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31-05-2013, 23:07
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#121
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Registered User
Join Date: Mar 2012
Location: Nelson NZ; boat in Port Stephens, NSW.
Boat: 45ft Ketch
Posts: 1,562
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Re: Choice of Material for Snubbers
Not meant to be a scientific report, just an illustration of how much sag increases with chain length. Pulling out this sag requires a good length and weight of chain to be dragged perpendicularly through some 6ft of water with a significant dampening effect. Short rodes don't have this.
The small change in length between chain end points indicates the vessel will be not be surging back and forth from the anchor with the large mass decelerations mentioned in the OP. The surge will be from the bow going up and down with the waves. This explains to me why anchor size graphs relate to boat length(size) more than weight. Also says to me that the snubber elasticity is less important than I thought.
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31-05-2013, 23:17
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#122
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cruiser
Join Date: Feb 2011
Location: Pittwater, Sydney
Boat: Lightwave, Catamaran, 11.5m (38')
Posts: 1,000
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by DumnMad
The small change in length between chain end points indicates the vessel will be not be surging back and forth from the anchor with the large mass decelerations mentioned in the OP. The surge will be from the bow going up and down with the waves. This explains to me why anchor size graphs relate to boat length(size) more than weight. Also says to me that the snubber elasticity is less important than I thought.
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Only if you are able to deploy 100m of 13mm chain. If space is tight and and your yacht is of a size for 12mm chain or you have lighter chain - maybe a different story?
In fact if I carried 100m of 13mm chain I might be encouraged to start a thread on 'anchors - smaller is better?'
Jonathan
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01-06-2013, 00:16
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#123
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Registered User
Join Date: Feb 2012
Posts: 2,441
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Re: Choice of Material for Snubbers
It's a matter of computing the area under a graph of load* vs extension:
This represents the work done lifting the chain;
When that area equals the kinetic energy the boat starts with, the boat will have been brought to a halt.
Work done = change in energy
This will tell you how high the chain's tension will have to rise to bring the boat to a halt.
The hardest thing, I reckon, is coming up with a realistic figure for the speed at which the boat starts its run. I don't recall ever seeing a higher figure on the knotmeter than something under half a knot, even in roadsteads.
* (strictly speaking, the horizontal component of chain tension, but for the purposes of this exercise, if I was being fussy I would simply call it a constant value of 5.9/6 x chain tension, assuming a constant scope of 6:1 , as 5.9 is the approx length of the horizontal leg of a 6:1 triangle)
It would not matter to disregard it altogether for the purposes of this exercise, and just use chain tension)
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01-06-2013, 01:33
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#124
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Registered User
Join Date: Mar 2012
Location: Nelson NZ; boat in Port Stephens, NSW.
Boat: 45ft Ketch
Posts: 1,562
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by JonJo
Only if you are able to deploy 100m of 13mm chain. If space is tight and and your yacht is of a size for 12mm chain or you have lighter chain - maybe a different story?
In fact if I carried 100m of 13mm chain I might be encouraged to start a thread on 'anchors - smaller is better?'
Jonathan
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Same principal applies for smaller boat with 8mm chain. Smaller forces involved on both boat and chain.
I've often read about the trouble that occurs when caught in a storm in crowded anchorages, boats dragging onto each other. Reinforces the story that short roads are not good in a real storm.
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01-06-2013, 01:54
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#125
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Registered User
Join Date: Feb 2012
Posts: 2,441
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Re: Choice of Material for Snubbers
When I submitted my previous post I didn't realise I was not on the last page, so I did not introduce it as I should have
Someone was asking how to turn chain catenary tension values into a peak load when snubbing a boat of a given mass moving at a given velocity.
Given a few points from a reliable tabulation of catenary for the chain you have in mind, (or a formula such as the one in Vryhoff's "Offshore Anchor Manual" p 156), a rough graph can quickly be laid out on squared paper.
The squares under the curve between two vertical lines tell you the work which has to be done on the chain to move it from the left line to the right line.
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01-06-2013, 02:11
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#126
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Registered User
Join Date: Feb 2012
Posts: 2,441
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Re: Choice of Material for Snubbers
I don't see the need to carry 100m of 1/2" chain, but I do plan to carry 70m of 1/2" chain, and go to a mixed rode when that length proves insufficient.
In such deep water, the catenary will be nearly as good for a mixed rode; it's the weight in the lower section which principally matters.
My chain locker will be amidships, to one side of the keel case, and I'll be able to open flaps at the bottom fore & aft) to spread the chain out in the bilge when off soundings, so it hugs the keel case as low in the hull as possible, serving as internal ballast. Naturally the sole in this area will bolt down, after lashings are cinched over the chain heap. (Secured using instances of the 'toggled Lark's Head' knot I posted on the sheetbend thread, so they can be remotely tripped without lifting the sole, in case of sudden need)
The ballast (along with other 'informal' ballast' is factored into the boat's formal ballast to arrive at a realistic CoM. This means that there is little penalty for carrying heavy chain.
Given that the windlass is back at the mast, I have half a mind to fit a shot of 16mm chain for the section from the anchor, when it's at the roller, to the windlass.
I wouldn't bother doing this with a newGen anchor, but with my Bruce, I know it sets and resets considerably more quickly and firmly with a heavy chain adjacent to it.
And the friction of such a heavy chain on the bottom is a considerable help when the boat is exploring a poor-holding bottom, in the manner of a tongue investigating a chipped tooth...
I don't expect this combination to routinely require a snubber, on a 7 tonne boat.
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01-06-2013, 02:41
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#127
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Moderator
Join Date: Jul 2007
Boat: Bestevaer.
Posts: 15,161
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by Andrew Troup
I don't expect this combination to routinely require a snubber, on a 7 tonne boat.
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As you realise, you will still need a snubber for stretch in stronger wind, and you will also want to use a snubber for noise abatement routinely in most anchorages.
With a custom boat, consider having a strong point on the bow as low down as possible. If you attach the snubber from this point you get increased scope, or reduced swinging room. Reducing the rode length by 4-5m, with no drawback in holding can at times be quite beneficial. (I am writing this from an anchorage where we only have a few meters clearance from rocks on two sides.)
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01-06-2013, 03:50
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#128
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Registered User
Join Date: Feb 2012
Posts: 2,441
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Re: Choice of Material for Snubbers
Noelex
Thanks v. much for the knuckle strongpoint idea. I've actually used it a few times on previous boats and can vouch for it, but don't plan to on this boat for similar reasons to those Evans traversed in a recent post.
I will however have a strong point in that location for haulout and salvage (and of course it will be available for situations where it makes sense to rig a snubber from it)
As regards your point
<<you will still need a snubber for stretch in stronger wind>>
If the wind alone were strong enough to cause issues with this rode (6m of 16mm & 70m of 1/2") on a 7 tonne vessel, I'm not sure what an elastic snubber would achieve.
A guy I built an anchor roller for was lying to a rode of that heft (less the 16mm leader) in Sth Georgia not long ago in conditions where he did not consider it seamanlike to rig a snubber.
His rationale was that if it got any worse he was going to have to leave, and the space was confined.
He was anchored alongside another rather experienced guy I know, who has spent his life sailing in those latitudes.
He also was not using a snubber, even though the wind strength was such that his anchor was slowly dragging, and had to be reset at intervals.
I should add that the first guy's boat is nearly four times the weight I'm looking at.
I use this example because I believe there's a youtube vid somewhere reputed to show the chain is coping just fine, and there is no appreciable snubbing. The owner was concerned about peak loads on the anchor in gusts, but remember this is a 27 tonne vessel.
I could ask for the URL if you'd be curious.
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01-06-2013, 04:13
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#129
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Moderator
Join Date: Jul 2007
Boat: Bestevaer.
Posts: 15,161
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Re: Choice of Material for Snubbers
I have used 13mm chain on a 14.5T boat and a snubber was essential for stretch from say 30-35k knots.
It is difficult to extrapolate this to a much smaller boat, but as the forces go up roughly as the square of the windspeed I don't think you will get much increase in windspeed before a snubber is necessary.
I also don't see any need to try and devise a system that will work without a snubber. As well as noise reduction a snubber serves other functions such as reducing the wear on the galvanising of the chain as it passes over the bow roller.
Keep in mind, with any system, even in storm force conditions, it is sometimes not practical to deploy 70m of rode.
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01-06-2013, 04:15
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#130
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 35,020
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Re: Choice of Material for Snubbers
I think it's clear that there is no way to calculate a priori what kind of energy a particular boat will need to snub for any given anchoring situation. We have been using hypothetical quantities of energy represented by the boat needing to be stopped from some particular speed, which is just a wild guess. In fact, the loading will be very complicated coming from all kinds of different factors.
I can say from experience that it is not too hard to exhaust the energy absorption ability of 100 meters of 12mm chain on a 25 ton yacht like mine. Someone mentioned that pitching down of the bow of the boat will also be an effective shock absorber -- I agree, but not always -- if the boat is pitching up at the moment a wave hits, then not at all.
And in fact, in my experience, it was pitching which seemed to cause the most violent snatch loads I experienced when I broke my (undersized) snubber in a storm. I thought my bow roller was going to be ripped off -- the forces were huge.
Andrew has got energy absorption of catenary all figured out . You can calculate how much energy absorption is left at any given stress, in the way Andrew mentioned. All I can say is that chain catenary starts to behave more and more like a solid rod at a certain point. Someone calculated that the more resistance the chain catenary provides, the more energy it will absorb -- I say again, the reality is exactly the opposite. That resistance is being applied more and more directly to the anchor, as the chain tightens.
Whether 70 meters of 12mm chain will do away with the need for any snubber on Andrew's boat, I have no idea. He will have to experiment, I guess. All I can say is that 100 meters of the same chain is not nearly enough on my 25 ton boat. I don't know how to calculate it (bet Andrew can figure it out), but I am guessing that by the time there is a force on the chain equal to its weight, there's practically no more energy absorbing going on. It seems to me that it's like a lever -- when the chain is hanging straight down, you are lifting it as if with the long end of a long lever; by the time it's straight-ish, you're lifting it with the short end of a long lever. In the middle somewhere, one kg of force will lift one kg of chain. The more and more unfavorable this "leverage" becomes, the less involved in the transfer of force becomes the catenary -- the more it behaves like a solid rod. But that's a guess -- maybe someone can figure out the math.
Most sailors know the "running into a concrete wall" sensation, when their chains pull tight, and know that this can cause damage or pull the anchor. That's why most of us (at least most, if not all) need snubbers which are capable of absorbing more energy, after the catenary effect is exhausted.
As to Andrew's chain locker -- brilliant! ! If your chain is forming part of the ballast, then there is practically no downside to having more of it. Why stop at 70 meters? If I had an arrangement like that, I would have 150 meters, maybe even 200 meters of chain -- which would weight 660kg. I can say that I've had 100 meters out on frequent occasions, and sometimes wished I could put out more.
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01-06-2013, 04:31
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#131
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Moderator
Join Date: Jul 2007
Boat: Bestevaer.
Posts: 15,161
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Re: Choice of Material for Snubbers
I think locating the chain further back is a great improvement on any cruising boat.
There are some difficulties particularly with drainage and smells.
Boreal has a similar system and there has been some complaints from owners about smell from mud and decaying sea life permeating the cabin, but these problems should be solvable. I think the benefits of centralised weight are appealing, particularly with smaller boats. I agree with Dockhead if you can achieve this I would carry lots of chain.
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01-06-2013, 04:32
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#132
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cruiser
Join Date: Jan 2012
Location: Pangaea
Posts: 10,856
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by s/v Jedi
Again, I disagree. A chain that gets lifted up from the seabed has in fact absorbed energy and stored it in the form of potential energy. It later transfers/releases it by lowering to the seabed again. The fact that this is a different method from storing energy than tensioning a rope, does not mean it is irrelevant or does not work. It is silly to compare stretch of chain with stretch of nylon just like it's silly to say nylon is cr@p because it does not weigh enough to absorb shocks. You must compare the stretch effect of nylon with the "lifting the chain" effect of chain. Better yet: look at how the two complement each other and start to understand why chain rode is always better than rope.
Same for polyester: it will stretch just fine and the same length, provided you make the snubber longer to compensate for the difference in stretch percentage. It all works but it all needs a slightly different approach.
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Exactly what I was trying to explain in my previous post, only put into words in a more precise way. I'm obviously in 100% agreement with Jedi.
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01-06-2013, 04:40
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#133
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 35,020
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Re: Choice of Material for Snubbers
To continue the thought about pitching:
I have no idea what a typical angular velocity of a pitching boat would be, but one radian per second doesn't seem preposterous. If I take my arm and pitch it up one radian (57 degrees) while counting "one-mississippi", I can well imagine. Of course one pitch cycle will rarely produce such a pitch angle, but it doesn't matter -- we're concerned only about the angular velocity.
That looks like it could produce quite a lot of force. One radian would be about 7 meters on my boat (the bow is about 7 meters from the center of gravity, and an angle of one radian is where the radius is the same length as the described arc). Seven meters per second is about 14 knots. That's the velocity right out at the bow, so obviously we're not talking about 25 tons all moving at 14 knots, but still -- even one-tenth of 25 tons moving at 14 knots would produce a fearsome amount of force, if the bow roller moving at that speed snatches up against the tightened chain, pre-stressed by wind forces.
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01-06-2013, 04:43
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#134
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 35,020
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by noelex 77
I think locating the chain further back is a great improvement on any cruising boat.
There are some difficulties particularly with drainage and smells.
Boreal has a similar system and there has been some complaints from owners about smell from mud and decaying sea life permeating the cabin, but these problems should be solvable and I think the benefits of centralised weight are appealing, particularly with smaller boats. I agree with Dockhead if you can achieve this I would carry lots of chain.
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I would simply seal it off -- make the chain locker like a tank, with its own sump and bilge pump (and obviously a large access hatch). This (separate bilge pump) will be necessary anyway if you need to wash the chain off while its stored and don't want all the silt and scum to wash into your bilge.
I wish my boat had something like that!
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01-06-2013, 04:53
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#135
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cruiser
Join Date: Feb 2011
Location: Pittwater, Sydney
Boat: Lightwave, Catamaran, 11.5m (38')
Posts: 1,000
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Re: Choice of Material for Snubbers
Andrew, I missed something, somewhere
You are going to carry 550lb of 13mm chain, plus some 16mm chain on a 7t yacht? Depends on the anchor - but your ground tackle weight, windlass included, will be 5% of the yacht weight.
If weight low down, on the seabed, is the 'answer', why compromise - why not just carry 16mm chain, and some cordage (if you are worried about abrasion (dyneema) otherwise nylon (or more, length than the nylon, dacron).
The bit I have missed is the 7t yacht.
You mention your Bruce, is this the same anchor you will carry - what sort of size is it (on the 7t yacht).
Excuse my querying.
Jonathan
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