Wire Rope Breaking Strain

[CFFrank35]

On a recent trip in 20 knot winds I noticed how slack my leeward shrouds were. I've splash out and bought the Loos tension gauge and want to get going on adjusting the rig on my CD28.

Here's the confusion: Loos have the breaking strain of 1x19 3/16" at 4700lb while Nigel Calders boat maintenance book has a breaking strain of 3,960lb.

Who's right here or should I just average it out?

Also, does anyone have any pointers on doing this job correctly?

Cliff

[John Vigor]

It depends on the grade of stainless steel. Type 302/304 commercial grade wire, most commonly used for standing rigging, has a breaking strain of 4,700 pounds for 3/16-inch-diameter wire.

Type 316 stainless steel wire, recommended for tropical use because of increased resistance to corrosion, is approximately 15 percent weaker.

To tension the rig give the upper shrouds and backstay a tension of approximately 10 percent of your boat's displacement. (Once again, displacement, not the breaking strain of the wire.)

You will induce slightly higher tension in the forestay because it makes a narrower angle with mast than does the backstay.

Then tighten the forward lower shrouds until the mast bows forward--slightly but noticeably--at the spreaders.

Tighten the aft lower shrouds to straighten the mast again.

Sail for a few hours in moderate winds to allow the rig to stretch.

Still in a moderate breeze, adjust the mast for straightness from port to starboard if necessary.

The old rule of thumb was that on a moderate beat, the leeward shrouds should feel slightly slack to the touch but not look slack.

Cheers,

JohnV.

[bottomscraper]

Your wire is probably the stronger 302/304 variety. I have been using a Loos gauge and Brion Toss's instructions from the Riggers Apprentice ISBN 0-07-064840-9. He recommends 15-20% for the uppers, 12-15% intermediates and 10-12% for the lowers. I have been using the lower end of the ranges. I usually do a rough tune and sail the boat for a couple of weeks and go back and do it again before installing any cotter pins in the turnbuckles. Interestingly enough JV's 10% of the displacement and BT's 15% of the breaking strength are rather close (at least on our boat). Coincidence? Probably not!

[CFFrank35]

John and Rich,

Thanks for your insight. I'll attempt this job over the weekend and let you know how I get on.

Cheers

[David L]

My Loos tension gauge says that it is calibrated for 304 stainless steel. My rigging is 316 st.st. so I use it for only relative measurements, rather than absolute. Has anyone found a multiplier to recalibrate this tool for 316 st.st.? Presumably, Loos thinks there's enough of a difference to mention it.
David

[John Vigor]

David, simply assume your grade 316 is 15 percent weaker than 302/304.

John V.