It is not beyond belief that I screwed up, though I did do the calculation twice. I agree that the break strength of 1/8" wire is about 2000 pounds. 34% stretch seemed high but was not unreasonable for the permanent deformation of the wire that would have happened before it broke. 3.4% is unreasonably low.
I used 1/8" wire because that is what was specced for the halyards of my 25. I have since dug out the backup copy of the owners manual for my 38 and find it uses 3/16 wire for the halyard, which has a break strength of 4700 pounds and will accommodate around 1200 pounds load before it starts to permanently deform. I do stand by my original point. High tech line is both lighter and far stronger than wire. And looking at the specification for elongation is not the way to make the choice of halyards. I'm essentially a cruiser. The halyard on my 38 is 7/16 Sta-Set X. It is inexpensive, strong, durable, and easy on the hand, winch, and rope clutch. I know it stretches about 1% at the max loads I typically use for the main halyard (probably around 400 pounds), and will stretch elastically to 3% or so with loads of 1000 pounds. But I consider that a safety margin that will cushion the shock loads you mention when making an accidental gybe or other mistake. After all, math isn't the only place I ever screw up. Rick Brass Sent from my iPad > On Sep 29, 2014, at 11:08, Steve Thomas via CnC-List <[email protected]> > wrote: > > Rick, > I think you made a typo, decimal in the wrong place, at least > according to their formula. Should be 3.4% (3.42144) for 1/8 inch 7 X 19 type > 302/304, according to my calculation using their formula for estimation. > > I checked because I found it difficult to imagine any wire surviving a 34% > elongation, and the point is moot anyway since the breaking strength that > size and type wire rope is 1760 pounds. Even my little C&C27 uses, and is > specified for, 5/32 wire in the halyards. I don't know how to calculate > halyard loads, but I would be surprised to learn that the actual loads ever > get anywhere near to the breaking strength of a halyard in good condition, > that is also the nominal size that the boat's designers called for. An > unintended jibe maybe? Seems to me it would have to be some sort of extreme > shock load. > > Steve Thomas > C&C27 MKIII > ----- Original Message ----- > From: Rick Brass via CnC-List > To: David Knecht ; [email protected] > Sent: Monday, September 29, 2014 9:42 AM > Subject: Re: Stus-List Wire-rope halyard > > You really need to do all the math, not just look at the elongation spec. > > 9mm Crystalyne (3/8 diameter is about the smallest practical for a winch or > your hand) has an average break strength of 11500 pounds. So you need 3300 > pounds of tension to get 7" of stretch. > > The average man can generate 50 pounds force on a winch handle, so you need a > size 65 winch. And I'd bet that 3300 pounds exceeds the strength of the luff > rope and fabric used to make your sail anyway. > > And, btw, according to the calculator on the Loos website, the stretch of > 1/8" annealed stainless 302/304/316 wire rip is 34% when subjected to a 3300 > pound load. So you need to do all the math when evaluation wire to rope > halyards as well. > > Rick brass >
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