How much anchor chain?

[bbg]

To elaborate on our collective wisdom would anyone like to comment on the reasons behind suggesting the OP invest of 50m of 8mm chain (for what I believe is a yacht with a performance pedigree)?

I don't pretend to be an expert, but I can see no good reason apart from that the person suggesting it has shares in Jimmy Green

I've always preferred a mixed rode for lightness and shock absorbing ability, but one would need either no windlass or the right gypsy to cope.

FWIW - I learned cruising on the west coast of British Columbia. Reasonably high tidal ranges, mixed ground of mud, sand and rock. Boat with a fine entry and overhang so not much buoyancy in the bow. Importantly, no windlass. My father used 3-5 metres of chain followed by a lot of rope, marked for depth at intervals. It never failed us, never chafed significantly.

I cannot imagine having to pull up all chain by hand. Nylon in water seemed virtually weightless. We would really only feel weight when the anchor came off the bottom.

I would say the only downside would be that in strong winds, the boat sailed around at anchor a lot more than I would have expected with all chain. We tried kellets with limited success. If I were to use that system again I would consider experimenting by hanging a second anchor off the bow - with just enough rope to let it reach the seabed and maybe a meter more. Not for the purpose of anchoring the boat, but for the purpose of reducing / limiting swing.

 

[Stemar]

I have about 55m of chain, and that has been plenty wherever I've anchored on the West Coast of Scotland. It's complete overkill on the East Coast!. I also have a long rope I could use to extend it, but have never used it.

Worth remembering that chain is heavy. I once deployed all of it (I was anchoring in 20m of water after the engine failed), and it was a very heavy job to recover it; two of us were blowing hard after getting it in, and the other guy is a big chap.

I think that's the issue. If you have a windlass, weight's no problem - until it decides it doesn't want to play, but that's another story and time for some ingenuity, as with any other breakdown. If you haven't, the ability to heave it all up again is the limiting factor and, even if you normally anchor in no more than 10m, you still need to be able to recover the lot from as deep as your scope allows you to anchor, because, sooner or later you'll need to, and a mixed rode makes that a lot easier.

I'll grant that, on coral, rope's a bad idea but, last I heard there wasn't a lot of coral in the N Sea or the Channel. As I understand it, it's been proven that the weight of the rode becomes irrelevant once the wind gets up; what matters is length, so the pull on the anchor's as near horizontal as possible, so the answer's more length rather than more weight. I think some chain's a good idea for benign conditions to reduce the amount the boat wanders around and, possibly, reduce wear, but nothing's yet convinced me that a mixed rode isn't the best compromise in most waters for someone without a windlass. Compromise is the important word; there are few absolutes and there will always be outlier situations where there's a better solution, but a mixed rode will deal with most situations adequately without busting your back.

 

[KellysEye]

I prefer chain to rode because I put out the length of chain required for the catenary to hold the boat and the anchor is the back up.

 

[CAPTAIN FANTASTIC]

I prefer chain to rode because I put out the length of chain required for the catenary to hold the boat and the anchor is the back up.

There are three types of anchoring. The first is relying on the anchor to dig in and hold the boat; the second is relying on lots of heavy chain deployment to provide the necessary weight and friction to hold the boat whilst the anchor is there to hold the chain!!; the third is the combination of the two.
I prefer the combination of the two with an anchor that is well balanced to dig in rather than whether the anchor has concave or convex flukes and a chain light enough to handle with additional rope to provide the necessary length.

 

[noelex]

A predominantly rope rode can be a good solution, but there are some drawbacks that need to be taken into consideration.

The major risk is that of chafe. A predominantly rope rode should obviously not be used in coral waters, but there are other possibilities that can chafe rode quite rapidly.

Isolated rocks such as this exist in some otherwise clear anchorages. This type of example is unlikely to be noted on any charts or pilot books, but it would have done a great job of cutting this boat’s rode if it was not chain:

The other problem is debris. Isolated rubbish is depressingly common in even remote locations. As an example, this large metal structure was very sharp:

These are only a couple of examples. To put this into perspective, the risk of cutting through the rope is not high and anchoring will never be 100% secure no matter what equipment is used. If the rope portion of the rode is cut the resultant drag (if we can call it that without an anchor attached) will be very rapid, increasing the risk of damage before steps can be taken. There are other drawbacks that are well known, notably the boat will swing and respond to windshifts much more rapidly than boats lying to all chain, which can be an issue in more crowded areas.

 

[lw395]

I have read that higher displacement to length ratios combined with higher hull curvature fore to aft (rocker) produces hulls that are more prone to hobbyhorsing (pitching) than boats with lower D/L ratios and flatter hulls - basically the difference between more traditional designs compared to modern flat hulls and fat sterns. Constant pitching would definately slow a boat down considerably, and I can understand that if you put weight in the ends of a boat prone to pitching this is a bad idea and would affect performance ...

However, what I have found is that my boat (a middle of the road 11,5m AWB from 1999 - a Bav36) has a tendency to slam rather than pitch - the wider stern resists pitching but the flat bow slams in the right conditions ... the slamming also affects performance, but it is remarkably reduced when the forward water tank (under the berths in the forward cabin) is full. This adds 150Kg of weight just behind where the bow meets the water (by modern standards my boat has a long overhang). Add to this the weight of 60m, 8mm chain, anchor and windlass and I normally have almost 270Kg sitting in the bows. I cruise under sail at 6-7 knots normally with roller furling main and genny, if I start going consistently over 7 knots, I reef because it is more comfortable.

My gut and my experience tells me that my boat sails better like this - slamming is reduced as the weight seems to help keep the bow down and cut through waves, pitching or hobbyhorsing is not something I've ever really experienced - laden or unladen.

I think hull design has a big influence on the effects felt by having more or less weight in the bows, and my personal experience is that my boat feels better with a bit of weight up front - it reduces slamming to almost nil and doesn't appear to slow my progress. Perhaps the accepted wisdom to avoid weight in the bow is not so significant in more modern hull designs?

I really don't know, would be interesting to hear others opinions. I know I will be keeping my anchor on the bows because it works for me and my boat.

Fore/aft trim is one thing.
Moment of inertia is something completely different.
Weight in the bow, like weight aloft, adds pitching inertia.
That means the boat is actually more resistant to pitching, a wave lifts the bow less, so it hits the wave harder with consequently more drag.

Keeping the weight out of the ends, and even more so, avoiding weight aloft, is almost always a good thing in a racing boat.
Trimming the bow down is also often fast.
Having a top class helm who can steer a path through the waves without slamming is good too, for the lucky few!

 

[noelex]

When comparing the holding ability of an anchor attached with chain vs rope, there are obviously differing views expressed in this thread.

On another sailing forum someone posted the results of this foreign magazine test that shows quite a dramatic difference in the holding power of an anchor with chain vs rope rode. I have been unable to find any other details.

Has anyone seen the test and can perhaps provide more information?

 

[Roberto]

On another sailing forum someone posted the results of this foreign magazine test that shows quite a dramatic difference in the holding power of an anchor with chain vs rope rode. I have been unable to find any other details.

Has anyone seen the test and can perhaps provide more information?

There you go:
https://www.giornaledellavela.com/2016/11/13/31622/
they used a 12kg Kobra, 8mm chain, straight pull from a fishing boat with a load cell, 4m depth, sandy bottom.

regards r

 

[noelex]

There you go:

Thanks Roberto. Perfect.

Time to break out the translators. Should be interesting reading.

 

[Roberto]

Thanks Roberto. Perfect.

Time to break out the translators. Should be interesting reading.

It s written with a very colloquial "style", automatic translators probably giving some funny output
ps do not expect anything overly technical, a lot is about how he got on the ferry to Sardinia, how he convinced his son to help him, etc etc

r.

 

[westhinder]

IIRC this was the OP’s first ever post to this forum. I wonder if he knew what it would lead to, but anyway, well done?

 

[Refueler]

Yes I'm reasonably familiar with the maths, have a look at the online calculator I posted a link to.
ISTR seeing a better one which directly varied the things we are interested in, but I couldn't immediately find it earlier.

For yachts, it's a gross simplification because yachts are often not hugely bigger than the waves they anchor in, real life is dynamic not static.
A few weeks ago I was stand on a beach looking at a small yacht anchored in some incoming swell. Might have been a Centaur? waves about 2ft peak to trough?
It was moving around a lot, but wasn't dragging as far as i could see.
I don't know exactly what the windspeed would have been, but OK for me to sail a Laser, but I'd not be surprised to have a swim or two. Maybe that's F4-F5 in metric?
It occurred to me that it was perhaps the kind of anchoring where we might need to understand whether it will work, but way harder to analyse in a useful way than any steady state constant wind stuff in still water.
Lots of stuff going on.
Rode stretching with each wave.
Rode deflected by wave action.
Boat moving up/down and forward/back.
Wind and gusts not in line with waves.

Analyse that and I'll be impressed!

Exactly ... that's why people who mock the Catenary believers ... are actually showing lack of appreciation of the factors.

How anyone can honestly disregard the effect the chain weight has and its catenary beggars belief ...

 

[Refueler]

I don't bother with all these worries and calculations. I just chuck the anchor over and let the anchor do what anchors do.

But I do have adequate anchors and adequate chain, and a boat which isn't too concerned about an extra few kilos.

Now that I can relate to !!!

 

[RupertW]

Exactly ... that's why people who mock the Catenary believers ... are actually showing lack of appreciation of the factors.

How anyone can honestly disregard the effect the chain weight has and its catenary beggars belief ...

Because the maths in a static system shows clearly that catenary disappears almost completely under very normal wind forces on a yacht, and experience bears this out for those of us who sail in clear warm waters and can observe the underwater behaviour of anchor and chain.

Adding a whole load of mobile forces to confuse the situation is a red herring to the catenary discussion because once it’s almost straight under a steady wind force then the mobile forces are hardly going to reduce the snatch loads which could break the anchor out.

 

[lw395]

Because the maths in a static system shows clearly that catenary disappears almost completely under very normal wind forces on a yacht, and experience bears this out for those of us who sail in clear warm waters and can observe the underwater behaviour of anchor and chain.

Adding a whole load of mobile forces to confuse the situation is a red herring to the catenary discussion because once it’s almost straight under a steady wind force then the mobile forces are hardly going to reduce the snatch loads which could break the anchor out.

I don't think the maths really show anything of the sort.
The difference between a rode being actually straight and looking pretty straight is fundamental.
You don't have to put much curve in the rode to increase the holding, see Noelex's post 85.
A very small amount of curve makes a significant difference to the angle of pull on the anchor.

Let's cheat and make the sums easy by approximating the catenary as an arc of a circle.
Consider a 6:1 scope, but you want a 1 in 12 slope on the rode at the anchor.
That only takes the middle of the rode to be deflected by about 2.5% of the rode length.
It might look damn nigh straight, but the curve helps!

As a rope-rode user, I therefore appreciate I need more scope than a chain user. Or my handy lump of lead....


Even a small deflection gives the effect of some 'spring', adding more tension removes some of the curve.

Adding a whole load of mobile forces does muddy the water, but unfortunately reality is like that.
The steady-state wind and tide force can be piddling small, but an anchor can still drag due to waves and gusts and windshifts.
To be fair, I do some anchoring for committee boat work where things are a bit more dynamic than anywhere I'd choose to anchor for the night, so I've seen the effects exaggerated somewhat.

 

[Refueler]

I do various types of anchoring ....

a) anchor to fish ... which basically means I'm not bothered if I drag a bit ... so I use an Anchorline Webbing reel to anchor.
b) Short lunch stop with relatively short chain scope ....
c) Long stay (eg overnight) where I need confidence I've done best I can ... which means a nice long scope of chain.
d) Stern anchor to hold her of a beach when bow moored to trees etc. Here I use a 4m length of chain to a rope rode - since getting the Anchorline Reel - I use that shackled to the 4m of chain. Here of course I drop the anchor as I slowly edge to the beach ... paying out webbing as I go .. snubbing it as I meet the mark I want.

Basically what I am illustrating - is Horses for Courses .... we can all benefit from the various methods depending on what we want to achieve and for me PEACE OF MIND that I've done my best.

As to Maths calcs and all that .... I prefer hands on experience over many years ..... what is they say about a Bumble Bee shouldn't fly ?

 

[Neeves]

At around 17 knots, depends on the yacht, and at 6:1 scope with 30m of 8mm chain deployed the last link of the chain is lifting off the seabed. How do I know - I measured it. But at a constant 17 knots your anchor has 70kg of force applied to it - assuming constant wind. Hopefully you power set (other wise the anchor will develop hold of 70kg - and move to do so) and for power setting think of about 100kg for every 10hp at full revs - note I say - about, varies with propellor etc etc.

Beyond 17 knots the last link is increasing applying tension to the anchor at an angle above the horizontal.

Now you can deploy more chain - but maybe you don't have more or there are yachts behind you and you are already comfortably close to them - and maybe there is a big rock behind you. You could of course move - but who would move when the wind is only blowing 17 knots.

For our catamaran the tension in the rode will be about 70 kgs. Its going to vary yacht by yacht - real estate built on the stern of your yacht and how you store your dinghy will all impact windage.

At 30 knots, or thereabouts, that same rode looks bar tight - it obviously is not bar tight - or its is bar tight but its not straight - but is as good as straight. In fact its actually like a long bit of steel rod (or heavy duty wire) and it is totally inelastic.

Now try to simulate this on land.

Have a short hanging chain held at one end by the members of a rugby scrum - and a 4x4 at the other end. With sag in the chain those beefy men can hold that chain as the car backs up - when the 4x4 backs up with the apparently straight chain, its not strait it does have catenary - those same beefy men cannot hold the chain - they either let go - or fall over. The amount of sag initially is such that much of the energy of the car is used to further straighten the catenary but when the catenerary is almost straight the energy developed by the 4x4s engine is unable to straighten the curve further and that energy is directed straight to the scrum - and they either let go, fall over or are pulled along.

Same with your yacht - at 17-30 knots there is enough sag that the catenary does its work - beyond 30 knots any increase in load is directed immediately and, can be, catastrophically to the yacht (and its called a snatch load).

As I mentioned at the outset you can mitigate the events at 30 knots by deploying more rode - but maybe you cannot.

If you have a nylon snubber, of course, it will continue to work until is snaps (at say 2t tension (unlikely anyone here will ever approach a 2t tension) - so no need to worry (except nylon (like people) ages and is a consumable.

Another scenario to think about is pulling or freeing a bogged car - why do the catenerary believers think they use nylon snatch ropes and not chain?.......

In real life your yacht is moving - it veers and hobby horses in the chop. It veers for a variety of reasons but one has nothing to do with the yacht - wind shear (think of those planes landing and 'veering' as they come in to land. Same happens with the yacht - the wind direction varies and the yacht scoots off - accelerating as it does so - and then it comes to the end of its tether (the rode). If the winds are gentle - does not matter - the energy of that moving yacht can be 'absorbed' by flattening the catenary - but if the catenary is already fairly straight - the energy (or movement, think momentum) of the yacht is much higher - where does that momentum go - well it goers straight down the rode and impacts both the man standing on the deck (who has to hold on and it hits the anchor in the same way (it holds (or not).

Beyond 30 knots the increase in tension needed to further remove any more sag - is huge - so if there is an increase in tension it impacts the ends of the chain, the anchor or yacht.

You can change the numbers, have bigger and longer chain - but usually your chain size is fixed and you hold a finite amount of chain. Yes you can add more - you can carry 200m if you want - but sailing performance will disappear. Most people anchor in well know anchorages (and anchorages tend to be constrained (not open) and are popular - and though you carry 100m of chain you might be unable to deploy same, other people read the same cruising guides - or its choppy further out. So think 'worst case' scenario - and you will not go wrong.

And its not horses for courses - all yachts will suffer from snatch loads - though the frequency will reduce if you don't stray far from home and only anchor under benign conditions.

If you still think this is all rubbish - wonder why most liveaboards or blue water sailors use long (10m or more), elastic snubbers.

Now a mathematician, JD where are you?, will come along and translate my laymen script into pure maths - but hopefully you get the idea.

Jonathan

 

[NormanS]

Yeah yeah, that's why we have an anchor fastened to the end of the chain. All these numbers. I sleep soundly at anchor. Do you?

 

[thinwater]

... It may surprise you to hear that even ships have brought in their anchor chain and found anchor missing ! Happened to a few ships that called in Indian ports due to using anchor chain for mooring ... therein are some stories !...

And you have leapt to to the conclusion (or implied as much) that the anchor has been missing for some time. In fact, it is 1000 times more likely that it broke loose during the weighing operation. It is also possible that there was very little wind. Either way, just a story.

 

[thinwater]

Sorry LW,

Have you any quantitative evidence to support that? I'd love to see the maths.

Jonathan

I've check the maths on that. The chain is moving too slowly to absorb much energy, once the wind is up.

Remember, behavior below gale force is not that important--anything decent should hold. It is when it blows that details matter.