The mystery of rig tuning

[JBP-PA]

I don't subscribe to the "as loose as possible" nor the "as tight as possible" schools of thought mentioned before. My preferred rig tuning lies somewhere in between.
On my single spreader fractional deck stepped rig, I tighten my cap shrouds until I have slightly more prebend than I'll want and then tighten the lowers to take some of that prebend back out to my final desired prebend. (In addition to laterally centered and straight of course.) I'll adjust the tension to ensure there is never any floppy slack in even high wind conditions. I also adjust to keep my head stay sag where I like for the wind I commonly sail in and tweak it with the adjustable backstay.

After the first few adjustments I can pretty much get the tension I like by feel and eye and don't use a guage, but it is a small simple rig. I'll check my tension every time I go forward by hand. I'll also check the mast straightness visually through the hatch while lying in the v-berth every morning.

 

[jviss]

to my surprise they don't make one big enough

Interesting, what size is your largest wire? I have the Loos PT-3 which goes up to 3/8" wire.

Many insurances companies stipulate 10 years and your supposed to replace the lot.

Wow, I had no idea! I will have to review my policy.

As far as measuring stretch with a LASER or tape measure, good luck with that.

Actually, measuring stretch with a steel tape is a very accurate way of measuring wire rope tension.

 

[dLj]

Actually, measuring stretch with a steel tape is a very accurate way of measuring wire rope tension.

You will have to explain this one to me. How do you make this measurement? Where and how are you measuring? What are your measurements points? What kind of accuracy are you able to obtain? I've never seen this done on a boat.

dj

 

[jviss]

You will have to explain this one to me. How do you make this measurement? Where and how are you measuring? What are your measurements points? What kind of accuracy are you able to obtain? I've never seen this done on a boat.

dj

I don't have the book I'm thinking of at home, it must be on the boat; I'll get the reference next time I'm there.

Wire rope stretch is directly proportional to tension. The tension per stretch for various wire ropes is well specified. If you measure across a meter or so you can measure tension quite accurately. I don't have the numbers in front of me.

I learned this in a book on rigging sailboats, and I recall having seen it elsewhere.

 

[jviss]

Here are a couple of discussions:
Rigging tension method
How to set up your rig: tension your shrouds on masthead or fractional - Practical Boat Owner.

 

[dLj]

Here are a couple of discussions:
Rigging tension method
How to set up your rig: tension your shrouds on masthead or fractional - Practical Boat Owner.

Reading through all these references, a number of questions come to mind.

The biggest question is when does one know when to start measuring? How do you know when you go from taking up "slack" and begin actually measuring tension? I surmise this is a rather approximate estimate at best. But then so is the loos gage, although that device has a more precise beginning and end.

I must admit, I find it very hard to see how the mm elongation is both diameter independent, and wire strength independent. I'll have to look into some of the mathematics to get my head around this one...

dj

 

[jviss]

Reading through all these references, a number of questions come to mind.

The biggest question is when does one know when to start measuring? How do you know when you go from taking up "slack" and begin actually measuring tension? I surmise this is a rather approximate estimate at best. But then so is the loos gage, although that device has a more precise beginning and end.

I must admit, I find it very hard to see how the mm elongation is both diameter independent, and wire strength independent. I'll have to look into some of the mathematics to get my head around this one...

dj

I'll ask my son, too; he's a PhD mechanical engineer specializing in materials characteristics. He spent a lot of time stretching steels to destruction in grad school.

 

[jviss]

How do you know when you go from taking up "slack" and begin actually measuring tension?

I would imagine when the wire is generally straight but not taught, the slack is out.

I've had a couple of Loos gauges; can't find the small one anymore. But, the big one covers all of my rigging.

 

[dLj]

I'll ask my son, too; he's a PhD mechanical engineer specializing in materials characteristics. He spent a lot of time stretching steels to destruction in grad school.

I'll be interested in his response. Just as a FYI, I've spent plenty of hours in test labs testing materials.

I've also had the opportunity to watch craftsmen doing things that were hard to explain, and they worked. So I'm not saying this is wrong, I'm just having a very hard time with visualizing how this works. I'll have to sit down and look at some specific cases and do the calculations. In this case, the gage length is 2 meters. That's a very long gage length. That may well play into how this works.

In any case, I'm sure your son will be able to answer far faster than I. I'm sitting in a cafe in Seixal having wine, fresh fish and delicious coffee... My mind is not into mathematics at the moment, nor will it be for at least a few weeks... ( or potentially a few months)

dj

 

[jviss]

I'll be interested in his response. Just as a FYI, I've spent plenty of hours in test labs testing materials.

I've also had the opportunity to watch craftsmen doing things that were hard to explain, and they worked. So I'm not saying this is wrong, I'm just having a very hard time with visualizing how this works. I'll have to sit down and look at some specific cases and do the calculations. In this case, the gage length is 2 meters. That's a very long gage length. That may well play into how this works.

In any case, I'm sure your son will be able to answer far faster than I. I'm sitting in a cafe in Seixal having wine, fresh fish and delicious coffee... My mind is not into mathematics at the moment, nor will it be for at least a few weeks... ( or potentially a few months)

dj

Ha, ha, enjoy!

Yea, I'll ask Will, and also find my rigging book when I go to the boat tomorrow or Friday to secure it for the storm.

 

[dLj]

By the way, in one of the links, @DArcy said he made two rods of the appropriate lengths to do this measurement. If this stands up to the mathematics, I could very well see making a 2 m rod and a rod with the marks for the % elongation values and making a simple support mechanism to use them on the cables. Now that would be really easy to deploy! The two rods with holders could be used to also align the cable so the pretension could be correctly established.

dj

 

[Davidasailor26]

Selden describes a similar process in the “folding rule method” on page 32 of their tuning guide at https://support.seldenmast.com/files/595-540-E.pdf. I used that approach once and felt like it was probably reasonably accurate, assuming you could get the top of the rule well adhered to the shroud. One downside is there’s not a good way to go back and check a tension - once you take the ruler off the shroud and change a different shroud you can’t re-check the original one without backing it all the way off and bringing it all the way up again.

 

[Will Gilmore]

I must admit, I find it very hard to see how the mm elongation is both diameter independent, and wire strength independent.

I thought the elongation was diameter dependent, that you measured on a scale that is specific to a certain standard wire size.

I'm not a proponent of using breaking strength or even tension as the measure in rigging because I feel it is all about the mast, not the stays. When the mast is correct and it stays within some acceptable tolerance while sailing, it is tuned correctly. Understanding, of course, that too much tension can cause unwanted stresses and deformation on the hull/rigging system.

Why use the breaking strength as the measure of correct tension? Many of these standards were set by naval engineers when choosing the materials for their design. These designs were often calculated decades ago. Today, we have improved materials, mono-wire and composite standing rigging as well as better cable wrapping techniques in the factory. When putting tension into a multi-strand cable, some of the elongation and narrowing of the cable would be due to the wire strands wrapping more tightly against each other. Better wrapping techniques may mean less tolerance to a cable that has an equal number of strands, and therefore an equal breaking strength as an earlier production, yet the stretch may be much less before higher tensions are reached. I don't know, but I can imagine such a scenario.

To do just a little basic math, a single strand of a set guage will have a stretch of s per n Newtons of tension on it. The question here is, is that a linear progression? After a stretch of s1 from n Newtons, does s2 = 2s when 2n is applied? Let's say it does. Stretch is a linear progression of tension on a cable (only for this exercise, because that's not obvious).

OK, one standard guage steel strand stretches by s units of length per n Newtons of tension. This should mean that 2 strands would stretch by s/2 for each n Newtons, and so forth. But, as far as the diameter of the cable goes, it does not grow by a straight linear progression with the addition of more strands. Diameter would grow the way diameters grow as the area of a circle increases. That is, as more strands are bundled into the cross section of the cable, the diameter increases more and more slowly. But, that isn't really the point. The question here is, is diameter independent of strength. Clearly not, but it isn't linearly related, as the cross sectional area is the measure that would be linearly related.

Three strands of a breaking strength b equals 3b, four strands equal 4b, and so on. this translates to stretch being inversely dependent upon diameter, and directly dependent on tension.

So, if the loos guage uses the same scale for a range of sizes of cable, it can't be accurate. The best way to be accurate would be to lay the cable out on the ground straight, measure a distance along the cable from the turnbuckle of, say, 4 feet, and mark that point. Next, install the cable and measure up along its length as you tighten it. You mark the start point and, assuming the cable loses no strength or tension as time and sailing pass, simply re measure from the turnbuckle to repeat the tension.

however, if continuous stretch over time can permanently distort and/or weaken a stay, then you can only measure tension by this method once. The next time, you will have to remeasure your baseline.

Just my musings. I don't really know.

-Will

 

[jssailem]

WOW… So many questions… No simple answers..

Removing any engineering hat and putting on my sailors watch cap… How much “precision” are you seeking? Do your worry about exact or good with a heavy dosage of safety margin?

It would seem the right tool is a loose gauge

Yes the Loos gauge is the tool to use. There are relative conditions identified as the target tensioning of a stay. You tune to the tension then assess the rig. From some experience and a lot of talking/watching good riggers the rough tuning and fine tuning go from science to art form. Then they adjust.

Then lets use a tape measure to measure the stretch..... really... So I will purchase a laser measuring tool.

A laser is engineering extreme precision. Perhaps used on the Americas cup boats, but then they go out and test the rig. Return to port and adjust as the performance dictates. For the majority of cruising sailors Americas Cup precision is not necessary to get very good performance. Once you sail the boat the slip side precision has been changed, anyway.

So when do you replace what.

You’re dealing with 2 conflicting challenges. Your Insurance company and your pocketbook. The Insurance company says 10 years, because they may not know the waters or the latitude you sail. Near the Equator, 10 years may be too long. Sail in Gale Force winds often 10 years may be too long. Day sail in above the 45th parallel and you may safely get 20 plus safe years from your rigging.

As the owner and captain you are the decision maker.

do you actually need to replace the turnbuckles or just the cotter pins. Do turnbuckles actually break, and what about the fittings on the end of the new wire are staylocks as good better or worse.

Simple answer…. YES. Chafe I the recommendations for Staylocks. They identify the recommended service time.

Given I have seen 60knots more than once and 8m (24ft) sea's I need to get it right and for me yea its not simple.

Yes it is not simple. You sail in gale force conditions and you likely desire a bit of extra safety. You ignore the rig after several such sails and you sail on borrowed time.

My rigger told me that of the 1800 sailboats in our marina greater than 60% of the rigs were at risk of failure. The saving grace was that most of the boats never left the marina after they were purchased. If you sail you need to understand your rig and inspect/repair/replace as necessary.

Good luck and safe sailing.

 

[dLj]

I thought the elongation was diameter dependent, that you measured on a scale that is specific to a certain standard wire size.

I'm not a proponent of using breaking strength or even tension as the measure in rigging because I feel it is all about the mast, not the stays. When the mast is correct and it stays within some acceptable tolerance while sailing, it is tuned correctly. Understanding, of course, that too much tension can cause unwanted stresses and deformation on the hull/rigging system.

Why use the breaking strength as the measure of correct tension? Many of these standards were set by naval engineers when choosing the materials for their design. These designs were often calculated decades ago. Today, we have improved materials, mono-wire and composite standing rigging as well as better cable wrapping techniques in the factory. When putting tension into a multi-strand cable, some of the elongation and narrowing of the cable would be due to the wire strands wrapping more tightly against each other. Better wrapping techniques may mean less tolerance to a cable that has an equal number of strands, and therefore an equal breaking strength as an earlier production, yet the stretch may be much less before higher tensions are reached. I don't know, but I can imagine such a scenario.

To do just a little basic math, a single strand of a set guage will have a stretch of s per n Newtons of tension on it. The question here is, is that a linear progression? After a stretch of s1 from n Newtons, does s2 = 2s when 2n is applied? Let's say it does. Stretch is a linear progression of tension on a cable (only for this exercise, because that's not obvious).

OK, one standard guage steel strand stretches by s units of length per n Newtons of tension. This should mean that 2 strands would stretch by s/2 for each n Newtons, and so forth. But, as far as the diameter of the cable goes, it does not grow by a straight linear progression with the addition of more strands. Diameter would grow the way diameters grow as the area of a circle increases. That is, as more strands are bundled into the cross section of the cable, the diameter increases more and more slowly. But, that isn't really the point. The question here is, is diameter independent of strength. Clearly not, but it isn't linearly related, as the cross sectional area is the measure that would be linearly related.

Three strands of a breaking strength b equals 3b, four strands equal 4b, and so on. this translates to stretch being inversely dependent upon diameter, and directly dependent on tension.

So, if the loos guage uses the same scale for a range of sizes of cable, it can't be accurate. The best way to be accurate would be to lay the cable out on the ground straight, measure a distance along the cable from the turnbuckle of, say, 4 feet, and mark that point. Next, install the cable and measure up along its length as you tighten it. You mark the start point and, assuming the cable loses no strength or tension as time and sailing pass, simply re measure from the turnbuckle to repeat the tension.

however, if continuous stretch over time can permanently distort and/or weaken a stay, then you can only measure tension by this method once. The next time, you will have to remeasure your baseline.

Just my musings. I don't really know.

-Will

@Will Gilmore Without going too deeply into your thought process, you are making a fundamental logic error. You are looking at this on a strand by strand basis but that is all normalized in the rating of the wire rope. The rating of the wire rope is based on it's composite construction. This measurement technique is based off the value of that composite construction.

Another question you bring up is why base off UTS (breaking strength were your words - UTS means Ultimate Tensile Strength). You shouldn't. Using UTS was, and often still is, used for design calculations. However, it has been shown to be an incorrect value in long term structures when designing to tight tolerances. You should base your design calculations off Yield Strength (YS).

While I agree in principal with your statement of it all being about the mast - that is directly related to rigging tension.

dj

 

[Will Gilmore]

Another question you bring up is why base off UTS (breaking strength were your words - UTS means Ultimate Tensile Strength). You shouldn't. Using UTS was, and often still is, used for design calculations. However, it has been shown to be an incorrect value in long term structures when designing to tight tolerances. You should base your design calculations off Yield Strength (YS).

My point about it being about the mast, and asking, why base the measurement off of UTS, is about setting the tension in a rigging for which those cable strength calculations have already been made by the designer. Assuming the designer has chosen an adequately strong cable to use for the stays, then holding the mast up against the forces of sailing isn't related to the breaking strength of the cable except as an incidental to the engineer's choice. A rigging tuner is interested in setting the tension based upon keeping the mast in form, not in when the cable will fail. That requires a specific tension or minimum to maximum tension between doing the job and effecting the boat. What is the tension necessary to hold the mast in form against the forces of sailing? It isn't ~15% of the breaking strength of a given cable. It is x tension. We change our sails, we switch from a 90% jib to a 130% Genoa. We add or remove roach, we rig for off-shore sailing with heavier stays, or pick out new, light weight composite rigging, add radar and antenna and anemometers to our masts. Going by the percentage of the strength of the cable to set your tension by is not what that rigging needs. That is what the design engineer needs. The rigger needs the mast to be held in column, straight, raked, with our without prebend. The rigging should not put more pressure on the chainplates/sheer, mastfoot/deck/keel, or the spreaders than the minimum to do the job.

I've read about riggers who seemed to think that if you upgraded to bigger cables, you have to still set them to the 15% and put more force on the whole system. Racers say all the time that they tune up to 20-25%. Why are we using a percentage? Is that some sort of safety feature? If you tune to less than 15% you are using oversized cables? If you tune to more than 15% you need bigger cables? Why use a percentage when measuring rig tension at all?

Why not say, it takes a tension of x to keep the mast where we want it.

I have very little experience running large boats. Our schooner had three 54' masts with one set of spreaders each, no rake, no baby stays, but I don't see the principles being any different for more complicated rigs.

-Will

 

[dLj]

My point about it being about the mast, and asking, why base the measurement off of UTS, is about setting the tension in a rigging for which those cable strength calculations have already been made by the designer. Assuming the designer has chosen an adequately strong cable to use for the stays, then holding the mast up against the forces of sailing isn't related to the breaking strength of the cable except as an incidental to the engineer's choice.

So like I said, you should not design based on UTS, you should design to YS. But in this discussion this is semantics.

The one thing you are missing in your logic is the conflicting requirements of as low of weight as possible and maintain adequate strength. That's one of the major reasons dyneema is now being used.

A rigging tuner is interested in setting the tension based upon keeping the mast in form, not in when the cable will fail. That requires a specific tension or minimum to maximum tension between doing the job and effecting the boat. What is the tension necessary to hold the mast in form against the forces of sailing? It isn't ~15% of the breaking strength of a given cable. It is x tension.

So again here, the rigger needs a tool that can be used to balance the strength/weight ratio of the rigging. The 15% is a number that is used, as you rightly state, by the design architect. But once the design is set - how to you adjust once built? How else do you know if you have the rigging set right for the mast to stay straight?

We change our sails, we switch from a 90% jib to a 130% Genoa. We add or remove roach, we rig for off-shore sailing with heavier stays, or pick out new, light weight composite rigging, add radar and antenna and anemometers to our masts. Going by the percentage of the strength of the cable to set your tension by is not what that rigging needs. That is what the design engineer needs. The rigger needs the mast to be held in column, straight, raked, with our without prebend. The rigging should not put more pressure on the chainplates/sheer, mastfoot/deck/keel, or the spreaders than the minimum to do the job.

I agree with the philosophy - but how to know in practice when your rigging it correctly tensioned? In your eloquent arguments, you fail to tell me how to provide a practical field measurement?

I've read about riggers who seemed to think that if you upgraded to bigger cables, you have to still set them to the 15% and put more force on the whole system. Racers say all the time that they tune up to 20-25%. Why are we using a percentage? Is that some sort of safety feature? If you tune to less than 15% you are using oversized cables? If you tune to more than 15% you need bigger cables? Why use a percentage when measuring rig tension at all?

Higher tension provides high stiffness. Racers use that for competitive advantage. Again - think about the strength/weight balance. Racers are looking for as light of rigging as possible.

I disagree with oversizing cables and maintaining the same % tension. Although there is likely a minimum tension required to keep rigging stiff.

Why not say, it takes a tension of x to keep the mast where we want it.

That is what is being done - we are simply measuring it through tension of the rigging. And that has been determined with the best balance falling in the 15% range of tension. Seems to me anyway...

dj

 

[jviss]

This is an interesting discussion. Thanks @dLj and @Will Gilmore for your detailed comments.

My goal with rigging is to keep the mast in column with the correct amount of rake, and any pre-bend as may be appropriate; and to have it tight enough that I don't ever have a shroud go slack, on any heading or in any wind in which I may find myself. This is so I don't experience shock loads, which can break things. 15% of breaking strength is a good and safe place to start.

 

[Will Gilmore]

In your eloquent arguments, you fail to tell me how to provide a practical field measurement?

I'm so glad you asked that question because it takes me right back to tension being all about the mast. Don't look at a guage or a tape measure, look at the mast. It will tell you when tension is right.

The plumb of your halyard hanging over your gooseneck will tell you when you are in column and how much rake you've set your forestry to.

Set your lowers to adjust the column. Lose tension, then tighten until it takes a little strength to deflect. Then set the uppers the same way.

The back stays bend the mast on a fractional rig, compress it on a masthead rig. If you have forward lowers, they can help put the pre-bend in.

Know your boat. Maybe pre-bend is not appropriate for your mast. Maybe rake isn't either.

Does the mast look right fore and aft, and laterally, smooth curves where there should be curves, straight, where it should be straight?

It takes x amount of tension to set these forms regardless of rigging composition. The mast will tell you. It looks right, it feels right, you haven't over tightened because you just tighten enough to over deflect, then you back off or counter deflect with a balancing stay.

Go sail and watch the rigging. Does it stay correct or do your leeward turnbuckles fall out of line? Is the too much sag in your jib luff?

There are plenty of sources for learning how to tune a mast without a guage.

-Will

 

[jviss]

Once I was sleeping on the boat and the mast pumping woke me up. I went on deck in a sleepy haze in about 20~25 kt. and tightened the baby stay. Problem solved!