Rigging Tension

Rigging tension seems to be more of a guess than engineering. The
guidelines I use are the mast should not deflect to either side nor pump
under load but do not have a good sense of when the rig tension is too loose
or too tight. I was wondering if someone could suggest a starting point for
setting the rig tension loading in pounds for the shrouds.

Fair Winds,

Rod

Rod

I think there are a number of parts to the answer to your question.

First, there’s the question of target tensions. I’ve not read Skene’s, nor do I know what rules Mr Hess used, but my understanding is that contemporary rig designers choose the diameter of wire rope to fit a target tension.

For a backstay, the target is a working tension that is 30 - 40% of the wire rope’s breaking load. For cap shrouds, the target is 20 - 25% of breaking load.

So the rig designer (we assume) calculates the working tension she/he wants and then chooses the wire diameter.

So for Zygote, when I wanted to get technical with the rigging, I worked backwards: I found the breaking loads of her stays and shrouds, calculated the tension that an assumed designer would have had in mind when specifying what wire to use, and then checked to see if the rig tensioning scheme that Roger Olson taught me, when we commissioned the boat, was in the ball park.

Second, there’s the problem of quantifying the tension, so that you have repeatability (ie you can re-commission the rig to the same tension after de-commissioning) and so know where you are, in terms of adjustment, relative to the safe working load of the wire.

I know of three ways - I’ll start with the trivial and then proceed to what I use.

  1. use masking tape to mark the position of the turnbuckle. This tells you nothing about tension, but lets you recover a tension with reasonable accuracy.

  2. measure the elongation of the wire (or rod, if that’s what you use). I’ll use metric measure because it’s the simplest, but recognize that by accident of birth you may be stuck in the imperial measure world. The standard length to measure is 2 metres (ie 2000 mm) - so on each stay or shroud, you take slack out of the wire (ie minimum hand tight) and measure 2 metres as accurately as possible. For stainless steel wire rope, regardless of its diameter and construction, the elongation of a 2 metre length by 1 mm corresponds to 5% of breaking load; so an elongation of 6 mm corresponds to 30% of breaking load. And so it goes.

I found that the measurement system didn’t work on Zygote - I’m only 184 cm tall and I’m not tall enough to do the job properly; your mileage may vary.

  1. use a gauge such as a Loos gauge. I use the cheapest Loos gauge on the market (I think two models are marketed, the more expensive of which retains the reading). My Loos gauge does not read tensions, in terms of pounds.force or kilogram.force; it reads Loos numbers and has a legend that translates Loos numbers to pounds.force.

I only have one Loos gauge and I’ve not had a way to test its accuracy. But it works for me (and, if - as I expect and hope - the master of White Wings III responds, you should get better guidance from his experience).

I started by tuning my rig using the technique that Roger taught. Then I did my calculations. I drew up the following table (apologies in advance, because the columns will get screwed up - I’ve previewed this post, but it didn’t matter whether I used tabs or the space bar, the column spaces collapsed; there should be 6 columns, not all cells are filled):

Stay Diameter Length (pin to pin) Breaking Strength (lbs)Tension (theory)Loos Gauge figure
(inches) (feet and inches) (lbs)

Headstay 9/32 40’ 2.75” 8700
Bobstay 3/8 8’ 0.5” 14800
Whisker stays 1/4 14’ 0.5” 6900
Staysail stay 9/32 26’ 6” 8700
Cap shrouds 1/4 37’ 9” 6900 1035-1400 39-40
Intermediate shrouds 1/4 26’ 7.5” 6900 1035 39-40
Lower forward shrouds 1/4 15’ 4” 6900 690 36-37
Lower aft shrouds 1/4 15’ 8.5” 6900 690 36-37
Backstay 9/32 8700 1740 48
Boomkin stay 5/16 3’ 10” 10600

Then I measured the tensions that were in the wire (from following Roger’s scheme). And sailed the boat. Then modified the tensions slightly to bring them closer into line with my theoretical figures. And sailed the boat to see if there was a difference (as noted above, White Wings has much better experience than me, because White Wings decommissions once a year, when her local water turns hard).

Third, there’s the question of whether all this makes a difference.

I like the quantification and repeatability that I now have. Sailing performance is harder to be objective about (each day is different). But I feel more confident about the pre-stress that the rig is under, so that might mean that I sail her a little harder than I used to.

Here’s a table showing my tensions (2002 figures were from just hand tuning; from 2003 on, I’ve hand tuned and then adjusted the hand tuning to fit my target tensions; my only regret is that I did not have a Loos gauge when Roger commissioned my boat, so I could see what tensions he had; or that I had a chance to run the gauge over Nereus). I’ve been remiss and not added figures from last year or this year (three columns).

Stay and Shroud tension: Loos gauge figures
Aug 2002 Jan 2003
Whisker – Port
Whisker – Stbd
Staysail 41 40
Cap – Port 41 42
Cap – Starboard 41 42
Intermediate - Port 42 39
Intermediate - Stbd 41 39
Lower Fwd - Port 33.5 36.5
Lower Fwd - Stbd 35 36.5
Lower Aft - Port 37 35
Lower Aft - Stbd 35 35
Backstay 43 47.5

You’ll see that the significant changes are that Zygote’s back stay and lower forward shrouds are more taut, and her intermediates are a tad less taut.

Okay, that’s enough from me. Over to White Wings III …

Cheers

Bil

Hello Rod, I have been following the BCC forum for the last three years
while building by BCC. It is a great source of information and inspiration
when the hours get long. I have been racing one designs for 35 years and
have not found two loos gauges that read the same. We have always used a
deflection test to calibrate our gauges and use them as a relative indicator
for set up only. When we have checked them with a strain gauge they were all
inaccurate. The way I have always tensioned a rig on a cruising boat is to
snug the rig up and make sure that it is in the middle of the boat. Then go
for a sail and load the rig up and make sure that the spar is in column. If
not make the necessary adjustments (with the pressure off) without getting
things to tight. As soon as you go on the wind and the leeward shrouds go
slack how tight and strait things were at the dock doesn’t matter. Head stay
tension is more a matter of matching the luff curve on the jib than the
numbers on a gauge. Of course this is just my opinion on the simple
approach.

Eric Pomber
----- Original Message -----
From: “BCC Forums” bccforums@samlmorse.com
To: bccforum@samlmorse.com
Sent: Saturday, May 14, 2005 6:40 AM
Subject: Rigging Tension

loose
for

I agree with Eric (and Rod) on the value of the empirical approach as
an alternative to “doing it by the numbers”. Whether setting up a
cruiser or a racing dinghy, after getting the mast in column and
“straight in the boat” I think the main variables are first the degree
of rake and after that the amount of headstay sag. Since the BCC has
a non-bendy mast a lot of other adjustments are probably not so
relevant. I recall some earlier postings in the archives about
weather helm sometimes being an issue with the BCC so perhaps mast
rake is worth special attention. Anyone with more BCC experience than
I have an opinion on that?

Scott
(S/V Itchen BCC #73)

PS. Right now Itchen is moored in Black Sound, Green Turtle Cay and
Dottie and I will be cruising the Abacos and maybe the Exumas until
the end of June or thereabouts. Any other BCCs in the area, or advice
on nice out-of-the-way gunkholes to visit?

S.

  • Hide quoted text -

Rod:

Now that Bil has put the pressure on, here is my experience/approach:

Rod:

Now that Bil has put the pressure on, here is my approach/understanding of our rig tension:

I initially set the rake with partial tension then center the mast in the partner with the lowers/intermediates (again, partial tension). As you know, our hulls are slightly asymmetrical so the “halyard to the chainplate” measurements are not correct. Tommy assured me that he goes to great lengths to center the partner on the centerline of the boat. (I now have a Spartite plug which does this for me each season)

I then start a progressive/incremental tightening of all the shrouds beginnig with the lowers and moving up. Once much past hand tight, I do 1/2 turn on each turnbuckle in a rotating cycle until I reach the desired tensions. It has never made much sense to me to start with the upper shrouds and work down. It is very easy to induce a compression bend in the mast and drive yourself crazy trying to tune it out.

Like Bil, I use the least elegant Loos Gauge. While it’s absolute accuracy may not be the best, it is relative tension, side to side, that is most important. The first couple of years with my boat, I used the Sam Morse guidelines but each haulout, found galling on the clevis pins on a couple of the lower shrouds and on one intermediate. That’s a sure sign of being too loose. Since using a Loos gauge for my ultimate tensions, the boat points better and the galling has quit. Also, having learned to sail in the eyeball and finger pluck days of rig tunig, the ultimate Loos settings simply feel right to me. Too loose is more of a concern than too tight. You would have to be a bit of a gorilla to accidentily overtighten anything but the lower shrouds.

I also check the mast for true and the leeward stays for sag while underway out of habit and have not seen anything that I thought needed changing from the static settings using the Loos tensions.

While not holding a candle to the depth of Bil’s technical research, this seems to work on WW III. I found Brian Toss’, “The Rigger’s Apprentice” to be helpful in understaning and developing my approach to rig tuning. I think I am attaching a MS Word document that Bil sent me several years ago with his Loos setting…mine are only slightly different from his and are in the third column “Harrer 2004”.

If you want more info, email me directly…or if the attachment doesn’t work…I’ve already deleted this reply by accident so who knows what will become of the attachment.

                        Best regards.............Tom

Rod:

Now that Bil has put the pressure on, here is my approach/understanding of our rig tension:

I initially set the rake with partial tension then center the mast in the partner with the lowers/intermediates (again, partial tension). As you know, our hulls are slightly asymmetrical so the “halyard to the chainplate” measurements are not correct. Tommy assured me that he goes to great lengths to center the partner on the centerline of the boat. (I now have a Spartite plug which does this for me each season)

I then start a progressive/incremental tightening of all the shrouds beginnig with the lowers and moving up. Once much past hand tight, I do 1/2 turn on each turnbuckle in a rotating cycle until I reach the desired tensions. It has never made much sense to me to start with the upper shrouds and work down. It is very easy to induce a compression bend in the mast and drive yourself crazy trying to tune it out.

Like Bil, I use the least elegant Loos Gauge. While it’s absolute accuracy may not be the best, it is relative tension, side to side, that is most important. The first couple of years with my boat, I used the Sam Morse guidelines but each haulout, found galling on the clevis pins on a couple of the lower shrouds and on one intermediate. That’s a sure sign of being too loose. Since using a Loos gauge for my ultimate tensions, the boat points better and the galling has quit. Also, having learned to sail in the eyeball and finger pluck days of rig tuning, the ultimate Loos settings simply feel right to me. Too loose is more of a concern than too tight. You would have to be a bit of a gorilla to accidentily overtighten anything but the lower shrouds.

I also check the mast for true and the leeward stays for sag while underway out of habit and have not seen anything that I thought needed changing from the static settings using the Loos tensions.

While not holding a candle to the depth of Bil’s technical research, this seems to work on WW III. I found Brian Toss’, “The Rigger’s Apprentice” to be helpful in understaning and developing my approach to rig tuning. My settings are similar to Bil’s as follows:

Bob stay - I set that with a tiny downward bend in the sprit (I can do this by
hand and the tensioning of the head/back stays seem to give the sprit
a nuetral position when I am done)
Whisker stays - 37
Staysail - 42 (now eyeball as I have a furler)
Cap - 41
Intermediate - 39
Lower - 37
Back - 47
Headstay (somewhere in the forum is a post from me regards my backwards approach to
setting the headstay tension with furler installed, theoretically the
same as the back stay…probably not but a great luff shape and nothing
has gone “twang” so far.)

If you want more info, feel free to email me directly.

                        Best regards.............Tom

Rod

Myapologies for being a slow thinker. I just realised that, thanks to Sumio Oya and John Cole, we have the luxury of space to post and store attachments again.

So I’ve quickly converted one of the pages of my Operating Manual for Zygote to Adobe pdf and attached it (rigtension.pdf, about 43K).

The file includes my calculations, starting with wire rope diameter, to derive a target Loos gauge number. The second table records my conversion from hand tensions to empirically based tensions, and adds the data from Tom Harrer’s White Wings III.

As always, cautions apply. Zygote and White Wings III have delrin blocks under their masts. And the pin to pin measurements for the fore and back stays should be adjusted (by around 3/4 inch, from my memory of a conversation with Sumio). And the usual disclaimer: I am not a professional rigger, just an informed boat owner. Use the Loos numbers and target tensions at your own risk!

Cheers

Bil

rigtension.zip (22 KB)

I would like to thank everyone who contributed to this thread. The replies to my rig tension posting are very helpful. Although more the norm for boats, it is interesting to learn the BCC hull is slighly asymmetric.

Based on the posting, the general approach to rig tension is more empirical than pure engineering. Further the Loos tension guage is best used to determine relative tension than absolute rig tension.

Being somewhat of an engineer, I thought Bil’s engineering approch was well thoughout and has merit but the application of the theory proved more difficult than initially anticipated. Still it would be nice if rig tuning could be reduced to an engineering science but I suspect the there are too many variable such as rigging age, ambient temperature, rigging interaction, etc., hence rig tuning will remain an empirical science based on one’s experience and the sharing of information from one sailor to another.

Bil, thanks for the pdf file.

Although I tuned IDUNA’s rig based on “feel” and limited experience, armed with the information provided in this thread, I feel better about tuning IDUNA’s rig this season. The relative Loos numbers provided in this thread will prove useful and serve as a guideline during the turning process.

Thank You and Fair Winds,

Rod
BCC IDUNA