chainplate calculations for a jordan drogue

[asdf777]

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

[Brion Toss]

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

[Stumble]

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.

[asdf777]

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

[Brion Toss]

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

[Stumble]

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.

[asdf777]

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