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JSD Chainplates - how many holes and where (stress cracking)

differentroads

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Joined
16 Apr 2012
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387
Location
Mediterranean
I might move the focus, where all the attention has been so far, to the plates themselves.

No mention has been made as to what they will be made from and it is not clear to me how far 'aft' beyond the hull and the discussed reinforcement the plates will extend.

But I would not skimp and would not use 316 but a duplex grade. At some point the tension will not be in straight line and all the tension, or most of it will be on one plate. I'd be worried it would bend (if it were 316). The weak point is the last or last two bolt holes.

Stainless anchor shanks, or good anchors, are made from duplex.

I'd also check the fit of the shackles in the proposed 'oval' holes. The edges of the holes need to be bevelled (or rounded - I don't know the term) so that you reduce the opportunity for the shackles to lock up in the slot (it happens with anchors it can happen with a chain plate) - and some times when it happens the shackle fails. If necessary I'd take any old piece of steel of the same thickness as proposed) and I'd cut the desired oval holes and actually try the actual shackle in the mock up of the holes - you will, maybe, be surprised at how easy they will lock and then the shackle is side loaded. Shackles loaded at 45 degrees lose 25% or strength (and 50% loss at 90 degrees) and I agree - I've tested it. The shackle holes might be larger than you show on your drawing (you don't mention size of shackle) and the 'least' bit of steel is at the shackle hole. So I'd work out how big the shackle holes need to be and that will determine the amount of steel for both the width and thickness of the plate.

I would pre-bend the chain plate to the appropriate angle to fit the dimensions of the bridle of the drogue.

The plate fits on the side of the transom, obviously, and I might consider welding an end plate, where the plate would be bent as it 'clears' the transom and weld a further plate at right angles to the top of the chain plate to give it more support - if that makes sense. You need a decent welder to deal with duplex.

Jonathan
Useful input that has made me look again at the straps on my own boat. They protrude just 80mm from the last of four bolts so have nothing like the lever arm of an anchor shank. I'm pretty sure 8mm 316 will be fine. But the side loading onto the shackle does look like a potential issue, though if you get beam on to the sort of seas that see you having a JSD behind you, it'll be the least of your problems!

I'm going to have a think about better articulation there - perhaps a C-link and a few links of 10mm chain or a steel hoop welding onto the strap or even an eyebolt.
 

differentroads

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16 Apr 2012
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387
Location
Mediterranean
Yes reinforcing the transom is on the radar, possibly tieing the plates via the backing pad to it as well. I know Roger Fitzgerald strengthened his as he was worried about pinching as the load came on. I have planned on adding g10 backing around the chainplates and the bilge pump outlets anyway. I could have some sort of bar across on the inside between the plates but I have to remind myself that there are a lot of boats out there that use drogues without issues with the hull falling apart. (Susie Goodall's drogue broke before the chainplate ripped out)

Although I went ott on bolts (and have since been corrected) I don't want to over complicate things though, pretty much all plates I've seen are 316 and I've not heard of any issues. The way I'm looking at them is it's a once in a lifetime event and if they keep me alive to finish the crossing then can be replaced/repaired once getting to port.

I do agree having a bend so it matches the bridle angle is a smart thing and somethingI'll be checking.
How many cones do you have on your JSD? Thats pretty critical to how strong the strap, bolts, backing plate and any reinforcement/load spreading needs to be.
I have 104 cones and took strain figures for that off, I think, Jordan's website. 2.7T if I remember right, all of which might come onto one fitting. He says his design recommendations are conservative and I certainly concur.
 

Yellow Ballad

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10 Oct 2013
Messages
1,428
Location
Shropshire - Sundance, Bristol Channel
DR I've not bought a JSD yet but I imagine it'll be similar to yours. Ballads weight is down as 3.5t but I would estimate loaded up for a long passage probably closer to 5t.

Mk2 plates.

60mm opposed to 75mm but longer, 625mm to accommodate the 4th bolt as I'm sure you can tell I'm a belt and braces sorta guy.

FB_IMG_1613596206512.jpg
 

Neeves

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20 Nov 2011
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7,392
Location
Sydney, Australia.
Useful input that has made me look again at the straps on my own boat. They protrude just 80mm from the last of four bolts so have nothing like the lever arm of an anchor shank. I'm pretty sure 8mm 316 will be fine. But the side loading onto the shackle does look like a potential issue, though if you get beam on to the sort of seas that see you having a JSD behind you, it'll be the least of your problems!

I'm going to have a think about better articulation there - perhaps a C-link and a few links of 10mm chain or a steel hoop welding onto the strap or even an eyebolt.
Just to put the maths into context.

Most anchors of the same weight have a similar shank length and width. The steel plate from which they are made is also similar, for a 15kg anchor around 15mm. You can check your own anchor for length and width of steel. But these dimension will give an idea of their lever arm characteristics.

Historically those made from a mild steel, say 350MPa, have tended to bend (think of Rocna and Mantus). When they used 800 MPa steel bending has not been reported. 316 stainless has a very low yield point. You can compare the lever arm length and decide what sort of stainless to use. The maximum tension on an anchor rode, for a 15kg anchor might be around 650kg. The shank of the hollow Ultra was 316 stainless, they now use Duplex and have installed a brace down the inside of the shank.

It seems a strange item for which the best material is shunned, to save very little - but then I have not done the maths.

Jonathan
 

differentroads

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16 Apr 2012
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387
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Mediterranean
My 10m boat is a heavy old girl of 7T displacement. So I tried to design the straps, bridle and drogue around the Don Jordan spec on About Jordan's Series Drogue
for a boat of 15,000lbs displacement. He says each leg of the bridle should be capable of withstanding, in my case, 70% of 10,000lbs, so 3.5T (I didn't remember right) ''ultimate, once in a lifetime peak transient load'' from a breaking wave that will be 10 times more than the working load during a severe storm.
For your boats displacement Jordan says each leg of the bridle should be designed around 70% of 8,000lbs, so 2.5T. From what you said here and previously, your strap and four 10mm bolts will be as strong as mine. So I'm going to say the metalwork is strong enough 😄 Neeves and Thinwater are far more qualified than I though.

I'm persuaded that the strength of the metalwork has to be matched with the strength of the topsides. The load in the most severe situations will be on the bolts in shear, so thickness of the grp and/or its bonding to other materials is critical. Your plans on that score sound right but I'm not a materials engineer.
It could be that the attachment ought to be able to withstand the same load on the beam trying to bend the strap and pull the bolts out. But imagine what a ''once in a lifetime'' wave in a violent storm is going to do to your boat if you get beam on to. A bent or pulled out chainplate is going to be the least of your worries as you get rolled, your cabin windows get stove in, the mast gets snapped and is trying to batter in through the topsides. But thats why we have JSDs, to keep our arses safely to the waves 🤣
 

differentroads

Active member
Joined
16 Apr 2012
Messages
387
Location
Mediterranean
Just to put the maths into context.

Most anchors of the same weight have a similar shank length and width. The steel plate from which they are made is also similar, for a 15kg anchor around 15mm. You can check your own anchor for length and width of steel. But these dimension will give an idea of their lever arm characteristics.

Historically those made from a mild steel, say 350MPa, have tended to bend (think of Rocna and Mantus). When they used 800 MPa steel bending has not been reported. 316 stainless has a very low yield point. You can compare the lever arm length and decide what sort of stainless to use. The maximum tension on an anchor rode, for a 15kg anchor might be around 650kg. The shank of the hollow Ultra was 316 stainless, they now use Duplex and have installed a brace down the inside of the shank.

It seems a strange item for which the best material is shunned, to save very little - but then I have not done the maths.

Jonathan
Good points well made. For me, it came down to familiarity with 316 and its availability. I'm not sure that I even knew about duplex when making my drogue and attachments three years ago. I wouldn't know if I can drill ten 10mm holes in it with by battery drill on a floating pontoon as I did with my 8mm 316 stainless (very slowly, going through four half decent bits and lots of cutting fluid)
Actually, many more holes cos I used the same steel strap to strengthen my inner forestay fitting, beefed up the backing pads for three foredeck cleats, the chainstopper and the bow roller. I got through a lot of drill bits that spring. My £100 Bosch battery drill handled it all and is still going strong.
 

Neeves

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20 Nov 2011
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Sydney, Australia.
316 as backing plates seems very reasonable to me. Its the overhang, the extension aft of the transom to which the shackle is attached with the big hole for the shackle eye, the shackle lying along the length of the chain plate and bearing on the end.......

There is an assumption the maths is correct - and its a good place to start. But I will guarantee the maths will omit some facet, some element of bad luck - its a yacht, things go wrong.

The reality is that few people have used a JSD in anger and no-one has had the nouse to carry a waterproof load cell and those that might have had one had better things to do (at 55 knots) than take actual load measurements. I can think of tether hooks that have failed (and I am sure the maths was right), I can think of keels that have fallen off on a number of different yachts (and am equally sure the maths was right).

In theory it is said that theory and practice are the same. In practice they are often different - because we don't know all the data.

If a duplex chain plate increases your safety factor by 2 (more likely 3) go for it. In the grand scheme of things they don't cost much - and it will be one less thing to think about. Why bother to buy Crosby shackles (that are the best money can buy) if you don't match the whole system. The shackles will have a safety factor of 5:1 - I wonder what the safety factor is for the chain plates.


I said earlier that 55 knots was awful - I mis-used the English. There is very little awe involved. I should have used the word terrible, not awful.

And 55 knots is not really exceptional - or it can be worse.





Jonathan
 

Bajansailor

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27 Dec 2004
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5,592
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Marine Surveyor in Barbados
I said earlier that 55 knots was awful - I mis-used the English. There is very little awe involved. I should have used the word terrible, not awful.
I would agree absolutely - I have been in 55 knots (June 2012) halfway between the Azores and Plymouth, with 9 metre high waves (albeit long wave length - we were not yet on soundings) behind us, and it was not much fun.
There was another occasion (November 1994) off the coast of Portugal heading south to Porto Santo when we were hit by a violent squall and the wind speed meter was pegged at 64 knots - that was not much fun either.
 
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