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Old 18-09-2017, 23:25   #1
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Anchor snubbers

Many boats I have seen anchored seem to use an anchor snubber - a short piece of line attached to their anchor chain and then to a bow cleat, but I am not sure of its value. Surely in all but the harshest conditions if a 4:1 scope is used then the catenary of the anchor chain should absorb any shocks caused by wave movement and the snubber is unnecessary. Can someone please explain why the snubber is useful?
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Old 19-09-2017, 00:00   #2
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Re: Anchor snubbers

There are several reasons for a snubber, and I'll just list a couple. I need to leave room for the other posters to add.

First, it takes load off of the windlass and transfers it to a strong point on the boat, such as a cleat. If you use a bridle then it transfers the load to two strong points, which I think is even better.

Second is the noise down below. At anchor we sleep in the forward cabin. Using the bridle quietens the noise of the chain fetching against the windlass and makes sleeping much easier.

There have been a number of posts on this topic, and if you have a few minutes you may wish to peruse some of them.

Cheers!

Steve
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Old 19-09-2017, 00:14   #3
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Re: Anchor snubbers

The primary purpose of snubbers on ground tackle is to absorb snatch loads. You use elastic rope -- almost always nylon -- and this absorbs energy when the boat moves back and forth with the sea and starts to snatch up. Think of it as a shock absorber.

Chain catenary may or may not perform this function. Mostly depends on how much weight of chain you have deployed and what the sea state is -- so the size of the chain is important. The problem is that when the chain starts to get pulled tight, the elasticity disappears. So if you "use up" the energy absorbing potential of the chain catenary, then you can have damage from snatch loads.

On my previous boat with 8mm chain, the snubber was absolutely essential -- the chain was too light to absorb enough energy to prevent snatching up in even in fairly moderate conditions.

On my present boat, with 12mm chain, I almost never use a snubber unless conditions are really rough and/or the water is quite shallow.

Please do NOT use a snubber for Purpose 1 in Steve77's post above. A snubber is not designed for or suitable for taking the whole load of the ground tackle -- it's just a shock absorber. The chain should be belayed by a different, strong means -- preferably as strong as the chain (otherwise it forms a weak link). A strong strop or a strong chain lock is best. The snubber is no good for this purpose because, to be elastic enough to fulfill its primary purpose, it cannot be as strong as the chain. Also nylon, when working to absorb energy, is less strong in practice than its rated strength, due to internal heating. On top of that, nylon loses strength when wet, and if that's not enough already, nylon is extremely vulnerable to chafe compared to other kinds of cordage. Your windlass won't hold your boat if a storm blows up -- don't leave your boat depending on an elastic snubber to stay attached to the anchor. Belay that chain!
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Old 19-09-2017, 00:56   #4
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Re: Anchor snubbers

A well-make anchor snubber will absorb the shock loads AND be strong enough to pull back on the anchor under tremendous loads. Serving two functions and providing a nice quiet restfull sleep.

We live on the hook in anchorages six months per year, been doing it for six years. Our anchor snubbers are directly connected between the oversized deck cleats and the anchor chain to take any load off the windlass.

Here it is in action with 10:1 scope in up to 40knots.

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Old 19-09-2017, 02:34   #5
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Re: Anchor snubbers

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Originally Posted by Paramotorgreg View Post
.. but I am not sure of its value. Surely in all but the harshest conditions if a 4:1 scope is used then the catenary of the anchor chain should absorb any shocks caused by wave movement and the snubber is unnecessary. Can someone please explain why the snubber is useful?
It takes a lot less force to lift all the chain than you might imagine, even with 10mm chain, and bear in mind that you want to be OK in those occasional big gusts.... With 10mm chain in 5m water at 4:1 it's something like 80Kg to lift the last link, 130Kg for 12mm chain. Not that much really.. And remember what benefit there is from the catenary in slowing the energy transfer still exists after fitting a snubber, the snubber helps in addition to the catenary, not instead of.

A lot of posts get the physics a bit wrong, it's the *rate* of energy transfer which is proportional to the force, so when you have a chain already close to off the sea bed and a boat veering about the energy in the moving boat gets transferred to the chain in a very short distance, so the force can get really high really quickly. With a good snubber setup this deceleration is over a longer distance so the force on the anchor is lower, it helps a lot!

I run a length of nylon down the side decks terminated near the cockpit through a block at the bow so there's not so much line in the water. Though usually only if it's gusting over 20Kts.

Some links..
Scope vs catenary (Rocna Knowledge Base)
https://www.spaceagecontrol.com/calc...tton=Calculate
https://www.desmos.com/calculator/ihht9akvm1
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Old 19-09-2017, 11:23   #6
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Re: Anchor snubbers

Thanks to everyone for your helpful and informative responses.
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Old 19-09-2017, 11:55   #7
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Re: Anchor snubbers

Just saw the thread, but I see it’s all be covered. Nothing else to add, except I love our bridle snubbers. Makes a significant difference to how we ride at anchor, dampens the sounds, ensures the loads are taken at the solid bow cleats and not the windlass, AND ensures any shock loads are managed without problems.

What interests or amazes me are some of the tiny strops some people use as snubbers. Sometimes as short as a foot or two. Guess it’s better than nothing, but can’t do much if it really starts to blow.
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Old 19-09-2017, 12:19   #8
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Re: Anchor snubbers

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What interests or amazes me are some of the tiny strops some people use as snubbers. Sometimes as short as a foot or two. Guess it’s better than nothing, but can’t do much if it really starts to blow.
Or the folks with static chain and slack snubber. WE get a kick out of that. They know enough to use them, but don't clearly understand their purpose or function.

We also get a kick out of the 2 foot snubber. We had two friends ask us why our snubber goes all the way into the water. They both claimed to have never seen that. Then I point out 4 boats in our proximity with the chain hook below the water line. 10 feet is the shortest we'll set our snubbers. Our snubbers are around 25 feet long.
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