Octaplait vs Doublebraid For Jordan Series Drogue

[toddedger]

The problem with using spin winches for attachment is they are usually forward of the stern, and the number one problem appears to be chaffing. So you would be inviting the lead to chafe on your deck somewhere. Thats why I went with straps on each side of the hull.

Zanshin Please consider adding sailors thimbles to the attachment points on your bridle.

[cwyckham]

Originally Posted by toddedger

The problem with using spin winches for attachment is they are usually forward of the stern, and the number one problem appears to be chaffing. So you would be inviting the lead to chafe on your deck somewhere. Thats why I went with straps on each side of the hull.

Zanshin Please consider adding sailors thimbles to the attachment points on your bridle.

How do you route the bridle from the winches to avoid chafe, Evans?

[estarzinger]

Originally Posted by cwyckham

How do you route the bridle from the winches to avoid chafe, Evans?

My biggest winches are the stern most ones. This happens to be a change we made from the NA plans when building the boat.

I put big blocks (big enough to take 3/4" line) on the stern quarters. Note I use these same blocks for shore ties and some mooring situations. I originally bought them to use in a "Pardey bridle" but after playing around with that . . . it is no longer among my "possible tactics" for hawk.

The bridle runs from the winches to the blocks and out to the drogue rode. The bridle does not touch anything except the winch and the block. The only possible point of chafe is at the block.

The bridle angle at the block is not huge (30 degrees?). A low angle here reduces loads and chafe potential. A longer bridle will create a smaller angle.

I might note that the stern quarter toe rail attachment was specifically designed and built for this. It's a lifting eye, joined to a ring frame, that us theoretically rated to the boat's displacement. I do not think it ever sees anywhere near this sort of loading (because the bridle deflection angle is small).

I have never seen any chafe at all.

[Zanshin]

Originally Posted by toddedger

...
Zanshin Please consider adding sailors thimbles to the attachment points on your bridle.

I spliced/whipped thimbles into the attachment points, but don't know what a "sailor's thimble" might be and google didn't help me, either.

[Bash]

Originally Posted by estarzinger

I had a long discussion with harken about it. They said the winch rating was mostly all about the pawl in moving operation. So, if I treated the winches as bollards, and did not load the pawl or spin the winch the effective rating would be way higher (they did not give me a number). So, I use a tug boat hitch with the bridle lines on the winches, which treats the winch as a pure bollard and does not load the pawl.

There's widespread ignorance about this, evidenced by people using winches to anchor spring lines at the dock. Much better to use a tug boat/lighterman hitch--or for that matter to make a loop with a bowline--than to load the winch in such a way that the pawls take the stress.

[estarzinger]

Originally Posted by Zanshin

I spliced/whipped thimbles into the attachment points, but don't know what a "sailor's thimble" might be and google didn't help me, either.

usually means a closed thimble





Usually called a 'sailmaker's thimble'.

Stronger than the more typical thimble with open throat

[Stumble]

I have been doing some further research into this, and I think we have all been making a fundamental mistake in ignoring fatigue issues when discussing rope selection. One of the primary failure methods of materials is thru fatigue, and just because these materials are rope instead of solid materials doesn't effect this issue.

So how does this effect the conversation?

Let's assume a storm that lasts for three days, with a wave period of 10 seconds. Over the course of this storm the line must be able to absorbe 6/min*60=360 cycles/hr * 24 hours= 8,640 cycles/day * 3 days = 25,920 total cycles. For most materials 30,000 cycles isn't really that big on a deal, but nylon is particularly susceptible to cycle fatige.

S/N curves of course deal with number of cycles at some load, so the cycles are only have the question. But for nylon at 30,000 cycles the fatigue limit is 30-35% of the original MBL of the line.

When you take a look at the same curve for a dyneema line however is it clear that dyneema is much less susceptible to fatigue. At 50,000 cycles dyneema has a cycle fatigue limit of 80% of the MBL.


So when speccing lines for this installation the nylon needs to have a MBL of about 3 times that of dyneema to reach the same cycle fatigue limit.

As an addition, this also means that we may need to be much more carefull about when to replace line. Since the number of cycles may be just as important as the visual condition of the line.

See:

figure 1 - http://www.barry.ca/publication/high...plications.pdf

[thinwater]

Originally Posted by Stumble

I have been doing some further research into this...

You must dig deeper before you start with calculations; fatigue is more complicated than the white paper suggests. The figures (I notice they couldn't keep to one comma convention, even within a single table) did not identify any specific rope construction. For example, it is well known that nylon double braid tries to saw itself to bits, while laid rope is less prone to this. Without stating the construction and wet/dry, the data is not ready to use.

In actual practice, I believe the load factors are even more in favor of high mod ropes than 3:1 because of internal abrasion numbers. This tends to be true with any low stretch fiber, because of reduction in rubbing. The specific reaction to the rubbing is also a factor.

However--there is always an however--we also need to consider whether the nylon will experience a lower load because of the energy absorption, and that is very difficult, perhaps impossible, to simulate with any accuracy. We've been discussing that.

As for the fatigue life of the rope, since modeling the stain is so difficult the only practical answer is to design for a fatigue life so long that the product would be replaced first for other reasons, perhaps 50-60% in the case of Dyneema.

What strain do we use? If we use the high factor for calculating fatigue, we insure failure unless we know the maximum strain (if the average is running at 65% we are going to break the line with one bad break, the only time, perhaps when we really needed it), so the reality is that we will design for 4-8 times the average storm strain. And we have circled back to the question of whether nylon will see a lower load. We know that chain vs. nylon are very different with a ground anchor, but the answer at sea will vary with the waves and the drogue. Smart money is guessing that with Dyneema the line and the attachment point will need to be stronger than for nylon... but how much?

Some vendors (Seabrake) recommend polyester as a good solid compromise; much better fatigue life than nylon, more immediate loading of the drag devise with the passage of a wave, and some give. Unfortunately, there is little space and weight savings.

http://www.tensiontech.com/papers/pa...wire_ropes.pdf

Handbook of Fibre Rope Technology - Henry A. McKenna, J. W. S. Hearle, N. O'Hear - Google Books

I don't think the group is unaware of fatigue. To me, that is what the thread is about. Lamentably we have no data on brait and fatigue life. I inquired through manufacturers some time ago--folks I've worked with before--but I guess a phone call is needed.

----

Very complicated with many variables. The best answer may prove to be a hybrid rode, using several materials. Or perhaps as staying with nylon and going up a size.

[CarinaPDX]

I just noticed this thread, so sorry for the late post. Hard drive failure has kept me distracted...

While in the UK in 1996 I discovered that double braid nylon was not available so I wondered if 8-plait nylon would serve (the OP's question). I wrote to Donald Jordan about using 8-plait and his response was:

"The increased stretch will not reduce the load, in a breaking wave strike. In fact it will increase it a bit. The drogue must pick up the load quickly before the boat broaches and must turn the boat into the wave. The cones near the boat perform this function and THE LESS STRETCH THE BETTER [emphasis added]."

I think that is a definitive answer.

Greg

[thinwater]

'I think that is a definitive answer.'

So why didn't they specify polyester? Amsteel, or course, did not exist.

[CarinaPDX]

I can only conjecture that dacron (polyester) was too expensive at that time to recommend.

I should point out that he was OK about my using 8-plait and made a specific recommendation as to the size for Carina. The above quote was in response to my suggestion that the increased stretch was an advantage for 8-plait. More of our communication had to do with the strength of the attachment points than the choice of line. He wanted strong eyes on the outside of the hull with no chance for chafe (e.g. at hawseholes).

This was part of a larger coversation via snail mail. I have his letters but I would have to look around on the disks for my half - needed to make sense of his replies. If there is interest I could haul out the scanner...

Greg

[Stumble]

Tom,

I actually had a study on brait nylon on my desk earlier today but can't find it. If I remember correctly it was from a study researching fatigue on wind farm mooring systems and for fatigue it was in the same general range as single braid nylon, and slightly better than double braid.

Certainly fatigue is a very complicated subject, and my engineering background is minimal. Other than staying in a Holliday Inn Express last night I can't claim any background other than reading a lot of studies. The reason I brought this up is that prior in the thread people (myself included) had been using line that just met the strength requirements, while in light of the issues of fatigue the line the MBL for specced line should have been much higher.

[CarinaPDX]

After thinking about it more (a dangerous activity) what I meant was that the answer was definitive regarding stretch: it is undesirable in this application. The other considerations are strength (well documented), friction (chafe and internal heating), to a lesser extent weight (but important for some) and finally cost.

It is clear that if cost is no issue then dyneema wins hands down in every other category. Polyester is a solid but more distant second, at a substantial cost savings. Nylon is the least desirable of these materials, except that it is also the least expensive and can certainly serve in this application. As for construction, braid is better than plait because of stretch (3 strand need not apply).

Ultimately having a Jordan series mini-drogue device is more important than the question of which line is best. Plait will do if designed correctly. OTOH I have never had a need to deploy one so first consider where and when you will be cruising and determine if there really is a need.

Greg

[estarzinger]

Just to make a practical/real world comment . . . If you follow Jordan's sizing recommendations . . . You are sizing for the "less than once in a lifetime loading" case (with some safety factor built in) . . . You are only going to get that wave once in a storm . . . . Usually the loads will be much less, by a factor of 4 or 5 if my experience in generalizable . . . . So, #1 I can't see fateigue being much of a problem and #2 pretty much any material is going to work, just some a bit cheaper and others a bit easier to handle.

The "real world" failures are with chafe, so I would suggest that's the primary performance factor and installation issue to focus on.

[Orchidius]

I thought I'd share ace sailmakers's reply to this question as they're the official producers of the JDS;

They state they have used various different ropes and combinations with great succes. Dyneema and dyneema with polyester sleeve too. They also used dyneema as the front section with double braid nylon as the last section. Also popular according to them is the use of dyneema as the bridle legs as it allows for much smaller and heavier duty closed thimbles.

Seems like it's very comparable to the anchor/anchor rode question...