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  #1  
Old 06-24-2010
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lifelines, why so strong?

My coated wire lifelines likely need replacing, and I've been searching and reading posts on the wire vs. synthetic debate. This question is not about that..I'm going synthetic when I replace them.

My question is about strength needed. Since I'm going with synthetic, I have many options, some with a much higher tensile strength than wire, some less.

Why do lifelines need a tensile strength of thousands of pounds? I've never seen a stanchion on a sub-40 foot boat that would hold anywhere near that when levered from the top of the tube. Even in a racing situation, I can't see how lifelines would be loaded or shock-loaded to that kind of force. Am I missing something? It seems to me that lifelines are there as a reminder or gentle nudge to keep you on board. On most boats it appears that the lifelines would guarantee you went in the water head first by tripping up your legs.

Sure I could see situations when conditions dictate scootching around the foredeck on your knees and a wave might knock you into the lifelines..but still..not that much force, certainly not needing thousands of pounds. If conditions are bad, I'm sure not relying on my lifelines to keep me out of the water, I'll be tied in.

I'm aware of racing rules that dictate equipment, but whats the reason behind it?

Am I missing something?
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Old 06-24-2010
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Stanchions do little more than hold lifelines in position. The restraint offered by lifelines arises from tension along the lines being transmitted into the pulpit at the bow and the pushpit at the stern or deck plates where lifelines are angled to the decks fore Ďn aft. They work like a bowstring. A horizontal load applied to a lifeline has to be resisted by the opposing horizontal component of the tension that develops in the lifeline. To give you an idea of the magnitude of that tension, wrap a length of string around each of your hands and pass the string through the center fold of a phone book or another weight of a pound or so. When your hands are together the weight dangles down and is easily supported. Then start moving your hands apart. The farther apart you move your hands, and the more the string approaches the horizontal, the more difficult you will find it to hang on to the string. At some point, you will not be able to do so for very long, if at all, because the tension in the string will be too difficult to endure. Now imagine the weight of a person hanging on that life-line, particularly if he/she is falling across a deck or swept by a wave.

FWIW...
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Old 06-24-2010
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Hey that's the best explanation I've seen yet on the subject. I understand that completely..didn't look at it that way. With my pulpit and pushpit as anchors, I question how strong they are.

Thanks!
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Momentum and shock loading is my answer to the question. A rope with 200lb breaking strength will hold if I lean against it, but not if I were to fall on it.

Anyway, I've gone to Dyneema lifelines and like them. I used 5mm and 6mm lines and they are far stronger than what they replaced.

The ISAF offshore racing regulations are pretty strict when it comes to specifications, and they state that wire diameter is to be 3mm(1/8), 4mm (5/32), and 5mm (3/16) minimum for boats of length <28ft, 28-43ft and over 43ft. At those diameters the recommended Grade 316 wire will have breaking strengths of 1700lb, 2400lb and 3700lb while the Dyneema has 2500lb, 3800lb and 5400lb for the same diameters.
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Old 06-24-2010
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How well do the Dyneema lines hold up to chaffe compared to wire?
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If there are sharp edges, dyneema will chaffe right through. If it's smooth which is should be, it takes years of abuse.
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Remember if using synthetic lifelines you will lose strength to UV, probably 20% over a couple of years.
As far as general strength, imagine a body weighing 180 falling the width of the boat into the lifelines. If they aren't strong they shouldn't be there.
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Old 06-24-2010
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Quote:
Originally Posted by Preserved View Post

Why do lifelines need a tensile strength of thousands of pounds? I've never seen a stanchion on a sub-40 foot boat that would hold anywhere near that when levered from the top of the tube. Even in a racing situation, I can't see how lifelines would be loaded or shock-loaded to that kind of force. Am I missing something? It seems to me that lifelines are there as a reminder or gentle nudge to keep you on board. On most boats it appears that the lifelines would guarantee you went in the water head first by tripping up your legs.

Sure I could see situations when conditions dictate scootching around the fo

Am I missing something?
What you are missing is something called MATHEMATICS, something rapidly and increasingly missing from USA education.

Trigonometry is the special branch of math, that in the case of lifelines, governs how an object reacts to SIDE LOADS placed against that object !!!! In engineering the 'science' of such force loadings is called 'statics'. It will govern or explain the resultant tension in music strings and the frequency that the string vibrates, after it is deflected away from its least tensioned or 'deflected' state, it governs how STRONG a bowstring must be ... an example would be for a bow that requires 50 pounds of draw/pull, the bowstring should have the strength of about perhaps 20-50 times the strength of the draw force to do that function without breaking.

Trigonometry dictates (and without getting into the details and calculations) that a 'typical' lifeline should be about 10 to 20 times stronger than the 'expected' loading from the side. A tighter than normal lifeline would need to be even stronger, a more loose lifeline could be less strong. A lifeline that has lots of stretch could be less strong but a life line that has 'less stretch' or elasticity would have to be stronger.

Moral of the story .... replace with equal or stronger materials, dont tighten them to 'bar-tight'; slightly 'more elastic' and 'slightly loose' is better .... its all in the 'math'.

To 'modernize' Id choose a modern UV resistant synthetic of equal or greater breaking strength ... because they are more elastic than stainless wire, thus 'mathematically' stronger for such a 'side loaded' application.

Last edited by RichH; 06-24-2010 at 11:32 AM.
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Manufacturer claims Dyneema is very UV resistant. For not much money you can buy Dyneema that's so much stronger than rope that 20% loss is still much stronger than steel.
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While I happen to like synthetics, and have replaced our wire rope running backs with Spectra, the fact that the material has virtually no "stretch" is a disadvantage in this application. Under a shock load you would want a modest amount of 'stretch" in your lifelines to suck up some of the energy of the load. Wire rope has more "stretch" than synthetics of similar strength.

One added note, when using wire life lines (which should not be covered with plastic) it is wise to have the connection between the end of the lifeline and the pushpit made with wraps of nylon or dacron (rather than Spectra or Dyeema) line lashings rather than pinned shackles. The lashings will suck up a lot of shock load and they can be cut when necessary to get the life-lines out of the way when trying to recover a crew overboard. (We keep an ultra-sharp "fishing net" knife in a sheath attached to our binnacle for just such an eventuallity.

FWIW...
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Last edited by svHyLyte; 06-24-2010 at 01:44 PM.
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