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Drogue Chain Plates Strength?

northcave

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Take a look at these chain plates in the picture below designed for a Jordon Series Drogue. Mr don Jordon said that you should ideally design each of your chain plates to hold half the boats displacement. In my case this would be 18/2 = 9 Tonnes.

Facts:
- My boat displaces 18T
- Hull top sides are solid GRP 10-12mm
- Current chain plates are 30cm long, 10mm thick and would allow approx 4 large M10/12 bolt holes to be drilled.
- Everything else in the system such as bridal, bow shackles and JSD are all rated to 9T +

The question I have is, would a chain plate of say 60cm with 8 larger bolts be twice reistant to tearing out of the hull as a chainplate of 30cm with 4 bolts?

The reason for asking is that I am trying to devise a way of ensuring my chain plates are up to the job and making them longer than my existing 30cm with more bolts, would be an easy solution as opposed to something like building up the inside with epoxy etc.



 
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60mm wide flat would be twice as strong in tension than 30mm wide for the same thickness.

I would use at least 6mm thick but I would also have a backing plate inside with both the inside sealed with sikaflex or similar.

If the tear out load that I would be concerned about .
I would also use hex head bolts with locknuts inside.

I have several 10mm U bolts bolted to my steel transom or I can use the chain plates of my twin back stays.
 

northcave

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It is the tear out load that I would be concerned about .
This is the question really. I am not concerned about the strength of the stainless chain plate.

If I made the chain plate twice as long with twice as many bolts, would this significantly increase the tear out resistance?
 

pvb

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This is the question really. I am not concerned about the strength of the stainless chain plate.

If I made the chain plate twice as long with twice as many bolts, would this significantly increase the tear out resistance?
You need to consider the weakest part of the whole set-up. If the drogue is shackled to the chainplate, as one pic shows, the weakest part is probably the shackle pin - what's the shear strength of that?
 

northcave

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You need to consider the weakest part of the whole set-up. If the drogue is shackled to the chainplate, as one pic shows, the weakest part is probably the shackle pin - what's the shear strength of that?
9T working load. Crosby. I'm really just trying to focus on the strength of the hull at the moment.
 

thinwater

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Give us the chain plate thickness anyway. Also I'm not clear on whether one of those boats is yours. You say there could be 4 bolts, but I only see 3 and the look longer than 30 CM in the first pic. Then you mention 45cm. I'm confused.

I've been doing a bunch of testing for an up-coming Practical Sailor article, so I have some very educated guesses.

First Pic. Good design. In that laminate, unless there is a lot of mat, 10mm holes well spaced should hold >5000 pounds each in shear. That gives you 15000 pounds, very close to what you need. With good laminate and a solid backing plate at least 2 inches wide (not just a fender washer--they do not count for anything), the strength is probably greater, about the 18,000 pounds you need. In other words, whoever installed them did their homework. You're good. As for the second picture, the bolts are far too close together and are probably only 60% strength because of it. Further, if one starts to crack, they will rip like a zipper, leaving a hole in the boat about 2-3 inches wide. If you do change for a larger plate, keep the holes 6" apart, minimum, or they will affect each other at peak load.

Some other things to consider:
* The strap has to be strong enough to withstand off-direction loading.
* The first bolt could see as much as 30% of the load; off-axis loads don't transfer well, unlike chain plates. The force could also be outwards. So use a huge backing plate, larger than the chainplate. A one-piece backing plate also helps distribute load.

30cm is really a little too short. I would go with 15 cm spacing plus 5cm for ends and 10cm for the eye = 45 cm. If I were going from scratch, I'd use 4 holes at about 130 cm and be very happy. 60cm will do that.

By the way, the likelihood of those loads is ridiculously low. To reach 20,000 pounds with a 90-cone drogue would require you to be going 30 knots, using Jordan's numbers. But this is a design basis, for working loads 5x lower (about 14 knots), which are believable.
 
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northcave

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This may not be feasible but if you could get the chainplate bolted through the hull/deck joint, it ought to be bullet-proof
Ok it is interesting you say that. I had considered bolting the chain plate through the iroko rails. My thinking is that it is up at the hull deck join, plus I'd benefit somewhat from the additional 15mm of Iroko maybe. Thats a totally guess to be fair.

 

pvb

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9T working load. Crosby. I'm really just trying to focus on the strength of the hull at the moment.
A 9t Crosby shackle will have a pin more an an inch in diameter, rather chunkier than the shackle in your pic! And too big to go through the hole in the chain plate you've linked to.
 
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thinwater

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Ok it is interesting you say that. I had considered bolting the chain plate through the iroko rails. My thinking is that it is up at the hull deck join, plus I'd benefit somewhat from the additional 15mm of Iroko maybe. Thats a totally guess to be fair.
I would NOT do that. The wood will move, is soft, and will weaken the bolts in shear. The shorter the bolts, from plate to nut, the stronger they will be.

Grade 2, about 5000 pounds for 10mm and and 7500 pounds for 12mm. 4 will be enough. Remember that going up in bolt size will not help; the FRP will fail at about 1 bolt diameter.

Jordan has a more conservative design, but remember he said it was conservative and he did not know the FRP lay-up.
 

Neeves

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A 3/8" G209 A Grade B Crosby shackle with a 7/16th" pin has a Min Break Strength of 9t. Note they also sell a G209 and I think its strength is 4.5t. Peerless sell the same sized shackles with a MBS of 12t.

If you want the better shackles ensure they are Grade B, which are twoce as strong as Grade A (Grade A are common in Eiurope, Grade B are like hens teeth)

I might be inclined to have a bigger hole in the chain plate so that the hole can take the eye of the shackles and then have the pin through the spliced eye. You will have much better articulation. This might mean a bigger plate - but I do not see having a bigger plate being disadvantageous. As Thinwater advises the limit is not the bolts nor stainless (you can go bigger and stronger/better) but the GRP. You might need to think of adding a big patch of glass inside.

I have not been following the drogue/para anchor threads but as Thinwater suggests you need to think the load might not be evenly balanced and then the loads might be, much, higher on one leg. Again its the glass that is the unknown, for us.

Jonathan
 

northcave

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A 9t Crosby shackle will have a pin more an an inch in diameter, rather chunkier than the shackle in your pic! And too big to go through the hole in the chain plate you've linked to.
Sorry I was wrong it is a G-2140 with WWL 7T and a 4.5x factor. Anyway there are a lot of threads debating shackle strengths so I'd really like to try and keep this thread on the topic of GRP strength in relation to the chain plate.
 

northcave

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You might need to think of adding a big patch of glass inside.
Yes this is dilemma and I wanted to avoid getting into the confines of the lazarette and layering up the GRP needlessly. Or if it was deemed necessary whether extending the length of the plate would do the same job.
 

Scotty_Tradewind

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With no practical knowledge of the Jorden or reading about it in advance or reading the postings below the OP,
it made me think hard as to what I would do if I didn't have research resources at all to help and if I hadn't been influenced by other postings.

My first thoughts, in very broad terms, with no material calculations at all..............
The longer the plates are along the boat, the more high tensile bolts you can put in with backing plates inside
the better.
The thicker the plates inside and out, the harder you can pull the sandwich of plates and hull
together with bolts increasing the 'bonded' strength of the three materials.

You have to support tons of weight but the snatch load could be extremely high.

The weakest link(s) is/are the more important part(s) to concentrate on and the nearer the boat you get with a

Jorden the more strength is required. Therefore the drogues at the furthest part away from the boat need less

strength in their webbing etc.. Any shackles boat end need to be stronger than at the far end,

Some GRP hulls are thicker in that area you show than others, so with the weight of your boat being challenged by high seas and gales, with the risk of very powerful 'tugs' and swings of the boat, you have to go stronger than risk weaker. Could the side of the boat be pulled away if a tug came from the side? Could a triangle of construction be made at the boat end to bear this side force or would internal plates have to be massive?

I wouldn't under estimate the strength of a sandwiched GRP+Iroko+steel plating each side. It would depend upon the quality of the iroko as to whether the sandwich could be pulled up tight enough and the steel thick enough so as not to buckle with the force applied to the bolts,

When it leaves the side plates what quality of materials is needed to go into the attachment of the drogue.

Is it a bridle? Is it of strapping, chain, rope? How could I arrange such a system so as to keep the shock out of the tugs?
A spring arrangement? Auto shockers?

I would need this to stay out of the way of any other equipment on the stern or the pushpit, otherwise that could get ripped off.

Pictures in your postings........ There looks to be a weakness in the ends of the plates where the drogue lines/bridle is attached. Any sideways force in that area needs a huge amount of strength to cope. With an over hanging plate can that be strong enough as shown?

At the very worst scenario, your boat is in Storm type conditions.
Plan for that and make it as resilient as you can and probably accept that in those conditions all the other gubbins hanging around the stern is sacrificial!

Do you really want to fit this thing?

Now I'll read the other posts.
S.

p.s. why?
 
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jwilson

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he two setups you show (assuming a strong enough GRP hull with some internal reinforcing) look pretty strong for a straight-aft pull. They look vulnerable though to a side-pull. Typically forestay fittings are not just fastened to the deck, but run down the bow a bit as well. If I was building this I'd look at an angled piece round onto the transom with another throughbolt and possibly using a hefty u-bolt on the transom bit as the attachment point.

I have no personal experience of Jordan drogues, but have experienced very bad weather, and have a healthy respect for the forces and damage seawater can inflict.
 

Graham_Wright

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I have no experience to back me up but all this fills me with horror.

I believe I have read that towing lines, whether to take forward or backward pulls should have their load distributed as widely as possible. Bolts in a line sound like a recipe for disaster. If one fails, the rest would surely follow.

My own solution would be to rig a bridle starting at my samson post (which passes through the grp deck reinforced with 4" of ply and then down through the keel with a stainless plate below) round both bow cleats, round four midships cleats and round both stern cleats. The bridle would be nylon.

Hopefully, this distribution of load would stop bits breaking off.

The same solution I would adopt whether towing or being towed (the latter is what the RNLI advise).
 

afterpegassus

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[QUOTE
My own solution would be to rig a bridle starting at my samson post (which passes through the grp deck reinforced with 4" of ply and then down through the keel with a stainless plate below) round both bow cleats, round four midships cleats and round both stern cleats. The bridle would be nylon.[/QUOTE]

and just how long, in the sort of weather where you are going to need this, are you going to spend on deck rigging all this to ensure proper load sharing of all these points? That prospect is a real horror.
I will trust the horizontal chain plates I fitted (500x50x6) on my own wee boatie and have the drogue attached in a couple of minutes.
 
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