Mathematic approach to anchoring scope

[MicHughV]

ah, I knew you were going to ask me that...let me root around and see if I can dig it up....it's here somewhere...

[MathiasW]

Originally Posted by MicHughV

ah, I knew you were going to ask me that...let me root around and see if I can dig it up....it's here somewhere...

Thanks so much!!!!

[Lodesman]

Originally Posted by Dockhead

Nothing wrong with the word "rope".

I have bolt-ropes, a bell-rope, sail-cover rope to name a few.

[MicHughV]

ok, try this.....I checked my store room for some of my old manuals, but most all my stuff is still in my old office....I'm retired now, but my office remains in place as I swing by from time to time.
I have oodles and oodles of technical data there...a veritable library...

but, see if you can download this online. I think you can...

(1) U.S Navy Mooring Design
Physical and Emperical Data.....

this volume relates to offshore moorings...

(2) Unified Facilities Criteria ( U.F.C.)
Moorings.

this volume relates more to tying a big ship up...

Let me know if this is of any use to you, otherwise, I will have to swing by my office and sort thru' stuff.

[Lodesman]

Originally Posted by rslifkin

On top of that, if anyone thinks they can realistically put out 135m (442 feet) of chain in 5m (16.4 feet) of water, they're absolutely insane. In many places, you just won't find enough space to do that. And even if you do, you'll be sailing around so wildly if the wind gets a bit gusty and shifty that it's all going to be a big mess and you risk sailing off fast enough to put a heck of a lot of load on the anchor when you pull things tight.

No, "insane" is anchoring into tight little clusters when there's hundreds of square miles of open roadstead available - but I've never understood the herd mentality of sheeple.
I did not suggest putting out 135m of chain every time you go to anchor, but if for whatever reason you're riding to hurricane force winds at anchor, then having had the foresight to have given yourself the room to veer to the full scope you have on board will not seem insane in the moment. I don't know how you think "you'll be sailing around wildly" - the forces required to move that amount of chain do not develop in gusts, and sustained winds are fairly steady in their direction - even if there is a change in direction, the chain would take most of the strain and slow you down as it has to drag over the seabed.

[MathiasW]

Originally Posted by MicHughV

ok, try this.....I checked my store room for some of my old manuals, but most all my stuff is still in my old office....I'm retired now, but my office remains in place as I swing by from time to time.
I have oodles and oodles of technical data there...a veritable library...

but, see if you can download this online. I think you can...

(1) U.S Navy Mooring Design
Physical and Emperical Data.....

this volume relates to offshore moorings...

(2) Unified Facilities Criteria ( U.F.C.)
Moorings.

this volume relates more to tying a big ship up...

Let me know if this is of any use to you, otherwise, I will have to swing by my office and sort thru' stuff.

OK, thanks a bunch!!! Really much appreciated!!!

I have downloaded two recent manuals and will browse through them soon.

[MicHughV]

yeah....if you are proficient with computers...more specifically with AutoCad...and even more specifically with 3D modeling, there are a variety of computer programs available now, that can do the number crunching for you.

3D AutoCad is the way to fly here, as 3D modeling can visually portray the many different angles of mooring/anchoring lines as influenced by tidal range, wind, passing ship effect and a myriad of other variables.
I was pretty efficient with AutoCad, even with 3D, but we had Autocad techs that could whizz around those programs with blinding speed...make me look like a blind man....

Nonetheless, a 3D program can relay in real time, visually, what happens to an anchor rode under varying conditions and a click of a button can lengthen or shorten a length of chain, increase wave height, and even model various types of different boats, as these are all available within the program. ie, you can keep the same circumstances, but go from a sailboat to powerboat just like that !!

However, I had the advantage, being " old school" where I could still figure stuff out by hand and a few calculations...

On top of all that, there is the practical matter of actually berthing or mooring a ship, whether by tug or thrusters..and port requirements, regarding ship movements, etc, pilotage, storm departures and so on.
Same applies to anchoring a sailboat. Anchor line scope is but only one component of the picture.

Where does this leave us regarding your "mathematical approach to anchoring scope"...well, it requires a thorough knowledge of maritime activity....not just sailboats...and hopefully the above recommended leads can send you on your way.

Finally, this is a cruising forum, where the objective is to share ideas, different perspectives, etc. but so often becomes a pissing match, where one poster will diss another poster by trying to be smarter...etc....at which point I loose interest.

Anyway, a bit of rambling on my part, which I'm allowed now, seeing as I'm retired....still sailing though....but more with one foot on the dock and one foot on the boat......

[MathiasW]

AutoCAD - I know the name, but that is about it. My wife had worked with it as engineer in her time.

My preference is to get as much as possible understanding by simplifying models and deriving analytical mathematical descriptions of them, i.e., formulas. Those allow me to understand how things depend on each other, which I find difficult when doing numerics only.

But all this is down to personal preference. I will study the manuals you have pointed me to and see what it means for my case.

Overall, I have found this thread to be very useful to understand where folks are in their thinking and how I need to improve 'my pitch', if you will.

So, thanks to all for sharing your thoughts.

BTW - I try not to visualise you with one foot on the dock and one on the boat... I always prefer that very moment to be a very brief, transient moment...

[barnakiel]

Originally Posted by MathiasW

Thanks!

And, oh yes, couldn't agree more on your conclusions!!!

To me, before doing all this exercise, I had somewhat naively assumed it is always better to flee to as shallow water as possible to ride a storm. But now, I know this is not necessarily the case...

Shallow, maybe, but how shallow?


We all know that in shallow water swell may be higher (typically on a lee shore).


Shallow water may imply proximity of the shore. And when this shore is to your windward, it will protect you from swell ...


So, as usual, shallow or not will depend on the location, wind direction and all the other factors - like our equipment, boat type, density of other boats, etc..


In fact, quite a riddle, and possibly the solution to which is not fully known until the blow has passed over you!


My lessons (from sailing and anchoring) are:


1. avoid swell,
2. avoid crowd,

3. use a big anchor and have plenty of chain (even you you only deploy part of it).



b.

[MicHughV]

Hope this is not considered thread drift.

When AutoCad first came on the scene, I was a bit dubious of it's capabilities.

But today, practically anything and everything on the planet is designed using AutoCad. In the hands of a skilled operator, it can do stuff to make you head spin.

You have to go back to school to learn it, and like school you must first go thru' grade 1, before you can go to grade 2. It can take many years to become proficient with it, as the system is constantly upgraded to ever more complicated usage.

The company I worked for, had several Ph.d's that did nothing but modeling with AutoCad. Wave modeling, beach erosion modeling, storm modeling, mooring modeling, ship movements, you name it, they did it. Every year they wandered off somewhere to take additional courses in it. I kinda tagged along at the rear end. I did go to back to school several times, but after a while I said enuff is enuff.

A good AutoCAd tech can write his own ticket and it is a highly sought after skill.

This thread, in the hands of good AutoCad techs, can spit out simulated 3D visual representations to make your eyes pop.

But, like you, I am still old school, and still like to work stuff out with pen and paper in hand, but we are no match for someone with good AutoCad skills.

[StuM]

Originally Posted by Lodesman

I have bolt-ropes, a bell-rope, sail-cover rope to name a few.

And assorted lengths of rope in the rope locker waiting to be put to some use.
Some of them used to be lines of various sorts, but they've been retired.

[Michael Cobbe]

A German mathematician(with way too much time on his hands) has turned his eye on trying to mathematically predict the best length of you rode when anchoring. Amongst other conclusions is that in deeper water, you need less rode than in shallow water and vice versa of course.


That is true we always did that because we viewed it was the weight the catenary that held the boat and the anchor was the back up when the wind got up. So shallow water needs more chain than deep water.

[Dockhead]

Originally Posted by Lodesman

No, "insane" is anchoring into tight little clusters when there's hundreds of square miles of open roadstead available - but I've never understood the herd mentality of sheeple.
I did not suggest putting out 135m of chain every time you go to anchor, but if for whatever reason you're riding to hurricane force winds at anchor, then having had the foresight to have given yourself the room to veer to the full scope you have on board will not seem insane in the moment. I don't know how you think "you'll be sailing around wildly" - the forces required to move that amount of chain do not develop in gusts, and sustained winds are fairly steady in their direction - even if there is a change in direction, the chain would take most of the strain and slow you down as it has to drag over the seabed.

Most of the places I know where people anchor chock a block are coves with good shelter, shelter which is not available in the open roadstead, so maybe not such a "sheeple" phenomenon as that.



Given enough room -- and that may be a big "given" depending on where you are -- I do agree that in bad weather there is hardly any such thing as too much chain (just like there is hardly any such thing as a too big anchor in such situations). You don't "sail around wildly" but nevertheless it takes a lot of room to deploy 135m of chain, which creates a swinging circle of almost 1.5 cables, which doesn't fit in a big majority of really sheltered places you might want to be anchored in in a big blow. That gigantic swinging circle also covers a lot of seabed and a lot of potential snags. A circle with radius of 135m covers more than 57,000m2 -- almost 6 hectares! -- almost double the area of a 100m radius swinging circle. It was Dashew who said he prefers anchoring on short scope, to limit the area of seabed you are dredging with your chain to limit the risk of snagging the chain.



I do agree with Mathias about deep water and here I think there might be some "sheeple" effect. People with a fetish for scope (like my Dad) might go into shallower water than necessary just to get more scope from the same amount of chain. Sometimes to get really good shelter you might need to be in shallower water, but I prefer to be in as deep water as practical, even if it means somewhat less scope. Deep water gets disturbed less in bad weather, and as everyone has pointed out (and thanks for the math, Mathias!) catenary works better in deep water. I seem to find better and better seabed in deeper water, too.

[MathiasW]

Originally Posted by Dockhead

I do agree with Mathias about deep water and here I think there might be some "sheeple" effect. People with a fetish for scope (like my Dad) might go into shallower water than necessary just to get more scope from the same amount of chain. Sometimes to get really good shelter you might need to be in shallower water, but I prefer to be in as deep water as practical, even if it means somewhat less scope. Deep water gets disturbed less in bad weather, and as everyone has pointed out (and thanks for the math, Mathias!) catenary works better in deep water. I seem to find better and better seabed in deeper water, too.

Thank you Dockhead!

If one takes the formula for a chain's potential energy above and asks, what are the conditions where the capabilities of the chain are best to store additional potential energy for a given, FIXED length L of chain, then one needs to differentiate this formula with respect to the anchor depth Y and set this to zero.

A bit tedious, but the maximum of this capability is reached when - in addition to being a fully developed catenary - the chain also fulfils the relation L =~ 2.2 .. 2.5 Y, where the precise factor depends slightly on the anchor load. It is 2.5 in the limit of vanishing anchor load.

Now, this is a differentiation, and so any FINITE amount of energy needs to be transferred by starting well before this maximum and ending well after it. So, this factor 2.2 to 2.5 is really not precise.

But still, it shows that there is a sweat spot for a chain of fixed length where it can absorb energy best. Fulfilling both requirements, catenary shape and this L =~ 2.2 .. 2.5 Y, usually means having to go to deeper water, which seems consistent with Dockhead's experience.

And finally, if we model swell as an energy burst impacting the vessel, then again, more chain has more room to store this energy, which is again pointing at going to deeper water. This argument assumes that the swell is the same in shallow water and in deep water, which often it is not. But the SAME swell is easier to deal with in deeper water.

Using good long snubbers or bridles will help you to stay in more shallow water, but at least the chain wants to go to deeper water... (I mean, not only gravity-wise... )

All this, I am sure, is obvious to the experienced sailor and is preaching to the choir, but for me it took me a little while to appreciate the nuances and implications...

[Dockhead]

Originally Posted by Lodesman

I have bolt-ropes, a bell-rope, sail-cover rope to name a few.

Absolutely. And foot-ropes, and a couple of miles of rope in the lockers. And we do rope-work. And if you want to refer to a piece of cordage which is neither cable nor small stuff, without referring to how it's being used, it's also perfectly fine to call it "a rope".

This allergy to the word "rope" is the hallmark of people trying to sound more salty than they actually are. Particularly common disease in the U.S.; not much noted in the fundamentally more nautical culture of the U.K.


The other thing which bugs me is referring to toilets -- the fixture -- as a "head". Also common in the U.S. But "head" or rather "heads" never referred to any fixtures; its the space where you put the fixture (or your bucket if you prefer). Nor does even the correct term "heads" correctly describe where you also bathe.