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post #31 of 38 Old 03-11-2011
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Originally Posted by RhythmDoctor View Post
You may want to analyze the force vectors of a 200-lb person pulling directly down on your lifeline. I would be concerned that with the way lifelines are routed around the stancions, the resulting tension from a 200-lb downward force might be much higher than you realize. Then compare that number to the line's maximum working load.

Also, do you do anything to protect against chafe at the stanchion tops?

While I think the dacron may stretch a bit too far for the aplication...

IMHO, it is you that need to review vector mechanics. The 1/2 dacron is as strong as typical lifeline material and the load will be less than 1/3 because the stretch will deminish the tight-rope effect.
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High school physics, perhaps colege engineering if we got into the fine points. Remember, mountain climbers take long falls on 7/16-inch nylon bent over rounded metal edges (carabiners); this does not aproach those forces.

Chafe is a separate matter, very dependent on the instalation. Certainly there should be no sharp edges. I'm going to guess that the lifecycle cost of SS lines is less than the 1/2-inch dacron because the lifespan is tripple. That is enough reason for most not to consider them. But if I like the look I would consider them safe.

(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




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post #32 of 38 Old 03-11-2011
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A less condescending reply would have been appreciated.

My concern was over the "amplification" of static tension because of the "guitar string" geometry of lifelines. This geometry is very different from a mountain climber pulling straight down on a lifeline described in your link:


As θ gets small, T can get very large - which I had always believed was why steel lifelines are typically used in this application. (Same reason why steel guitar strings last longer than other materials.) Stretching of dacron line would affect θ in a way that reduces T, but that benefit may be offset by severe chafing and cutting that could occur at the stanchions when the line is under tension (and which was pointed out in the link you provided).

My comment was related to static tension only. The dynamic effects described in your link are a separate issue from what I was mentioning. But when you consider the dynamic effects, my concern becomes even greater. Take the extra dynamic stress caused by decelerating a falling person from his rate of fall to zero and divide it by sin θ, and you are even more likely to exceed the maximum break load and/or cut through the line at the edge of the stanchion.

My point is still the same - I believe many people neglect the unique geometry of the rigging of lifelines, and how that can cause very high amounts of tension. You can belittle it by calling it high school physics if you want - it still seems to be neglected by many.


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post #33 of 38 Old 03-11-2011
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For what it's worth, if one intends to do an analysis of a lifeline arrangement, one must look at the entire cable system, from end connection to end connection. In most cases, lifelines terminate at pulpits at the bow and stern although on some smaller yachts, they may angle downward from the last fore’n aft stanchion to deck fittings near the bow and stern. For the most part, stanchions are not intended to take horizontal loads (in bending) but rather to simply to hold the lifelines in position at height and take vertical loads much as do the struts in a tent or the spreaders on a mast. When a load is thrown against the lifeline from inboard, it will move outward and tension up just as does a bow-string when the bow is drawn. The tension in the line is carried by the connections at the bow and stern pulpits. Any loading out of plane is carried by the stanchions acting as struts, hence a little “flex” at the top of the stanchion is desirable. Accordingly, however, one wants tight lifelines so that there is as little “give” or horizontal displacement in the lines before they are tensioned.

Where lifelines and stanchions may fail is with an inward pull rather than an outward push as the lines generally follow the curvature of a hull and hence cannot go into tension when loaded inward. An inward push on a stanchion can seriously load up a base with “prying action” and can easily damage a deck. (I vigorously oppose anyone grabbing or pushing on a stanchion when we're moving the boat about by hand in a marina!)


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post #34 of 38 Old 03-11-2011
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hmmm interesting. my solution would be to have more lines higher and beefier, also stretch a net throughout.

Remember that cables are often used to keep car on roads so it should not be hard to rig a system to keep a person, on board a boat. the forces are smaller.

Given that everything is over engineered with a safety factor (esp boat items i.e a triple inter braided tubing with an max load of 80psi on a fitting (double clamped) with a working load of 2psi and a max of 10psi) Really if you do your inspection and maintenance there should be no problems just make sure you use the right material and if its non standard do the math on it.
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post #35 of 38 Old 03-11-2011
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A no life line experience

This is an interesting discussion.

It reminded me of an experience I had setting a cruising chute on one of the new fancy "day sailors." The boat was beautiful, with teak decks, and no life lines. I found myself crawling around the foredeck like I was in a gale in calm conditions. The perception that you are about to fall overboard was significant. I decided I like life lines
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post #36 of 38 Old 03-14-2011
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The stretch may indeed be more, but the slight increase in sag would be negligible. UV resistant Dacron will last about as long as swaged SS fittings, and doesn't have the issues of the vinyl covering hiding corrosion and even enhancing it. Chafe is not a factor as the lines are run inside of the stanchions and seized to each of them. The deciding factor for me is still the "cheese slicer" effect. Try it for real some time. I'll match my grip on 1/2 inch 3-strand soft rope against even a professional bodybuilder's grip on a 3/16 SS line, with or without salt water on them. If my wife slips, I want her to have something she can hang on to...
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post #37 of 38 Old 03-28-2011
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I can't say I feel more secure with life lines. I'm pretty tall and they hit me right around the knee. If I loose my balance and hit the life line I am going head overheels overboard no question. ALso there is a seriously good chance of impaling myself on the stanchions. As for holding on value, what could be worse than thin wire? And a wave is more likely to wash you under the line then into it. I think I'd rather have a beefy rail about 3 inches in diameter about 5 inches high. You could brace your foot on it securely and if you do start to go overboard you could sprawl out have something to get a firm hold on with both hands.
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post #38 of 38 Old 03-28-2011
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Be careful folks, someone (most likely in the insurance, lawyer or lifeline making business) may decide all recreational vessels need 42 inch handrails, access ramps and handholds surrounding the head.
Sailing involves more risk than sitting on a couch.
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