chainplate calculations for a jordan drogue

[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

[asdf777]

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

[Stumble]

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?

[asdf777]

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