Who Needs a Snubber, Anyway?

[S/V Alchemy]

Originally Posted by valhalla360

I have mixed feelings on the trip line. In an isolated anchorage they may be OK but I've seen a few run over in busy anchorages as people are trying to use up any available space.

You may know what you are doing but no guarantee the next guy is smart enough to realize what the float is and that he shouldn't run it over.

True, which is why careful consideration should be given to visiting crowded harbours full of the unskilled or unaware in the first place. When the sea and wind seem predictable, why throw in the completely unknown of "other sailors"? I'd rather heave to outside or spend on a dock if I really wanted to see a place. Having said that, you can tell pretty quickly if a place is full of duffers or moderately skilled sailors just by looking at how their boats are put away.

[thinwater]

Another place we have become lost in the woods is confusing scope (is the last link still on the bottom) and shock absorption (is there enough sag in the chain to actually do anything). Although they sometimes happen at the same time, they are different issues. For example, in shallow water it is perfectly possible at long scope to still have chain on the bottom (or at least a very low angle), but have no meaningful shock absorption.

Just sayin' the math is different.

I use relatively long snubbers (40' bridle) because I anchor shallow a lot. I cleat to the midships and have carefully evolved chafe elimination gear (yes, the snubber pumps over the bow).

The problem with short snubbers is that while they are fine much of the time, they have a well-earned reputation for failing when the wind gets up. Just when you need them, they fail to reduce the force sufficiently and fail due to fatigue. Sail had a silly article a few months ago on this, which reached all of the wrong conclusions.

[Dockhead]

Originally Posted by conachair

I'm loosing the plot here about what you're actually arguing, sorry

So my point - the catenary curve changes as the water depth gets greater for a given scope.

Maybe you missed the edit, but from this equation..

Catenary Force in kg = Specific chain weight x (chain length)^2/(2 x Depth)

So for 10mm chain (about 2kg/m) in 4m water at 5:1 scope to just lift the last link:
F=(2*20*20)/(4*2)=100Kg

Same in 10m water:
F=(2*50*50)/(10*2)=250Kg

Or a scope of 3.16:1 for a horizontal force of 100Kg.
Water depth makes a big difference, you need less scope for the same horizontal force compared to shallower water.

In very shallow water forget the catenery effect , it hardly exists unless you have a ridiculous scope out. Though the angle to the anchor is so shallow that it doesn't matter much with our lovely new gen anchors

Some maths:
An Analysis of Anchor Rode Length versus Depth
ANCHORWATCH- THE THEORY OF ANCHORING

We're just talking past each other.

You're talking about deeper water with constant scope. Which means more chain at greater depths. I have no problem with your calculations, but they don't support the idea that deeper water improves holding.

Obviously, and trivially, a greater quantity of chain (which you get in deeper water with constant scope) will give a greater catenary effect. But this is an effect of more chain, not of more depth.

The same quantity of chain is more effective, the shallower the water is. The worst case (bar tight) angle of pull is better. It is true that catenary works over a smaller range in shallower water, but catenary has more effect in shallower water once the chain is off the bottom. Any in any case, the chain also works when it's lying on the bottom.

So more depth is of no benefit in anchoring. You need less scope in deeper water (as Peter Smith proved) but you don't need less chain. The same amount of chain will work better, in shallower water.

How effective chain lying on the bottom is, most cruisers know from experience. I was anchored a few days ago in some strongish weather, gusting in the high 20's. I was all alone in a vast open bay and so let out every inch of my 100 meter chain (330 feet). The water depth varied between about 5 and 8 meters.

Besides the strong wind, this is a strong tidal area, with currents up to 4 knots. But with 100 meters of chain out, the boat never went more than 60 meters from the anchor as it swung back and forth with the tide. Even when 4 knots of tide was combine with 20-odd knots of wind.

It means no force was ever put on the anchor at all -- I was lying to the chain alone.

[a64pilot]

Originally Posted by Dockhead

For the record -- not using a snubber is NOT good practice, because, as someone said, once the chain starts snatching hard up -- where the snubber will not only improve comfort but will be preventing actual damage -- it's too late to rig it.

Why would it be too late, connect the hook to the chain and let out enough chain so that now the lines and not the chain are tight.

But to be completely honest I have never had the chain snatch hard up, I've always attached the snubber before we got there.
It's not rocket science, wind picks up or you see the black wall of clouds coming, hook up the snubber. Like reefing?
But last weekend, with 0% percent chance of rain, and never had more than 5 kts of wind at the greatest, I didn't bother with the snubber. This weekend, we may get enough wind off of the Hurricane so that I will use it.

[Dockhead]

Originally Posted by thinwater

Another place we have become lost in the woods is confusing scope (is the last link still on the bottom) and shock absorption (is there enough sag in the chain to actually do anything). Although they sometimes happen at the same time, they are different issues. For example, in shallow water it is perfectly possible at long scope to still have chain on the bottom (or at least a very low angle), but have no meaningful shock absorption.

Just sayin' the math is different.

I use relatively long snubbers (40' bridle) because I anchor shallow a lot. I cleat to the midships and have carefully evolved chafe elimination gear (yes, the snubber pumps over the bow).

The problem with short snubbers is that while they are fine much of the time, they have a well-earned reputation for failing when the wind gets up. Just when you need them, they fail to reduce the force sufficiently and fail due to fatigue. Sail had a silly article a few months ago on this, which reached all of the wrong conclusions.

I'm not sure why you wouldn't have any shock absorbing ability with chain on the bottom -- of course you will. More scope increases the energy absorbing capacity of the chain, just like heavier chain does.


But in any case, I sure do agree with you about short snubbers. It is inherent to nylon that it heats up internally as it stretches and absorbs energy, and in a storm with a lot of cycling, it tends to fail at far below its rated strength. A short snubber has to be light in order to be stretchy enough. Get the same amount of stretch with heavier but longer line, but with far greater ability to absorb energy without breaking.

[valhalla360]

Originally Posted by S/V Alchemy

True, which is why careful consideration should be given to visiting crowded harbours full of the unskilled or unaware in the first place. When the sea and wind seem predictable, why throw in the completely unknown of "other sailors"? I'd rather heave to outside or spend on a dock if I really wanted to see a place. Having said that, you can tell pretty quickly if a place is full of duffers or moderately skilled sailors just by looking at how their boats are put away.

In general that's a great idea but in some areas, a dock is extraordinarily expensive and all the local anchorages are packed.

Often these are some of the most interesting places to visit (hence the packed anchorages)

[conachair]

Originally Posted by Dockhead

We're just talking past each other.

You're talking about deeper water with constant scope. Which means more chain at greater depths. I have no problem with your calculations, but they don't support the idea that deeper water improves holding.

I'm not sure anyone said that? The point is in deeper water the catenary curve is different in your favour, water depth most certain effects the catenary.

Obviously, and trivially, a greater quantity of chain (which you get in deeper water with constant scope) will give a greater catenary effect. But this is an effect of more chain, not of more depth.

Any why is there more chain.... because...... it's deeper.

The same quantity of chain is more effective, the shallower the water is.

but catenary has more effect in shallower water once the chain is off the bottom.

Err, what exactly does "more effect" mean? And prove it. Mathematically

You need less scope in deeper water (as Peter Smith proved) but you don't need less chain. The same amount of chain will work better, in shallower water.

Off course you need more chain. It's deeper!

And for example, a boat in 20m water with 5:1 scope just lifting the chain would need 15:1 in 2.5m water for the same force.

It means no force was ever put on the anchor at all -- I was lying to the chain alone.

A load cell would have been interesting..

https://www.youtube.com/watch?v=ZVDrvRaifOA

[conachair]

Originally Posted by Dockhead

More scope increases the energy absorbing capacity of the chain, just like heavier chain does.

Not quite, or not in that wording, when a boat is stopped from moving that energy needs to go somewhere - to the anchor chain the the anchor then the seabed. It's the speed of the energy transfer which is important as far as force on the anchor is concerned. Which is why snubbers work better than catenary when it all gets a bit exciting, it only takes a very short distance for a catenary to go from pretty full to bar tight.

[Dockhead]

Originally Posted by conachair

I'm not sure anyone said that? The point is in deeper water the catenary curve is different in your favour, water depth most certain effects the catenary.


Any why is there more chain.... because...... it's deeper.





Err, what exactly does "more effect" mean? And prove it. Mathematically


Off course you need more chain. It's deeper!

And for example, a boat in 20m water with 5:1 scope just lifting the chain would need 15:1 in 2.5m water for the same force.

OK, it looks like we are not actually disagreeing about very much anymore! However, I do think that the proposition that putting out more chain in deeper water gives more catenary effect is fairly trivial. Of course it does. The real question was whether deep water in itself improves catenary effect. It does not.


As to more catenary effect from the same length of chain in shallow water, that's easy to show --

Once the chain is off the bottom (you get a head start with deeper water, but I'll come back to that), you get greater horizontal force from a given piece of chain, in shallower water.

Why? Simple -- geometry. The catenary calculator is for chain hung between two points of the same elevation. But the anchor in the seabed will not be at the same elevation as the bow roller.

The greater the difference in elevation, the less horizontal force and more vertical force. At the limit -- once the chain is hanging straight down -- the weight of the chain imparts zero horizontal force and 100% vertical force.

On the contrary, the closer the anchor is to the height of the bow roller (so the more it looks like the catenary calculator) -- the shallower the water -- the greater proportion of horizontal force.


Now why does the same length of chain work better in shallower water, than in deeper water. I've already explained that, but for those who were not paying attention:

1. The whole problem is easier from the beginning -- the angle of pull even when the chain is bar tight is better. Simple geometry.

2. If a significant part of the chain is lying on the seabed, there will be less chain to hang and impart horizontal force, absorbing shock and improving the angle of pull. But you don't need to improve the angle of pull if a significant part of the chain is in the seabed -- it's already ideal. And besides that, the chain engages the seabed and will be doing beneficial work if it's lying on the seabed.



Net net -- deep water is no benefit in anchoring, except possibly in terms of shock absorption. For a given length of chain, you will get better holding, the shallower the water is -- because you will get either a better angle of pull, or you will have more chain on the seabed generating holding forces.

For shock absorption -- the same amount of chain in deeper water MIGHT give you better shock absorption in some conditions when you have more chain hanging in the water as opposed to lying on the seabed. But not at the limit -- this advantage disappears once the chain is substantially off the seabed, and because you can get more horizontal force from hanging chain in shallower water, the ultimate force will be greater.


Looking at it in terms of constant scope makes no sense -- anchor chain is not sold by the "scope", it's sold by the meter. You don't magically get more chain in your locker just because you're anchoring in deeper water.

[S/V Alchemy]

Originally Posted by valhalla360

In general that's a great idea but in some areas, a dock is extraordinarily expensive and all the local anchorages are packed.

Often these are some of the most interesting places to visit (hence the packed anchorages)

Yes, but hardly as expensive as the sort of damage a dragging boat can cause. Part of my calculation of risk management in boat management is knowing when to spend...and when to leave.

My opinion has been influenced by seeing how many boats dragged and sank in Antigua during Hurricane Gonzalo in '14 and this article, to name a few similar ones. There are many lessons to be learned here about tackle sizing, anchors away from others, keeping an ear open for radio, monitoring the weather, keeping your stuff secure and having a game plan if hit by a squall: Safety at Sea: Storm in the Harbor | Cruising World

[S/V Alchemy]

Originally Posted by conachair

A load cell would have been interesting..

Does this mean I've finally seen MaineSail? We appear to wear equally worn shorts...

[thinwater]

Originally Posted by S/V Alchemy

Does this mean I've finally seen MaineSail? We appear to wear equally worn shorts...

I ran exactly the same chain drag tests in the Chesapeake a few days ago and got basically the same result--less on sand, about the same in soft mud.

Of course, the real answer is even less, because it's unrealistic to think you have more than a small amount of chain lying undisturbed on the bottom when there is enough wind to matter.

But it does buffer the effect of shifts, since it has to cut a grove through the mud to a well-buried anchor. Harder to quantify. A bigger chain would have more friction, but the anchor would not be buried so deep and thus the grove not cut as deep.

[S/V Alchemy]

Originally Posted by thinwater

I ran exactly the same chain drag tests in the Chesapeake a few days ago and got basically the same result--less on sand, about the same in soft mud.

Of course, the real answer is even less, because it's unrealistic to think you have more than a small amount of chain lying undisturbed on the bottom when there is enough wind to matter.

But it does buffer the effect of shifts, since it has to cut a grove through the mud to a well-buried anchor. Harder to quantify. A bigger chain would have more friction, but the anchor would not be buried so deep and thus the grove not cut as deep.

Yes, all that makes sense. You can't get far rolling on a shock absorber. That said, the differences between all-rope rode, rope rode with chain leader, and all chain are pretty noticeable even in benign to moderate conditions.

[thinwater]

Originally Posted by Dockhead

...It is inherent to nylon that it heats up internally as it stretches and absorbs energy, and in a storm with a lot of cycling, it tends to fail at far below its rated strength. A short snubber has to be light in order to be stretchy enough. Get the same amount of stretch with heavier but longer line, but with far greater ability to absorb energy without breaking.

In my view the heating thing is 1/2 true. Yes, the line heats, but ONLY if it is already working well above it's SWL of about 10% BS. If the line is sized properly (<10% BS) it will never heat. If it is heating, it is overloaded enough that it will break soon even if cooled.

Also, if you break a new, cold nylon line in a test rig, the ends fuse slightly from the heat released at rupture. Not all of the heating has to do with cycling.

Sail Delmarva: Can Nylon Rope Melt Due to Load Cycling --- Myth Semi-Busted

(this was part of a project with New England Ropes)

[thinwater]

Originally Posted by S/V Alchemy

Yes, all that makes sense. You can't get far rolling on a shock absorber. That said, the differences between all-rope rode, rope rode with chain leader, and all chain are pretty noticeable even in benign to moderate conditions.

Oh yeah. I've had both on the same boat.

I think the reason cat sailors are sensitive to the issue is that the boats are light and we anchor shallow. The first day I anchored with my all-chain rode I had a polyester bridle I had used with a rope rode. When the wind started gusting... bam, bam, bam.... I spliced up a nylon bridle that afternoon, at anchor, while getting jerked around.

For me, chain with a long bridle is the best of both worlds.