Hammocks and stress on standing rigging - SailNet Community
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post #1 of 13 Old 08-06-2011 Thread Starter
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Hammocks and stress on standing rigging

I see alot of hammocks strung up on decks, attached to the standing rigging (e.g. forestay and shrouds, etc.) and while it's clear that this works, I've been wondering what kind of stress and possible reduction in lifespan this may cause to the rigging. Since the stresses on standing rigging are expected to be vertical along the length of the wire, and the hammock places a bending stress horizontally, what is that doing to the wire? Is it causing or increasing work hardening at the point of stress/bend?

I'm wanting to string a hammock up myself, but am concerned about the long term affects/damage to the rigging.

Any experts care to comment?

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post #2 of 13 Old 08-06-2011
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Originally Posted by patrickstickler View Post
I see alot of hammocks strung up on decks, attached to the standing rigging (e.g. forestay and shrouds, etc.) and while it's clear that this works, I've been wondering what kind of stress and possible reduction in lifespan this may cause to the rigging. Since the stresses on standing rigging are expected to be vertical along the length of the wire, and the hammock places a bending stress horizontally, what is that doing to the wire? Is it causing or increasing work hardening at the point of stress/bend?
I string a hammock between the mast and the forestay. If you have a genny, then you know that the force applied to the forestay isn't only vertical. I use a line with beads on it to wrap around the furled genny, and then attach the other end to the spinnaker pole fitting on the mast. Works great.

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post #3 of 13 Old 08-06-2011
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The horizontal stress would be less than 70kg. If you rigging cant cope with this - guess what!!


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post #4 of 13 Old 08-06-2011
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The horizontal stress would be less than 70kg. If you rigging cant cope with this - guess what!!
Yes, but pointed in about one inch of the forestay/shroud. For starters, I would multiply it by the number of inches. However, I think this is a good question, since they weren't intended to take load strictly at one point. Intuition suggests it won't collapse, as you point out, but the OP questioned impact on longevity.


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post #5 of 13 Old 08-06-2011
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Minnewaska,
Standing rigging has a life. The 'sum' of a few hours of a hammock would be less stress than one gybe. If the rig is too light, make it heavier.


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post #6 of 13 Old 08-06-2011
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I'd use a selvagee around the furled headsail to distribute the load, but to take care not to bend the foil.

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Minnewaska,
Standing rigging has a life. The 'sum' of a few hours of a hammock would be less stress than one gybe. If the rig is too light, make it heavier.
A gybe puts stress along the length of the forestay, not one spot. An ATN tacker for an asym spinnaker is a good example of a pointed load on the forestay, so I suspect this has been tested over time. On the other hand, just because a business sells a product, like ATN, doesn't necessarily mean it doesn't have a negative impact on the longevity of your forestay.

Just think the OP's question was a good one. Not sure of the answer other than through intuition.


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post #8 of 13 Old 08-06-2011
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A gybe puts stress along the length of the forestay, not one spot. An ATN tacker for an asym spinnaker is a good example of a pointed load on the forestay, so I suspect this has been tested over time. On the other hand, just because a business sells a product, like ATN, doesn't necessarily mean it doesn't have a negative impact on the longevity of your forestay.

Just think the OP's question was a good one. Not sure of the answer other than through intuition.
Yep. I think its all good. Hammocks are very enjoyable on the boat.


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post #9 of 13 Old 08-06-2011
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Great question,


Assume that the horizontal load is attached in some benign fashion ( difficult but not impossible )

Rule of thumb for shrouds ( and chainplate ) design is 2.5x displacement of boat.

A 8,000 lbs curser shroud is designed for 20,000 lbs tension.


Does a 70kg horizontal load harm shroud ?


Unlikely
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post #10 of 13 Old 08-07-2011
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Where does this 70 kg load figure come from? If it is the weight of the person in the hammock then the real load is far greater. When a line is taut and you apply a point load to it the force is roughly multiplied by the length of wire from the load to the support, divided by the sag caused by the load. So if you have a 6 inch sag and the load is applied 6 feet above the attachment point then the load is 72/6 x 70 Kg or about 840 Kg or 1848 lbs! Don't believe me? Get out your Loos tension gauge and check it for real.

Standing rigging is usually made from 1 x 19 construction. The wires are thick and stiff, and should never be bent in a tight radius, except once if formed around an eye permanently. So a point load on this kind of wire can easily cause a permanent kink that you may not notice. That kink will be loaded and unloaded constantly as you sail. So it could fail from metal fatigue.

So if you are going to do this I highly recommend slack rigging, and something to spread the point load. The larger the sag, the less the loading on the rigging.

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