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.
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.
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.
- 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 …