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29-05-2013, 09:39
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#16
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Choice of Material for Snubbers
If the OP's had said "my opinion is that nylon is the best snubber material" . . . .
I would have replied that my current opinion is that a hybrid solution seems to me to be the best over all solution: taking cost into account Dacron from the cleat to just over the bow roller (with the 'normal' amount of snubber line out), and then nylon from there to the chain attachment, and then dyneema (either sling to chain hook or soft shackle). That's a relatively inexpensive, easy to make snubber line, that combines both stretch and chafe resistance.
I should note, this is not my idea . . . I am just adapting it from New England Ropes. They suggest dyneema instead of the Dacron section (and I am also using dyneema). That obviously generally adds cost while also increasing chafe resistance. I think that probably makes sense if you are anticipating storm or hurricane use (which NER in fact is for mooring buoy use) but less cost effective than Dacron for 'normal use'. I might note that quite a number of these NER hybred lines were used in New Bedford thru the last two hurricane hits and zero broke, while apparently (according to the harbor master) all other constructions had at least one failure. I happen to use dyneema for my mainsail reef lines, so it's 'free' for me to use 'retired' lines from that application in the snubber.
I am not sure what sort of Dacron or nylon construction would be best for those two parts - there are various sorts of constructions with a range of stretch vs chafe vs breaking strength alternatives. I think I personally would start with a Dacron 8 plait and nylon climbing line . . . but I doubt you would really go far wrong with any of the typical constructions.
However, the OP essentially said: "nothing but nylon can work, everything else will break almost immediately (including Dacron & straight chain)". That's simply and provably false by real world observation. . . .just a 'real world sniff test' says the OP is wrong. My Dacron snubber did not break last summer. People who only use chain and a metal chain lock without any snubber at all almost never break their chain. Those are just simple facts. As to 'better calculations'.....these sort of dynamic load calculations are incredibly difficult to get right. Several people have tried quite hard and put a lot of effort related to anchor rodes, and so far, none (that I have seen) have come close to accurately modeling the actual load dynamics. . . . they all have failed both 'the real world load cell' check, and the much more simple 'sniff test' (do things actually break in the real world roughly when this model says they will).
Again, Dockhead and I will probably simply have to disagree on this topic.
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29-05-2013, 09:45
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#17
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Registered User
Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by Dockhead
My calculations were done carefully and are correct, as far as I know. If someone wants to dispute them, they could provide figures and their own better calculations, and not just a statement that they are false.
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From the example in the pdf you posted:
"Here’s some numbers. If we put over 100 feet of line and hold it, a typical nylon line will stretch 20%, to about 120 feet, before reaching it’s safe working load at which point we hope the ship is stopped – beyond that we risk damaging or parting the line. A high strength/low stretch line such as Spectra or Kevlar may stretch only 2% or about 102 feet. If the ship is moving 1 knot, the dynamic load on the nylon line as it stretches to 120 feet would be about 5 lbs per ton of vessel displacement while the dynamic load on the Spectra line as it stretches to 102 feet (a ten times faster rate of deceleration) would be about 50 lbs per ton."
The way the author arrived at 5 pounds per ton was by plugging the numbers into the formula of Load in pounds = Tons displacement * 100 * Velocity^2/Distance, or 5 # = 1 * 100 * 1^2/20. Plugging your numbers into the same formula you get a static load of 25 * 100 * 4 = 10,000#. Put a snubber on that stretches by 10 feet, and you're only dealing with 1,000# force.
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I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
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29-05-2013, 09:51
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#18
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Registered User
Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by estarzinger
However, the OP essentially said: "nothing but nylon can work, everything else will break almost immediately (including Dacron & straight chain)". That's simply and provably false by real world observation.
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I guess that depends on what you mean by "can work". If you mean not break, then any strong line that doesn't break meets that criteria. If you mean provide deceleration of the acceleration imposed by wave and wind on a vessel anchored so that you introduce a "distance" value into the formula for Force converting static force into dynamic force, then only a stretching line will do.
All depends on what you want your snubber to do.
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
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29-05-2013, 10:04
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#19
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by Delfin
If you mean provide deceleration of the acceleration imposed by wave and wind on a vessel anchored so that you introduce a "distance" value into the formula for Force converting static force into dynamic force, then only a stretching line will do.
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two things:
#1 as Andrew has pointed out every material will stretch to some degree. The two interesting questions are 'how much stretch do you want' and "what/where should you make trade-offs for other factors (like chafe, UV resistance, etc)". And there may well not be a 'one size fits all' answer to these two questions.
#2 the OP purported to show that both Dacron and Chain would in fact actually break (due to the shock loads) in a relatively typical situation, which is (as I said) just plain false.
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29-05-2013, 10:13
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#20
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,745
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by Kettlewell
My non-scientific observation is that stretch is good in a snubber, and I have always used 3-strand nylon for the purpose and never broken it in bad weather. I use 3/8" nylon on my 38-foot 20,000 lb. boat, and I used the same thing on my 32-foot catamaran. I have never had a problem with "too much stretch," and in the past I often used mixed rodes with a short length of chain and long lengths of nylon (still very common here on the East Coast). In recent close brushes with the remains of Sandy and Irene I had a couple of Fortresses out on long lengths of nylon--really long--and there was no feeling of "too much stretch." I have also hung on a parachute sea anchor offshore in Force 9-10 on 400 feet of 1/2" nylon. The ride is noticeably smoother and easier when on mostly nylon, in comparison to being on chain with a snubber--maybe my snubbers are too short. Plus, read your Van Dorn and your Frayse. A mixed rode (chain and nylon) is best for the lowest loads on the system and therefore the best holding power.
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for the non scientific approach. Geez.... is it really that complicated? Nylon 3 strand is the ticket. I used 1/2" on 44 mono and 42 catamaran. Had a 5/8 snubber also. Both stretched quite a bit in a blow. That's what I wanted, something to absorb the energy....
#2 the OP purported to show that both Dacron and Chain would in fact actually break (due to the shock loads) in a relatively typical situation, which is (as I said) just plain false.
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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29-05-2013, 10:14
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#21
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Registered User
Join Date: Aug 2010
Location: Anacortes, WA
Boat: 55' Romsdal
Posts: 2,103
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by estarzinger
two things:
#1 as Andrew has pointed out every material will stretch to some degree. The two interesting questions are 'how much stretch do you want' and "what/where should you make trade-offs for other factors (like chafe, UV resistance, etc)". And there may well not be a 'one size fits all' answer to these two questions.
#2 the OP purported to show that both Dacron and Chain would in fact actually break in a relatively typical situation, which is (as I said) just plain false.
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In storm conditions, more stretch coupled with appropriate breaking strength is best. The stretch determines the ability of the snubber to add deceleration distance and the length determines how big the "d" in the force formula actually is. Double the length of stretch before the snubber breaks and you half the force you're dealing with. It's as simple as that.
The catenary of the chain provides lots of "d" until it can't provide anymore. Whatever stretch in the chain exists after the catenary is used up is of almost zero import in the calculation.
And no, neither typical chain nor dacron will break under typical storm conditions, although without a stretchy snubber, you'll approach the values of static loading as "d" goes to zero, which will place a heck of a load on everything else.
__________________
https://delfin.talkspot.com
I can picture in my head a world without war, a world without hate. And I can picture us attacking that world, because they'd never expect it. - Jack Handey
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29-05-2013, 10:40
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#22
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Registered User
Join Date: Jul 2006
Location: Massachusetts
Boat: Finnsailer 38
Posts: 5,823
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Re: Choice of Material for Snubbers
Even if a non-stretchy snubber or no snubber "works," it will certainly result in increased peak loads on the boat's fittings and the anchor. Even without a snubber the anchor will have some give in the bottom, reducing the load, or the anchor might even pop right out, further reducing the load until (hopefully) it recatches. I think we have all witnessed how a sudden tremendous gust can cause an anchor (hopefully on someone else's boat) pull right out of the bottom, setting the boat free. So:
#1: There is no argument that greater stretch = reduced peak loads.
#2: Reduced peak loads = less strain on deck gear and anchor, meaning whatever anchor you have will hold better.
#3: There obviously has to be a balance between adequate strength, stretch, and chafe resistance.
__________________
JJKettlewell
"Go small, Go simple, Go now"
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29-05-2013, 10:45
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#23
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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 Delfin
Excellent analysis, Dockhead, although there must be a decimal placement issue going on in your conclusion. I am certainly no engineer, but I think (and this being a forum if what I think is wrong, I'll find out from 38 people), is that if a 24 ton boat were moving at 2 knots and you brought it to a stop instantly, the force would be around 9600 pounds. Since the force goes up exponentially to the acceleration, if you make the speed 3 knots, the effective weight of the vessel in terms of the load on the gear goes to over 21,000# of force.
But that's not what happens with boats at anchor - there is always some distance they travel under increasing load on the ground tackle before they stop. If the distance is 5 feet, then the 2 knot, 24 ton example yields a force of 1,920#; for the 4 knot example, 4,200#. This is why the snubber is so important - after the catenary of the chain would have been lost you have the stretch of the snubber that increases the distance the vessel travels before it comes up hard, and this reduces the force by a factor of the distance traveled.
That's why I wonder if Evans 15' of nylon is long enough (did I get that length right?) since that allows for around 4' of distance to travel, reducing the force to 1/4 of what it would have been, which is great but less reduction than he would get if the stretchy part of his NASA designed snubber were twice as long. Maybe that's why most recommendations are for a minimum of 30'? Beats me, but if my math is correct, then if Delfin is bucking on a wave at 2 knots acceleration (seems reasonable, but I don't know) the force I have to deal with would be 65 tons * 100 * 4 / 7 (stretch of 30' of nylon) = 3,714#. The key to accuracy is clearly the effective displacement of the vessel - (does a 65 ton vessel place a 65 ton load on the ground tackle when bucking at anchor in waves?), and the actual speed of acceleration (2 knots seems reasonable, but is it?)
These relatively manageable forces is the reason why no one much seems to break anchor rodes/chains, and why you can use pretty small diameter snub lines even for bigger vessels.
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Well, I can tell you this much -- if an even 1 ton boat is moving at even 0.01 knot, and is stopped instantly, I mean literally in 0.000000 nanoseconds, then the dynamic force is infinite -- would explode the whole universe. That's mere mental masturbation, because nothing with any mass can be accelerated or decelerated in 0 time.
The principle is that one G of acceleration will produce one gravity of force -- so a 25 ton boat accelerating or decelerating at 9.8 meters per second per second will be subject to a force of 25 tons. Two knots are 1.029 meters per second. So it's a simple calculation to show that to go from two knots to zero knots in one-tenth of a second will produce a force of 1.05 G, or 26.25 tons. Here's a handy calculator: Online Acceleration Calculator (flexible units)
Distance travelled during this tenth of a second (I am guessing that the average velocity during the deceleration will be (v0 + v1)/2, assuming a constant rate of deceleration, so 0.5145 meters per second; someone please correct me if I'm wrong), is 5.1cm.
So you'll need total stretch of 5.1cm (including the whole system) and strength of more than 26.25 tons, to slow the boat down from 2 knots to 0 in one-tenth of a second.
The more stretch you have, the more time you have, the less deceleration, and the forces go down dramatically. That is the basic principle of a snubber.
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29-05-2013, 10:53
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#24
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Registered User
Join Date: Feb 2013
Location: Oregon
Boat: Seafarer36c
Posts: 5,563
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Re: Choice of Material for Snubbers
So my bil was not exaggerating when he described running into a rock straight on at 8.5 knts. in a 50' Bene. ( it did not sink.)
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29-05-2013, 11:20
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#25
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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 estarzinger
two things:
#1 as Andrew has pointed out every material will stretch to some degree. The two interesting questions are 'how much stretch do you want' and "what/where should you make trade-offs for other factors (like chafe, UV resistance, etc)". And there may well not be a 'one size fits all' answer to these two questions.
#2 the OP purported to show that both Dacron and Chain would in fact actually break (due to the shock loads) in a relatively typical situation, which is (as I said) just plain false.
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Well, let's get straight what I purported to show. I am a techno-ignoramus so not able to express these things very well.
I did not purport to show that dacron or chain would break "under ordinary conditions". What I showed was more theoretical -- that the amount of force produced by a 25 ton yacht travelling at 2 knots would break a 50 meter 12mm G40 chain or a particular 6 meter piece of Dacron, but not a 6 meter piece of nylon. In reality, it's not so clear, because anchor move, anchor rollers bend, and there is elasticity in different parts of the system.
But no one has refuted the calculations. If you don't like my calculations -- I will be interested to see your own.
In reality, of course, you can use Dacron as a snubber, and if you make it thin or long enough, you can even make it as elastic as a nylon snubber. No one disputes that. You could even use a steel cable as a snubber if you could make it long enough.
Maybe let me state it another way --
Do you want to use Dacron as a snubber? If you are not willing to reduce the total strength of your snubber, then your Dacron double-braid snubber will have to be roughly 8 times as long as the same strength snubber in nylon octo, in order to get the same elasticity. So you will need 48 meters instead of 6 meters; 80 meters instead of 10 meters (my heavy snubber is 10 meters). Here is a handy calculator of acceleration vs. distance vs. speed which you can use to satisfy yourself: Online calculator: Kinematics. The Equations for Uniform Acceleration. Better than the other one because all the variables can be used.
So it's your choice, what material to use. I see that Evans has ultimately chosen nylon for the elastic component of his snubber -- same as my choice (and I'm going to copy his super-snubber, too, which is the best snubber I hae ever seen ). I think the choice of material is pretty obvious but obviously everyone will make up his own mind.
Do people break chains and so forth at anchor? Chains, rarely, but that's only because there are softer things to be found in the system, like anchor rollers and bowsprits. I have myself had a broken bowsprit after a snubber failure -- a very rude lesson in shock loads.
Are 2 knot speeds "ordinary conditions", that your anchor gear should cope with? I think 2 knots is a very big force which would never exist for cruisers who anchor in reasonable conditions and in good shelter. But in stormy conditions and when the shelter disappears (wind shift), things can go to s&*t in the blink of an eye. The forces your snubber will have to deal with can go through the roof. Snatch loads -- shock loads -- are nothing to trifle with. The curve of force versus stopping distance is very steep -- and I don't think anyone can argue with that, at least.
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29-05-2013, 11:27
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#26
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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 estarzinger
. . .
I would have replied that my current opinion is that a hybrid solution seems to me to be the best over all solution: taking cost into account Dacron from the cleat to just over the bow roller (with the 'normal' amount of snubber line out), and then nylon from there to the chain attachment, and then dyneema (either sling to chain hook or soft shackle). That's a relatively inexpensive, easy to make snubber line, that combines both stretch and chafe resistance.
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You and I are in violent agreement on this point. Sounds to me like the perfect snubber
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29-05-2013, 11:33
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#27
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Choice of Material for Snubbers
Quote:
Originally Posted by Dockhead
The more stretch you have, the more time you have, the less deceleration, and the forces go down dramatically.
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I am happy that you have discovered that F=MA, but unfortunately engineering does not end there. Believe it or not, I do in fact quite well understand the basics physics of force and deceleration and I still disagree with you.
#1if you don't like shock loads, and think they are BAD, I can tell you from first hand experience that you do not want your snubber to chafe thru in 50-60kts (happened to me on an oyster 72 with an all 3 strand nylon rode with fire hise chafe protection). You should in fact add some significant value into your engineering trade-offs for chafe resistance. That makes it at least a two factor analysis, while you only want to consider one factor.
You can of course try to compensate for chafe "after the fact" with fire hose or leather, but nylon systems with chafe protection can and do in fact fail. From a QA perspective it is useful to think about how to avoid that failure mode in the basic design rather than how to minimize it later.
There are actually even more factors. Just for instance, Jedi has said he does not use a good stretchy type of line because it is too time consuming to splice.
#2 if a snubber was all & only about elasticity, why are you not using one of the industrial "bungee cords" rather than nylon? Why are you not putting out twice a much snubber as you in fact putting out? Why do you not have one of the various designed shock absorbers in the snubber? You have in fact made a decision to limit the elasticity of your snubber when there are still easy things you could do to further increase it - why? You are in fact not really acting as if you really believe what you are saying that it is in fact NOT all about more elasticity is always better.
You tell me, what concerns/downside do you see to a hybrid design of Dacron between the cleat and roller, with nylon from there to a short dyneema chain attachment? You seem to be implying that this is a completely unreasonable system because it is not all nylon and has a little less stretch than an all nylon one would. Do you really think say 2m of Dacron in the system is a bad trade-off to minimize the likiihood of the system chafing in 50kts?
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29-05-2013, 11:39
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#28
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always in motion is the future
Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 19,779
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Re: Choice of Material for Snubbers
I disagree with it all, nonsense!
If I take 1/8" nylon with bubble gum core, and it stretches a hundred yards, it is not going to absorb any shock; instead, it is allowing my boat to accelerate and make it worse.
The snubber *must* be tuned to the boat. Static tests and theories don't stand up to reality, there are much more crucial factors involved that are being ignored.
Some consider a snubber a device to keep forces off the windlass like a replacement of a chain stopper and use whatever they like while others consider a snubber a shock absorber. The term is used for different things and everybody is talking from his/her pov regardless if it has anything to do with what others post. I could post about the flycatcher I just saw and it would be as relevant as the rest.
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29-05-2013, 11:41
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#29
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Marine Service Provider
Join Date: Apr 2005
Location: Salish Sea
Boat: Tayana 37
Posts: 249
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Re: Choice of Material for Snubbers
On any boat I have sailed, I now and I think always have used rather heavy three-strand nylon line secured to chain by a rolling hitch. Perhaps it is not perfect, but has always worked well for me.
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SalishSeaPilot.com cruising guides
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29-05-2013, 11:46
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#30
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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 estarzinger
I am happy that you have discovered that F=MA, but unfortunately engineering does not end there. Believe it or not, I do in fact quite well understand the basics physics of force and deceleration and I still disagree with you.
#1if you don't like shock loads, and think they are BAD, I can tell you from first hand experience that you do not want your snubber to chafe thru in 50-60kts (happened to me on an oyster 72 with an all 3 strand nylon rode with fire hise chafe protection). You should in fact add some significant value into your engineering trade-offs for chafe resistance. That makes it at least a two factor analysis, while you only want to consider one factor.
You can of course try to compensate for chafe "after the fact" with fire hose or leather, but nylon systems with chafe protection can and do in fact fail. From a QA perspective it is useful to think about how to avoid that failure mode in the basic design rather than how to minimize it later.
There are actually even more factors. Just for instance, Jedi has said he does not use a good stretchy type of line because it is too time consuming to splice.
#2 if a snubber was all & only about elasticity, why are you not using one of the industrial "bungee cords" rather than nylon? Why are you not putting out twice a much snubber as you in fact putting out? Why do you not have one of the various designed shock absorbers in the snubber? You have in fact made a decision to limit the elasticity of your snubber when there are still easy things you could do to further increase it - why? You are in fact not really acting as if you really believe what you are saying that it is in fact NOT all about more elasticity is always better.
You tell me, what concerns/downside do you see to a hybrid design of Dacron between the cleat and roller, with nylon from there to a short dyneema chain attachment? You seem to be implying that this is a completely unreasonable system because it is not all nylon and has a little less stretch than an all nylon one would. Do you really think say 2m of Dacron in the system is a bad trade-off to minimize the likiihood of the system chafing in 50kts?
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Well, we have a deep failure of communication. I am actually finding it hard to find anything that we actually disagree about. Either I am expressing myself extremely badly, or you are really not reading what I am writing.
In no particular order:
1. I never said the more stretch the better. And anyway, a bungee cord of a given diameter will not have nearly as much energy-absorbing capacity as nylon octo. We need "enough" energy absorbing capacity. How much is that, I am not able to calculate. I took stopping the yacht from 2 knots as a random example.
2. You keep fiercely defending your hybrid snubber, apparently oblivious to the fact that I have said at least 10 times that I think it is fantastic; I'm even planning to make one myself. I never said that energy absorption is the only value; in fact I explicitly said that I worry about chafe and worry about the susceptibility of nylon to chafe. If you have "enough" stretch in the system, the of course -- who could argue -- other parts of the system can be designed with other values in mind, like chafe resistance, and quality of attachment to the chain. I think you've really nailed those things, and said so a number of times.
3. I never said that either stretch or energy absorption is the only value in designing a snubber. Of course it's not. Of course if it chafes through, no amount of stretch will save you. I would think that this would be obvious to anyone.
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