go through a small hole in a piston. They get pretty hot.

The constant of the wind and current is stored until you release the anchor, and results in an equalibrium comparable to spring compression from the weight of a car. You're dealing with the variable forces of waves and gusts, just as bumps in the road.

I don't know of anyone manufacturing shock absorbers for boats. It would be pretty big (your boat weighs more than a car) and need quite a long travel. You might not want to carry it around.

But then, a boat in water has a bunch of damping built in. The water itself is damping motion in the boat, as you've noticed when you pull it to the dock so you can climb on board.

Hey, I enjoyed thinking about it. Thanks for posting my Saturday afternoon cognitive exercise.

Did I leave anything out?

[MathiasW]

Yeah, your points are spot on!

I have a pair of heavy-duty springs on board which are used when moored stern to dock. These come in different sizes and mine are for 25t vessels. Essentially, they are two interleaved metal cages that compress some "rubber" balls. Not sure how inelastic that ism, though.

But I have been thinking of adding that to my short snubber line, which is an emergency snubber line in case the main bridle fails (this being a trimaran). See picture enclosed.

This solution does not address the hunt for better non-linear characteristics of snubbers / bridles, though.

[thinwater]

Interesting question. After 100 years, cars are still suspended with spring + damper. In the case of a boat, the damper is the water we are resting in.


Dampers can be made adjustable. You can add a drogue to the boat, according to weather. You could replace the snubber with a damped spring system. Sounds complicated, bulky, and heavy.


Ropes are less ideal the more rapid the load. Watch a climber take a whipper (long fall) on nylon rope and you will see there is little rebound. Some of this is friction over carabiners, but much of it is internal absorption, released as heat.

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

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

I think the practical answer is that with sea anchors in ocean swells and open rodesteads with long rope rodes, you can see some rebound, but in harbors and with snubbers up to about 80 feet, you do not. The sea anchor problem can be avoided by using a JSD in most cases (no long rode with single loading point and you can use a non-stretch rode). The open rodestead problem is solved by using chain plus conservatively sized rope rode (larger than the minimum = less stretch). Snubbers themselves won't cause enough rebound to be important, or at least that is what expereince tells us.

[MathiasW]

Originally Posted by thinwater

Ropes are less ideal the more rapid the load. Watch a climber take a whipper (long fall) on nylon rope and you will see there is little rebound. Some of this is friction over carabiners, but much of it is internal absorption, released as heat.

Oh! If I didn't know climbing wasn't for me, I would know now...

[Discovery 15797]

Interesting brain exercise.

For a vessel at anchor in gusty winds with some chop the Force is never constant.

By using Hooke's Law I am assuming you are equating the catenary of the chain to a spring.

But, you don't seem to be factoring the elastic properties of a 3 strand bridle including diameter or length which effect its elasticity.

Finally, a kellet can act as a "shock absorber" to help counter-act surge forces.

[MathiasW]

Originally Posted by Discovery 15797

Interesting brain exercise.

For a vessel at anchor in gusty winds with some chop the Force is never constant.

By using Hooke's Law I am assuming you are equating the catenary of the chain to a spring.

But, you don't seem to be factoring the elastic properties of a 3 strand bridle including diameter or length which effect its elasticity.

Finally, a kellet can act as a "shock absorber" to help counter-act surge forces.

No, I was referring to the snubber / bridle for elasticity, not the chain. And yes, a kellet helps. But kellets of any size that are still manageable have only a rather small effect. 10 kg or so raised by a metre or two is not a lot of energy.

[StuM]

How does the concept of "too much snubber" work with mixed chain and rope rode?

[Benz]

You could have a snubber made of very elastic line, with a second, slightly longer parallel leg made of something less stretchy. When the first line stretches enough, the second line will take up the strain. The first line will have done all the damping necessary--essentially it will have eased the shock, and the second line, being more static, will prevent the first line from stretching to where it wants to bounce back hard. If you put both lines inside a loose woven cover, they won't get all tangled.

[MathiasW]

Originally Posted by Benz

You could have a snubber made of very elastic line, with a second, slightly longer parallel leg made of something less stretchy. When the first line stretches enough, the second line will take up the strain. The first line will have done all the damping necessary--essentially it will have eased the shock, and the second line, being more static, will prevent the first line from stretching to where it wants to bounce back hard. If you put both lines inside a loose woven cover, they won't get all tangled.

Yes, I had thought along those lines... (pun intended ). But isn't this the opposite of the desired non-linearity? With such a configuration I would require initially a very small force to create a big stretch, and only when the force becomes larger, would the additional elongation get smaller. But it seems I want something like F = c'' * sqrt(s), so the force should initially ramp up very quickly with small elongations, and then flatten out.

[MathiasW]

Originally Posted by StuM

How does the concept of "too much snubber" work with mixed chain and rope rode?

I wouldn't call it a 'concept', but I agree with your comment and I have no answer. It is just that I have seen posts on CF in the past that said they were having problems as described by me.

[Pete7]

Originally Posted by MathiasW

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

You know I skip read your post until I found this bit, then the glazed look must have appeared across my face.

Originally Posted by MathiasW

So what can we do?

Ok, that's easier You could do what most folk do, go to the chandlers and buy a length of 3 strand rope that looks about right then use it. Mine is 10mm dia and 10m in length on a 31 ft yacht. Most of the snubber stays on deck, from the winch to the chain just above the water line.

Seems to work and if it ever wears out, well its back to the odd bins in the chandlers for another piece

[Pete7]

Originally Posted by MathiasW

I wouldn't call it a 'concept', but I agree with your comment and I have no answer. It is just that I have seen posts on CF in the past that said they were having problems as described by me.

The chap who is trying to flog Viking anchors recently suggested using bungy cord for a long snubber. Don't think the forum really took to the suggestion, but it might be interesting to watch, from a distance.

[MathiasW]

Originally Posted by Pete7

You know I skip read your post until I found this bit, then the glazed look must have appeared across my face.



Ok, that's easier You could do what most folk do, go to the chandlers and buy a length of 3 strand rope that looks about right then use it. Mine is 10mm dia and 10m in length on a 31 ft yacht. Most of the snubber stays on deck, from the winch to the chain just above the water line.

Seems to work and if it ever wears out, well its back to the odd bins in the chandlers for another piece

Yeah, I know that path! My question was more to figure out whether there are alternative, smarter ways of dealing with this...

[Pete7]

Given the work you did on catenary a while back, if I wanted the answer to that question, I would be asking you