Neeves
Well-known member
Roger you are too knowledgeable for me 
Might burrs fit on angle grinders?
Jonathan
Might burrs fit on angle grinders?
Jonathan
Drogue Chain Plates Strength?
- Thread starter Tim Good
- Start date
Neeves
Well-known member
I think angling the chain plate to the theoretical direction of pull is sensible but as the pull angle will vary, it might be higher (or lower) - not such an issue with a vertical plate but it might also be a widely different angle in the horizontal (and the yacht rolling) so to have the backing plate (and chain plate) taken round to the transom itself and deck with fillets for strength seems eminently sensible.
If the chain plate is angled and has flanges and fillets attached then care needs to be taken to ensure the shackle can articulate and nothing impinges on the bridle itself.
It has been pointed out to me there are no reports of chain plate failure - this might all be a step too far, belt and braces to the extreme.
Jonathan
Kukri
Well-known member
We had a good look at Tony Curphey’s “chain plates” on “Nicola Deux” at SIBS and mentally scaled them from Nic 32 to Nic 55. Pretty massive! It’s also fair to say that Tony has built a whole system to suit deploying a JSD, including a very different companionway hatch, and a reduction in the size of his cockpit.
Now, thinking about lateral pull on the chain plates, I think that this is most likely to arise when the boat is caught by a breaking sea and thrown down the face. Just the conditions where you would be knocked down if lying a-hull, in fact, and the JSD is intended to stop it happening, but all sorts of funny things happen at sea.
I think that when a lateral pull does come on the chain plates, it will tend to be towards the centreline of the boat, rather than away from the centreline, because the arm of the bridle nearest the load will go slack. If I’m right, the lateral load condition will not put a huge tension load on the bolts holding the chain plates.
Have I missed something?
Now, thinking about lateral pull on the chain plates, I think that this is most likely to arise when the boat is caught by a breaking sea and thrown down the face. Just the conditions where you would be knocked down if lying a-hull, in fact, and the JSD is intended to stop it happening, but all sorts of funny things happen at sea.
I think that when a lateral pull does come on the chain plates, it will tend to be towards the centreline of the boat, rather than away from the centreline, because the arm of the bridle nearest the load will go slack. If I’m right, the lateral load condition will not put a huge tension load on the bolts holding the chain plates.
Have I missed something?
Neeves
Well-known member
If there is too large an overhang and the plates are a strap made from 316 - the strap may bend.
'Funny things happen at sea' ( maybe not funny things) is a very pertinent comment.
The yacht is not going to simply lie vertically - it is going to roll. The chain plates need to take the tension from any angle and with the shackle locked in its slot and extending the length of the overhang - and then there is different tension put on the bolts. If the strap bends toward the centre line then later that bend will, presumably try to straighten out.
Jonathan
'Funny things happen at sea' ( maybe not funny things) is a very pertinent comment.
The yacht is not going to simply lie vertically - it is going to roll. The chain plates need to take the tension from any angle and with the shackle locked in its slot and extending the length of the overhang - and then there is different tension put on the bolts. If the strap bends toward the centre line then later that bend will, presumably try to straighten out.
Jonathan
zoidberg
Well-known member
As did I.... and discussed his thoughts about JSDs with him. He'd used his rather a lot. It was 'ragged at the edges' and he acknowledged he was due to change/upgrade his cones for something more robust, during his refit. I gathered from him, and from Roger Taylor, the idea that the smaller your boat the more you need to reduce the volume of your cockpit.... so that heavy seas which leap on board are very swiftly thrown overboard again.
Neeves
Well-known member
You also need to ensure that the sea that fills the cockpit cannot run into the cabin. Keeping the washboards in place and climbing in through the hatch is one way to minimise the risk. If you have an open transom then having a net is also useful (otherwise your shoes, or one of them, goes - never to be seen again - and one shoe is not much use by itself ).
I was also looking at shackles - my experience with shackles is a bit limited to the bottom end of the market (or smaller applications). Suddenly we are talking about shackles that are going to be huge if they are to accept the tensions implied. My 3/8th" shackles are but toys (WLL of 2t). We are now talking about 1/2" (as still being small) and more along the lines of 5/8th" or 3/4". These shackles are, too me, huge - and are seriously going to impact the 'lever arm' effect. A Crosby G209a 3/4" shackle has a WLL of 7t (3.5t if loaded at 90 degrees) and if loaded at 90 degrees its going to be locked into the chain plate - and the shackle is 5" long (they are made by an American coy ). Your over hang is now large. I suspect a 3/4" shackle is a toy in this application for Kukri?
Shackles might need a re-think - there must be something else - or there are, plural. Dyneema comes to mind, not restricted to rope but also tape..........? a longish tape loop cow hitched onto the chain plate.... Harking back to the lifting industry - they use sewn loops to make dyneema 'chains' for lifting - so sewing loops is quite acceptable.
Synthetic Link Chains
Jonathan
).I was also looking at shackles - my experience with shackles is a bit limited to the bottom end of the market (or smaller applications). Suddenly we are talking about shackles that are going to be huge if they are to accept the tensions implied. My 3/8th" shackles are but toys (WLL of 2t). We are now talking about 1/2" (as still being small) and more along the lines of 5/8th" or 3/4". These shackles are, too me, huge - and are seriously going to impact the 'lever arm' effect. A Crosby G209a 3/4" shackle has a WLL of 7t (3.5t if loaded at 90 degrees) and if loaded at 90 degrees its going to be locked into the chain plate - and the shackle is 5" long (they are made by an American coy
). Your over hang is now large. I suspect a 3/4" shackle is a toy in this application for Kukri?Shackles might need a re-think - there must be something else - or there are, plural. Dyneema comes to mind, not restricted to rope but also tape..........? a longish tape loop cow hitched onto the chain plate.... Harking back to the lifting industry - they use sewn loops to make dyneema 'chains' for lifting - so sewing loops is quite acceptable.
Synthetic Link Chains
Jonathan
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Laminar Flow
Well-known member
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.
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.
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Neeves
Well-known member
Laminar Flow.
I think you are correct - in fact the chance of many of the JSD's in existence ever being used in anger is, I hope, very low. Our opportunity thus to gain insightful knowledge is limited even though there might be many JSDs on yachts.
However I am reminded that Susie Goodall was equipped with a JSD that had, I hope, a considerable amount of skilled and knowledgeable input. It was recent and should have enjoyed the benefit of decades of sensible input. Yet - she might not have been far off losing her yacht and her life. Consequently if I can raise questions about specific parts of the design that might merit a second look I will do so - even though the chances of failure are not high. As has been said, referring to chain plates - no-one has complained about bent chain plates. But sticking with the chain plate theme - the cost of removing a chain plate failure, just one failure, by upgrading from 316 stainless ro duplex is peanuts - so I find it very difficult to understand why it might not be a consideration. The chances of you locking up a shackle in a chain plate (or anchor shank) is not high - yet routinely I (and many others) recommend the use of the best shackles money can buy. I know people who did not follow this advise (on buying the best shackles money can buy) and I have a retrieved shackle - that failed - and caused the loss of a A$1,000 anchor.
Duplex stainless cost about 2 times that of the same size/weight of 316. Frankly, whether I need it or not, I think my life is worth investing that little bit extra.
But then no-one complained about the tether hooks used on a Clipper yacht - and when tested - were found wanting (but only after a man lost his life).
It took Susie Goodall to raise some of the questions - do we need to wait for another failure. I would rather the questions are asked before someone dies, or loses their yacht. I appreciate that being sufficiently prescient to know a hook or a JSD is going to fail requires considerable skill - So I'm terribly supportive of thrashing it about here and never having failures as I for one lack the necessary skills of prescience. I'm also very supportive of those who sit down, try to simulate possible scenarios and develop data - this has not been done as far as I am aware for a JSD.
Now - if you are willing to guarantee the integrity of the designs of JSD chain plate that have been aired on this thread and other threads - I'd like to see the data.
Jonathan
I think you are correct - in fact the chance of many of the JSD's in existence ever being used in anger is, I hope, very low. Our opportunity thus to gain insightful knowledge is limited even though there might be many JSDs on yachts.
However I am reminded that Susie Goodall was equipped with a JSD that had, I hope, a considerable amount of skilled and knowledgeable input. It was recent and should have enjoyed the benefit of decades of sensible input. Yet - she might not have been far off losing her yacht and her life. Consequently if I can raise questions about specific parts of the design that might merit a second look I will do so - even though the chances of failure are not high. As has been said, referring to chain plates - no-one has complained about bent chain plates. But sticking with the chain plate theme - the cost of removing a chain plate failure, just one failure, by upgrading from 316 stainless ro duplex is peanuts - so I find it very difficult to understand why it might not be a consideration. The chances of you locking up a shackle in a chain plate (or anchor shank) is not high - yet routinely I (and many others) recommend the use of the best shackles money can buy. I know people who did not follow this advise (on buying the best shackles money can buy) and I have a retrieved shackle - that failed - and caused the loss of a A$1,000 anchor.
Duplex stainless cost about 2 times that of the same size/weight of 316. Frankly, whether I need it or not, I think my life is worth investing that little bit extra.
But then no-one complained about the tether hooks used on a Clipper yacht - and when tested - were found wanting (but only after a man lost his life).
It took Susie Goodall to raise some of the questions - do we need to wait for another failure. I would rather the questions are asked before someone dies, or loses their yacht. I appreciate that being sufficiently prescient to know a hook or a JSD is going to fail requires considerable skill - So I'm terribly supportive of thrashing it about here and never having failures as I for one lack the necessary skills of prescience. I'm also very supportive of those who sit down, try to simulate possible scenarios and develop data - this has not been done as far as I am aware for a JSD.
Now - if you are willing to guarantee the integrity of the designs of JSD chain plate that have been aired on this thread and other threads - I'd like to see the data.
Jonathan
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Kukri
Well-known member
OCC members will have spotted that Steve Brown has put up an excellent best practice guide to JSD deployment and retrieval.
“Novara” is a big boat! I’ve watched Steve’s video of his visit to South Georgia which includes deployment and recovery of her JSD.
Useful Trick - powered self tailing sheet winch - in “Novara”’s case a Lewmar 55.
Sailing an AeroRig - Yachting Monthly
“Novara” is a big boat! I’ve watched Steve’s video of his visit to South Georgia which includes deployment and recovery of her JSD.
Useful Trick - powered self tailing sheet winch - in “Novara”’s case a Lewmar 55.
Sailing an AeroRig - Yachting Monthly
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zoidberg
Well-known member
I'd rather like to see that, but would probably require to be approved for membership first. And before that, I'd need to get out ocean-sailing again....
zoidberg
Well-known member
Agreed. A proper sense of proportion is warranted. As is avoiding effort in solving non-existent problems.
However, I challenge the 'urban myth' that use of a Drag Device is only to be considerd by deep-ocean wanderers. The JSD was invented to halp prevent more capsize losses of the Fastnet 79 type, and there have been numerous incidents in home waters since that justified the deployment of an anti-capsize device - had one been carried.
Similarly, that an incident is believed unlikely should not prevent us carrying kit to help deal with it. For example, liferafts, lifejackets, personal tethers, EPIRBS, etc...... While it is hard to justify a 'gold-plated' standard of expenditure on kit, there is no justification for carrying clearly inadequate safety/survival kit..... but there are plenty of boats still carrying known-inadequate tethers and UV-degraded webbing jacklines, year after year.
The kit carried MUST be up to the job, otherwise I/you are deluding ourselves - and deceiving those who come with us who rely on our judgements and integrity to keep them safe.
thinwater
Well-known member
When the boat is thrown in this way (and I think you are 100% correct about this being the likely worst case senario, because now the wave is striking the boat from an angle) it is also heeling sharply, which means the pull is downwards . The boat is also going down a steep slope, and the pull from the JSD is from inside the wave. I think 30-45 degrees is quite realistic. This concentrates 30-50% of the load on the last bolt (or two bolts if they are side by side).
Thus, the optimum shape is probably more triangular (or really and asymmetrical diamond if we consider the overhanging part), with a double row of bolts at the aft edge and more material in the plate at the transom break to deal with the odd angle forces. But I would probably just go up a size one everything, which is what Jordan did (his design is much stronger than his stated rating, and he knew that), and go with the simpler straight construction. Then I would evaluate the layup for sheer and crush strength around the transom bend. Order of magnitude, it should probably be twice the thickness of the chainplate design or 1.5 times the bolt diameter, or I would add some local reinforcement--easy to do. This is a shear load, and it will bear far more on the fiberglass than on the backing plate.
Just remember, if you are going to scale up the Jordan chain plate design, you need to scale everything up together; distribution area, bolt sizes, layup, and a little something extra for bending of the overhang (if you have double the force and double the overhang, you will have four times the bending moment).
Kukri
Well-known member
Thank you; really good points.
A steel or aluminium hull probably doesn’t need to worry about additional strengthening, because the topside to transom area is strong due to its shape.
A GRP, wood or ferro one might. And getting a backing plate in, and secured well, in what is always a confined space, isn’t easy.
So, yes, diamond shaped. Perhaps with longer sides against the topsides. Or, given a really good welder, and a means of checking the weld, an eye welded to a triangular plate?
My boat is pretty beefy in and around the counter because she was designed as an offshore racer in the masthead rig and symmetrical spinnaker era, with massive spinnaker turning blocks on the deck there. But I reckon a chat to a good laminator is indicated!
A steel or aluminium hull probably doesn’t need to worry about additional strengthening, because the topside to transom area is strong due to its shape.
A GRP, wood or ferro one might. And getting a backing plate in, and secured well, in what is always a confined space, isn’t easy.
So, yes, diamond shaped. Perhaps with longer sides against the topsides. Or, given a really good welder, and a means of checking the weld, an eye welded to a triangular plate?
My boat is pretty beefy in and around the counter because she was designed as an offshore racer in the masthead rig and symmetrical spinnaker era, with massive spinnaker turning blocks on the deck there. But I reckon a chat to a good laminator is indicated!
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rogerthebodger
Well-known member
Roger you are too knowledgeable for me
Might burrs fit on angle grinders?
Jonathan
No they will fit in drill but most drills are not fast enough. You need something like 30 000 rpm.
thinwater
Well-known member
Question. Has anyone every seen a drogue pull...
The average pull seems to be 2-3 degrees down, or about 20:1 scope.
The steepest down pull I have seen is about 20 degrees (2:1 scope), but thankfully, I have never sailed in seriously breaking waves (tips don't count) and never got sideways or hit strongly sideways. And yes, this is absolutely when it pulled the hardest.
So is down 3-20 degrees, or down 12-15 degrees the design window?
(this is a survey--I don't claim to know the answer)
- Upwards at any angle, even small?
- Horizontal (or is it always downwards)?
- Largest downward angle?
The average pull seems to be 2-3 degrees down, or about 20:1 scope.
The steepest down pull I have seen is about 20 degrees (2:1 scope), but thankfully, I have never sailed in seriously breaking waves (tips don't count) and never got sideways or hit strongly sideways. And yes, this is absolutely when it pulled the hardest.
So is down 3-20 degrees, or down 12-15 degrees the design window?
(this is a survey--I don't claim to know the answer)
zoidberg
Well-known member
I have great faith in 'engineers' - especially Scots engineers, and I'd really, REALLY like it if one of you could design for me one of those 'Beam me up, Scotty' badges I've seen on telly!
rogerthebodger
Well-known member
I have great faith in 'engineers' - especially Scots engineers, and I'd really, REALLY like it if one of you could design for me one of those 'Beam me up, Scotty' badges I've seen on telly!
Only half Scottish I think the top half.
The best beam me up is " Beam me up Scotty there is no intelligent life here.
thinwater
Well-known member
How about something like this:
Ignore the exact dimensions. and bolt locations for the moment. It is only a concept sketch. The key ideas are these:
Ignore the exact dimensions. and bolt locations for the moment. It is only a concept sketch. The key ideas are these:
- Horizontal is arbitrary and probably wrong. What is the right angle?
- Loads will be off axis, so we need more bolts near the transom, but still well spaced.
- The maximum bolt diameter is guided by the laminate thickness. There is no point in a large bolt that will just shear through.
- Minimum bolt spacing depends on the laminate type, but 10 bolt diameters is probably a bare minimum, based on reports I've read (some say 5x is acceptable with a backing plate).
- the first two rows of transom bolts should probably be able to handle 30-50% of the total design load. We cannot count on efficient transfer to the remainder of the strap, and if they fail, the strap will come off like Velcro.
- The plate needs to be wider near the transom to better support bending forces and to reduce the crushing force on the transom joint in the event of a side strike. A wider plate also reduces or eliminates the need for reinforcement of the layup. The optimum depends on the design and construction of the boat.
Kukri
Well-known member
Thank you!
I really like that! I would make the profile of the part aft of the transom rounded, just to eliminate any possibility of chafe.
I really like that! I would make the profile of the part aft of the transom rounded, just to eliminate any possibility of chafe.
rogerthebodger
Well-known member
You also need to consider the directions of pull in the other plain to reduce the bending on the chain plate.
