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Old 18-03-2011, 22:56   #1
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Jackline tension

I didn't want to hi-jack the tether thread but this one is very closely related.

In the tether thread there are comments to the effect that your tether should not let you go over the side so there should be no need for a single-hand release mechanism.

The above raises the question of how tight to keep the jack-lines and how to keep them tight. Is it even reasonable to think that you could have a practical jack-line / tether setup that would let you move from cockpit to bow but not have enough slack to let you go over the side ? Would you do it by breaking the jack-line into short segments forcing the user to clip and reclip while moving around ? Would you do it with tightness preventing slack ? If it is done with tightness over a non-flat deck how do you keep it down so it doesn't act as a trip-wire ?

We'll still make sure to have single-hand release mechanisms on our harnesses but setting things up to prevent anyone going over the side in the first place would still be really desirable if it is at all practical.


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Old 19-03-2011, 02:55   #2
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Re: Jackline tension

I keep my jacklines 'hand-tight'. No significant slack. In my situation they run inside the inboard shrouds.

I think slack in the jacklines causes problems with tangles and twists as one moves along the deck.
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Old 19-03-2011, 03:16   #3
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Re: Jackline tension

If you keep your jacklines tight, they will break under load. Theu must have some slack, or the geometry of the pull on them will magnify the force.
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Old 19-03-2011, 07:44   #4
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Re: Jackline tension

I have the jack lines that most say not to have but they seem to work well in my case. I have stainless cable that runs from the bow to the stern along the deck.
The way the system is designed, it is kept tight with a turnbuckle. It attaches to dedicated padeyes. This keeps it tight, tight and flat against he deck and tight against the junction of the flat horizonal side deck and the vertical rise of the cabin, keeping it from being under foot. It is also a clean run from bow to stern with no need to remove your tether for the whole trip. You can easily clip in before you exit the cockpit. FYI, I am inexperienced. The boat is a 46 Morris that I bought in 2008.
Now everywhere I read it says do not use round jacklines. Have I been fooled?
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Old 19-03-2011, 07:48   #5
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Re: Jackline tension

Quote:
Originally Posted by Dockhead View Post
If you keep your jacklines tight, they will break under load. Theu must have some slack, or the geometry of the pull on them will magnify the force.
And the evidence in support is . . . . . .
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Old 19-03-2011, 07:56   #6
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Re: Jackline tension

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Originally Posted by RDW View Post
I have the jack lines that most say not to have but they seem to work well in my case. I have stainless cable that runs from the bow to the stern along the deck.
The way the system is designed, it is kept tight with a turnbuckle. It attaches to dedicated padeyes. This keeps it tight, tight and flat against he deck and tight against the junction of the flat horizonal side deck and the vertical rise of the cabin, keeping it from being under foot. It is also a clean run from bow to stern with no need to remove your tether for the whole trip. You can easily clip in before you exit the cockpit. FYI, I am inexperienced. The boat is a 46 Morris that I bought in 2008.
Now everywhere I read it says do not use round jacklines. Have I been fooled?
rdw
You have not been fooled at all. The jackline need only be strong enough to halt a human body flying through the air and snapping the jackline taut. The shape of the line is irrelevant provided the slider matches the line. When selecting think around 2000kg swl NOT break load.

I wish I could afford a Morris.
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Old 19-03-2011, 08:51   #7
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Re: Jackline tension

Quote:
Originally Posted by RDW View Post
......
Now everywhere I read it says do not use round jacklines. Have I been fooled?
rdw
Quote:
Originally Posted by savoir View Post
..... The shape of the line is irrelevant provided the slider matches the line. When selecting think around 2000kg swl NOT break load......
Hmm... I have to disagree regarding the shape of the jackline if it lays on the deck or anywhere it may be stepped on.

There is a significantly higher chance of rolling your foot if you step on a round line as opposed to a flat one, especially on a heaving deck in heavy weather. Try stepping on a flat ribbon and then a round rope and see for yourself.
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Old 19-03-2011, 08:58   #8
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Re: Jackline tension

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Originally Posted by Wotname View Post
Hmm... I have to disagree regarding the shape of the jackline if it lays on the deck or anywhere it may be stepped on.

There is a significantly higher chance of rolling your foot if you step on a round line as opposed to a flat one, especially on a heaving deck in heavy weather. Try stepping on a flat ribbon and then a round rope and see for yourself.
In RDW's original post:
"tight against the junction of the flat horizonal side deck and the vertical rise of the cabin, keeping it from being under foot"
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Old 19-03-2011, 09:07   #9
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Re: Jackline tension

Quote:
Originally Posted by cal40john View Post
In RDW's original post:
"tight against the junction of the flat horizonal side deck and the vertical rise of the cabin, keeping it from being under foot"
Well I can't speak for RDW's boat but my jacklines are in the same place and somehow I manage to step on them when the weather is up, the boat is healed up and pitching and its wet and dark; but I guess the Morris may be a much more stable platform or he picks better weather to sail in.
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Old 19-03-2011, 09:41   #10
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Re: Jackline tension

My jacklines (flat) run from a center bow cleat tight to port & starboard pad eyes near the stern. A problem I've yet to solve is the best way to get past the dodger. On my narrow old boat, it is the trickiest spot going forward. There really is no handhold until past the dodger, which is flimsy to be depending on with the deck at a steep angle underfoot. It's a real balancing act. Have been thinking about maybe installing some kind of permanent s.s. arch in place of the flimsy dodger frame so that there is something to rely on at this point (supports the idea of an ugly hard dodger). An under-boom center s.s. cable from this point might work??? Also, am installing additional eyes to clip into close to centerline at mast and headstay.

The problem as I see it with jacklines, sailing single-handed, is that even when tight, the port/starboard jacklines will NOT prevent going over the side. Would feel a lot more secure attached to a short leash at all times to eliminate this possibility. I have my doubts about the feasibility of actually getting back on board being dragged along, especially on the windward side. Don't want to test this out!

By the way, a great product as far as foot slippage goes is Kiwi-deks. Covered my decks with the stuff, including slippery rounded edges to get rid of all slick spots. It is tough as nails and VERY aggressive.
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Old 19-03-2011, 09:47   #11
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Re: Jackline tension

the jack line is tight in a 90 degree angle of the structure of the boat. You can not get your foot over it because the vertical section is right over the line. To step on it you would have to pull it out of the angle to get your foot on it. It is possible but not easy. On the bow beyond the cabin trunk it is exposed on the deck.
The jack lines came on the boat and I have not been convenced to change them.
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Old 19-03-2011, 09:49   #12
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Re: Jackline tension

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Originally Posted by smurphny View Post
The problem as I see it with jacklines, sailing single-handed, is that even when tight, the port/starboard jacklines will NOT prevent going over the side. Would feel a lot more secure attached to a short leash at all times to eliminate this possibility.
Exactly, that's what I'm thinking too and I was hoping someone had a solution that just wasn't obvious.



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Old 19-03-2011, 10:07   #13
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Re: Jackline tension

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I have stainless cable that runs from the bow to the stern along the deck.
A time tested solution, that is not fashionable today. The cited problems are (a) the wire rolls underfoot and can cause you to slip (I see yours runs right along the cabin trunk, so out of the footpath for most of its run, but I imagine it must be exposed to stepping on/rolling when it goes onto the foredeck), (b) the wire will beat up your decks (when stepped on and when used), and (c) for the grand prix races boats (the cut the tooth brush in half crowd) the wire was considered heavy (compared to webbing). The wire is the longest lasting solution but spectra will last almost as long.

Personally, I think wire is perfectly acceptable but it is not the optimal choice. The optimal first choice today (money excluded) would be spectra webbing, and the second choice would be hollow webbing (note polyester is preferred to nylon because of uv sensitivity) with a spectra cord threaded down the middle of the webbing. These would tend to be stronger than the normal size wire used (3/16" or 1/4"), and not roll under foot, and not beat up the deck, and are light in weight.

Quote:
Originally Posted by RDW View Post
The way the system is designed, it is kept tight with a turnbuckle. It attaches to dedicated padeyes.
This is OK, IF AND ONLY IF the payeyes and wire splices are REALLY strong. There was a dynamic fall test done in the UK of several boats with this system and NONE passed - on all the boats either the padeye, or its thru deck attachment or the wire swages failed.

A poster above has mentioned this - its due to simple engineering principles - force vectors. If the wire is quite tight then the load on the ends will equal very roughly 10x the fall/snatch load. (math: if the wire only deflects by 5 degrees when under load then, then the load on the ends is = load on the middle/sin (5 degrees) = load x 11.4).

The current ISO and OSR spec is that the tether/jackline system be able to take a 1430 ft-lb fall (=100kg x 2m). That means with a tight (5 degree deflection) wire, the padeyes and thru deck bolts and swages need to be able to take 1430 x 11.47 = 16,407 ft-lbs. That requires the sort of mounting that is under a winch base and rarely under a padeye.

You can do two things to reduce this loading: (a) make the jack lines slacker so the angle under load is not so small. Just for example, if you make them slacker so the angle under load is 10 degrees rather than 5, the load multiplier goes from 11.4 down to 5.7. and/or (b) put some elasticity in the system. That does increase your chance of going overboard very slightly Even a small amount will help a lot absorb the short loading. You could put short slightly elastic pieces (say dacron) at the ends of the wire - that does increase the number of connections likely to fail. My solution has been hollow polyester webbing with spectra cord somewhat loosely threaded down the middle. The webbing is set up tight, but is able to stretch somewhat (to absord some shock loading) before the spectra cord comes tight.

But that's a worst case calculation (which is required for the OSR's) and in practice the tight jacklines don't break very often. The practical advice from the British testing was to tighten up your life lines to a bit tighter than you normally do, find the strongest guy in the boat yard and have him pull up on the middle of the life lines as strongly as he can, and then inspect everything. If there is nothing broken or damaged you are ok, but if there is something stressed you needed to fix/strengthen it in any case.
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Old 19-03-2011, 10:13   #14
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Re: Jackline tension

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Originally Posted by savoir View Post
And the evidence in support is . . . . . .
Geometry. Divide the ltransverse force by the sine of the angle made by the jackline at the supports.
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Old 19-03-2011, 10:37   #15
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Re: Jackline tension

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Originally Posted by RDW View Post
the jack line is tight in a 90 degree angle of the structure of the boat. You can not get your foot over it because the vertical section is right over the line. To step on it you would have to pull it out of the angle to get your foot on it. It is possible but not easy. On the bow beyond the cabin trunk it is exposed on the deck.
The jack lines came on the boat and I have not been convenced to change them.
rdw
Then I can't see how you are wrong, then. Certainly, a jackline that can't break trumps a jackline I might step on. Besides, what is this? A quarter-inch of 7 x 19? Stepping on that in seaboots would be more uncomfortable than slippery if it is as taut as you say and is in the turn of the cabin, so to speak.

Besides, even in good weather, if I am clipped on, I tend to move like an ape with a bad back, so my CG is nice and low. I expect to slip, so I make sure I can't fall far or fast.
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