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casey1999 10-18-2011 06:48 PM

Rigging Tension
 
Been reading up on sailboat rigging and don't quite understand rigging tension.

Most documents I read state tension shrouds and stays to 15% of wire breaking strength. Why 15% and say not 5%?

Also, say a boat has shrouds and stays that are 316ss and 1/8 inch diameter with break strength of 1,780 lbs. I would tension to 15% or 267 lbs.

Now say I want to get some safety factor and I up the size to 1/4 inch with a break strength of 6,900 lbs. Why can I not just tension the stays and shrouds to 267lbs (so I do not hog the keel), and then have a bunch of safety factor- all turnbuckels and other fittings would be sized for 1/4 cable.
Regards

smurphny 10-18-2011 06:59 PM

The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.

casey1999 10-18-2011 07:22 PM

Quote:

Originally Posted by smurphny (Post 787750)
The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.

Thanks, I have a copy of the Selden instructions. I understand the stretch part, but if I up the size to say 1/4 inch, the stretch will probably be insignificant (stretch based on wire size and load applied), so then why not tension to only 267 lbs?

tommays 10-18-2011 08:13 PM

You still need that 10 to 15 % on any size wire to get the sloppy out of it

dsmauney 10-18-2011 08:28 PM

Resist Metal Fatigue
 
One of the reasons for tension in rigging is so the wire is always in tension on Windward or Leeward. When loads fluctuate on metal the higher the average load that the varying load fluctuates around the longer the fatigue life. This of course within limits that the hull can handle. The same reason you tension bicycle
spokes so tight.

Good Winds
DaveM

mitiempo 10-18-2011 08:38 PM

I agree, the rigging will be sloppy loose if 1/4" wire is tensioned to 267 lbs (3.8% of breaking actually).

Also, if you are planning on upsizing the wire, remember to upsize everything else from chainplates to turnbuckles to mast tangs to match otherwise it is a waste - and regardless the load on the boat is greater.

Wire seldom has a load problem - mostly it is a corrosion issue. Larger wire will corrode as fast as smaller wire.

casey1999 10-18-2011 09:09 PM

1 Attachment(s)
Quote:

Originally Posted by mitiempo (Post 787790)
I agree, the rigging will be sloppy loose if 1/4" wire is tensioned to 267 lbs (3.8% of breaking actually).

Also, if you are planning on upsizing the wire, remember to upsize everything else from chainplates to turnbuckles to mast tangs to match otherwise it is a waste - and regardless the load on the boat is greater.

Wire seldom has a load problem - mostly it is a corrosion issue. Larger wire will corrode as fast as smaller wire.

I do not plan to increase my wire size (5/16inch) but just trying to get an understanding of the dynamics of the rig tension- Thanks.

One thing I find interesting is that it seems sailboat rigs seem so fickel, when all around us are power poles and guyed antennas that get almost no inspections and still stay up. Seems they are using galvanized wire. Do large antennas need to have their guys replaced evey so many years? Some of these antennas are huge and the wind load must cycle them just like a sailboat rig. The below tower is 1000 feet tall and has an elevator to get to the top- located in Baltimore MD.

GaryHLucas 10-18-2011 09:21 PM

I'm not sure I'd agree that you need to increase the tension so much when going to a larger wire size. Lets say the new wire size is twice as strong. For a given load on the mast it will stretch about half as much, consequently the slack side won't have to take up nearly as much either, it equals out. Putting more load on the wire just sends more load into the hull and chain plates. However, it is possible that the hull and deck move so much that it is that motion that the rigging needs to absorb, not cable stretch. In that case you may need to stretch the larger cable by an even greater amount, because the hull and deck are stretching farther too! I'd love to see someone measure before replacing the rigging and then again afterwards.

Gary H. Lucas

GaryHLucas 10-18-2011 09:26 PM

Quote:

Originally Posted by casey1999 (Post 787797)
I do not plan to increase my wire size (5/16inch) but just trying to get an understanding of the dynamics of the rig tension- Thanks.

One thing I find interesting is that it seems sailboat rigs seem so fickel, when all around us are power poles and guyed antennas that get almost no inspections and still stay up. Seems they are using galvanized wire. Do large antennas need to have their guys replaced evey so many years? Some of these antennas are huge and the wind load must cycle them just like a sailboat rig. The below tower is 1000 feet tall and has an elevator to get to the top- located in Baltimore MD.

Yes the fatigue life of galvanized wire is better than stainless. However sailboat rigging also sees lots of other loads, not just wind, from people, other boats etc. You also have to be aware of conditions where the pull is not exactly fair. Anything that bends the wire rope during loading and unloading really shortens the life of the wire.

Gary H. Lucas

casey1999 10-18-2011 09:26 PM

Quote:

Originally Posted by GaryHLucas (Post 787800)
I'm not sure I'd agree that you need to increase the tension so much when going to a larger wire size. Lets say the new wire size is twice as strong. For a given load on the mast it will stretch about half as much, consequently the slack side won't have to take up nearly as much either, it equals out. Putting more load on the wire just sends more load into the hull and chain plates. However, it is possible that the hull and deck move so much that it is that motion that the rigging needs to absorb, not cable stretch. In that case you may need to stretch the larger cable by an even greater amount, because the hull and deck are stretching farther too! I'd love to see someone measure before replacing the rigging and then again afterwards.

Gary H. Lucas

That is why I was saying if the wire size is increase, maintain the original tension, that way the keel will not hog, which would deform the deck and boat. I am still trying to understand all of this.


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