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Old 12-02-2016, 13:13   #31
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Re: Broken jib sheet questions

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Originally Posted by poiu View Post
Good point about apparent wind, I'd overlooked that. So the sheet loads would be about 1,300 kg instead of 700 kg. A lot higher, but still a fraction of the 8,000 to 11,000 max load.

I'm surprised you think it OK for a 6 year old sheet to break at such a low load especially when not particularly high miles have been done.
Don't recall saying I think it's OK. But these ropes are made of plastic. After 6 years in the sun you have to expect some degradation.

I would have thought your sheet loads would be higher than that. When mine broke I estimate there was about 2000kg on it. Still far less than the breaking strain for new rope, but that's the point - it wasn't new.

I would have thought, with a far bigger headsail there would be much bigger loads on your boat.
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Old 12-02-2016, 13:28   #32
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Re: Broken jib sheet questions

I think several things contributed to the failure including age, UV, and most importantly the relatively small diameter of the block compared to the diameter of the rope. Is there a way to increase the diameter of the block? If so, I would go with that. Then decide if you like the feel and stretch absorption of the large polyester ropes or would like smaller rope in which case go with covered Dyneema sheets of some sort. Note that there is virtually no give with this line which can be both a plus and a minus. It is incredibly strong but does not stretch to absorb shock loads which transmits the shock to other parts of the system.
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Old 12-02-2016, 13:42   #33
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Re: Broken jib sheet questions

What I am seeing here is that you folks with the boats that have higher loads on everything, have different issues with sheets than those of us with lighter boats. It was extremely interesting to me that 44'CruisingCat replaces his 14mm genoa sheets every three years. One of our 14 mm genoa sheets broke once where the eye splice bore on the (at the time) s/s shackle that attached them to the clew. Feeling that that was a chafe issue, we decided to end for end them. They are now tied on with bowlines.

Accomplice's remark about watching the stretch in his 5/8" (~16 mm), accompanied by the visible shrinking while stretched, leads me to wonder if this is what poiu's sheets experienced. When line is under that kind of tension, it is really vulnerable. You don't want to be standing near there when it breaks and releases all that stored energy. I'm thinking this is a big boats kind of issue, with their higher loads.

So a possible solution for poiu might be to consider line that is lower stretch than the polyester double braid, but not the super expensive dyneema racing line. Maybe a discussion with a couple of different manufacturers of rope could point in a constructive direction.

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Old 12-02-2016, 13:50   #34
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Re: Broken jib sheet questions

I'd suggest that any boat that sails fast to windward will generate large sheet loads.


You have the sail sheeted in very tight, and big apparent wind numbers.
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Old 12-02-2016, 14:20   #35
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Re: Broken jib sheet questions

They do make dynemma/polyester blend line. I used one for my mainsheet. Could be an in between line as Ann mentioned.

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Old 12-02-2016, 14:23   #36
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Re: Broken jib sheet questions

Also, Defender is having a Miami boat show sale going on right now and I believe line is on there as well.

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Old 12-02-2016, 15:38   #37
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Re: Broken jib sheet questions

Poiu, before you burn that old line, you might consider cutting out the failed portion plus a couple of feet each side of the break, and sending it to the manufacturer for analysis. They have the expertise to determine cause of failure, and might, in the event that there was a defect, replace it for you! At the least, they could advise as to replacing sheaves, etc, from a knowledgeable position.

All this speculation by us amateurs is pretty useless IMO.

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Old 13-02-2016, 02:52   #38
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Re: Broken jib sheet questions

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Poiu, before you burn that old line, you might consider cutting out the failed portion plus a couple of feet each side of the break, and sending it to the manufacturer for analysis. They have the expertise to determine cause of failure, and might, in the event that there was a defect, replace it for you! At the least, they could advise as to replacing sheaves, etc, from a knowledgeable position.

All this speculation by us amateurs is pretty useless IMO.

Jim
Good suggestion. I will do that if I can discover who made it.

Anyone know from the original picture who made it? It looks like a Marlow or an English braid product, but the internal make up doesn't correspond to their spec. I can't find anything in the Samson or New England Ropes range to match. Ditto Maffioli and Kingfisher and Liros.

I don't share your sentiments about amateur speculation. If it were pretty useless then this site loses a lot of its appeal. I get an awful lot from the wide range of amateur thoughts and knowledge here and have on this matter too.
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Old 13-02-2016, 04:21   #39
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Re: Broken jib sheet questions

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Originally Posted by poiu View Post
Good suggestion. I will do that if I can discover who made it.

Anyone know from the original picture who made it? It looks like a Marlow or an English braid product, but the internal make up doesn't correspond to their spec. I can't find anything in the Samson or New England Ropes range to match. Ditto Maffioli and Kingfisher and Liros.

I don't share your sentiments about amateur speculation. If it were pretty useless then this site loses a lot of its appeal. I get an awful lot from the wide range of amateur thoughts and knowledge here and have on this matter too.
Looks just like the sheets which were originally delivered with my boat. Even the color. Mine were made by English Braids and were a type no longer produced. I know because I tried to source the very same rope and failed. They had a really lovely soft "hand" which I was loathe to lose when I retired the old sheets.

I believe the core shown in your photo is fairly normal for double braid polyester rope. Marlowbraid (what I'm using now) has a three-strand core, but I think that's unusual.

The Marlowbraid is inferior to the old ropes. The cover is not as densely woven, and has a "plasticky" feel to it, compared to the old ones, which felt almost like cotton in the hand. The only good thing about them is they are less affected by salt in the weave so don't need washing as often.

I'm about to replace my sheets again, so I'd be interested if anyone has any tips for what to use. Like Poiu I was thinking about dyneema, but got a nosebleed when I saw the cost.
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Old 13-02-2016, 04:32   #40
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Re: Broken jib sheet questions

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I had the same thing happen to me 15 months ago -- a large, double braid (5/8") sheet parted where it went around a sheave in the genoa car. After bringing the sail back under control and tying a now-shorter jib sheet to the clew, I set about trying to figure out how and why this could happen, and why there.

What I observed was that on most points of sail in a good breeze, the jib sheets not only stretch, but stretch by an amount that varies with an amplitude of up to several inches. The way our boat is setup, the sheets go from a cockpit winch back to a turning block at the stern, forward on deck a genoa car, and then up to the clew. The distance from the winch to the turning block to the car was about 30 feet, with another 5 to 20 feet to the clew. Watching a point on the sheet where it passed over the sheave, it would move forward and back several inches. There was enough strain on the jib sheets that they were noticibly thinner. We had been on the same point of sail, a beam reach, for over 48 hours. The wind was in the high teens.

What I believe happened was that although the line hadn't chafed, that the sheave was of large diameter, that the line working itself back and forth, under strain, pressing hard against the sheave gradually weakened until it parted. I do not know whether it was a process of work hardening, where the continued flexing made the fibres more brittle, or whether there was heat buildup internal to the line, or what mechanism was at work, but I do know it parted in the middle of the line, at the car, not the bowline on the clew.

I asked my sailmaker (Mack) their opinion about the sheets, and in particular if I should use a less stretchy higher-tech line. Their response was something to the effect that there were large, and varying forces acting on the rig/sail/sheets, that when the sheets stretch they were absorbing that extra energy would otherwise be taken up somewhere and that I should stick with double braid.
Interesting you observed the stretch. I had never noticed it. Never looked for it either. According to Samson's catalogue, their polyester braid/braid will typically stretch 1.8% when set and aged for a 10% load, so on a 40ft length of rope a stretch of nearly 4.5" is to be expected if the load goes from say 10 to 15%. A likely sailing scenario. If it could be measured well this stretch would be a fair proxy for a strain gauge.
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Old 13-02-2016, 04:36   #41
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Re: Broken jib sheet questions

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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.

That's about a 3 D/b (bend ratio). That gives you a 30% reduction in strength. The normal recommendation is a 5 D/d (but I might note that 5:1 is not as common in that particular application because people are trying to keep the sheet blocks 'low profile', and they are giving up strength in return).

The angle from deck to the clew at the sheave on the car was about 60 deg,

That is quite normal

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.

As mentioned you have to look at apparent, but still 'normal' conditions.

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.

The cover will be about 50% strength after that UV exposure. The core will be higher, hard to say exactly because it depends on the cover braiding. In a polyester double braid, the cover and core each carry 50% of the load. So you could be down 25-40% strength here on UV. As a note - in a dyneema line, the core takes 100% of the load and the cover is for non-load bearing purposes.

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.

The low bend ratio also increases fatigue. There are graphs on my load testing page on all this.
I would not have expected that break. The only thing I can see from you post that looks out of 'recommended' is the bend ratio and 3 is low but not so very bad or too unusual. After combined bend ratio and UV you could be down to 40% of initial strength but that should have still been enough. My personal guess is that sheet got shock loaded at some point - damaged it but did not break it until now.
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Old 13-02-2016, 04:51   #42
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Re: Broken jib sheet questions

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I would not have expected that break. The only thing I can see from you post that looks out of 'recommended' is the bend ratio and 3 is low but not so very bad or too unusual. After combined bend ratio and UV you could be down to 40% of initial strength but that should have still been enough. My personal guess is that sheet got shock loaded at some point - damaged it but did not break it until now.
Evans, what do you think about Dyneema sheets for this application?

Poiu and I have the same sails (in fact the same sailmaker, thanks to his introduction), which are carbon laminate and pretty low stretch. Does stretch in the sheets harm performance for our usage, as much as the racers say? Some of them have said that it is sacrilege, using ordinary double braid sheets, with such sails. What sayest thou?
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Old 13-02-2016, 05:41   #43
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Re: Broken jib sheet questions

^^

I would go with dyneema definitely. For two reasons - one is you have those expensive sails, a shame to "crippled" them with a stretchy sheet. And at least my experience was the total cost of ownership was not much different because the dyneema lasted much longer (because no stretch means no chafe).

There are various ways to splice the dyneema lines, but the simplest way (a core only splice) is actually much easier and diy-ish than Dacron double braid. I would put eyes in the end and connect to sail with soft shackle so you don't wear the sheet at the contact point.
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Old 13-02-2016, 05:42   #44
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Re: Broken jib sheet questions

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I would not have expected that break. The only thing I can see from you post that looks out of 'recommended' is the bend ratio and 3 is low but not so very bad or too unusual. After combined bend ratio and UV you could be down to 40% of initial strength but that should have still been enough. My personal guess is that sheet got shock loaded at some point - damaged it but did not break it until now.
I appreciate your comments, especially useful as you have done a lot of good testing work in this area.
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Old 13-02-2016, 05:44   #45
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Re: Broken jib sheet questions

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^^

I would go with dyneema definitely. For two reasons - one is you have those expensive sails, a shame to "crippled" them with a stretchy sheet. And at least my experience was the total cost of ownership was not much different because the dyneema lasted much longer (because no stretch means no chafe).
So you agree with the racers that these sails are "crippled" by double braid sheets?

Damn, it must be so, then. Bad news for my bank account, and maybe Poiu's, too.
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