chainplate calculations for a jordan drogue
 

If somebody could verify my calculations for jordan drogue chainplates, it would be really great!

The boat is 30' sloop, 8600 lbs official displacement, but probably around 9,500-10,000 lbs fully loaded.

I'm thinking of the following:

1.5" x 0.25" x 12" with a 1/2" hole and 4-5 bolts. Crosby 7/16 209a shackle - the pin is 1/2". WLL ~= 6000lbs. The chainplate strength should be:

(1.5" - 1/2") * 0.25" * 30,000 lbs/in^2 (tensile strength for 316 without yielding) = 7,500 lbs.

Does it look right?

Many thanks!
Gleb

In a simplistic world yes...
But...
This is pure tension, statically loaded... You start adding dynamic flexural fatigue, take into consideration the stress induction of the pin hole, mounting holes, area reduction for chamfering, etc... maybe 2/3 of the 7.5klbs. Now don't forget your FS, 1.5x Together reduces the load capacity to = 3300 lb...

Of course this is overkill reduction wise but...

Thank you!

I thought the ultimate tensile strength was the worst case scenario, and by using the yield strength, we don't need to do use any further reductions?

70,000 should be the breaking strength of 316, so the breaking strength of 1.5"x0.25"x12 316 chainplate would be ~ 17,500 lbs, in which case, 7,500 lbs gives that >50% safety margin.

The topsides are 1/8" thick, so not sure if going with a bigger chainplate would make a lot of sense. Fiberglass/bolts will probably rip out first before the chainplate breaks.

Has anybody installed jordan drogue chainplates? Have you used a backing plate? Does it have to be steel or could I get away with starboard/aluminum or G10?

Many thanks!
Gleb

Variables
 

Hello,
Let's start with that shackle. The 9,000lb working load is, I believe, one-fifth of the rated break strength. So right away there's a bit of a strength discontinuity with the chainplate numbers you have.
Next, while the yield strength is an important detail, and needs to be considered when determining scantlings for a given material and application, ultimate tensile strength is of primary importance. Imagine a material with an extremely high yield strength. This will break very shortly after it reaches yield, so there is basically no reserve of strength before failure. by contrast a material with a low yield strength (like 316) has a huge reserve, and can be scaled such that the design load never approaches yield.
In your case, the strength of the proposed chainplates would approach 19,000lbs. This is still way short of your shackle strength, so the we need to ask, is the shackle correct, the chainplate correct, or neither? Which of course brings us tot the question we really need to start with: what is the load? This is followed by: What is the appropriate factor of safety? So far we only have vessel displacement and weight, which are related, but not nearly definitive.
Fair leads,
Brion Toss

Hi, Brion,

This is helpful - thank you!

From the drogue design notes, the peak load would be:

Peak line load for both chainplates: 10,000 lbs (displacement) * 80% = 8,000 lbs
Peak load per chainplate would be 8,000 lbs * 70% = 5,600 lbs (that's a breaking wave load on the quarter)
Working load limit in a severe storm = 8,000 lbs * 10% = 800 lbs.

The breaking load on the 1/2" pin 209a Crosby shackle according to the specs should be at least 3xWLL, so the shackle should break around the same load as the 1.5"x0.25"x12" chainplate =~ 18,000 lbs.

Now that I'm looking at these numbers, keeping in mind your note about yield vs ultimate strength, it may be that I don't need dedicated chainplates on the topsides and the cleat and eye pad may be able to handle all but the most severe conditions.



What do you think?

Thank you!
Gleb

Glen,

You are doing it backwards. The first question is what load does the drogue apply, then everything is sized to this. 800lbs may be perfectly sufficient, it may be highly conservative, or it may be incredibly low. Until you know the expected loads nothing else can be worked out.

In this case the peak load is 8,000lbs. The working load for the drogue is ignored, and everything needs to be sized to handle the 8,000lbs worst case scenario. Because of the way Jordan has engineered this however it is reasonable to step down the safety margin however. Where normally a life saving piece of gear should be 10:1 here it looks like in their worst case they have already assumed a substantial safety margin, and something more like 1.5 or 2:1 may be reasonable.

And keep in mind you can't just cut the expected load in half for each chainplate. First because they will be offset which even in a steady state will result in different loads, but for something like a drogue because there is a real likelyhood that one side may bear the brunt of a shock load substantially higher than half. I would like to design around each plate carrying the full load if I could. Giving complete redundancy, but I might be ok with each assuming 75% of worst case value.

So my ideal would be two chainplates each sized for 24,000lbs. If that was considered unreasonable then I would be willing to accept something down to a minimum of around 9,000lbs. Then size the rest of the gear to this. The fact that drogue plates are usually installed outboard and at deck level means that it isn't normal to have a problem with installing them. So oversizing is a pretty easy step to take, and no one has ever complained their gear was too strong.

Of course I am at best an amature, and would be happy to accept correction.

Hi, Stumble,

Sorry if I was unclear in my previous post - these numbers represent forces exerted by the drogue on the chainplates. The maximum force on the boat of this displacement would be 8,000 lbs (on both chainplates). These percentages - 80% and 70% - came from the design notes.

The maximum load on one bridle leg would be 70% of the total maximum load. So the absolute maximum load on one chainplate when a breaking wave hits on the quarter is 5600 lbs for a boat of this size. These calculations have already been done by the drogue designer. Assuming they're correct, the question is what kind of chainplate or attachment can handle this load...

The working load of the drogue, per designer, is 10% of the peak load - 800 lbs for both chainplates, so <560 lbs per bridle leg/chainplate under "normal" operation. In this case, normal means a "severe storm", so I'm assuming force 10-11: <= 64 knots of wind and up to 52 ft waves.

Basically, the question is this: how likely can the existing cleats/padeyes hold the peak load of 5600 lbs? Also, what is the best chainplate/shackle to work with a regular load of ~500 lbs and be able to handle a peak load of 5600 lbs?

Thank you!
Gleb

Clearer...
 

Hi again,
I am enjoying this conversation. Assuming that we need to design the gear around the peak load, what factor of safety shall we use? I am assuming at least 3:1, which would mean a break load for the chainplates of at least 6 tons, and preferably more, as you noted below. Not hard to get.
It is the proof load of that shackle that is about 3 times the work load; the breaking load is about 5 times, and industrial standard.
If your cleats have 4 fasteners of .25", you might get an average break of just under 7,000lbs (see http://www.boatus.org/findings/16/). What sizes are your fasteners? 5/8" single-braid nylon has a break strength of about 10,000lbs. Is that what you hope to use?
Fair leads,
Brion Toss

Brion,

I'm curious what if any weight you ascribe to the maximum peak load as determined by Jordan? I wouldn't quibble with 3:1 but I could also argue that 10;1 would really be proper.

It's also worth noting that current best practice is to use dyneema for a JSD not nylon. The reduced stretch helps keep chaff down, and the minimal stretch actually reduces shock load since the entire drogue picks up loads more evenly with less delay between when the first cones start pulling and when the last ones do. It also prevents the boat from accelerating down waves since load is applied much earlier in the wave cycle.

Of course this also means a smaller, lighter, and easier to handle unit, which is always nice. And on a price/MBL dyneema is actually about the same price as polyester. Of course it does demand splices instead of using a cleat, but that isn't a huge deal.

That's great! Yes, the cleat fasteners are 1/4". Unfortunately, there is no backing plate under the cleats, but the pad-eye has a SS backing plate of about 1/8" in thickness and 2.5" diameter.

From the Crosby specs for 209A shackles:

Maximum Proof Load is 2 times the Working Load Limit (metric tons). Minimum Ultimate Load is 4.5 times the Working Load Limit based on metric tons.

So the limits on the 7/16 209A Crosby shackle (1/2" pin) would be:

WLL: 2.7 tons ~= 6,000 lbs
Proof load: 5.4 tons ~= 12,000 lbs
Breaking load: 12 tons ~= 27,000 lbs

The jordan drogue specs call for 5/8" double braid nylon to be used as a bridle and lead line.

Not sure about the safety margin... Does the 3:1 safety margin mean that each part is designed to handle 3x maximum drogue load on the system? In this case, I doubt that the topsides or deck will hold 18,000 lbs. Is it worth considering a load greater than the boat's displacement or design a system around once in a lifetime occurrence?

Just wondering... To me, it looks like the cleats will hold with a pad-eye as a backup in all, but the most dire events, in which case something else fails before the cleats will. But it's still worth considering chainplates to avoid chafing, which, people say, can be huge in severe storm conditions.

Am I wrong? I'm curious how chainplates and rigging are usually designed.

Thank you!
Gleb

Details
 

Hi again,
First, I very much like the idea of using Spectra instead of nylon, though it is likely that I am simply inclined to use Spectra whenever possible. But in this application, I can see how it could reduce both shock loading and chafe. Stumble, any data on this?
If nylon is used, I would strongly recommend a single- instead of double-braid, as it has far superior energy-absorption properties (see Yale's paper on their Brait product for more on this).
I also agree that a 10:1 SF would be reassuring, but not practicable here, or at least not for every component. Gleb, I understand that the SF's we are discussing might seem high, but 18,000lbs is approximately what the shroud load is, with safety factor, for your boat. If indeed the deck structure and belays cannot support these loads, that level of strength might still be needed.
Bear in mind that it is vanishingly unlikely that your drogue will put a 9-ton stress on the hull, but (a) it could happen, especially in an accelerated load, so it is prudent to design around the possibility, and (b) the safety factor is also there to take some degree of materials degradation and flaws into account. Regarding (a), you ask if it is worth designing a system around a once-in-a-lifetime occurrence. I don't think that is the correct question. You want this drogue precisely because you think that odds are fairly good that you will be in circumstance where reefing or heaving-to will not suffice for survival, but you seem willing to bet that, horrible as the conditions might be, they won't be maximally horrible. I recommend making this system as strong as you can -- or can afford -- and hope that things never get as bad as they could be.
Fair leads,
Brion Toss

Brion,

I don't have the data, but Beth or Evan Leonard may. I assume you know them (or of them) but I know they did some of the testing on the JSD with Steve Dashew. Ever since then both Estar and the Dashew's are using Dyneema drogue lines.

There is also this from Donald Jordan (quoted from someone else, quoting him). When discussing nylon vs polyester in 1996.

"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 should also mention I have a huge preference for dyneema as well, and keep having to keep myself from using where it may not be sutable.

Thank you, Brion and Stumble - it all makes sense.

The cleat will definitely NOT hold 18,000 lbs...

Yesterday, I found this in a local rig shop for $75 a piece (2 plates)



It's CSJ-16-210 (stay extender) with dimensions 8.5"x1.5"x3/16". It has a WLL of 5900 lbs according to the manufacturer and about the name number in my calculations for a single plate holding the shackle. The breaking strength of one plate should be around 11,500... At least there is 2:1 safety factor.

Unfortunately, the holes are 5/8". Two of these can be installed with 3 bolts (every other hole), while using the other place in reverse as a backing plate.

What do you think?

For the dyneema vs nylon, I'll email oceanbrake people your suggestions. I think the primary reason for choosing nylon is cost, but will enquire about single braid.

Many thanks!
Gleb

P.S. Do you think a backing plate will be just as important? Would you choose a thicker plate without the backing plate? Can this be calculated? Is it possible to estimate how much load (in sheer strength) the top sides can handle? Not sure how to approach it.

Thanks!
Gleb

I wouldn't use those extenders. One solid piece is stronger than two plates due to load shedding issues. And frankly I don't think $75 is a very good price.personally I would go with a single wide plate, with the attachment point hanging just off the transom. So there is nothing that could contact the legs once installed. For a backing plate I would use G10 at least 1" wider than the plates, epoxy bonded to the hull, with fender washers backing the nuts.

Without knowing the layup schedule in this area I couldn't guess how to calculate the size (Brion probably does). But a wider strap provides more contact area and thus more friction, as well as spreading the load over a larger area. So the question in my eyes is what other constraints do you need to deal with? Is there other equipment that needs to be worked around, aesthetic issues that need to be adressed, or is the entire side of the hull available?

Thanks! I'm hoping to put chainplates on the blue strip, but the hull to deck joint may interfere - the bolts are sticking out quite a bit, at least an inch. The only opening to this hull area is through lazarette, and access may be problematic.



The only other consideration is using these chainplates for anchoring from the stern.

The hull thickness (top sides) is 1/8" - 3/16". Most likely 3/16" the way I remember it.

G10 seems like an excellent suggestion! I'm thinking 1/4" thick, 2" wider as you said.

So would 316 SS 1.5" x 3/8" x 12" be better than 2" x 1/4" x 12?

How about grade 2 titanium? SS 316 or grade 2 Ti? I'd be curious to play with titanium, but grade 5 is not practical cost wise.

Thank you!
Gleb

Why not 2"x3/8x12? Here bigger is always going to be better. And frankly the extra weight and cost would be minimal.

While G2 titanium is fine, you would need to resize the mess of it. G5 is substantially stronger, so much so that you can go much smaller and keep the same strength. So small in fact that when the company I used to work for has a set of chainplates redesigned for G5 everyone (including the engineer) decided they looked to small and put in bigger ones. They were probably fine, but it just looked wrong.

Give Allied Titanium a call. For a couple of 12" straps with no bends I would be suprized if G5 would really be that expensive. And at roughly four times stronger than 316, you could go silly small, if we assume a design load of 18,000lbs, in G5 you would need about .15 square inches of working metal total (ie the area to the side of the bolts). I would also recommend G5 bolts, just to prevent a dissimilar metal issue. But not required.

Here is a follow up. I got titanium chainplates - really cool stuff!



My shackle pin seems slightly less than 0.5" and AT made a hole slightly larger (0.520") to account for tolerance, etc., so in the end, there is almost a millimeter of free play.





Is there any shock load to worry about or it's too small? Do you think there is a way to calculate it?

Thanks,
Gleb

No worries
 

Hi Gleb,

I wouldn't worry about a shockload from the hole being slightly big. But I'm glad to see you decided on chainplates instead of the cleats, since the big issue with cleats is not with them but with leading the bridle (or dockline) for that matter, over the rail with no chafe. Many drogues and parachute anchors have been lost due to chafe, so the ability to lead a bridle fairly to your drogue is of great importance.
Ben