If you like to know the anchor load and minimal chain length... here is the tool to do it...

[Neeves]

When you mention the word snubber there are a number of responses:

'What's that' being common

or 'I use a snubber - who does not - mine is 2 m long and takes the load off the weindlass'

or 'I use 4m of 15mm nylon'

and for the very few, like Sandy - 'I use a deck length snubber (or longer) and its 10mm nylon and I also use a back up in case the snubber fails so as to protect the windlass' - and in an anchoring context the best answer is the last

But each of the three options is a correct use of the word snubber. Mathias and I are, evangelical?, over people using long skinny snubbers. The snubber does not replace, and is not as strong as, the chain. It simply offers elasticity and if push comes to shove you can use a short rode and a decent snubber will replace the catenary you cannot use (because there is not enough room).

I don't think Mathais is suggesting that every time you anchor you sit down with his link and make a new calculation. I would use it at home - look at the various scenarios that you have experienced, plug in the numbers, and then look at the impact of those variations - and take note. You really only need to do this once to see the impact of altering the snubber (though I'd have preferred it if Mathias had put numbers on his snubber options (maybe he did and I missed it). The characteristics of your yacht are not fully defined in Mathias link - so the numbers it generates will not be exact - but they are ball park.

You quite happily add waypoints for you intended passage and when you receive the forecast it dictates whether you follow through with the passages or cut the grass. Think of the link as a forecast - you will indeify what might or might not be useful. If you know it all - leave it for those who have had 30 years of anchoring.

You don't need to do any of this if you know what you are doing but it would be invaluable if with your ingrained expertise and experience you did realise those who don't anchor much, or have had issues, might find the link useful. NormanS has a 'heavy' yacht and beefy chain - I (and some others) do not and a snubber is a boon. There is nothing 'new' about a snubber - its old school and was called a 'mixed rode'. Now a snubber has been taken into the 21st century and Mathias has provided a link to a free resource that allows you to customise conditions from the comfort of your fireside.

I went through a swift learning process using a cross section of different snubbers - If I had had Mathias link he would have allowed me to save playing around with multiple dog bone things (and cheap variants made from scrap rubber).

But if you start with a deck length snubber, see Sandy's post above - start at the transom, and its (depends on yacht 8mm - 12mm) but have some excess so you can play with extra length AND use Mathias link to set up different scenarios, depth, wind speed etc - then you have the basis to design a snubber specific to your needs. But like sail trimming you need to put in some effort, practice makes perfect.

A snubber will make a good anchor much better, will turn an average anchor into a good one - assuming normal skills of seamanship.


Finally - I don't think Mathias links is perfect - but it is free and can be used as YOUR basis for developing the answer that suits you.

To Mathias - thank you for the work, thank you for allowing access to the link to be free. I wish there were more like you allowing your skills and expertise to be made freely available to anyone. Vyv Cox does this with his website - I wish there more of you.

Take care, stay safe

Jonathan

 

[MathiasW]

Oh the irony ???

My “go-to” reference on this subject usually starts with the Admiralty Manual of Seamanship. These publications regularly appear on well known auction sites for just a few pounds. Volume 2 covers “Seamanship” and Chapter 9 is dedicated to “Anchors, Cables and Anchor Work”. Of course, it’s not aimed at recreational sailing vessels but it gives an awful lot of thought-provoking information.

Yes, I am aware of those, and indeed excellent material. But it takes some time to become aware of them, and for non-native speakers there is even one more hurdle to overcome.

What I am missing in those references, though, is the topic of anchoring in shallow water with a lot of swell. Swell hardly matters to big ships and so it is not covered there, but it has a tremendous effect on small vessels. If you play around with my tool and use a lot of swell (say 0.6 kn of vessel velocity) and shallow water (3 m, 5m, and 9m), you will see that the anchor load goes up dramatically the more shallow the water is. One the other hand, if you use an excellent snubber, you can keep this increase in check, mostly.

So, even if folks do not want to use the tool for their actual anchoring, I believe it still has a huge educational upside in allowing everybody to analyse what happens in certain unfavourable scenarios. If you can see yourself that you can half the anchor load when using an excellent snubber, it may then seem prudent to want to have an excellent snubber. The tool only states the properties such excellent snubber must have (in terms of how much it stretches at 8 Beaufort), but it does not give any purchasing recommendation...

Cheers, Mathias

 

[MathiasW]

It’s a strange world where universal approval of one’s work, regardless of content, is the required norm.

Not approval, but appreciation of an effort. A subtle difference. I think that is not asking too much.

If you have ever gone to a British school, you will remember the encouraging "Oh how lovely, dear! Well done!" by the teacher. They even have stickers for this that the children bring home. My children went to school in the UK and had this feedback all the time. I think it is a lovely approach and it is a pity to see that it is not honoured anymore when people get older.

But it is ok, I can live with that. I went to a German school and I just shoot back. No problem with that.

 

[MathiasW]

Good to see that @MathiasW is doing something in the anchor modeling space.

For me it is too precise and I do have a professional engineering qualification. I am very much of the old school. Chuck Place the anchor on the sea bed and payout the selected scope for the depth then add a snubber. Following @Neeves 's comments a few years ago this is attached to the cleats on the transom and run forward for a bit of spring. I chuckle to myself and always think of the scene from one of Crocodile Dundee films when Dundee says, 'now this is a knife', and think of @Neeves saying, 'now this is a snubber'.

I once crewed on a boat that was skippered by a ex-racer, we arrived at the destination and the skipper was unable to get a marina berth. So we anchored. On attempting to set the anchor it refused. After the fourth attempt I enquired of the depth of water and was a tad surprised that a extremely experienced chap had calculated the scope on the charted depth when we had almost another five meters under the keel. An extra 20 meters of chain and the anchor bit first time and did not move for three days. Later in the pub, he admitted that he never anchored. It is a skill that needs practice.

Thank you Sandy, most kind! If you can help me understand where you are having difficulties with the tool, perhaps I can improve it a bit. My original tool was the Expert Mode only, but it was clear that this would not fly at all with the vast majority of users. It took me a long time to come up with the definition of the snubber characteristics as they currently are in Basic Mode, but I am sure there is room for making it more simple. I just have no clue at the moment how to and would need some guidance.

Thank you!

Mathias

PS: And yes, I have been living on the hook now for more than 2 years and I have seen horrible scenes at anchor, which can only be explained by complete ignorance. And 3 vessels at the beach is the current count. Very sad, and it does not have to be, really.

 

[MathiasW]

Awelina of Sweden Welcome Page

It's just some stuff I've made for myself, but put there in case useful to anyone else. All programs can run locally, ie once saved locally don't need an internet connection.

I do like this site! Well done!

As said before, you do not include the effect of swell, nor snubbers. In shallow waters that will make a big difference.

I actually also provide some graphs on my web page, but not interactively. I have published a list of short digests for a range of windage areas and chain thicknesses. So, if you know your windage area (which you must with your tool), then you can search for the closest match in my list, and then see how the graphs change when swell and snubber are included.

So, have a look at

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

and scroll almost half down the page.

You can then find a digest for instance for a vessel of 10 square meters windage area:

https://trimaran-san.de/wp-content/uploads/2020/07/Anchoring_digest_Aeff10_v.5.pdf

You'll find graphs similar to yours, but now including swell, and a little bit the effect of different snubbers.

But, my graphs are not interactive as yours are...

Cheers, Mathias

 

[MathiasW]

When you mention the word snubber there are a number of responses:

'What's that' being common

or 'I use a snubber - who does not - mine is 2 m long and takes the load off the weindlass'

or 'I use 4m of 15mm nylon'

and for the very few, like Sandy - 'I use a deck length snubber (or longer) and its 10mm nylon and I also use a back up in case the snubber fails so as to protect the windlass' - and in an anchoring context the best answer is the last

But each of the three options is a correct use of the word snubber. Mathias and I are, evangelical?, over people using long skinny snubbers. The snubber does not replace, and is not as strong as, the chain. It simply offers elasticity and if push comes to shove you can use a short rode and a decent snubber will replace the catenary you cannot use (because there is not enough room).

I don't think Mathais is suggesting that every time you anchor you sit down with his link and make a new calculation. I would use it at home - look at the various scenarios that you have experienced, plug in the numbers, and then look at the impact of those variations - and take note. You really only need to do this once to see the impact of altering the snubber (though I'd have preferred it if Mathias had put numbers on his snubber options (maybe he did and I missed it). The characteristics of your yacht are not fully defined in Mathias link - so the numbers it generates will not be exact - but they are ball park.

You quite happily add waypoints for you intended passage and when you receive the forecast it dictates whether you follow through with the passages or cut the grass. Think of the link as a forecast - you will indeify what might or might not be useful. If you know it all - leave it for those who have had 30 years of anchoring.

You don't need to do any of this if you know what you are doing but it would be invaluable if with your ingrained expertise and experience you did realise those who don't anchor much, or have had issues, might find the link useful. NormanS has a 'heavy' yacht and beefy chain - I (and some others) do not and a snubber is a boon. There is nothing 'new' about a snubber - its old school and was called a 'mixed rode'. Now a snubber has been taken into the 21st century and Mathias has provided a link to a free resource that allows you to customise conditions from the comfort of your fireside.

I went through a swift learning process using a cross section of different snubbers - If I had had Mathias link he would have allowed me to save playing around with multiple dog bone things (and cheap variants made from scrap rubber).

But if you start with a deck length snubber, see Sandy's post above - start at the transom, and its (depends on yacht 8mm - 12mm) but have some excess so you can play with extra length AND use Mathias link to set up different scenarios, depth, wind speed etc - then you have the basis to design a snubber specific to your needs. But like sail trimming you need to put in some effort, practice makes perfect.

A snubber will make a good anchor much better, will turn an average anchor into a good one - assuming normal skills of seamanship.


Finally - I don't think Mathias links is perfect - but it is free and can be used as YOUR basis for developing the answer that suits you.

To Mathias - thank you for the work, thank you for allowing access to the link to be free. I wish there were more like you allowing your skills and expertise to be made freely available to anyone. Vyv Cox does this with his website - I wish there more of you.

Take care, stay safe

Jonathan

Thank you Jonathan!

This is exactly my intention to provide sailors with a tool so that they can do many different scenario plays before the shit actually hits the fan.

And yes, the tool is not perfect. I have tried quite hard to make it simple, but this comes at the expense of it not allowing snubber details to be punched in as per data sheet. In Expert Mode you have more options and can be more precise. In Basic Mode, perhaps the results will only be qualitatively correct, if you do not match the properties of your snubber correctly. But you can still compare different scenarios and see for yourself that some scenarios are really bad and you do not want ever to be in such a scenario.

For instance: Many people will think their snubber is good and so they will punch in GOOD as snubber characteristics. And some will then observe that the tool gives a much larger snubber stretch than they have actually ever observed with their own snubber. This is not an indication that the tool is incorrect. Rather it should be a red light as it means the assumption their snubber is good is far from the truth. Perhaps, if they punch in POOR instead, the snubber stretch the tool comes back with is more in line with what they see. That is then the time, perhaps, to realise that they might want to go for a really good snubber, like the ones Jonathan has been describing for years.

And finally, I think I now know why some folks see all Zeros in their results. This is when the input data do not allow to calculate a chain length and anchor load. For instance, the swell was assumed to be too large, and the chain simply cannot absorb it. In such a case you should always find a red error message at the bottom of the page. Actually, the page should also not show all zeros, but say "error" in each cell, but for some reason this functionality stopped working. We will fix that again.

So, feedback how to make this tool simpler and more intuitive to use is very welcome.

Cheers

Mathias

 

[[70521]]

Thank you Sandy, most kind! If you can help me understand where you are having difficulties with the tool, perhaps I can improve it a bit. My original tool was the Expert Mode only, but it was clear that this would not fly at all with the vast majority of users. It took me a long time to come up with the definition of the snubber characteristics as they currently are in Basic Mode, but I am sure there is room for making it more simple. I just have no clue at the moment how to and would need some guidance.

Its not a difficulty with the model, just feel it is overkill for a simple question to a complex issue.

Lets look at the variables:

1. Weight of vessel;
2. Hull shape;
3. Type of anchor;
4. Type of rode;
5. Speed of wind;
6. Angle of wind;
7. Range of wind speeds;
8. Rate of tide;
9. Set of tide;
10. Range of rate of tide;
11. Rage of set of tide;
12. Type of sea bed;
13. Sea state; and
14. Change of sea state

I am sure others will think of more. I have the same comments about Steve the YouTube anchor testing chap.

The consensus of opinion based on hundreds of years of anchoring vessels is is that four or five times the height of tide at high water will cope with 90% of anchoring conditions. I am not convinced that any skipper will spend time with a model working out to the decimeter how much scope to lay.

My own boat is about five tonne all up, I also crew on a vessel that is about 100 tonne and both use the five times height of tide at high water. Both vessels have sat comfortably in a F8 in a moderate/sea state 4.

 

[NormanS]

Its not a difficulty with the model, just feel it is overkill for a simple question to a complex issue.

Lets look at the variables:

1. Weight of vessel;
2. Hull shape;
3. Type of anchor;
4. Type of rode;
5. Speed of wind;
6. Angle of wind;
7. Range of wind speeds;
8. Rate of tide;
9. Set of tide;
10. Range of rate of tide;
11. Rage of set of tide;
12. Type of sea bed;
13. Sea state; and
14. Change of sea state

I am sure others will think of more. I have the same comments about Steve the YouTube anchor testing chap.

The consensus of opinion based on hundreds of years of anchoring vessels is is that four or five times the height of tide at high water will cope with 90% of anchoring conditions. I am not convinced that any skipper will spend time with a model working out to the decimeter how much scope to lay.

My own boat is about five tonne all up, I also crew on a vessel that is about 100 tonne and both use the five times height of tide at high water. Both vessels have sat comfortably in a F8 in a moderate/sea state 4.

Yes, I think that's a fair range of the variables. The amount of yawing that a vessel does at anchor can also have an effect on security of holding. Some can't be avoided - a lightweight catamaran will skitter about, while a heavier, full keeled (old fashioned ?) yacht will lie more docilely. Help can be achieved by, for instance, rigging a riding sail, or simply by reducing forward windage. I don't often see a riding sail rigged, but I do know that it works.
Maybe just as Neeves has his long running campaign about snubbers, I could have a campaign in favour of riding sails. ??

 

[MathiasW]

Its not a difficulty with the model, just feel it is overkill for a simple question to a complex issue.

Lets look at the variables:

1. Weight of vessel;
2. Hull shape;
3. Type of anchor;
4. Type of rode;
5. Speed of wind;
6. Angle of wind;
7. Range of wind speeds;
8. Rate of tide;
9. Set of tide;
10. Range of rate of tide;
11. Rage of set of tide;
12. Type of sea bed;
13. Sea state; and
14. Change of sea state

I am sure others will think of more. I have the same comments about Steve the YouTube anchor testing chap.

The consensus of opinion based on hundreds of years of anchoring vessels is is that four or five times the height of tide at high water will cope with 90% of anchoring conditions. I am not convinced that any skipper will spend time with a model working out to the decimeter how much scope to lay.

My own boat is about five tonne all up, I also crew on a vessel that is about 100 tonne and both use the five times height of tide at high water. Both vessels have sat comfortably in a F8 in a moderate/sea state 4.

OK, that is fair enough. I do not expect a sailor to check his/her anchoring with such a tool every time.

My point, though, is that it makes a lot of sense to play around in advance with such a tool to see how different parameters affect the anchor load and the required minimal chain length. I consider this Good Seamanship, actually.

If you say a scope of 5:1 has always worked for you, then forgive me for saying, but you were lucky. The required scope is definitely a function of the water depth. In deeper water you get away with less scope than in very shallow water. So having a general rule stating that a scope of 5:1 will always work can only be true if you also always anchor in about the same water depth every time. Just look at the catenary curve of the chain, it is non-linear, and so a one-fits-all scope rule cannot possibly exist. In extremely shallow water you may need a scope as large as 10:1.

And then there is the trouble with chain in very shallow water. Most folks believe it is best to anchor in as shallow water as possible when hiding from a storm. But that is not true. A chain works by its elasticity to absorb shock loads. Now, this elasticity vanishes when the chain is pulled horizontally. It has a maximum at a scope of as little as 1.4:1. In practice, one needs a larger scope so as also to maintain a horizontal pulling angle at the anchor. All this can be understood when looking at the physical model of the chain and doing the mathematical analysis. As this analysis is too taxing for most folks, an alternative approach is to use the tool I have provided. One can then see that one can overcome the disadvantages of an almost horizontal chain in shallow water by using a very good Neeves snubber . I am enclosing a graph with calculated results for 3, 5, and 9 metres of anchor depth, and for various qualities of snubbers. You can clearly see how large the anchor load is in strong swell (vessel velocity at anchor is as high as 0.6 kn), when you anchor at 5 metres. In fact, without a snubber, the tool returns an error for 3 metres of anchor depth, as in this case the chain cannot cope with the swell energy anymore. It simply fails to absorb it completely. What then happens is that at the very least you will get an awful shock load at the anchor and the bow cleats. Or the anchor starts to slip...

So, yes, if you have a very massive anchor, then 1322 daN (more than a metric tonne) at 5 metres of anchor depth is something your vessel can perhaps cope with. But it is not comfortable with such load peaks, and it may still be dangerous if the wind starts to blow much stronger. In contrast, if you escape to 9 metres water depth (and assuming you have not more swell there), and use a very decent snubber, the anchor load goes down to a mere 156 daN, and you even need less chain. What a massive difference!

It is this kind of insight that a physical modelling can provide, and my tool can assist those that cannot do the modelling themselves. After 30 years of anchoring experiences and having tried out all kinds of anchoring situations, you may well have achieved the same insight. But this tool offers a short-cut to all those, who do not want to wait for 30 years and, more importantly, who do not want to test potentially dangerous anchoring scenarios themselves.

Next question is, of course, what is an excellent snubber, and here I refer to Jonathan Neeves. As far as my tool goes, an excellent snubber is defined as a snubber that stretches by 1.6 metres in 8 Beaufort wind and no swell and very shallow water.



And finally, a success rate of 90% would simply be not good enough for me. It would mean that every 10 days I get into trouble at anchor, or 35 days in a year. For my peace of mind and safety, I need to have a method that is much more reliable than that.

Cheers, Mathias

 

[fredrussell]

This thread is not going well.

 

[pandos]

This thread is not going well.

May be there is an app/ tool to assess how well a thread is going so you don't have to guess?

 

[PaulRainbow]

To Mathias - thank you for the work, thank you for allowing access to the link to be free. I wish there were more like you allowing your skills and expertise to be made freely available to anyone. Vyv Cox does this with his website - I wish there more of you.

There just might be more people willing to share skills and knowledge if certain people didn't make accusations about having some sort of ulterior financial interest. I know of several skilled and knowledgable people, variously involved in the marine industry, who will not post on PBO because of accusations that they only post here to gain some financial gain. Ironic that you have posted the above, in a thread where the OP makes several posts/comments about how the "commercial" version of his app' has more features, when you are one of those who make such accusations about other people.

 

[NormanS]

Mathias, You say that "most folks believe it's better to anchor in shallow water............ for a gale". Have you asked most folks, because I for one believe the exact opposite, largely for the reasons that you give about achieving a useful catenary. I would have thought that fairly obvious.
On another point: You go on about the effect of swell in shallow water. Now it may be a language difference, but swell cannot exist in shallow water. Wind generated waves, yes, but genuine swell cannot be transmitted in shallow water. Unless you are meaning swell piling up as it reaches shallow water, as in breaking on a beach. Surely nobody would anchor in that!

 

[MathiasW]

Mathias, You say that "most folks believe it's better to anchor in shallow water............ for a gale". Have you asked most folks, because I for one believe the exact opposite, largely for the reasons that you give about achieving a useful catenary. I would have thought that fairly obvious.
On another point: You go on about the effect of swell in shallow water. Now it may be a language difference, but swell cannot exist in shallow water. Wind generated waves, yes, but genuine swell cannot be transmitted in shallow water. Unless you are meaning swell piling up as it reaches shallow water, as in breaking on a beach. Surely nobody would anchor in that!

I simply see how crowded it is at most anchorages in the shallow part, and that the deeper parts of the anchorage, which are still well protected, are avoided. It is also the feedback I have received over the past year or two by many that their resort is and will remain the shallow part.

And sure, breaking waves you will want to avoid. By swell I mean any kind of waves that make your vessel move. Those also exist in more shallow water when there is a real blast. Of course, in the open there will be more swell. My point is that all other things being equal, the exact same swell energy is more difficult to absorb in shallow water than it is in deeper water.

Actually, a gust will also move the vessel, and in this sense my tool could also be used to analyse gusts. And gusts for sure are also possible in more shallow water.

Chains are great at withstanding static forces, in whatever water depth. But dynamic loads in shallow water, whether generated by swell or gust, a chain without decent snubber finds it hard to deal with.

Cheers, Mathias

 

[MathiasW]

There just might be more people willing to share skills and knowledge if certain people didn't make accusations about having some sort of ulterior financial interest. I know of several skilled and knowledgable people, variously involved in the marine industry, who will not post on PBO because of accusations that they only post here to gain some financial gain. Ironic that you have posted the above, in a thread where the OP makes several posts/comments about how the "commercial" version of his app' has more features, when you are one of those who make such accusations about other people.

Well, the fact remains that this tool is free for you to use, whilst I have an annual running cost to support its existence. My main objective here is to make this free tool visible to others and help them anchor more safely. In the process, I might pick up a feedback or two that will help make this free tool more user-friendly, and I am grateful for that. Should this at some point spill over to the professional version, then it will help towards paying the annual server costs of the free tool. So, people buying the professional tool are then supporting the free tool. I think this is a fair give and take, feedback for free usage, and I cannot see anything wrong with that. In particular, as nobody is forced to provide any feedback to begin with, but still can use the tool. Nobody is getting rich with this tool and the buck earned per hour spent on it just sucks. Luckily, I do not have to for the rest of my life. But I do like to help those that appreciate help.

 

[thinwater]

Thanks for this feedback. You do not mention which numbers were too high in Basic Mode? Anchor loads? Or snubber stretches? The latter depend on the quality of the snubber.

For a monohull with average properties my Basic Mode is based on measurements of the windage area / anchor load done by the late Robert Smith. For other vessel types I have added a fudge factor to the windage area.

Cheers, Mathias

I must have entered something incorrect the first time. Maybe a decimal error. The numbers are all very close to what I have measured (+/- 15%) and observed (chain angles).

I assume 8 BFT means 8% of breaking minimum strength. I don't understand the abbreviation.

---

The vessel velocity is a problem in shallow water with a short or no snubber. The boat will only be moving a very short distance, too little for the GPS to register, and yet that few feet back against a tight chain can be a huge force. I find myself thinking the velocity number needs to be fudged using a combination of windspeed, snubber type, and depth, since most sailors will not want to measure that or can't.

I got similar numbers with a good snubber. But when I went to chain, without exact knowledge of velicity, the results seem touchy regarding velocity. Yes, I understand that V makes a BIG difference when the chain is horizontal. It's a tricky problem, which is why I recommend a hidden calculation. I simply built tables based on tests, and velocity was part of the conditions, not measured separatly.

---

Yes, one of the big picture concepts is that depth has a major effect on chain and on chain vs. chain-rope rodes. Sort of obvious... but not to all. Since multihull sailor can anchor very shallow and because they have some sort of snubber anyway (bridle) they are naturally more in tune with this aspect. I consider "deep" anything over 7 feet, since I need only half that.

 

[Wing Mark]

Mathias, You say that "most folks believe it's better to anchor in shallow water............ for a gale". Have you asked most folks, because I for one believe the exact opposite, largely for the reasons that you give about achieving a useful catenary. I would have thought that fairly obvious.
On another point: You go on about the effect of swell in shallow water. Now it may be a language difference, but swell cannot exist in shallow water. Wind generated waves, yes, but genuine swell cannot be transmitted in shallow water. Unless you are meaning swell piling up as it reaches shallow water, as in breaking on a beach. Surely nobody would anchor in that!

Indeed, unless you're anchoring in surf, you're not going to get the same wave height in shallow water.
The presumption that the boat's 'speed when anchored' stays the same seems like bollox to me.
Shallow water is very bad if you're on a lee shore, with swell coming in.
As it happens, this Summer I sailed around a yacht anchored in quite fierce swell in shallow water, they were waiting for enough tide to get into port and I was racing my Laser. I spoke to the crew in the pub later. Their CQR held quite nicely, they were not using a 'snubber' and a lot of chain, just a short chain and a long nylon rode.

 

[Wing Mark]

I must have entered something incorrect the first time. Maybe a decimal error. The numbers are all very close to what I have measured (+/- 15%) and observed (chain angles).

I assume 8 BFT means 8% of breaking minimum strength. I don't understand the abbreviation.

---

Yes, one of the big picture concepts is that depth has a major effect on chain and on chain vs. chain-rope rodes. Sort of obvious... but not to all. Since multihull sailor can anchor very shallow and because they have some sort of snubber anyway (bridle) they are naturally more in tune with this aspect. I consider "deep" anything over 7 feet, since I need only half that.

8 BFT means Beaufort gale 8 I guess,
it's between F2F as in 'fresh to Frightening' and
DOC as in 'blowing dogs off chains'
HTHBIDI.

 

[MathiasW]

Indeed, unless you're anchoring in surf, you're not going to get the same wave height in shallow water.
The presumption that the boat's 'speed when anchored' stays the same seems like bollox to me.
Shallow water is very bad if you're on a lee shore, with swell coming in.
As it happens, this Summer I sailed around a yacht anchored in quite fierce swell in shallow water, they were waiting for enough tide to get into port and I was racing my Laser. I spoke to the crew in the pub later. Their CQR held quite nicely, they were not using a 'snubber' and a lot of chain, just a short chain and a long nylon rode.

Well, there you have it. A long nylon rode will normally be much more effective than any snubber. So that rode saved them.

And clearly, the speed at anchor does not have to be the same in shallow and in deep water. But when it does, the effect is as described. It is not a general presumption, but an if then statement to help explain what is going on.

 

[MathiasW]

I must have entered something incorrect the first time. Maybe a decimal error. The numbers are all very close to what I have measured (+/- 15%) and observed (chain angles).

I assume 8 BFT means 8% of breaking minimum strength. I don't understand the abbreviation.

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The vessel velocity is a problem in shallow water with a short or no snubber. The boat will only be moving a very short distance, too little for the GPS to register, and yet that few feet back against a tight chain can be a huge force. I find myself thinking the velocity number needs to be fudged using a combination of windspeed, snubber type, and depth, since most sailors will not want to measure that or can't.

I got similar numbers with a good snubber. But when I went to chain, without exact knowledge of velicity, the results seem touchy regarding velocity. Yes, I understand that V makes a BIG difference when the chain is horizontal. It's a tricky problem, which is why I recommend a hidden calculation. I simply built tables based on tests, and velocity was part of the conditions, not measured separatly.

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Yes, one of the big picture concepts is that depth has a major effect on chain and on chain vs. chain-rope rodes. Sort of obvious... but not to all. Since multihull sailor can anchor very shallow and because they have some sort of snubber anyway (bridle) they are naturally more in tune with this aspect. I consider "deep" anything over 7 feet, since I need only half that.

Thanks for coming back on this! I am glad to hear that the results are so close to what you measure. (I was getting nervous when somebody like you with all your measurement setup gets something completely different! One cannot expect more from the model, really.

As to BFT - I am sorry, it is the abbreviation of Beaufort, and I am realising now that it is not so common necessarily in other parts of the world. In the documentation I explain it in more detail and I also give the equivalents in kn and in m/s.

Take it as my little revenge for all the feet and pounds I see everywhere...

And yes, the velocity at anchor is a very difficult beast. In the absence of any precise measurements, I have not found anything better than the almost universally available chart plotter. Luckily, it does not matter a lot, most of the time, whether I key in 0.3 kn, or 0.4 kn. And if I want to be on the safe side, I put in a value larger than I have observed on the plotter. Ballpark, I will be ok then.

Cheers, Mathias