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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 10-18-2011
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Quote:
Originally Posted by casey1999 View Post
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.
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VERY simple answer .... its because the sails are cut for operating on 15% tension loaded stays.
Ultimately the general rule of thumb of 15% will wind up reacting with the forestay. A sailmaker EXPECTS that the forestay will be AT 15% tension for sailing in 12-15kts. and so CUTS the leading edge of luff in a smooth curve 'hollow' to be equal and MATCH the EXPECTED and very predictable sag in the forestay at 15% tension, not 5%, not 20%. The tension in the shrouds/stays must be at an operating tension so that the FORESTAY is at 15% ...... and the 'luff curve' (luff hollow) cut into the windloaded sail ***MATCHES*** the sag in that forestay.
15% ..... This is the basic tension that the (plain vanilla, cruising) SAILS are designed and cut for. For ultimate simplicity, set all rig tension at 15%, go sailing on a hard beat and THEN 'tweak' the tension by minor tension adjustment so that the mast stays perfectly 'straight' and "in column" ... but remember that the sailmaker EXPECTED that the forestay will be operating at 15% tension when you are sailing in 12-15kts.

Last edited by RichH; 10-18-2011 at 09:57 PM.
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  #12  
Old 10-18-2011
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Actually, the larger shrouds will stay tighter if tensioned to the same load as the smaller ones. Reason is the shrouds on the windward side will stretch less at the same load.
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  #13  
Old 10-19-2011
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Quote:
Originally Posted by smurphny View Post
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.
This method is, in my experience, near universal. I wouldn't have to take my socks off to count the number of times I have seen anyone setting up a rig with any kind of tension gauge, even one as simple as a Loos.
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Old 10-19-2011
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Quote:
Originally Posted by SloopJonB View Post
This method is, in my experience, near universal. I wouldn't have to take my socks off to count the number of times I have seen anyone setting up a rig with any kind of tension gauge, even one as simple as a Loos.

I have a Loos guage just because of the "slightly loose" statement... I have no idea how loose that really is. Can I flex that with my hand? Is that starting to flap in the breeze? I like hard and fast numbers... I wish they had those for all rigs.

My Hall Spar doesn't seem to bend much at all. I kept loosening the shrouds, but it never seemed to bend either way.
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Last edited by funjohnson; 10-19-2011 at 10:27 AM.
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Old 10-19-2011
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You create bend by tightening a shroud or stay, the backstay for example.
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  #16  
Old 10-19-2011
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Quote:
Originally Posted by casey1999 View Post
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
In sailboat rigging the FACTOR OF SAFETY is already built-in in the design and the OEM selection of the wire. For you to add safety factor on top of the OEM safety factor would cause you select larger diameter wire and larger diameter connection terminals etc. Since the OEM wire at ~15% loading will produce a very predictable 'stretch' and 'sag' when windloaded by sails, the increased size to produce the OEM designed/defined strain / elasticity will have to operate at MUCH LESS applied load/tension .... hence the mast will be 'loose' and subject to increaesd impact values as the mast 'rocks' back and forth sideways, the jib, etc. will now be operating on a very slack wire and will no longer take its designed shape that the sailmaker cut into the sail AND the mast will no longer be 'dynamically' as strong because it will no longer be set with the amount of proper 'pre-bend' (far-aft bowing).
If you want your boat NOT to be able to 'point' well and want it to heel over aggressively while being exceptionally SLOW, have a LOT of 'helm pressure' (boat 'skidding off' to leeward when attempting to 'point').... be at less than 15% static forestay tension.

Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig, and ~1/2" forward bow per each spreader set on a multi-spreader rig ... and the sailmaker expects that the mast will be set up for that designed pre-bend and the forestay to be ~15%. Without normal expected prebend a mainsail will set up in a powered-up (increased draft) shape because in 'good' mainsails the sailmaker ALWAYS adds a smooth curve to the front of the luff to accommodate the expected 'prebend'. Prebend mathematically makes a spar MUCH stronger (by increasing the geometric 'moment of inertia' or "I" to the third power to prevent/retard the mast from flexing or oscilating due to 'induced harmonics' ... called 'mast pumping'.

Rigging size, mast stiffness, etc. are not a 'black art'. Typically the boat designer selects the rigging/mast based on typical 'scantlings' that include normal SAFETY FACTORS that historically 'work' ... for safety and long service life. An inshore design will be at 1.5, A coastal design will usually have an inbuilt Safety factor or 2, an offshore design 3 ... or more. The wire load bearing capacity is selected so that when the rig is set at 15% tension ..... and then later when the boat is 'pulled over' and heeled, the rig tension doesnt (much) go over 30% rig tension, 30% being the limiting load factor that unduly promotes 'fatigue' in stainless components. It is important to realize the all 300 series stainless quickly fatigues when loaded beyond 30% stress (normal 'endurance limit' of 300 series SS is 30,000 psi, although normal 300 series has an ultimate load value of 90,000psi) .
So, the 'typical' method by a designer to arrive at 'correct scantling' wire size, etc. is to mathematically/theoretically pull the mast horizontally from the top until the boat is at a ~45° angle of heel, calculate the resultant rig tension that is needed to get the boat to that 45 degree heel angle ... then multiply by the applicable safety factor to arrive at the proper scantling sized wire. ..... YOUR need is to keep the rig at near the 'design' static (boat upright) loading is YOU must set the rigging to a basic 15% of tension so that the mast remains 'straight' (side to side), mast has a proper amount of 'pre-bend' ... and the forestay IS operating at 15% static tension for sailing in 12-15kts.
Guessing and By-Goshing the proper rig loading using eyeballs, wire pushing, 'What John does', .... will get you NOWHERE. Its all in the numbers .... basic ~15% tension for normal wind and seastate conditions. ........ All the rest is 'myths & mysticisms".
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Last edited by RichH; 10-19-2011 at 11:38 AM.
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  #17  
Old 10-19-2011
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Also, if you increase rig size it will also increase rig weight. Wouldn't that make the boat more tender, as in wanting to heel more?
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Old 10-19-2011
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Quote:
Originally Posted by smurphny View Post
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.
This is the correct method of rig tensioning. After done take measurements and use these measurements for next rig tensioning. 15 % is just about right and might change. Considering the length of lowers one can easily decide that the lowers should be tensioned less than the others.
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Old 10-19-2011
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So what kind of gauge do you use to measure said rig? Great thread!
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Old 10-19-2011
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I agree that 15 to 20% of breaking strength is the right ballpark, a loos guage is not much money compared to a lot of boat expenses, probably a good investment. I also think going up 1/32 as in from 1/8 to 5/32 is ok. It won't add much weight and should be ok at the lesser tension. Maybe the boat was designed as a daysailer and is now used as coastal cruiser and island hopper. The slightly larger wire may provide some extra peace of mind.
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