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Old 02-07-2009, 12:33   #31
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But it will only stretch about 10% at the working load that will endanger your anchor

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Originally Posted by Tom Blalock View Post
New nylon can stretch about 40% before breaking. Allowing 30% (age and attachments), a 15 foot snubber can stretch 4.5 feet. That should be sufficient for a 5 foot wave, considering the catenary of the chain and the boat dipping before rising.
Example: a boat would use ~ 8000# rope/chain attached to an anchor that might hold 1500#. The SWL of the rigging is ~ 2000#, and so everything is ballanced. However, the stretch at 1500# load is what matters, and it will not be 30%, but closer to 10%. Thus, in 5' waves, by that reasoning, a 50' snubber is needed, if the same diameter as the rode.

I have over-simplified. However, anchoring in 5' waves in a blow - that is really something to avoid. If not, you're going to need a lot of anchor, chain, and a lot of snubber, I think.

I am quite sure that a rope that stretches 30% will only do that a handfull of cycles before it snaps. They are scary to watch, when loaded that hard. Imagine a 1/4" snubber - it would probably last through 5-10 waves.
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Old 02-07-2009, 13:06   #32
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Quote:
Originally Posted by thinwater View Post
Example: a boat would use ~ 8000# rope/chain attached to an anchor that might hold 1500#. The SWL of the rigging is ~ 2000#, and so everything is balanced. However, the stretch at 1500# load is what matters, and it will not be 30%, but closer to 10%. Thus, in 5' waves, by that reasoning, a 50' snubber is needed, if the same diameter as the rode.

I have over-simplified. However, anchoring in 5' waves in a blow - that is really something to avoid. If not, you're going to need a lot of anchor, chain, and a lot of snubber, I think.

I am quite sure that a rope that stretches 30% will only do that a handfull of cycles before it snaps...
Id calculate the SWL at the lowest figure in the chain (in your example the anchor holding @ 1500#).
Notwithstanding, as you imply, nylon rope will only stretch about 40% prior to breaking (near its breaking - not working strength).

Thus, in 5' waves, by that reasoning, a 50' snubber is needed, if the same diameter as the rode, slightly less, if smaller, as most (including I) recommend.

Longer snubbers (as far as practical) are better.

Not an advocate of mediocrity, I'm less interested in minimums, than in practical exemplars.
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Old 02-07-2009, 18:45   #33
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If you are anchored in 5 foot waves, you can expect to have a larger one now and then, perhaps a freak 10 footer sometime. Set your length accordingly. As for diameter, which I made no mention of, I select a breaking strength about equal to the weight of the vessel.
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