Is my chain too thin?

Eren said:
In my personal case, I think that we reach to the highest tension on the chain when we are moored in Med style, with 90 degrees angle to the side wind. Then the full side profile of the boat, which I guess would be around 100 sqm, is exposed to wind. However in such a case the tension is shared between the chain and mooring line. Of course I take the necessary precautions to avoid such an exposure, but I should make my calculations with such non-ideal conditions.
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Sunseeker have done it for you that’s what I,am saying - the OEM stuff is not a finger in the air or because Robert Braithwaites dad did that way .
Yup the force will shared somewhat but normally it’s bow on to the wind @ anchor a lot less area. - plus the catenary effect assuming you have let enough of the 120m out .
 
Portofino said:
The bow rise in an anchored swell of say 1-2 M acts on the catenary lifting
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An anchorage in gale force wind is only tenable if sheltered at least by waves motion (as it was in my previous example).
With 2m waves (which btw are bound to be breaking waves, in a blow!), you'd better run elsewhere pdq, with any vessel whose length in meters takes only 2 digits to be measured... :ambivalence:

Portofino said:
Any veering to swing both hulls to the “ end “ before returning on the arc to the other side ( if I understand you example ^^^ ? ) is actuall damped towards the final position , ie the boat decelerates before what feels like what you call a snatch
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Trust me, it doesn't decelerate one bit, till the chain pulls strongly. When the swinging is constantly feeded by very strong wind, it happens very fast.
Can you imagine walking around a solid and heavy boat like BA and risk to fall when the boat is "braked" by the chain pull?
And mind, this was with no waves at all, since we were anchored right in front of the coast from where the wind was blowing, hence extremely well sheltered from sea motion - thanks God.

Portofino said:
I mean according to your “ momentum “ theory at max arc and snatch loads , then the breaking strain needs to be greater than the displacement as you infer all the 45 tons +any snatch addition will be applied at max arc ?

But why is the 5 or 7 ton chain not snapping then ?
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LOL, well, it's not like the whole boat is hanging from a chain attached to the sky, of course! :D
You should think of a 5T force applied sideways to one cheek of the bow roller in the following way:
Let's assume to put the boat resting on its side, and hang a 5T weight to the anchor chain.
That is a 5T force pull, and (admittedly without doing any math) I just refuse to believe that it wouldn't be enough to bend even a very strong bow roller.
 
Portofino said:
Xmas present but they are sought after - supply demand etc

MapisM - ck out the customer reviews on this “Lord “ guy :)

Calling his work “BS “is a bit harsh I think .
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With all due respect, if the principle that "The heaviest pull on an anchor is a function of mainly topside windage of Sq M of boat profile - not the displacement" comes from his work, well, at least that bit is plain BS, imho. Can't comment any other part of his work - in fact, I didn't.
 
I have done some homework and found out with some calculation that 40 knot wind coming perpendicular to my side will apply a force around 2300 kgf. As this is shared by chain and mooring line at Med style mooring, I am in the safe zone, just as both MapisM and Portofino had indicated. When at anchor, this force is much lower as the boat acts like flag and wind exposure is minimized. Of course catenery effect will add up in both cases but, that also doesn't effect the result much.
 
Eren, of course I don't want to contradict a conclusion which I've been the first to envisage, even if based on gut feeling/experience alone... :D
But I believe that a proper calculation of the forces involved is much more complicated than applying a constant perpendicular wind force to a given surface, and then split the result between stern and bow lines, for at least a couple of reasons.

Firstly, with the boat moored Med style, the chain and/or lines resist the wind force perpendicularly to it, so on top of splitting the load between chain and lines there must be some trigonometric calculations involved, to translate the 2300 kgf (or whatever) into chain/lines UTS.
Don't ask me how - too lazy to engage brain and try to apply concepts which I haven't used for ages, but if you managed to make that type of calculation I believe you can see what I mean.
Anyway, this effect is bound to make the linear force applied to the chain/lines pull even lower, so all is well and good.

Otoh, secondly and imho most importantly, if as I understand you made a sort of static calculation (constant wind speed applied to windage), you are in practice neglecting the boat weight, which I believe is a big mistake, as I already argued with that Lord - through Portofino acting on his behalf... :rolleyes:
Now, as you surely know if you've been moored Med style in strong Xwind, the boat is NOT constantly pushed by the wind in one direction, staying there for as long as the wind lasts.
There is rather a constant push/pull effect, due both to wind gusts and to some chain/lines inherent elasticity.
Therefore, when the boat is pushed say to port by the wind, rather sooner than later she's bound to come back to her centered position, if not even "rebound" to stbd side a bit, as soon as the wind releases for a moment and the lines pull her back.
Trouble is, the following gust will push the boat to port again, but with a higher force, because on top of the "normal" wind pressure, you will have also the boat inertia working together with the wind, once the boat starts moving.
This is where the boat displacement enter into the equation, acting as a multiplier of the wind force.
But while for the previous point it's only laziness that restrict me from at least trying to do the math, for this second dynamic effect I have no idea of how to calculate it.

Experience tells me that these peak/snatch forces are the ones capable to actually make damages, though.
While I never came across a broken chain, I've definitely seen mooring lines broken, and even whole cleats ripped off from decks... :ambivalence:
 
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I am surprised there has been no mention of using a rope to take the load off the chain. This is common practice with sailing boats. Once wind force gets up to about force 6 or so the chain will be straight and it above this point that a stretchy rope has value in absorbing the loads. With 10mm chain a long strop (20m+ ) of 16mm nylon attached to the chain just below the waterline, over a bow roller and back to a strong cleat will take the load off the chain and allow you to use the smaller chain size in higher wind conditions. This reduces the weight of the chain significantly and means a smaller windlass (hard work lifting 100m 12mm chain plus a 60kg anchor!)
 
Tranona said:
hard work lifting 100m 12mm chain plus a 60kg anchor!
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That's an hard work which the windlass should do only if anchored in 100m of depth, which obviously never happens.
Even if you drop 1 mile of chain, the max load upon recovery will always be equal to the chain length to the seabed+anchor.
 
MapisM,

Both you and the Lord is right. The lord says "The heaviest pull on an anchor is a function of mainly topside windage of Sq M of boat profile - not the displacement", however he doesn't claim that it is the ONLY pull. The second biggest pull comes from "Moment of inertia" as you have already very well explained. The wind starts to move the boat and there should be a force to stop it, which is the tension applied to the chain in this case. Rough calculation of it is not that difficult, if the formula of Force = mass x acceleration is remembered. As can be seen from the formula, mass (weight of the boat) is directly related with the force. So it is more difficult to stop a 45t boat than a 500 kg exact balsa maquette of it. However, it will still be less than the wind load, as the Lord claims. And this puts us still within the limits of the chain, when I consider a side wind of 40 knots. Above that, I should leave the Med style mooring and go for anchoring out, where the forces will be much lower.

I am now more glad that I fitted the anenometer with connection to my Raymarine system. I will set the alarm to 40 knots before going to sleep :)
 
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Regarding anchor weight and holding power there’s been work done graphs and scales using these two - the surface area , and weight .
Here’s a typical graph .It uses a Danforth and scope 3/1 .Its just an illustration of how they calc anchor weights and pull out forces - there may be others too .

You simply ratio you number ( after converting the units if necessary) for any other scope as shown in the example in the pic .

There’s no accounting for displacement, that’s a red herring it’s mostly windage as I said .
With other anchors you can say move a wind scale by 1 to up or down if you are comparing anchor / anchor types .But we are doing that .

So Eren ,
You know your anchor kg,s , you know your surface area , you can rework the various scopes / ratios - thus you may be able to back work .
If not there will a suitable chart for your boat @ Sunseeker , this is just an example of how they calculate anchor pull out forces from the Sq M , windage to arrive at the correct size .This is just to illustrate the methodology how they do it .

With regards to F= Ma ( where M = the displacement and a acceleration)
Sure if the a is large then you see snatching to some extent , but it’s not that large , think say throwing a stone zero to what ever as your arm swings through as you let go the a is large relative to the M , then a boat bow swaying about at anchor - Ok the M is large but the a is tiny .

And consider this as already explained-
Your chains breaking strain is 5 or 7 tons , so if you apply F=Ma ( logically I might add ) then even if a is 1 you are at 45 T - chain s still there has not snapped —- ok you could take a vector off the 45 T , buts even so it’s gonna have to reduce a helluva lot to dip 45 min under the chain breaking strain of 5/7
Not trying to rubbish this just putting the right proportions on it — it’s tiny the D factor ,compared to windage
so Your 45 tons or what ever D doesn’t enter inter in to the equation to make any meaningful difference, on the chain size calcs .Its in there’s somewhere but tiny in relation to the pull generation from windage / sq M Surface area .

As said the chain does not know what it’s attached to ,just feels mostly the windage .
Infact one could attach a parachute, or open sail to a rode after chucking an anchor in ,in a blow, after say picking up a buoy .
No boat ,no displacement, here - small M the parachute, variable a gusting .
Then watch the wind , gusting or not - via the parachute/ sail —— drag it or snap the rode .
So here we have removed the boat and it’s displacement all together. Killed the M component of the equation- using just windage alone !

Interestingly for the benefit of everyone that graph shows the effect of a bigger anchor with increasing wind states .
Indeed the Dashews with there 83 ftr use 10 mm , but a with huge anchor ,and of course plenty of if - scope .

As far as the rode is concerned or the breaking strain of the chain I’ve covered that in my post #34 ^^


With regard s to snubbers ,
Here’s an extract from ( Sorry MapisM - MIT again , I feel your arrhg :) )
After all the Naval architecture dept has great alumni
Although I think we are a million miles away from anchoring in these conditions,interesting never the less .
An example of nylon being the snubber of choice due to the elasticity.
Again another possible example of “ my dad did it this way “ or MapisM ,s “ gut feeling “
Passed down - no science , —- nylons best etc etc cos Bob said so , etc etc

Chafe of nylon Rodes
When it comes to an extreme blow, research by Massachusetts Institute of Technology and Boat US into the use of nylon mooring pendants that failed during hurricanes Bob and Gloria suggests that a primary cause of failure is heat generated in the pendants as they stretch and contract over the relatively tight bend that occurs where a pendant (or rode, or snubber) comes over a bow roller or chock. The failed rodes had melted strands in the interior of the lines; i.e., the heat was generated by the fibers stretching and contracting rather than by chafe. The researchers speculated that this heat buildup is exacerbated when a hose is used for chafe protection because the hose traps the heat while at the same time preventing cooling from the wind and wind-driven spray.

Anyhow I think with your ananometer you are kinda ahead of most on here ,
You just need to work out as said knowing the anchor Kg, and rode out which line on the wind speed graph you are .
 
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Thanks for the valuable input with scientific backup!

Portofino said:
Not trying to rubbish this just putting the right proportions on it — it’s tiny the D factor ,compared to windage so Your 45 tons or what ever D doesn’t enter inter in to the equation to make any meaningful difference, on the chain size calcs .Its in there’s somewhere but tiny in relation to the pull generation from windage / sq M Surface area .
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I think we are all speaking more or less the same language now.

Portofino said:
Indeed the Dashews with there 83 ftr use 10 mm , but a with huge anchor ,and of course plenty of if - scope .
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I also read several articles that put bias more on anchor size than the chain size. The common idea is, "anchor drags long before the chain breaks".

It is a pleasure to discuss practical boating issues at this level.
 
Bit late to this thread but I have upgraded the gypsy and chain size on previous boats as well as swapping the anchor for a more modern and heavier design. I currently have 120m of 12mm chain with a 55kg Rocna on my 40t Ferretti 630. IMHO you cannot have too much chain and too big an anchor if you want to sleep soundly on the hook at night.
I may be in a minority here but I also believe that the friction between the chain and the seabed provides a significant amount of resistance to dragging as well as the holding power of the anchor itself. Of course, the heavier the chain and the bigger the chain links, the greater the friction. I have snorkelled out to observe how my anchor and chain is set on many occasions and I have never seen all of the chain lifted off the seabed even when the boat is shearing from side to side heavily in strong winds
In your case I would definitely upgrade to 12mm chain. Within the overall cost of running a 73 footer the cost is peanuts
 
Portofino said:
There’s no accounting for displacement, that’s a red herring it’s mostly windage as I said.
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I see no justification at all for that, in the pages you linked. In a sense, it's rather the opposite, because the whole analysis is, yet again, static.
You don't need to convince me that assuming a perfectly constant crosswind (and no other effects like the chain/lines elasticity), windage is the only dimension affecting the equation, because I already agreed with that.
In fact, when the Lord says that the problem "must be related largely to windage rather than displacement", I would rather say ONLY to windage, because displacement is totally irrelevant in the context of the analysis he's making.

My point is that in real world there's no such thing as a crosswind force creating a constant push against a static windage.
What really happens is that the boat creating an X sqft profile above the water moves back and forth with an amplitude that depends on several factors, and when this happens, its weight makes a HUGE difference on the peak loads that chain/lines must bear.
Sorry, but anyone who believes that a 150T steel trawler and its own maquette made in balsa (weighing 150Kg or whatever) can be even remotely compared, obviously has never seen the behavior of boats moored in a proper blow.
 
Deleted User said:
In your case I would definitely upgrade to 12mm chain. Within the overall cost of running a 73 footer the cost is peanuts
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I fully agree with all the reasoning behind your conclusion - minority part included, fwiw!
Otoh, I would NOT do that just for fear of breaking the chain.
 
Deleted User said:
In your case I would definitely upgrade to 12mm chain. Within the overall cost of running a 73 footer the cost is peanuts
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You've got a good balanced ground tackle:encouragement: I would definitely upgrade my chain, had I not replaced my gypsy and added a 10 mm chain stopper just a few months ago...
 
MapisM said:
Hang on, I might buy that! :D
Is it by chance one of the models I mentioned in this thread?

PS: why did you already replace the gipsy, btw? I can't believe that OEM chain was less than 10mm...
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Yes it is exactly the Lewmar, which is not the basic one. I kept silent for the thread as I still haven't used it in practice. I think it is a good one, but will be able to report about it in a couple of weeks. Actually I have a lot of new things on the boat to check and report about, such as a new 1000W solar kit, new Webasto system with some smart insulation solutions, slurry blasting, new JL Audio music system for the fly are a few to count. I haven't slept in my cove during winter :)

I changed the gipsy because the old one had worn teeth.
 
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