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Old 12-08-2014, 08:43   #46
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Re: Snubbers - Or the Lack of Them

As with all things boating...eventually you will be given a lesson why it's good to be in the habit of using the snubber.
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Old 13-08-2014, 00:35   #47
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Re: Snubbers - Or the Lack of Them

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Originally Posted by valhalla360 View Post
A nylon snubber doesn't have to be as strong as the chain (2 meters is not long enough though).

Let's assume a 10,000kg boat (21,000lb):

Let's say in a bad gust, when the cantenary runs out, the boat is moving at 10ft/sec. Technically, there is a small amount of elasticity in the chain. Say the chain stretches 1/2 an inch over 0.1 seconds. When you run the math, the chain and the attachment point take approximately 300,000newtons of force (approx 67,000lbs)

Now add in a 50' snubber that can stetch 5' over 0.5 seconds while bringing the boat to a stop and you will find the force is cut by a factor of 5 or approximately 60,000newtons (13,500lbs).

There are some complications that make the exact forces not quite the same but the principal holds true. The snubber greatly reduces the forces involved.
Somethings not right with the numbers here? I can't see a boat moving at 10 ft/sec, thats near 6 knots, unless you are in some serious surf, and in that case any snubber made from nylon will rapidly overheat and break unless it's so large that stretch is minimal. With those sort of loads the nylon will stretch much more than 10%, and the deceleration caused by the snubber will extend the time period it takes. I figure 3 knots would be about the most we would normally see. The catenary of the chain will also decelerate a vessel before it goes bar tight. Saying that I really appreciate the use of numbers rather than random guesses, and I think the 6:1 ratio is probably representative of the benefit of a longer snubber. Cheers

One of the times we broke the chain was when a gust blew the gaff mainsail up the mast and we took off. We were probably doing 3 or 4 knots when the chain came up tight with a bang. It didn't break completely, just opened up a link. As I said it was crappy stuff, and too short (only 30m) so we where often at short scopes of 3 or 4:1.
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Old 13-08-2014, 00:51   #48
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Re: Snubbers - Or the Lack of Them

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Originally Posted by Dockhead View Post
Someone on here - I think it was Andrew Troup - questioned whether we even need snubbers at all, if we have a heavy chain. I have been cruising continuously since the end of April, and I decided to try leaving off the snubber. I have 100 meters of 12mm chain.

I would never leave off the snubber if I ever thought that the wind would get up enough to take all the catenary out of my chain, but in the benign Baltic, this was not a problem (not a single gale in 3 months here). Guess what - it was fine! Never missed the snubber. Made no difference at all. The reduction of effort was welcome, too.
Dockhead, with over 9,000 posts in this forum, I cannot believe (well maybe I can believe) that you really need an answer to this question...I am sure that you must know the answer.

So, wait for it...here it comes...wait for it...what anchor do you use and do you think that snubbing or not snubbing has anything to do with your anchor?

Laughing out loud!

Please have a really good day.

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Old 13-08-2014, 06:08   #49
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Re: Snubbers - Or the Lack of Them

when i left off the snubber in a calm flat anchorage, i twisted my sprit and had to readjust it, i now use snubber line always..and i keep an eye on it .. my original chafed thru, so i roll hitched another on to the chain and when i was ready to come into marina, we just pulled it up with the chain and i sorted the mess when i got into slip.
winds picked up a tad in spring, preparing for summer blows, i came in when the winds were at 30 kts, having walked backwards a few feet with each new blow, i decided it was time. ... our holding here sucks, and when i lifted anchor i learned why i was walking back--- there was only 2 inches of anchor set into anything worth setting into . not unusual here. the set did fine to 30 kts then we walk 2 ft... botttom is silt over wtf,and no one eknows what the wtf is..is a river bed and gets righteous in summer with runoff and storms.. every year the bottom is different here, and bad bad bad holding.
it has beeen found that when one backs down on their anchor here, there will be an unsetting of that anchor and that boat will drag. here one needs to lay down more than 150 ft chain to stay put. please remember this when ye come to barra lagoon, as we (all the cruisers here) do save ye, is best when we dont have to.
we did tests after the third boat dragged the second time.
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Old 13-08-2014, 10:45   #50
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Re: Snubbers - Or the Lack of Them

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Somethings not right with the numbers here? I can't see a boat moving at 10 ft/sec, thats near 6 knots, unless you are in some serious surf, and in that case any snubber made from nylon will rapidly overheat and break unless it's so large that stretch is minimal. With those sort of loads the nylon will stretch much more than 10%, and the deceleration caused by the snubber will extend the time period it takes. I figure 3 knots would be about the most we would normally see. The catenary of the chain will also decelerate a vessel before it goes bar tight. Saying that I really appreciate the use of numbers rather than random guesses, and I think the 6:1 ratio is probably representative of the benefit of a longer snubber. Cheers

One of the times we broke the chain was when a gust blew the gaff mainsail up the mast and we took off. We were probably doing 3 or 4 knots when the chain came up tight with a bang. It didn't break completely, just opened up a link. As I said it was crappy stuff, and too short (only 30m) so we where often at short scopes of 3 or 4:1.
Mostly picked 10ft/sec to make the math simple to demonstrate the principal of why an elastic system doesn't have to be as strong.

It could certainly happen in storm conditions and while the rope might break, a chain only system with 67,000 lb yank would likely the pull the cleat out of the deck if the chain doesn't break first.

If it helps, run the math with 5ft/sec (a very realistic situation in much more common conditions) and you still see a huge increase in forces compared to a system that allows for stretch.

Plus if the snubber gives way, you will likely hear or feel the difference. The anchor chain will hopefully hold and give you an oppurtunity to take action (run a new snubber, use the engine to take some of the strain off, pull anchor and get away). If you are purely on chain, when it fails, you are adrift immediately.
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Old 13-08-2014, 18:00   #51
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Re: Snubbers - Or the Lack of Them

Fair enough. I assume it will 1/4 the loads. Interesting to do a proper anaysis. Need to look at alain's site again.
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Old 17-08-2014, 10:01   #52
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Re: Snubbers - Or the Lack of Them

In an anchoring system, there are other parts than the chain that need protection from snatch loads. The bow roller is one.

Last winter, I had to anchor in a gale, to wait for a bridge to open. The bridge was downwind, the wind was too strong for motoring head to wind and the channel was so narrow that I couldn't realistically beat to windward. The scope was very short because I was almost aground on the mud bank (rising tide) and I saw the bow roller flexing sideways as each wave struck the bow.

I had to surge the chain manually on a cleat (no windlass, 8mm chain) each time, until I managed to add 2 meters of 14mm line as a snubber that saved the situation.

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Old 17-08-2014, 15:25   #53
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Re: Snubbers - Or the Lack of Them

Alain, you're not paying attention, dont you know that 2 meters of snubber is not enough...

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Old 17-08-2014, 15:35   #54
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Re: Snubbers - Or the Lack of Them

One small issue with a snubber being set ALL the time is that it can mask any chain noise that can indicate dragging, or even wind and movement. Around here the nights are often glassy calm due to decoupling, with no snubber and sleeping in the forward cabin I can hear if the boat is dragging the chain across the seabed. This means some wind, maybe 10 knots. Normally this is enough to wake me up for a check.

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Old 18-08-2014, 13:42   #55
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Re: Snubbers - Or the Lack of Them

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Alain, you're not paying attention, dont you know that 2 meters of snubber is not enough...
Yes, I know 2 meters of snubber is not enough generally to absorb wave action but I hadn't room for more in the beginning, for I was almost aground then.

I wasn't afraid of dragging (the bottom is very stiff mud there, tested it with keels on other days) or breaking the chain (8mm, less than 2 year old, on a 3.5t boat).

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Old 18-08-2014, 23:25   #56
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Re: Snubbers - Or the Lack of Them

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.

I wasn't afraid of dragging (the bottom is very stiff mud there, tested it with keels on other days)
Alain
Interesting way to test bottoms - I usually use other methods. Is there an advantage to testing with your keel?

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Old 19-08-2014, 11:47   #57
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Re: Snubbers - Or the Lack of Them

When aground, you are "anchored by the keel": no need to deploy an anchor and wash it later.

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Old 19-08-2014, 22:51   #58
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Re: Snubbers - Or the Lack of Them

well damn! why didn't I think of that? You also save a lot of money - no need to even carry an anchor - or chain for that matter. the anchor well can then be used for something more useful like more gin for sundowners

hell, we can close down all the threads on this board about "does size really matter?" Is bigger better? Rocna versus manta versus et al.

Sir I shall be happy to second any nomination for you to the Nobel committee
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Old 19-08-2014, 23:30   #59
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Re: Snubbers - Or the Lack of Them

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well damn! why didn't I think of that? You also save a lot of money - no need to even carry an anchor - or chain for that matter. the anchor well can then be used for something more useful like more gin for sundowners

hell, we can close down all the threads on this board about "does size really matter?" Is bigger better? Rocna versus manta versus et al.

Sir I shall be happy to second any nomination for you to the Nobel committee
Yeah, it might kill the anchor threads, but it will fuel up the keel type threads: my full keel is a better anchor than your piddley little fin keel...

Ain't no escape, mon!

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Old 20-08-2014, 00:44   #60
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Re: Snubbers - Or the Lack of Them

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Yeah, it might kill the anchor threads, but it will fuel up the keel type threads: my full keel is a better anchor than your piddley little fin keel...

Ain't no escape, mon!

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We can rename the technique.. No more "anchoring".. it be a "wedgie!"
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