Snubbers

[MathiasW]

Thanks Jonathan! And greetings from Mexico! Banderas Bay.

We had recently a hurricane with 70-80kn wind pass right over our head, but I preferred to relocate to the marina for two days, rather than testing all my knowledge and theory at anchor... I did not want to push things, really... Two vessels ended up on the beach, two managed to stay at anchor.

A number of folks have said so in this thread, but just to make it absolutely clear: Snubbers are there to absorb shock loads from gusts and swell. In fact, in shallow water, they are the only means that help, as an almost horizontally spanned chain cannot store shock load energy. It only holds static force. For this reason it can even be better to relocate to deeper water, so that the chain can be more effective. This is only true, of course, as long as the swell and gust is similar at this deeper anchorage. And you have enough chain...

I have a long treaty on the physics of anchor chains and snubbers / bridles at

Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN

No worries, link is in German, but the treaty is in English...

There are a number of insights in this treaty that some will find surprising, but it is backed up by reports by very experienced sailors (or should I rather say anchorers ) In any case, it will hopefully help the reader to make better informed decisions at anchorage.

If you ARE interested in working out the anchor and snubber loads, there are a number of tools out there to do this:

Free, but VERY powerful online tool by Bjarne:
http://svamanda.dk/anchor/intro

Free, but very simple Anchor Chain Simulator for Android (you'll need to know the windage area of your vessel, and it has only metric units):
https://play.google.com/store/apps/d...ae.anchorchain

Not free, but rather sophisticated AnchorChainCalculator for iOS and Android (mention by Jonathan already):
https://play.google.com/store/apps/d...haincalculator
https://apps.apple.com/us/app/anchor...r/id1533741243

The latter tool is by me

.

All of these tools will calculate the anchor load given wind force and windage area of the vessel, and some, like Bjarne's tool or mine, will also cover the effect of snubbers and bridles, and allow you to use the length of your vessel plus some vessel characteristics instead of the more challenging windage area. Bjarne and I use the same physical model, so our results should always be very similar, except that he also includes the stretch of the metal links in a chain, which I have ignored.

For my app I am currently working on extending my snubber model to be non-linear if needed, which is more realistic. With this, I want to provide a better model of the fact that pulling a snubber with twice the force will not lead to twice a stretch, but rather a little less.

As to using springs or similar, I have to confess I have not made up my mind yet.

A) Normal springs as used for mooring lines will not be able to store enough energy to make a real difference.

B) But there is also the fact that a very soft snubber / spring that allows the vessel to move back a lot when a gust / swell hits the vessel, also has a price to pay: The vessel will gain energy simply by moving in the force field of the wind gust, very much like a rolling ball gains energy on a slope it is rolling down on. And this energy will have to be stored in the chain or snubber as well. So, from this point of view, it seems prudent not to allow the vessel to move backwards in the wind by more than is really needed.

C) But then again, energy is force integrated over distance, and when you need to store a given amount of energy and you do not want the force to be excessive, the only way to achieve this is to increase the distance. This will decrease the force. So, overall, it would appear to me that springs that extend perhaps only by 10 or 20 cm under load, are not what you want as a snubber at anchor.

Cheers, Mathias

 

[thinwater]

Honest, the simplest way to get a good number for your boat is to buy a cheap load cell. Up to breaking waves, the force ~ wind speed squared.

There are really too many differences in yawing behavior to make calculation practical. I've seen up to 50% change based on yawing (extreme vs. <20 degrees). Within the normal range, 25-35% variation between boats of slightly different designs (same exact length) is normal.

The calculations are a great learning tool, but they won't get you to a number in the same way looking at a load cell will. Much or most of engineering is this way. Why do you think they have to instrument the rig on AC boats? They surely have calculated everything first.

 

[MathiasW]

Honest, the simplest way to get a good number for your boat is to buy a cheap load cell. Up to breaking waves, the force ~ wind speed squared.

There are really too many differences in yawing behavior to make calculation practical. I've seen up to 50% change based on yawing (extreme vs. <20 degrees). Within the normal range, 25-35% variation between boats of slightly different designs (same exact length) is normal.

The calculations are a great learning tool, but they won't get you to a number in the same way looking at a load cell will. Much or most of engineering is this way. Why do you think they have to instrument the rig on AC boats? They surely have calculated everything first.

Yes, sure, but for once, the vast majority of folks will not go to the bother of getting a load cell and installing it. Let alone reading it out in a storm. Most folks use a simple scope rule they have have been taught in the past. THAT is the reference point we are talking about, not some fancy AC racing folks.

And yes, these tools are also to allow us to work out what if scenarios. The tools have helped me to realise when my bridle will be overpowered. So, in my view, it is good seamanship to get an idea of what the loads will be before the shit actually hits the fan. If you fancy installing a load cell, great. If you are not that keen on that kind of stuff, why not use other methods?

And by playing with the tools I have also gotten a better sense of what scenario to avoid. This will be true regardless of whether the numbers are accurate to 20% or not.

Cheers, Mathias

 

[Neeves]

I would agree with Mathais that most people are not going to be that interested and motivated to buy a load cell. The other factor is that when the wind is blowing 50 knots you have other things on your mind other than measuring the tensions in your rode for the sake of science as your own interests will be focussed elsewhere.

Now I'm not sure what the difference is between a crane scale and a load cell, except that crane scales are cheaper and seem to cover lower loads or tensions.

I have both a 2t load cell and a 300kg (or 500kg - don't recall) crane scale

Here is a link to crane scales. If you are interested you want one that will ideally measure 500kg (even 1,000kg) and you want a manufacturer who will sell one unit, minimum order quantity of one unit). I had a quick look, and then my powers of concentration evaporated - and maybe you will find one to meet my rough specification. The other function is price - and my guess is you will find one that is not particularly expensive. My crane scale is powered by AA batteries, my load cell works of 240v AC, has a computer interface etc etc. My crane scale is simply a Digital display and the unit is one piece - whereas my load cell has an independent display on a very long cable.. My load cell is more accurate (I think - I did have it calibrated) but the level of accuracy of the crane scale is going to be more than sufficient. A crane scale is a bit like a bigger luggage scale. You could calibrate the crane scale yourself with big and many buckets of water.

Digital crane scale for Measuring Weight Selections Of Featured Suppliers - Alibaba.com

As implied you need to instal the scale before you need it - installing one when its blowing 50 knots would not be particularly easy. You need to be able to record maximum tensions and maximum windspeed, the gust, causing that tension. having a display remote from the cell itself makes this easier. A crane scale, which all seem to be integrated units, makes this difficult as you will have the scale as part of the rode and you will not have wind data available there. I am sure you could integrate the 2 tension and wind - but I did it manually and simply measured the peaks. Sometimes peak load and peak wind did not overlap - because the peak wind occurred when the yacht was 'returning' from the previous peak - so I discarded those results and simply worked with peak load and peak speed - and plotted the peaks.

Anchor Testing and Rode Loads - Practical Sailor


I did a crude analysis of windage. I was given by the local Bav agent a drawing of a Bav 45 and a Bav 35 (on A3 paper, and I already have one for a Lightwave. I simply resized them to the same scale. I then printed them onto graph paper (remember graph paper) and counted the squares - tedious. I then thought laterally - I had printers packing card which is made to a precise weight (thickness). I printed the outlines onto the card, cut out the shapes and weight them. From your scale you can work out the areas. You need to decide how you are going to proportion the beam vs frontal area. I found that often our cat could be beam on to the wind - so I simply took full frontal and full beam area. A Bav 45 monohull and a Lightwave 38 are 'similar'. Of course you need to think of biminis, dinghies, furled sails

I know its crude - offer a better way!

There are lots of unknowns - my attitude was - do the best you can. Forget about streamlining etc etc - every yacht is different any way, how the sail is furled, the dodgers, Bimini etc. Don't worry - if you have an order of magnitude and Mathias' links - you have the basis to consider what you can do to reduce the impact of veering/yawing/horsing.

You, or we, only need a few people do make some measurements (area and load cell readings) on a cross section of yachts, say 5' increments from 25' to 55' - that's 7 yachts and we have the basis to allow extrapolation for anyone. I always wanted to make measurements on a mono hull but never found anyone willing to allow me to take their yacht for a day and make the measurements - I understand their reluctance as I would want to anchor when the wind is going to reach 35 knots and anchor in full exposure to the wind.


Finally

I agree with Mathias - when the forecast is for 50 knots (and it is only a forecast not a certainty - and might be worse) you go and find somewhere where the wind is not going to be 50 knots. It might be a different anchorage it might be a marina - but bragging rights don't impress me (one jot). You can have the best gear available but there may be a burried towel 1cm beyond the toe of your anchor and when you can only power set to, say, 35 knots, then your anchor will move forward, be clogged by the towel - and it will drag. Better to be in a marina or more shelter than on a beach and have the need to be lifted off by helicopter (the recent Scillies storm). Now sometimes you don't have a choice - but when you do -take it - its called, seaman ship. In the interim hone your skills, take the best advice have the right anchors (note the plural) snubber, and spares - etc etc. The weather is only a forecast (based on very expensive computational resource (and can still be wrong), the seabed - its a guess (basically - you have no idea - and diving on your anchor will not give the answer - you cannot see that old towel buried just forward of your anchor).

Fortunately your chances of being caught out in 50 knots or more - are very small - as for most a wind of 30 knots will be enough to encourage cutting the grass - not venturing afloat.

Take care, stay safe,

Jonathan

A further thought

Mathias underlies that in shallow water there is little catenary and a snubber offers a very sensible and efficient alternative. He omits to mention that if push comes to shove - there is no-where else to go and limited space - then you cannot deploy more chain - there is no room and a snubber (a long snubber but commenced at the transom) offers a replacement to catenary and finally if you have seen the light and realise the limitation to chain and have down sized - then again you have limited catenary and a snubber offers an alternative. I have 'managed' the replacement of 4 rodes, ours, a 50' cat, a 50' tri and a 45' Ovni and am about to make one for a 9.97m yacht half way through their circumnavigation all down sized from 10mm to 8mm (saving 85kg) or 8mm to 6mm, saving 45kg) - for 100m rodes. Small chain needs less power to retreive, takes up less room in a locker, reduces towering and if everything goes pear shaped - is less less strain on your back if you have to retreive by hand. Here in Oz - its also cheaper than buying the bigger chain (what's not to like?) - but you do need to believe in snubbers, or bridles - and that's the big issue

Read the July issue of YM! for a monohull or the August special edition of Multihulls World. There are a whole host of articles in Sail magazine - google 'SAIL magazine and snubbers' OR send me a PM and I have some pdfs.

 

[thinwater]

Yes, sure, but for once, the vast majority of folks will not go to the bother of getting a load cell and installing it. Let alone reading it out in a storm. Most folks use a simple scope rule they have have been taught in the past. THAT is the reference point we are talking about, not some fancy AC racing folks....

Cheers, Mathias

I meant something like this. $50-150 dependent the range required. The sensing element is a load cell.

We could say "the calculator is close, 50%"... but then we argue over this anchor vs. that, or 35# vs. 45# at length, which is only 20-25% increase in holding capacity. Perhaps it would be smarter to say "If your boat has a rode tension, including yawing, of 250# at25 knots, you need X size NG anchor." In fact, it is possibly more complicated than that. At some (15-30 knots, depending on depth, scope, and exposure) wind speed chain catenary stops working and the type of snubber matters. Boats start to yaw more as the chain lifts off the bottom.

Just sayin', sometimes we argue over fine points when we just don't know the basic numbers, don't understand all of the interactions (yawing, for example), and thus can't really calculate the bottom line.

 

[Dragon461]

I app

Thanks Jonathan! And greetings from Mexico! Banderas Bay.

We had recently a hurricane with 70-80kn wind pass right over our head, but I preferred to relocate to the marina for two days, rather than testing all my knowledge and theory at anchor... I did not want to push things, really... Two vessels ended up on the beach, two managed to stay at anchor.

A number of folks have said so in this thread, but just to make it absolutely clear: Snubbers are there to absorb shock loads from gusts and swell. In fact, in shallow water, they are the only means that help, as an almost horizontally spanned chain cannot store shock load energy. It only holds static force. For this reason it can even be better to relocate to deeper water, so that the chain can be more effective. This is only true, of course, as long as the swell and gust is similar at this deeper anchorage. And you have enough chain...

I have a long treaty on the physics of anchor chains and snubbers / bridles at

Catenary Anchor Chain Length - Die Kettenkurve - Fun Facts - SAN

No worries, link is in German, but the treaty is in English...

There are a number of insights in this treaty that some will find surprising, but it is backed up by reports by very experienced sailors (or should I rather say anchorers ) In any case, it will hopefully help the reader to make better informed decisions at anchorage.

If you ARE interested in working out the anchor and snubber loads, there are a number of tools out there to do this:

Free, but VERY powerful online tool by Bjarne:
http://svamanda.dk/anchor/intro

Free, but very simple Anchor Chain Simulator for Android (you'll need to know the windage area of your vessel, and it has only metric units):
https://play.google.com/store/apps/d...ae.anchorchain

Not free, but rather sophisticated AnchorChainCalculator for iOS and Android (mention by Jonathan already):
https://play.google.com/store/apps/d...haincalculator
https://apps.apple.com/us/app/anchor...r/id1533741243

The latter tool is by me

.

All of these tools will calculate the anchor load given wind force and windage area of the vessel, and some, like Bjarne's tool or mine, will also cover the effect of snubbers and bridles, and allow you to use the length of your vessel plus some vessel characteristics instead of the more challenging windage area. Bjarne and I use the same physical model, so our results should always be very similar, except that he also includes the stretch of the metal links in a chain, which I have ignored.

For my app I am currently working on extending my snubber model to be non-linear if needed, which is more realistic. With this, I want to provide a better model of the fact that pulling a snubber with twice the force will not lead to twice a stretch, but rather a little less.

As to using springs or similar, I have to confess I have not made up my mind yet.

A) Normal springs as used for mooring lines will not be able to store enough energy to make a real difference.

B) But there is also the fact that a very soft snubber / spring that allows the vessel to move back a lot when a gust / swell hits the vessel, also has a price to pay: The vessel will gain energy simply by moving in the force field of the wind gust, very much like a rolling ball gains energy on a slope it is rolling down on. And this energy will have to be stored in the chain or snubber as well. So, from this point of view, it seems prudent not to allow the vessel to move backwards in the wind by more than is really needed.

C) But then again, energy is force integrated over distance, and when you need to store a given amount of energy and you do not want the force to be excessive, the only way to achieve this is to increase the distance. This will decrease the force. So, overall, it would appear to me that springs that extend perhaps only by 10 or 20 cm under load, are not what you want as a snubber at anchor.

Cheers, Mathias

WHat people are missing in my, "use a spring theory" is that, it's actually acting out its duties in a length of snubber warp controlling snatch in the warp.
From what I see in the eastern med short snubber length are used, these 12m +are new to me and I haven't seen

 

[Neeves]

I app

WHat people are missing in my, "use a spring theory" is that, it's actually acting out its duties in a length of snubber warp controlling snatch in the warp.
From what I see in the eastern med short snubber length are used, these 12m +are new to me and I haven't seen

What are these 12m snubbers to which you refer - when the chips are down - we have a 30m snubber available.

You certainly don't see ours as most of our 30m is inboard and many people do use snubbers that commence at the transom - and if you have a 10m deck - then you will see 2m outboard beyond the bow - a 12m snubber.


I am happy to try new ideas, that's how I got where I am now. But in order to attract me to try a new idea I need some convincing - I need some data. So far, you are not alone, I have seen nothing to suggest that short spring offers any advantage.

I don't recall the detail but I tested one of these rubber dog bone things. The dog bone thing has a limited and finite ability, it is limited by the cordage wrapped round it. Allow the rubber dog bone thing more freedom - it fails. But 2m of the appropriately sized nylon offers the same elasticity as the rubber dog bone thing, nylon is cheaper, offer elasticity well beyond the limits of the dog bone thing, is lighter.

Snubbers need stretch, distance, to be effective. Springs and rubber dog bone things actually stretch very little. Working on a 4:1 safety factor then 10m of nylon offers 1m of elasticity, 30m of nylon offers 3m of elasticity - a dog bone thing offers a few centimetres (because you have not offered data - I don't know what your spring offers......

Springs and dog bone things work very, very well in a mooring or securing a yacht in a pen/marina where rope lengths are short and movement needs to be restricted.


A big gust hits my yacht - my yacht moves back and is restrained after 1m. The same gust hits your yacht, you yacht moves back 6cm (say). I don't feel a thing - you feel a jolt. Your anchor feels the same jolt (and I don't notice). Imagine a bungy jumper with my nylon and then jumping again reliant on your spring and a long inelastic rope.......or imagine a car running down hill restrained by dyneema and your spring or my long nylon - which is the more gentle to to the occupants (or the securement point).

Jonathan