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ContinuousWave: Whaler Performance
1/4" Spectra lifting harness..Creepy
|Author||Topic: 1/4" Spectra lifting harness..Creepy|
posted 06-02-2003 12:22 PM ET (US)
It was almost too creepy to watch...an Outrage 18/19 being lifted and then lowered with that skinny line. The owner had the thing made up by a local rigging company. Apparently the harness holds up to 2500 lbs SWL, and storage is easier than stainless cable. It just looks wrong!
|Over the LINE||
posted 06-02-2003 01:04 PM ET (US)
I use high tech line for all kinds of sailing aplications, but my 13' bridle is cable. The problem with high tech line is it gives little or no warning before it lets go and is much more subject to abrasion than its cable counterpart. As far as being strong enough to do the job, Spectra, Vectran, and the like are plenty strong enough for the job.
posted 06-02-2003 03:49 PM ET (US)
I'm also a sailor.
However, if the rating is for 2500lb, there is not much margin for error. An outrage 18/19 has to way a very significant fraction of that.
I wouldn't use this without a SWL of much more than that! If there was any shock loading of this harness, the boat would come down.
Too, anytime you tie a knot, you dramatically reduce the strength of the line/knot combination. If I recall, the best of knots reduce the overall strength to something like 70-90%.
Let me put it to you like this - I wouldn't pick my boat up with that and I wouldn't want to be around anywhere near when someone picks it up. That rig sounds like a disaster waiting to happen.
posted 06-02-2003 05:09 PM ET (US)
Interesting info from the Spectra web site: SpectraŽ fiber, one of the strongest and lightest fibers in the world, is pound-for-pound 10 times stronger than steel. And like steel, SpectraŽ fiber has low elongation. It also has high resistance to abrasion and flex fatigue. In addition, SpectraŽ fiber resists corrosion and degradation from many kinds of chemicals. SpectraŽ fiber can be an ideal substitute for wire and steel and can also be used as the strength member of lifting slings. Slings made with SpectraŽ fiber weigh approximately 60 lbs. each versus steel wire rope lifting slings at nearly 400 lbs. each. Lighter slings made from SpectraŽ fiber reduce installation and recovery costs. In addition, slings made with SpectraŽ fiber do not twist and kink like steel wire so they can generally be used multiple times for a variety of jobs
|Over the LINE||
posted 06-02-2003 07:21 PM ET (US)
I'm assuming the rigger rated it at 2500# SWL not the manufacturer (with all the built in safety factors he should have). I'll also assume that it was spliced not tied (so the loss was minimized) I looked up 1/4" Spectra cored T-900 and it has a break strength of 4400#. 1/4" single braid Vectran has a break strength of 6600#. 1/4" Stainless wire comes in at 6000-9000#. There are three pieces of line holding the boat up, not one. So I would think the line is adequate but not overkill.
All that said, I have enough vectran in my garage to make a bridle for my 13' and I'm still going to stick with cable. Can't bring myself to trust the stuff holding up masts or boats (My leatherman goies through it a little too easily). I guess overkill is more my style.
posted 06-03-2003 12:18 AM ET (US)
Don't forget to allow for the tension in the sling to exceed the simple weight of the boat.
The lines of the sling typically carry more tension than just the boat weight because they do not run vertically. Usually there is an angle to the sling, and this causes the tension in the line to be greater than the weight. Some simple geometry will work out how much, but it is too late tonight for me to work out an example.
|Over the LINE||
posted 06-03-2003 08:25 AM ET (US)
I was curious enough to open a West Marine wish book to get some numbers, but not enough to geustimate actual weights of the boat or do any real math.
posted 06-03-2003 01:31 PM ET (US)
The new lines are in common use now by tugs and towboats for moving barges. So much so that whole new product lines of winches have been developed just to deal with the new lines. Just treat it like any other material and design a safety margin into the application.
posted 06-03-2003 02:08 PM ET (US)
Yep, there were splices in the line. I'll try to get a picture next time I see the guy launching, just for kicks. I also have a plastic coated stainless lifting harness, and don't plan on changing over any time soon, although it is somewhat of a pain to store. I recall a formula that a manufacturer sent me when I was looking into purchasing a harness, and Jim is correct...the lower the angle to the lifting eye, the lower the SWL became. Unfortunatly I tossed it.
posted 06-03-2003 05:54 PM ET (US)
The Specs for all these hi-tech lines presume that the there is no degradation such as chemical, UV, knots etc... If you use this stuff you should make sure that you have a very, very healthy overage on the SWL.
I have these lines all over my sailboat. Last year the jib halyard let go - even though it was sized to pick up the entire boat but would never, ever see anything near those loads.
It let go at the splice where the shackle attaches. The splice didn't fail, the line did.
The other thing that is really scary about these is that there isn't much to chafe anymore. So, you can get chafe in a very small area and it will be unseen - until the line parts.
Not to say that you shouldn't use these lines - they are wonderful. However, for things like slings - were the care of them may not be the greatest - coming into contact with gasoline, oil UV ( an especially insidious problem with these lines) and poor storage practices, you need to be extra extra careful. I would have a sling made out of the old thick nylon webbing that they use on cranes etc... There is so much extra redundancy in that there is plenty of margin just because of the construction. That is not the case with a sling that is made of hi tech spectra (or whatever) because the trade off is weight aloft and bulk vs strength.
All in all, I would not recommend this kind of line for that sort of application.
posted 06-12-2003 11:39 AM ET (US)
I would be just as worried (if not more so) by that plastic covered stainless. If you can't see the metal you have no way of knowing if it is corroded, frayed or otherwise compromised. That is why replacement lifelines on sailboats are now typically uncoated.
I don't know how much an Outrage of this size weighs but I have used a self-made spectra sling (around 10 loops of spliced 1/4" spectra line) to attach a 6000lbs SWL nylon sling as a one point lift point for my 3300lbs sailboat. That would be a safety factor of at least 4 so you know where I'm coming from.
Ultimately, a lifting strap costs around $50 and cost is almost entirely independant of design load (more related to length), so why underkill? Just because you CAN?
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