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Old 29-05-2013, 13:57   #46
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Re: Choice of Material for Snubbers

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With the above, the problem that I see most often is that people don't have a clue what size snubber they need. I see boats half my weight with 3/4" snubbers. That is why I keep stating that your snubber is too thick until you break one in the middle (not due to external chafing) and go up one size. I broke 1/2" so I'm with 5/8" and it never broke until in a full cat4 hurricane. It survives 60-80 knot squalls that last 20 minutes, although the 80-knot squall resulted in hard spots, where the fibers had melted together. This gives an exact data point and IMHO, one that confirms 5/8" 3-strand nylon is the right size/type combination for my boat.
I like it--empirical evidence from actual on-the-water experience! However, my own philosophy is to size the snubber for 95% of my anchoring situations, which do not include 80-knot gusts. In general (there have been exceptions) I do not see those type of gusts without some warning, so my usual snubber is sized to be able to handle up to and including the 40-50 knot gusts we sometimes see in thunderstorms. If I am expecting heavier weather, I will often slap on a another snubber or one that is longer and/or heavier than the first. If I am getting a lot of wind another anchor will almost always be going over the side, reducing the load too.
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Old 29-05-2013, 15:03   #47
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Re: Choice of Material for Snubbers

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Originally Posted by Chas Erwin View Post
Jedi

When suggesting a snubber of 30' in 5/8 diameter do you refer to a bridle - thus two legs each 30' - or do you mean a single line at 30'?

Charles
Single line. I don't like bridles at all except for catamarans.
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Old 29-05-2013, 15:08   #48
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Re: Choice of Material for Snubbers

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I like it--empirical evidence from actual on-the-water experience! However, my own philosophy is to size the snubber for 95% of my anchoring situations, which do not include 80-knot gusts. In general (there have been exceptions) I do not see those type of gusts without some warning, so my usual snubber is sized to be able to handle up to and including the 40-50 knot gusts we sometimes see in thunderstorms. If I am expecting heavier weather, I will often slap on a another snubber or one that is longer and/or heavier than the first. If I am getting a lot of wind another anchor will almost always be going over the side, reducing the load too.
Well, that is basically what I have. It's just that we got a rare 80 knot squall and my snubber didn't break although it had half melted midway.

This is still something I need to express better: in my experience, the snubber does not break or fail from regular chafing. They break from internal shafing in a part that is beyond the bow out in the open not rubbing anything. The inner part melts and then the fibers on the outside is not enough and it breaks.

No amount of Dyneema for the inboard end is going to solve that. May be polyester (Dacron) will solve it, I don't know.
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Old 29-05-2013, 15:21   #49
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Re: Choice of Material for Snubbers

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This is still something I need to express better: in my experience, the snubber does not break or fail from regular chafing. They break from internal shafing in a part that is beyond the bow out in the open not rubbing anything. The inner part melts and then the fibers on the outside is not enough and it breaks.
You expressed it very clearly, though I have never experienced that problem of line melting, even when hit by an apparent tornado in Chesapeake Bay. It takes some pretty severe wave action and/or gusty conditions to create that kind of heat in the line.
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Old 29-05-2013, 15:24   #50
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Re: Choice of Material for Snubbers

I believe that Practical Sailor came to the conclusion that snubbers and dock lines that don't chafe though; fail due to internal heat.

Also I'm not going to do the cals, but I would bet people (I do normally but that isn't in a storm shock situation) use too long of a snubber/have too much slack chain so that the snubber can in fact stretch beyond its breaking point. Which is probably way after it would have been safe to transfer the load back to the chain.
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Old 29-05-2013, 15:29   #51
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Re: Choice of Material for Snubbers

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Originally Posted by Dockhead View Post
Sorry, I didn't say that.

I never said that you need 90,000 pounds working load (!!) on your ground tackle.
Huh?

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Originally Posted by Dockhead View Post
A 25 ton yacht (like mine) moving at 2 knots will generate the following dynamic loads at various “stopping distances”:

Nylon octo, 1.5 meters – 923 kg
Polyester braid, 24cm – 5773 kg
Chain, 3,4cm – 40,749 kg
I appreciate your posting the article from Prof. Mariner, as I think it provided real world calculations on dynamic loading on rodes in different conditions. Those calculations seem to correlate pretty well with actual data generated by Practical Sailor in the test described here: Anchor Testing and Rode Loads - Practical Sailor Article

They used all chain rode on a 5.5 ton 38 foot Lightwave cat in winds up to 30 knots. Those winds generated veering through 75 degrees, and they report that all the catenary was removed during these veers bringing the boat up hard on repeated shock loads and the maximum load measured was.....900 pounds. Interestingly, if you use the displacement of the Lightwave cat and make some guesses about the distance moved by the vessel before the catenary of the chain was gone, you'll come up with about this loading value using the easy to understand formula in the Prof. Mariner article.
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Old 29-05-2013, 15:34   #52
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Re: Choice of Material for Snubbers

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Also I'm not going to do the cals, but I would bet people (I do normally but that isn't in a storm shock situation) use too long of a snubber/have too much slack chain so that the snubber can in fact stretch beyond its breaking point. Which is probably way after it would have been safe to transfer the load back to the chain.
There are so many variables that it is impossible to make a definitive statement about all boats in all situations, but I have numerous times hung on a very long length of nylon attached to a short length of chain in conditions up to and including hurricane strength winds--certainly over 100 a few times, and never experienced melting or breaking issues, even when on the size of rode and/or snubber I would use day-to-day. So, I don't think you can say a long snubber is worse than a short snubber in severe conditions. Certainly, snatch and peak loads will be reduced the longer the snubber. I would venture to guess that if you could put out enough length you could hang on a very thin piece of line even in hurricane conditions. The average load is not all that high--it is the peak loads you need to avoid.

However, the loads on a big boat like Nick's are much greater, and he has experienced and seen this problem first hand, so these issues are not well understood.
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Old 29-05-2013, 15:39   #53
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Re: Choice of Material for Snubbers

I think there is a big difference between a rode that is a section of chain with a long length of rope, and an all chain rode with a 20' section of snubber on it.

And the only definitive statement I can make about boats is that it is not good when they go under water (even on submarines as we sounded an alarm each time before doing it).
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Old 29-05-2013, 15:45   #54
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Re: Choice of Material for Snubbers

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Originally Posted by Cotemar View Post
I have never lost a snubber to chafe and have never lost a chain hook.

I have tested numerous snubbers and chain hooks and you know what, they all worked well.

Sailing is far more fun than worrying about the zinchoid cravostat molecular line cordage on leap year.
I'm nearly in complete agreement with you but it's a decent enough discussion point. This is my first time spent in a hurricane zone during the season and if we encounter any cyclical storms it will be at anchor. So if there are some simple modifications I can make to my ground tackle I'll do it.

What spooks me out though is making a decision on a laptop about what "should" work. With a nylon snubber that's roughly your boat length, it's a fairly known quantity where the problems will be and what to do about them. If the elasticity is too much either go thicker or shorter, or both.

I don't want to be the guy breaking new ground with what on paper looks like the best idea only to find out in the middle of a hurricane some interesting quirk that puts the shortcomings of nylon into prospective.
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Old 29-05-2013, 16:02   #55
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Re: Choice of Material for Snubbers

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Originally Posted by Delfin View Post
Huh?



I appreciate your posting the article from Prof. Mariner, as I think it provided real world calculations on dynamic loading on rodes in different conditions. Those calculations seem to correlate pretty well with actual data generated by Practical Sailor in the test described here: Anchor Testing and Rode Loads - Practical Sailor Article

They used all chain rode on a 5.5 ton 38 foot Lightwave cat in winds up to 30 knots. Those winds generated veering through 75 degrees, and they report that all the catenary was removed during these veers bringing the boat up hard on repeated shock loads and the maximum load measured was.....900 pounds. Interestingly, if you use the displacement of the Lightwave cat and make some guesses about the distance moved by the vessel before the catenary of the chain was gone, you'll come up with about this loading value using the easy to understand formula in the Prof. Mariner article.
The tests with the 38ft Lightwave would have been carried out by JonJo on his vessel.
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Old 29-05-2013, 16:31   #56
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Re: Choice of Material for Snubbers

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Originally Posted by downunder View Post
The tests with the 38ft Lightwave would have been carried out by JonJo on his vessel.
Affirmative,

A catamaran is not ideal (or not to anyone with a monohull), but its all we had.

Jonathan
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Old 29-05-2013, 16:38   #57
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Re: Choice of Material for Snubbers

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Affirmative,

A catamaran is not ideal (or not to anyone with a monohull), but its all we had.

Jonathan
Ideal for someone such as myself planning for a catamaran.

The discussion and information on this thread has been great.
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Old 29-05-2013, 16:43   #58
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Re: Choice of Material for Snubbers

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Ideal for someone such as myself planning for a catamaran.

The discussion and information on this thread has been great.
The tests were conducted with no bridle, so no snubber. We will eventually repeat with a snubber, or snubbers and we are also aiming to repeat with a 50' yacht (limited by what we can have access to).

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Old 29-05-2013, 17:09   #59
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Re: Choice of Material for Snubbers

This is my last thought for now on this topic, until I have more direct experience with and have fined tuned my 'new solution' . . . .

(1) Clearly an all nylon snubber works and is a perfectly adequate solution. It's the traditional and well proven solution. I don't think anyone will say that all nylon is 'unreasonable'.

(2) I personally think that if you want to move from 'adequate' to 'best' than some sort of hybrid will be the solution. I have some significant engineering support in that thinking from both MIT and New England Ropes. As mentioned in some posts above the difference between 'adequate' and 'best' will probably only matter in significantly severe conditions, OR when you are thinking about perhaps leaving your boat on anchor alone/unattended for a length of time.

This thread (and the other one) mostly talks about dyneema and Dacron and nylon very generally, but each of those comes in a number of very different constructions, which have very significant differences between them. So, the question of which line construction is 'best' is almost as interesting/important/difficult as the fibers.

(3) In all this, I think the 'soft shackle' is the most useful/practical development . . . . more so than the rode discussion. Both the rolling hitch and chain hook obviously generally work, but IMHO the soft shackle is just simply day to day "easier and better" than both with zero downside.
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Old 29-05-2013, 17:10   #60
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Re: Choice of Material for Snubbers

JonJo, when you set up your next test series, how about checking out Dockheads theories on peak loading from a moving boat? One could, under calm conditions, first motor up creating a large slack in the rode, and then motoring briskly in reverse until the slack takes up and jerks the boat to a stop. I suspect that the numbers thus generated would be far lower than his calculations suggest, but real data would be useful to test the theory.

Another query, slightly off thread: in calculating the frontal area of a catamaran, can one really ignore the "tunnel"? I believe that at higher velocities one might find that that apparently open area added significantly to the total drag... don't know about the kind of wind speeds that we (hopefully) will be experiencing.

Cheers,

Jim
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