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Old 11-02-2016, 07:56   #1
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Broken jib sheet questions

I broke my jib sheet and need to deal with it. Following the break I am now using an old, previously retired sheet.

I have a 20mm diameter braid on braid polyester jib sheet tied with a bowline to the clew of the yankee and running through a 65mm diameter sheave on the genoa car to a large cheek block, then to the winch. The angle from deck to the clew at the sheave on the car was about 60 deg, so quite steep. Whilst beating under full sail in barely moderate conditions and with a steady wind of 16kt the sheet snapped at the jib car sheave.

The sheets are about 6 years old and spend most of their life in the Caribbean sun, but show no significant sign of wear or aging. Mileage is not large. The sheet loads should have been somewhere around 700kg from the Harken calculator and the sheet breaking strength should be somewhere over 8000kg. I am not sure of the brand of my sheets, so am assuming a low strength figure there.

I am surprised it broke. I can't work out why. I would have expected the sheet to go at the bowline as that ought to be the weakest spot.

I wonder if the repeated running over the tight bend at the sheave on the car weakened the rope, plus of course there would be some UV damage. But all combined, surely not to weaken it to less than a tenth of its rating?

So, I am wondering what to do. If the breakage is due to a fatigue failure over the jib car I could just shorten the rope to work a different area. I do end-to-end it already, so any fatigue will be less than it otherwise would be.

I know it is a good idea to have a large diameter sheave in relation to the rope diameter, so I could go for a dyneema sheet, where I could have the same strength with 14mm instead of 20mm and get a 4.6x ratio over the sheave instead of the 3.25 that I have now. Plus lighter weight and easier handling. Or maybe a larger size for reserve strength. On the other hand would a thin rope work OK on the winch? Would more wraps deal with it OK? Would it be too inelastic and transmit heavy loads to the boat? My sails are already stiff string sails, so there will be little shock absorption in the system.

Any suggestions as to what happened and what to do about it?
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Old 11-02-2016, 08:16   #2
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Re: Broken jib sheet questions

I believe friction occurs between the inner and outer braid under loads. That friction would certainly increase at the sheave with that kind of angle. The UV damage didn't help either.

What to do? Implement a preemptive replacement strategy. New sheet every five years?
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Old 11-02-2016, 08:17   #3
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Re: Broken jib sheet questions

How often do you you switch your sheets end-for-end?
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Old 11-02-2016, 08:34   #4
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Re: Broken jib sheet questions

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How often do you you switch your sheets end-for-end?
Once a year, but then I may well lose track. After hurricane season storage I look for the end with rust marks (from the clew) and put it on the other end. Not vigorously done as there is clearly room for error. I must tag it.
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Old 11-02-2016, 08:37   #5
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Re: Broken jib sheet questions

End for end as well as cutting a foot off the end periodically to move the wear points. Sheet size might depend on your winch and clutches.
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Old 11-02-2016, 08:37   #6
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Re: Broken jib sheet questions

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Originally Posted by Fiveslide View Post
I believe friction occurs between the inner and outer braid under loads. That friction would certainly increase at the sheave with that kind of angle. The UV damage didn't help either.

What to do? Implement a preemptive replacement strategy. New sheet every five years?
Well, really my concern comes from it breaking at so low a load, which indicates it really was weakened badly. If the effect was linear, I could break it at 2 years use in high loads from heavy weather.
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Old 11-02-2016, 08:40   #7
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Re: Broken jib sheet questions

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End for end as well as cutting a foot off the end periodically to move the wear points. Sheet size might depend on your winch and clutches.
Good points there. It does make sense to cut down.

I have no clutches, just the jaws on the main winch, which only serves the jib. They can go right down to a very small size.
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Old 11-02-2016, 08:49   #8
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Re: Broken jib sheet questions

I'd probably swap for 16mm braided poly over dynema for the strength and handling comfort. 2m additional length should allow for 8-10 yrs of cutting and swapping. I don't think I'd trust older running rigging than that.
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Old 11-02-2016, 13:12   #9
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Re: Broken jib sheet questions

Replace your jib sheets now. With what, I will not go there (usually a can of worms and disputed recommendations), but the time "in use" of the old ones is to me impressive. If you keep the surviving retired one, mark it so you can reverse it if you ever need to reinstall. Just a personal observation: A too small diameter line has drawbacks for the hands as well as the winch, but I am not a racer in any formal sense, just short term competitiveness.
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Old 11-02-2016, 13:36   #10
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Re: Broken jib sheet questions

It is just a piece of rope. Replace with new quality piece.

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Old 11-02-2016, 13:53   #11
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Re: Broken jib sheet questions

stuff slaps on other stuff n breaks. what is the big deal with changing out sheets occasionally. one would think no one ever had items on board that actually fail. this happens.
is one reason why i never dispose of line--is handy when ye dont have any ..... .
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Old 12-02-2016, 02:59   #12
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Re: Broken jib sheet questions

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stuff slaps on other stuff n breaks. what is the big deal with changing out sheets occasionally. one would think no one ever had items on board that actually fail. this happens.
is one reason why i never dispose of line--is handy when ye dont have any ..... .
I'm sorry if you think I am making a big deal out of it. I think I am giving it the attention it deserves.

I do think it important to understand what is going on otherwise it will only happen again and to loose 90% of my line strength again in little time is not good enough for me.

Think again about keeping old line unless you know you don't have line like mine that has been damaged such that its strength is 90% lost. If you do then those lines are pretty much useless and in fact dangerous if you use them to protect your boat or your life.
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Old 12-02-2016, 03:10   #13
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Re: Broken jib sheet questions

I have worked some more on researching what might have gone wrong and whilst I can't find information relating to textile rope there is a lot of information about the fatigue life of wire ropes decreasing with decreasing sheave diameter to rope diameter ratios. It does so quite dramatically. A D/d ratio of 60:1 giving 90% fatigue life and 3:1 like with my set up giving a 3% fatigue life. (Pythonrope). If polyester works to any degree like steel then this is a very important issue.
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Old 12-02-2016, 03:58   #14
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Re: Broken jib sheet questions

Totally bizarre. Even if chafed at that spot, it should have had a huge reserve of strength.

I have never heard of such a case and can't imagine why it happened other than, perhaps, a defect in the rope. That kind of deflection angle is no problem at all. Have you checked for any sharp edges where the rope might have been chafing unbeknownst?

If it were my boat, I think I would check carefully for sharp edges, then just replace the rope and forget about it. I have a lot of dyneema rope on my boat, but I use double braid polyester for my sheets -- 18mm so one size down from yours. These are under a lot of load, and I like for them to be fat and easy to handle. The bulk of a larger rope is also a certain proof against chafing -- more material which can be sacrificed.

But that much diameter is not needed for the winch, so if you wanted to go to something smaller and dyneema cored, that wouldn't be any problem other than cost. The racers on here (paging Exile) will tell you that you actually need dyneema sheets, for sail trim -- so that the sheet doesn't stretch and spill wind. IIRC you have the same sails as I do (it was you who introduced me to my sailmaker!), and I guess that could be a valid point, since the carbon lam sails are pretty low stretch.

Marlowbraid 20mm has an average breaking strength of 11 (!) tons; you could go to something like 16mm D2 Racing 78 which has the same strength. 16mm sheets handle fine and will be fine on your winches. They will be vastly lighter, less windage, easier to handle, and will run better through the blocks. 11 tons seems like massive overkill to me, but you could do 18mm dyneema if you wanted even more.

Note that with dyneema you don't want to use knots, because the cover pulls off, so you can't use bowlines. You will want spliced soft eyes and soft shackles, or one of another of the various attachment methods, which the racers on here will know about better than I do.

The cost of dyneema sheets of that size will be pretty huge, which is a good reason to stay with double braid, however.
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Old 12-02-2016, 04:05   #15
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Re: Broken jib sheet questions

What chemicals do you put on the Deck? Ever sprayed the block with lubricant? This might have weakened the line locally.

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