November 1, 2011

Tuning the rig

EVERY YEAR toward the end of the sailing season Old Wotsisname makes the same request. He wants to borrow my Loos tension gauges. After twelve months of straining to keep his masts upright, his shrouds and stays have stretched somewhat, and he wants to set about screwing up the turnbuckles to the correct tension.
   Now, while I admit I don't like lending my best tools, I'm not a total curmudgeon about it. I do lend my expensive gauges to OW, and I do eventually get them back after I've nagged a few times. But there has to be a limit.  Why should I be the perpetual sucker? How many years is this going to go on?
   Perhaps not many more. I have found a method of tensioning a sailboat's standing rigging that doesn't need gauges. It may well take a few years to soak into OW's rather dense gray matter, but there is hope on the horizon.
   It's a convenient fact that that the elastic stretch of stainless-steel wire increases in rough linear proportion to the load, up to about half of the wire's breaking strength. Therefore, stretch is a good indication of load.
   In fact, when a 33-foot-long 1 x 19 wire (of any thickness) is loaded to half its breaking strength, it will stretch 2 inches. Little wonder, then, that the leeward shrouds sometimes look a little slack.
   Nevertheless, you can use this principle to tune your rig. Here's how:
   Take all the load off a wire and mark on it as accurately as possible with tape or a marking pen a length of 1,980 mm.  Do this anywhere along the wire, where it's most convenient.
   Now tighten the turnbuckle and measure the length again as you do so. You will find that every extra millimeter of stretch (up and above 1,980 mm) induces a load in the wire of 5 percent of its breaking strength.  In other words, an increase of 2 mm, with a space between your marks now of 1,982 mm, indicates a 10 percent load.
   You can find out the breaking strength of the wire from the manufacturer's or retailer's catalog, and from that you can calculate the load in actual pounds or kilograms if you like. But it's just as easy to reckon that a moderate pre-load for the average rig is about 25 percent of the breaking strength.  In which, case, you need to stretch your marked length by 5 mm to 1,985 mm.
   That's it.  All you need is a tape marked in millimeters. I think I'm going to have to buy one for Old Wotsisname.
Today's Thought
Often ornateness goes with greatness;
Oftener felicity comes of simplicity.
— William Watson, Art Maxims.
“Doctor, I think I’ve got water on the knee.”
“No problem, I’ll just give it a tap.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Don P said...

Thanks John,
After seeing all that math I realized that this is the perfect time to ask Santa for a tensioning gauge. What do you hope for in your stocking?

John Vigor said...

Well Don, seeing as how I've got two sets of Loos tensioning gauges, one for me and one to lend to Old Wotsisname, I don't need any more of those. I think I would like one of those cute little electronic instruments that records barometric pressure and shows the past few days in graph form. It won't replace an ordinary aneroid for obvious reasons, but it's ability to show you the general trend of the barometer, and the latest range of highs and lows, is a great safety factor.
(If Mrs. Santa is reading this, please check with me first about exactly which one I want. Please don't try to surprise me again, okay? Thanks.)

John V.

Brion Toss said...

25% tension is extremely high, even for continuous upper shrouds, jibstays and backstays. The method you describe appears to be lifted directly from Selden's tuning guide, but they give much more moderate recommendations, with varying tension based on wire length, and show principles to explain how tuning works in different rig configurations, not just how to calculate tension. Following your directions, just about every sailboat afloat would be horrendously overtuned...
Brion Toss

John Vigor said...

To tell the truth, I'd never seen the Selden guide before. Selden claims to be the world's leading manufacturer of masts and rigging systems in carbon and aluminum for dinghies, keelboats and yachts.
Their website is very interesting:

John V.