Stealth mode - New Generation Ropes

[Seaworthy Lass]

I have just mocked up the proposal put forward in post #244 using 8 mm UHMWPE loops. Length of loops was 132 cm. The best I could set up the separation between the loops at the “boat end” was 54 cm. This gives 2.44:1 , which is fairly close to the required 2.5:1. I used the winch on the other side of the cockpit to apply load. It would not have been high compared to the breaking strain of the set up, but the line felt like steel, so it was a moderate amount.

It looks extremely promising to me. Rock solid when load was transferred from two legs to one (the legs were jammed on top of each other so no shift occurred) with no throat issues if the bridle leg eyes have a generous length.

I think it looks good enough that it is worthwhile Thinwater and Breaking Waves reviewing this if they have the time. Cow hitching the leader to the bridle legs may well be better than the current technique of cow hitching the bridle legs to the leader.

I think we need the “best method” of dealing with this junction, taking into account simplicity. Cruisers with not a great deal of experience with UHMWPE may be putting this drogue together, so the less to go wrong with construction, the better.

A few more photos will follow in the next post.

SWL

[Seaworthy Lass]

Side views of the junction.
I put some white cardboard behind so the image was not cluttered.

The throat on the bridle results from the thickness of two lots of line (as opposed to two lots of cow hitches if the bridles are hitched onto the leader).
I would just go overboard and make the eyes nice and long (eg in this instance half a metre or so).
If care was not taken with eye length it would be far less detrimental than hitching the other way around, as the throat is governed by the wider width of two cow hitches. There is also no need to construct a loop. Keeps it simple and less likely to be stuffed up.

[weavis]

er.

Maybe I did something wrong here..

[weavis]

But thank you seriously for your additions to boating knowledge... I struggle to remember a clove hitch at times.

Big Hug

[Seaworthy Lass]

This is what occurs when load is transferred to just one leg. I could not perceive any shift in position of the bridle leg that took up the load.

When load was removed completely, the junction felt like it had been glued together.

Very promising I think.

[Seaworthy Lass]

Originally Posted by weavis

er.

Maybe I did something wrong here..

Looks like you have hit on the only type of undressed knot to hold in UHMWPE .

SWL

[Seaworthy Lass]

One other advantage of hitching the leader to the bridle legs may be:
The cow hitch is made over two lots of line so the turns are less sharp with most likely a lesser reduction of strength.

[thinwater]

Originally Posted by Seaworthy Lass

... The Shark manufacturer said they used thimbles with no exact indication of thimble or shackle size etc, then disappeared from the conversation just when they were needed :

I am sticking my neck out on a block saying I think this is wrong, but that is my opinion.

The bridle junction is the point that sees the highest loads, and they are flipping from one leg to another. Techniques that are perfectly reasonable under low loads may fail here. This is one spot I think we really need to get it right. As a starting point, simply a calculation of throat angles and resulting tearing forces for different proportions of line diameter to eye length would be a useful guide if the two bridle legs are being cow hitched on....

SWL

Agreed, and I don't think you are sticking your neck out.



MC posted images of a JSD bridle that nearly failed because the thimbles shifted. Starzinger studied the deformation of thimbles at high load. I've published images of them coming loose at high load. No matter how carefully installed, it is VERY difficult to install a thimble so that it can not shift if the load approaches BS. The line stretches and the splice stretches as it sets. This is unavoidable and it is an unnecessary risk that has been proven to be material.


Thimbles make perfect sense in applications below the WLL, which is where anchoring systems and rigging operate. Stretch is less and the thimble does not deform. But this does NOT extrapolate to applications far above the WLL that also include cyclical loading. Different. I've gotten away from thimbles for practically everything and gone to webbing.

[oldbilbo]

This is excellent. There's a sense of 'we're getting there'.
Thanks, guys and gals. This matters....

[thinwater]

Originally Posted by Seaworthy Lass

This is what occurs when load is transferred to just one leg. I could not perceive any shift in position of the bridle leg that took up the load.

When load was removed completely, the junction felt like it had been glued together.

Very promising I think.

Brilliant and thank you for your effort! That is exactly how I was going to set it up.



The "glued together" result is what you are looking for. No movement. However, before we say that the conventional 2 half hitch method is inferior, we need to prove it. In other words, my guess is that it will be "glued together" as well, suggesting that both methods are safe. So long as the angle does not exceed the coefficient of friction. DSM gives this as 0.04, but that is smooth surface and does not include the effects of braid texture. Additionally, the slide angle, if you will, is very low because the eye is long. It would be good to warn people, if the two half hitch methods moves.


Good work. I love playing with low friction rings, but I think this is a place where simple is best.


I once was given a 3-strand bridle to test that was made by splicing the second leg into to the other leg in a Y, like a big, open eye splice, I could not find any data on the strength of this construction, so I pulled one apart. In fact, it was very close to 100% strength and broke at one of the eyes, not the Y splice. Although it looked funny to my eye, it worked and I have not heard of a failure. Just an example offed for no particular reason.

[thinwater]

Originally Posted by Seaworthy Lass

One other advantage of hitching the leader to the bridle legs may be:
The cow hitch is made over two lots of line so the turns are less sharp with most likely a lesser reduction of strength.

I see one potential weakness. I haven't thought this through, so take it easy on me.

As the boat yaws from side-to-side the rope will flex where it comes out of the cow hitch. The flexing will be shap and localized. The standard arrangement does not do this, because the pitching is vertical and the swiveling of the chain plate shackles takes care of it. I think about 20-40 degrees is pretty normal, but I will defer to people how have "been there. If the legs are "glued in place and the legs are not quite singly loaded, the rope exiting the knot has to flex, or rather that will be the easiest place.



I had a Polyester cover/Kevlar core sheet cow hitched to a genoa. When the sail flogged it flexed the same place, at the exit of the knot. After ~ 5 years of occasional cruising, the core failed internally, while leaving the sunburned polyester cover intact and undamaged. The rope did not fail, but I noticed this during a walk-around inspection at anchor. Granted, Kevlar is FAR more susceptible to fatigue due to flexation.I wish I had taken a photo before I took it off the sail. In fact, I cut out the bad slot and reinstalled it, with a sewn eye, chafe gear, and a soft shackle through the clew.



For Dyneema (from Marlow, https://www.yachtsandyachting.com/ne...pe-performance) the bending fatigue life of Dyneema is given below. The "life factor is defined as the safety factor/(D:d) ratio. If we assume we are working at 50% of the WLL during the storm (10:1 safety factor) and the D:d ratio at the end of the knot is one, the life factor is 10. That gives a really pathetic life expectancy, perhaps only dozens of cycles. Obviously this is somehow not fair. On the other hand, we know that nylon climbing ropes take serious damage after 100 rappels or so, with similar bending and a safety factor of about 35:1. That suggests a fatigue life of 1000, which is not crazy, based on mountaineering experience.

But this is still a serious concern. That rope will bend sharply, side-to-side, under high load.



Please find better data and explain what I have missed. There is a fatigue factor here, greater than normal loading, but I don't know how to evaluated it.

[Seaworthy Lass]

Originally Posted by thinwater

I see one potential weakness. I haven't thought this through, so take it easy on me.

As the boat yaws from side-to-side the rope will flex where it comes out of the cow hitch. The flexing will be shap and localized. The standard arrangement does not do this, because the pitching is vertical and the swiveling of the chain plate shackles takes care of it. I think about 20-40 degrees is pretty normal, but I will defer to people how have "been there. If the legs are "glued in place and the legs are not quite singly loaded, the rope exiting the knot has to flex, or rather that will be the easiest place.

Hmmm. Thinking here. As the boat yaws, doesn’t the load get transferred from one leg to the other? It is not the same situation as a genoa flogging. (Edited to add: maybe it is).
Shock loading would still apply though.

I am just speculating too. Conachair may be able to help here.

Regarding the attachment to the boat, in our case it is fixed (monster double bollards welded under the deck to the underlying aluminium framework), so we won’t get any benefit from swivelling action of shackles.

This Y junction really is a nasty point when you consider what is occuring there. If knots are being used, bend ratios are high. Compression forces are huge. Fatigue I just can’t get my head around. Replacing the knot with a LF ring would solve a lot of issues (apart from shock loading if this occurs during the transfer of load between the legs). Any knot is horrid in this application, particularly in UHMWPE.

If we stick with a knot/knots at this junction, a couple of things would help:
Firstly, way over specifying the line diameter of the bridle and leader (to decrease cost, make the first section of the leader short).

Secondly, replacing the bridle and leader after use. Consider it a disposable portion of the drogue. For sailors racing in the Southern Ocean who may need the drogue multiple times, carry several bridles. They are extremely lightweight in UHMWPE and take up little space.

This is where I can’t offer anything else useful. The engineering side is way out of my depth (medical background here).

SWL

[Seaworthy Lass]

Or, maybe, given the problems with any knot, it is less important to have the junction “glued” and instead go back to the loop idea (twirled to make it small) and just connect eye splices with chafe protection to it. The soft loop is a soft version of a solid LF ring and pretty bullet proof if the same diameter of line as the bridle is selected and it is tripled with twirling.

If the bridle needs to slide on the loop, maybe let it do so unrestricted. I doubt a cow hitch can be stopped from moving on slippery UHMWPE with the high loads that could occur here. The friction would then be high.

A loop and eye splices looks more and more appealing to me. Way back in post #106 I ditched the idea of cow hitches on this loop. Conachair suggested this as well. My thoughts are going back and forth on this, as I don’t know what is the lesser of two evils. Getting rid of a knot at this Y junction seems the best solution possibly. I hate cow hitches when an eye can be spliced on.

The more I look at the photos and read Thinwater’s comments, the more and more I dislike the notion of any knot at the Y junction.

SWL

[thinwater]

Originally Posted by Seaworthy Lass

Hmmm. Thinking here. As the boat yaws, doesn’t the load get transferred from one leg to the other? It is not the same situation as a genoa flogging. (Edited to add: maybe it is).
Shock loading would still apply though.

I am just speculating too. Conachair may be able to help here.

Regarding the attachment to the boat, in our case it is fixed (monster double bollards welded under the deck to the underlying aluminium framework), so we won’t get any benefit from swivelling action of shackles.

This Y junction really is a nasty point when you consider what is occuring there. If knots are being used, bend ratios are high. Compression forces are huge. Fatigue I just can’t get my head around. Replacing the knot with a LF ring would solve a lot of issues (apart from shock loading if this occurs during the transfer of load between the legs). Any knot is horrid in this application, particularly in UHMWPE.

If we stick with a knot/knots at this junction, a couple of things would help:
Firstly, way over specifying the line diameter of the bridle and leader (to decrease cost, make the first section of the leader short).

Secondly, replacing the bridle and leader after use. Consider it a disposable portion of the drogue. For sailors racing in the Southern Ocean who may need the drogue multiple times, carry several bridles. They are extremely lightweight in UHMWPE and take up little space.

This is where I can’t offer anything else useful. The engineering side is way out of my depth (medical background here).

SWL

Perhaps I was not clear.


When the pitching is vertical, the shackles will swivel up and down, because the holes in the transom chain plates are drilled horizontal. This is fine, they can take it, Jordan expected this.



When the boat yaws horizontally, within the range of the triangle formed by the bridle, the Y bends at the join, which is the exit of the cow hitch. The load varies from leg to leg, but the geometry of the triangle does not change unless one leg goes slack, which mostly it will not. The bending, thus happens at the knot. The bridle legs eyes do not bend because they still have load on them. I think the bending happens just as the leader leaves the cow hitch.


How serious this fatigue factor is, we do not know.


Something like this must also happens with conventional connection. There is bending, but where is not as obvious. In fact, the bending MUST happen with any connection that does not have free movement, and thus chafe potential.


Try bending the test rig from side to side and see where it flexes. I think there are several modes that are possible, depending on the details of how it draws up.

[Seaworthy Lass]

Originally Posted by oldbilbo

This is excellent. There's a sense of 'we're getting there'.
Thanks, guys and gals. This matters....

I am certainly learning lots here.

SWL

PS I interpret “guys” as gender neutral

If we start getting politically correct here, you may need to add all sorts of other terms other than “gals” .