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Rigging Tension

17K views 45 replies 16 participants last post by  tommays 
#1 ·
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
 
#2 ·
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.
 
#3 ·
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?
 
#5 ·
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
 
#6 ·
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.
 
#7 ·
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.
 

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#8 ·
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
 
#11 · (Edited)
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
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.
 
#16 · (Edited)
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".
;-)
 
#23 ·
Thanks, there is a lot of good info here, I will need to read many times to understand all of this.

One question, what do you mean by "Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig ( I have single spreader). Also, how would I measure the forestay tension if the forestay is covered by roller furler? Also, the Selden manual (down load hints and advice from below link)

Seldén Mast AB

states tension forestay up to 40% breaking strength. Based on you statement that anything above 30% could lead to fatigue breaking, why would Selden say 40%.
Regards
 
#20 ·
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.
 
#21 ·
I just got the Loos PT-3 guage I ordered last night. Has a manual with it (I tried to find a copy on internet that I could post here but no luck so far). Manual states to use the gauge then under sail make final adjustments making sure leeward shrouds do not go slack. The thing I like about a gage is that I can go to the boat and measure what the tension is now before I start to play with things, and I can make sure I am not overtension (or under) when I have the final set up (I like facts). I was also going to use the gage under sail just to get an idea as to what the tension is on all shrouds and stays- hey I'm an engineer, what can I say. For $200 not a bad deal.

One question, how do I measure forestay tension if I have a rolling furler? At the base not enough cable and at the top (where attaches to mast) I don't think there is enough cable.
Regards
 
#24 ·
Hey that is a good idea. However I do have both forward and aft lower shrouds and a baby forestay. I would need to measure tension on all of these and then use some geometry to calculate the forestay tension correct?
 
#27 ·
For what its worth, a bicycle wheel with thin gauge spokes is tensioned higher that the same wheel with thicker spokes and a longer spoke would be at greater tension than a shorter one (though that would make a strange wheel, like the one I made for a clown's bicycle)
When tensioned correctly (and given no damage has been done to the rim) the rim will be round and true and the tension on the spokes will be equal. Since the wheel receives dynamic rotational (radially and laterally) loading any unequal tension will walk around the wheel until the spokes loosen to the same tension.
While we don't sail bicycle wheels, our sailboat rigs share some engineering factors. So thinner gauge wire will be at a higher tension that heavier ones and opposing wires should be of equal tension and gauge. Also stress risers like odd bends at fittings must be relieved or the wire/fitting will fail from fatigue prematurely.
Keep on pedaling
John
 
#28 ·
Quote "So thinner gauge wire will be at a higher tension than heavier ones..."


According to the experts, like Selden, rigging wire should be tensioned to between 15% and 20% of the wires breaking load. This will be a higher tension on larger wire, not on smaller wire.
 
#41 ·
There is probably 1 1/4" of solid glass on top of the header now, so I'm really not worried about any failure there BUT, as you say, any deflection would have an effect on rigging tension. I'm just thinking about when I go offshore and run into a real extended pounding if a post would offer any additional measure of safety. It would sit right in the forward V-berth doorway but I use this mostly for storage anyway. If anyone wanted to sleep up there they'd have to be skinny:)
 
#43 ·
Used the Loos PT-3 guage and found fore and aft stay at 20% breaking strength and shrouds at 5%. Gauge works great, took 5 minutes to do this. Also measured tenstions easily while sailing.

Now I need to work on bringing tensions up in the shrouds.

One question for those that have used a Loos gauge. The gauge is calibrated for 304 stainless. If I have 316 wire, do I need to use a correction factor?
 
#45 ·
For the Loos gauge, you read the number then go to the chart (sticker on gauge) that gives you % breaking strength and also the actual tension (in pounds) on the cable. The instruction manual says the gauge is for 304 wire. So when unsing with 316 wire do you read the % breaking then calculate tension based on that (since 316 has lower breaking strength than 304 stainless)?
 
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