Drogue Chain Plates Strength?

thinwater

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Obviously rounded.

Bending in the horizontal "plane" is a consideration. If the angle is less than 30 degrees, it won't if the overhang is short. This is one advantage of the wider plate. But you would need to calculate for each case. I would try to make the plate wide and thick enough that it would not bend, since that is the simplest option (bending plate that thick is not easy, and there is no "known" angle.

stress = WLh/8I, where I = bh^3/12. You can Google it.

If you lose one bridle leg, I wonder if you will just cut the JSD loose, since the boat will now be at a bad angle. It's a good question. But it's probably easier to over engnieer the bridle so that cannot happen than to engnieer the plate for that case. The boat will also be taking a horrible beating it probably cannot endure. Make the bridle strong and chafe-proof. Dyneema with chafe covers comes to mind.
 

thinwater

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Note that the bridle and drogue are not straight, suggesting rather light tension in the trough/back slope. It is also common for the boat to get a little angled like this in the trough.

Yes, there is upwards pull, but probably only a few hundreed poinds at this instant. The rode has been lifted by the wave, but the bulk of the drogue is in deeper water, behind the wave.
drogue upwards angle.jpg
 

differentroads

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Good reading.

I'd bet folding money that the thimble cut the eye, rather than the splice pulling out, because I have seen that on JSD bridles.
View attachment 109731
I just dragged my bridle out to have a look. Open thimbles with sharp cutty edges. Looks like I'll be replacing them.
What's people's thoughts about chainplate to bridle attachment? 16mm octoplait in my case. Shackle, closed thimble and splice? Spliced eye and cowhitch to a shackle? Any other options?
 

differentroads

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Although some of the contributers to this and the other similar thread can be perceived to be overthinking the challenge, I for one appreciate th
What strikes me the most about many of these discussions is a distinct lack of a sense of reality.

JSD states that the common loads on gear from his drogue are 10% of the ultimate maximum load while encountering a "survival" wave. The average storm rarely lasts more than five days; less than three of which will likely ever be under survival conditions. I doubt very much that we need to be concerned about stress fatigue.

The concern whether the OP's chainplates might bend under extreme loading, is about as relevant as worrying about whether you might be able to deflate the life raft and restore it to it's container after having had to use it after a sinking.
Well, yeah, there might be some overthinking going on. But I for one appreciate the time and brainpower that people like Neeves and Thinwater put into this and other subjects. I learn loads, some of which I put into action (I definitely will sort out the shackle and thimble weakness in my system), some of which I'd like to but can't or won't (today I checked the grp layup at my drogue chainplates -8mm hull layup thickening to 12mm before the transom turn, plus 2mm-ish of extra biaxial and some epoxy thickened with silica to bed a 50mm wide x 3mm thick backing plate onto, with four 10mm bolts. Should be wider and a bit thicker) and some of which I appreciate the theory but don't see the actual problem. The latter category will be the risk of the 8mm stainless strap bending or being pulled out if I get hit beam on by a wave that is big enough to do that sort of damage. My JSD either failed to keep my arse into the waves because it broke already or I've encountered a monster rogue wave. Either way, I'm in such a world of trouble that the JSD is way down the list of things to deal with.
The more regular problems appear to be, from my admittedly haphazard reading, the cones themselves chafing and shredding or the metal to warp connection chafing (eg open thimbles, windvane steering gear.)
 

Kukri

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I just dragged my bridle out to have a look. Open thimbles with sharp cutty edges. Looks like I'll be replacing them.
What's people's thoughts about chainplate to bridle attachment? 16mm octoplait in my case. Shackle, closed thimble and splice? Spliced eye and cowhitch to a shackle? Any other options?

Closed thimbles can be hard to come by.

I remember reading a paper on drogues and parachute anchors by an American oceanographer; he was quite clear that open thimbles are not up to the job, and he recommended either solid pear shaped thimbles or round thimbles, in both cases with the eye seized with a racking at the throat of the splice and with each leg of the eye seized to the thimble to stop it rotating out under heavy load.
 

thinwater

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I think it is helpful to look at experiences, since the operation of a JSD is different from a mooring or anything we might easily compare it to.

  • Cow hitch/lark's head. Yes, they weaken the rope, but is there any history of failure? I am not aware of any, and the thought is that it eliminates a number of failure modes, the most serious of which is chafe.
  • What are the histories of chafe? I've read of wear on bridle arms; chafe gear and avoiding contact. I've seen several instances of sharp thimbles causing either serious chafe/cutting, or in this case, probably failure. Sailmaker's thimbles and tube thimbles are alternatives. Also threading webbing over the eye before splicing as a chafe guard. I have done the latter; no wear, since it seems to reduce movement. Certainly, you would want to use that option only in combination with SS shackles.
  • The bridle/rode join is complicated because it is 3-way and can jerk a good bit. Jordan suggested cow hitching the lines together. I wonder if a spliced Dyneema (multiple passes, about 5 inches in diameter) would be strong, chafe-proof, and winch-friendly.
  • Shock loading probably can only occur on the bridle and bridle/rode join. Farther down the string there is too much damping.
 

Tim Good

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I just dragged my bridle out to have a look. Open thimbles with sharp cutty edges. Looks like I'll be replacing them.

I asked for hard eye thimbles and it wasn’t an issue. Just a little more $$$. I think it should be standard to be honest. In the grand scheme of the JSD cost why bother with open thimbles.

39BB4D61-FCE7-49EC-8963-D9EB446508C8.jpeg
 

thinwater

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Closed thimbles can be hard to come by....

Google "tube thimble" and "enclosed thimble". They are common in large sizes. Samson makes them up to 3 3/4" diameter rope.

920_thimble_bronze.jpg
 

thinwater

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Fatigue

Assume:
  • The design load for my boat (Jordan page) is 8000 pounds.
  • 48 hours and two storms, so 96 hours.
  • wave period 15 seconds or a total of 24,000 cycles.
  • Typical load from a non-breaking wave is about 16% of design load. The wind load only on my PDQ is on the order of 800 pounds and I have seen anchor rode tension as high as 1300 pounds with much smaller waves.
  • The fatigue limit of 316 SS is about 30% of the tensile strength (I looked it up). At 24,000 cycles it's probably more like 35%. However, that 16% figure would be a COMMON occurrence. There will also be a large number of waves at 20-40% and a few at >80%.
However, his chain plate design is about twice this figure. The bottom line is that his design is fundamentally appropriate.

(not stainless, but similar)
S-N_curves.PNG


Fiber fatigue is similar. Nylon will be challenged at that load cycle, so I would not use it, but polyester and Dyneema should be fine. If you are going to use nylon (parachute sea anchor) you need to up-size to avoid the fatigue failures that occasionally occur (folks claim it is heating, but it is mostly fatigue, followed by fibers fusing from the energy release at rupture--nylon ruptured at high load always fuses, even if only one cycle, ask rope testers).
Comparison-of-the-results-of-fatigue-tests-on-the-new-nylon-rope-with-the-best-data.png

And please point out any errors. This is just a conversation.
 

zoidberg

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It is in the public domain via Dropbox: -

JORDAN SERIES DROGUE Best Practices Guide 2.0

Thank you for that....

I'm minded that thare are numerous other factors in the mix to get right. One of them is the arrangement for securing the 'leader' end of the rode to the bridle arms. Thereare questions regarding what could be best practice, and what is clearly not. I recall that Susie Goodall was unable to be definitive regarding her equipment failure, but she seemed to think the failure was at the bridle/leader connection - or thereabouts.

Anyway, rummaging in one of my 'bins' this afternoon, I unearthed a number of these, intended for wire rope. I think they may be suitable for 12-strand Dyneema. My sources tell me that 10mm standard Dyneema is good for 13500kg, while 12mm will give 20000kg..... or rather more if the extra treatment is worth paying for.

50967222492_476ac0e408_z.jpg


Now, there are smarter examples of braced thimbles, and smarter prices, too, from the likes of Colligo Marine, Harken and others. That pair above still carries the price-label of £10....
 

Neeves

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To introduce an idea.

I'd drop the shackle and the thimble.

I'd over size the hole through which I'd attach the bridle and insert a 2 part threaded LFR (or thimble), made from stainless steel. I don't know how big (internal diameter) the thimble needs to be. In fact it will vary (of course) as the JSD will also vary with size of yacht. So its the proportions I'm not sure of. I'd attach dyneema through the LFR, tape or rope - again not sure which. I'd then attach bridle arm to the short strop that is through the LFR. If you use tape it could be a sewn loop.

The short strop proposed can be monitored for chafe but chafe should be minimal because the LFR is highly polished.. The bridle should not suffer from chafe - its a dyneema to dyneema joint.


Don't give me the excuse you cannot get the LFRs - I am using them on my anchor bridle plate. The LFRs can be any size, the 2 unknowns are the internal diameter and how long they need to be. Aluminium 2 part LFRs can be bought off the shelf but mine were specially made to suit the size of the hole in the plate itself.

I run my snubbers through the 2 part thread LFRs (316) and the bridle plate - 2205 duplex. I did do this in 7075 aluminium alloy and a 6xxx for the LFRs but went off the idea - mainly because the anodising on the LFTs wears and I was not sure of longevity in an anchoring environment. My stainless LFRs are a bit long and I'm using Sikaflex to pack out, on a bridle plate (chain hook in mono speak). The Sika is standing up well - but I could make the length of the LFRs shorter so they screw tight. I'm using my bridle plate with climbing rope, kermantle. In fact I'm typing this whilst using the plate at anchor.

If you want to see my bridle plate send me an email address and I send a pdf. I don't know how to post a pdf here

Jonathan
 

rogerthebodger

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I think that will be across the stern, rather than away from it, unless you lose a bridle leg to chafe.

I'm talking as a mechanical design Engineer where loads and stress analysis is a major consideration.

I would put a bend on the chain plate such that the pull when the bridle is balanced with equal angle so any divination from that as there will be in either direction will put the least bending load on the chain plate.

I don't know much about Jorden drogues but I do know strength of materials and loading.

One issue is the bridle a fixed leg length of is there a block that will move the load as the boat veers side to side

This is the approach I proposed early. Joanne Socerates middle picture with oval hole for a bow shackle maybe a downward bent so as in better line as thinwaters diagram

p.png
 
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Tim Good

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The two legs of the bridle are of equal and fixed lengths, because you want the pull of the series drogue to pull the stern back into line with the seas if the boat is knocked askew by a sea, so you want all the pull of the drogue to come on the far corner of the transom.

Absolutely. Pulling it back in line is really important. When I used mine I expected it to veer off a little, but the stern was kept really very straight. Perhaps this is down to my hull / keel or transom shape I dunno. But it really didn’t beer off anywhere near being worrying in terms of causing a pull in the chainplates any more than maybe 30 degrees.
 

Yellow Ballad

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Surprised to see Jeanne Socrates bolts so close together (I'm assuming her hull is GRP) certainly closer than 120mm.

I like the thought of a wide diamond shaped plate however on a smaller (Jester size) boat I fear the hull curvature both along and down the hull won't allow this. Possible a Y shape but then again for such a light boat a single strap seems to suffice?

I'm sold on the oval hole for the bow to sit through like an anchor shank.
 

differentroads

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To introduce an idea.

I'd drop the shackle and the thimble.

I'd over size the hole through which I'd attach the bridle and insert a 2 part threaded LFR (or thimble), made from stainless steel. I don't know how big (internal diameter) the thimble needs to be. In fact it will vary (of course) as the JSD will also vary with size of yacht. So its the proportions I'm not sure of. I'd attach dyneema through the LFR, tape or rope - again not sure which. I'd then attach bridle arm to the short strop that is through the LFR. If you use tape it could be a sewn loop.

The short strop proposed can be monitored for chafe but chafe should be minimal because the LFR is highly polished.. The bridle should not suffer from chafe - its a dyneema to dyneema joint.


Don't give me the excuse you cannot get the LFRs - I am using them on my anchor bridle plate. The LFRs can be any size, the 2 unknowns are the internal diameter and how long they need to be. Aluminium 2 part LFRs can be bought off the shelf but mine were specially made to suit the size of the hole in the plate itself.

I run my snubbers through the 2 part thread LFRs (316) and the bridle plate - 2205 duplex. I did do this in 7075 aluminium alloy and a 6xxx for the LFRs but went off the idea - mainly because the anodising on the LFTs wears and I was not sure of longevity in an anchoring environment. My stainless LFRs are a bit long and I'm using Sikaflex to pack out, on a bridle plate (chain hook in mono speak). The Sika is standing up well - but I could make the length of the LFRs shorter so they screw tight. I'm using my bridle plate with climbing rope, kermantle. In fact I'm typing this whilst using the plate at anchor.

If you want to see my bridle plate send me an email address and I send a pdf. I don't know how to post a pdf here

Jonathan
These sort of screw together LFRs to make the chainplate hole smooth enough to attach the bridle to directly? I didn't even know they existed ?
How wide would a strap or diamond shaped chainplate have to be to accomodate one to thread a polyester bridle though like my 16mm Octoplait? 58mm outside diameter for a 23mm inside diameter from what I could find, with a 35mm hole in the chainplate. That would give 23mm of chainplate around the hole. I'm guessing that would be substantial enough.
VMG low friction ring to screw on - Boutique en ligne Ino-Rope
A thinner dyneema bridle or strop would be ok with a smaller LFR. 35mm OD with a 24mm hole.

Gets rid of the chafe point of a thimble and the risk of a shackle jamming in the hole. But I'm not sure how one would attach the bridle or strop in the LFR. Cowhitch? Spliced or sewn loop? These rings are low friction but not no friction, so isn't any fabric moving in them around a 180° bend going the risk chafe?

And they are designed for going through bulkheads - 18mm minimum in the case of the larger one on the VMG site - so are much wider than the chainplate and there'll be a need to pad them out to fit snugly.

Its academic to me but interesting nontheless. And I'll be able to use the knowledge gained some way, one day, I'm sure
 

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